DESIGN AND CONSTRUCTION OF A THREE HUNDRED kA BREECH SIMULATION RAILGUN by BRETT D. SMITH, B.S. in M.E. A THESIS IN -ELECTRICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN ELECTRICAL ENGINEERING

Jtpproved

Accepted

December, 1989 ACKNOWLEDGMENTS

I would like to thank Dr. Magne Kristiansen for serving as the advisor for my grad­

uate work as well as serving as chairman of my thesis committee. I am also appreciative of the excellent working environment provided by Dr. Kristiansen at the laboratory. I

would also like to thank my other two thesis committee members, Dr. Lynn Hatfield and

Dr. Edgar O'Hair, for their advice on this thesis.

I would like to thank Greg Engel for his help and advice in the design and construc­

tion of the experiment. I am grateful to Lonnie Stephenson for his advice and hard work

in the construction of the experiment. I would also like to thank Danny Garcia, Mark

Crawford, Ellis Loree, Diana Loree, and Dan Reynolds for their help in various aspects

of this work.

I owe my deepest appreciation to my parents for their constant support and encour­

agement. The engineering advice obtained from my father proved to be invaluable.

11 CONTENTS

.. ACKNOWLEDGMENTS 11

ABSTRACT lV

LIST OF FIGURES v

CHAPTER

I. INTRODUCTION 1

II. THEORY OF RAILGUN OPERATION 4

III. DESIGN AND CONSTRUCTION OF MAX II 27

Railgun System Design 28

Electrical Design and Construction 42

Mechanical Design and Construction 59

Diagnostics Design and Construction 75 IV. MAX II OPERATION 89 v. EXPERIMENTAL RESULTS 97 VI. CONCLUSIONS 118

LIST OF REFERENCES 120

111 ABSTRACT

The objective of this project was to build a rail gun materials test bed which could be used for the relatively quick and inexpensive testing of promising rail and insulator materials. This report discusses the design, construction, operation and testing of the rail- gun facility. The facility is called MAX II which stands for Moving Arc Experiment II.

The basic design requirements of the railgun system were decided upon first. These requirements included, among others, a minimum peak rail linear current density of 300

.kNcm and projectile velocities in the range from 500-2,000 m/s. A crowbarred RLC cir­ cuit was chosen as the power system for driving the rail gun. A model of the power sys­ tem and the railgun ballistics was developed and used to determine appropriate system parameters. Each component of the system was then carefully designed to withstand the large magnetic forces which exist in the railgun system. Upon completion of the con- struction of the facility, a series of ten test shots were made in order to determine the abil- ity of the system to meet the design requirements. It was found that the system could meet these design requirements. Improvements to the system were also recommended.

The materials problems which presently plague railgun technology must be addressed in order for the use of rail guns to become practical. The testing of new rail and insulator materials is therefore essential in the development of rail gun technology. The

MAX II facility should prove to be a helpful tool in the testing and development of these new materials.

. IV LIST OF FIGURES

2.1 Railgun System Diagram 5

2.2 Ideal Railgun 7

2.3 Railgun Circuit Model 12

2.4 Railgun With RLC Power Circuit 14

2.5 Railgun With Crowbarred RLC Power Circuit 16

2.6 Rogowski Coil Diagram 20

2. 7 Rogowski Coil Circuit Model 21

2.8 Rogowski Coil Circuit Model With Integrator 23

2.9 B Dot Probe Configuration 25

3.1 Railgun Bore Configuration 30

3.2 Circuit Diagram For Simplified Model 35

3.3 Sample Model Armature Current 39 3.4 Sample Model Projectile Velocity 40

3.5 Sample Model Projectile Displacement 41

3.6 Picture Of Capacitor Bank 45

3. 7 Picture Of Capacitor Bank Fault Protection 46

3.8 Elementary Ignitron 47

3.9 lgnitron Trigger Generator Schematic 49

3.10 Ignitron Pulse Generator Schematic 50

3.11 Ignitron Trigger Circuit Block Diagram 51

v 3.12 Picture Of Voltage Probe And Pulse Transformer For Ignitron Trigger 52

3.13 Picture Of Trigger Generator And Pulse Generator 53

3.14 Ignitron Tube Configuration 54

3.15 Picture Of Ignitron Tube Setup 55

3.16 Schematic Of Parallel Plate Railgun Connections 57 3.17 Picture Of Parallel Plate Railgun Connections 58 3.18 Railgun System Equivalent Circuit 60

3.19 Simulated Capacitor Bank Voltage For VP(O) = 4.2 kV (Uncrowbarred) 61 3.20 Simulated Armature Current For VP(O) = 4.2 kV (Uncrowbarred) 62

3.21 Simulated Capacitor Bank Voltage For Vp(O) = 5.8 kV (Uncrowbarred) 63 3.22 Simulated Armature Current For Vp(O) = 5.8 kV (Uncrowbarred) 64 3.23 Simulated Capacitor Bank Voltage For VP(O) = 4.2 kV (Crowbarred) 65

3.24 Simulated Armature Current For VP(O) = 4.2 kV (Crowbarred) 66 3.25 Simulated Capacitor Bank Voltage For Vp(O) = 5.8 kV (Crowbarred) 67

3.26 Simulated Armature Current For Vp(O) = 5.8 kV (Crowbarred) 68

3.27 Drawing Of Muzzle End View Of Rail gun 70

3.28 Side View Picture Of Railgun 71 3.29 Rail And Insulator Sample Drawing 72

3.30 Picture Of Breech Flange 73

3.31 Picture Of Muzzle Flange 74

3.32 Picture Of Parallel Plate Support Structure 76

3.33 Picture Of Rogowski Coil 77

Vl 3.34 Integrator Simulation Circuit 79

3.35 Simulated Armature Current For Integrator Test 80

3.36 Simulated Measured Armature Current For Integrator Test 81

3.37 Picture Of B Dot Probes For Sensing Rail And Armature Current 82

3.38 Picture Of B Dot Probes For Sensing Armature Current 83

3.39 B Dot Probe Mounting Detail 84

3.40 Break-wire Schematic Representation 86

3.41 Break-wire Circuit 87

3.42 Picture Of Break-wire System 88

4.1 Picture Of Hydraulic Stud Tensioners 90

4.2 Picture Of Hydraulic Pump 91

4.3 Projectile Drawing 93

4.4 Picture Of Projectile With Fuse Strip In Place 94

4.5 Picture Of Catch Tank 96

5.1 Capacitor Bank Voltage Trace For Fourth Shot 98

5.2 Armature Current Trace For Fourth Shot 99

5.3 Armature Current Trace For Fifth Shot 101

5.4 Composite Armature Current And B Dot Traces For Fifth Shot 102

5.5 Break-wire Trace For Seventh Shot 103

5.6 Armature Current Trace For Ninth Shot 105

5.7 Composite Armature Current And B Dot Traces For Ninth Shot 106

5.8 Break-wire Trace For Ninth Shot 107

• 0 Vll 5.9 Armature Current Trace For Tenth Shot 109

5.10 B Dot Probe #1 Trace For Tenth Shot 110

5.11 B Dot Probe #2 Trace For Tenth Shot 111

5.12 B Dot Probe #3 Trace For Tenth Shot 112

5.13 Composite Armature Current And B Dot Traces For Tenth Shot 113

5.14 Break-wire Trace For Tenth Shot 114

Vlll CHAPTER I

INTRODUCfiON

The use of electromagnetic forces to accelerate projectiles is an attractive technol­ ogy with several practical applications. The railgun is one of these applications. The rail- ..... gun uses the Lorentz (J x -B) force to accelerate projectiles, unlike conventional guns which use the expansion of combustion products. Because of inherent velocity limits in the expansion of gases, projectiles fired from conventional guns are limited in velocity to the 500-1300 m/s range [1]. Railguns, however, do not suffer from this limitation.

Muzzle velocities as high as 20,000-50,000 m/s have been theorized as obtainable while velocities in the 6,000-7,000 m/s range have been reported [2]. The relatively high veloci­ ties obtainable by railguns make them a promising means for the advancement of gun technology.

Problems, however, have been encountered in the development of rail guns. One such problem is poor efficiency. It has been shown that under ideal conditions only half of the energy delivered to the railgun goes into kinetic energy of the projectile [3] while experimental results show typic-al efficiencies of existing railguns to be less than 30 per cent (usually less than 15 per cent). This poor efficiency has hurt the chances of rail guns to be deployed in space as part of the SDI program.

Another problem encountered in the development of rail guns is that the high veloci­ ties predicted early in the development have not been achieved. One explanation offered

1 2

on this problem described the arc restrike well behind the projectile as the major cause of velocity restrictions [2]. This arc restrike is in tum blamed on bore material ablation.

There are, however, promising approaches under investigation for resolution of this prob­ lem.

The bore materials, two conducting rails and two side wall insulators, used in rail­ guns present another problem which needs to be solved before railgun technology can

become practical. The railgun bore environment is a harsh one. Bore pressures can be as high as 414 MPa (60,000 psi) with plasma temperatures as high as 30,000° K [2]. Materi­

als used in railgun bores must not only withstand mechanical and thermal loadings induced by this environment, but must also retain good electrical properties during

operation. Material ablation causes the need for most existing railgun bores to be either

honed or taken apart and cleaned after only a very few shots. This mode of operation is not satisfactory for practical use. New materials must be found which can survive the mechanical loads while resisting material ablation and maintaining good electrical prop­ erties.

Work is being done on understanding the thermal and chemical processes involved in material ablation and the roles of key material properties are being investigated [4, 5].

Metallurgists and ceramicists are working on the development of new materials which can withstand the railgun bore environment [6]. The culmination of this work will require testing of materials in the bore environment to verify the results .

.. 3

The MAX II (moving arc experiment) facility has been designed and constructed for use as a railgun bore materials test bed. This small railgun is designed for the rela­ tively quick and inexpensive testing of new rail and insulator materials. The following

chapters outline the basic theory of rail guns, discuss the design, construction, and

operation of the MAX II facility, and present testing results obtained from several MAX

II shots. CHAPTER II

THEORY OF RAILGUN OPERATION

A simple diagram of a railgun system is shown in Fig. 2.1. This diagram shows the current flow in the loop which induces the B field. Current flow through the armature is in the negative y direction while the induced B field is in the negative z direction. The

Lorentz force on the armature can be written as

.....J xB- = -F, where ..... J=-Jy

and

-B =-Bz.

Therefore,

..... J X -B = (-Jy) x (-Bz) = JBx. (2.1)

Equation (2.1) shows that the force on the armature has magnitude JB and direction x.

Thus the armature is accelerated by the Lorentz force in the x direction. This simple

description of railgun operation does not lend itself to calculations, however, because of ..... the difficulty in calculating J and -B due to the geometry of the system. A more useful

description can be found from energy considerations.

4 ~·

.;.

,( t = 0 ARMATURE ------~

LAUNCH PACKAGE

ENERGY 1 I __, ( .. F = J X B STORAGE -~- 0__, \ K B I RAILGUN zLx

Fig. 2.1. Railgun System Diagram

Vl 6

A force F is directly related to the change of energy, E, with position, x, by the well known relation

dE -F=­dx. (2.2)

The energy stored by the railgun magnetic field is given by

(2.3) where 4 is the railgun inductance and I is the armature current. Consider the three­ dimensional case depicted in Fig. 2.2. Assume a current, I, is flowing and all conductors have zero resistance. The problem [7] can be divided into three regions. Regions I and III are assumed to have fixed lengths. The fields in these regions are complicated. However, they can be characterized by fixed inductances L1 and Lm. Region II is chosen so that the magnetic field distribution is identical at all axial points. The inductance af region II can be characterized by

(2.4) where d4/dx is a constant. The total gun inductance, 4, can now be written as d4 Lp_ = ~+ Lm+ dx X. (2.5)

Substituting Eq's. (2.5) and (2.3) into Eq. (2.2) gives

-FL =![~(~+Lm+ ~x)~ 7

II __ _y I t::l w X ® -L...

c: ::I bl,) ·--~ ~ -~ 0 "'0...... ~ N . - X bl,) 0 t ! ·-~ -

t::l w X G L...

(I') ~ D I- 1- uu W:J ~.....~ ~z WD o...u 8

-F =--(~11 d 2)+--(Lml 1 d 2 )+-- 1 d (~--xi 2J L 2dx 2dx 2dx dx

di di 1d~ 2 ~ di =~1-+Lml-+--1 +-xl- dx dx 2dx dx dx

=--11~ 2 +1-dl(~+Lm+-x ~ J (2.6) 2dx dx dx ' where FL is the force on the armature. The equation for the flux, , of this system can be written as

(2.7)

Because flux is conserved, taking the derivative of Eq. (2.7) with respect to x yields

di di d4 ~dl (2.8) 0=~-+Lm-+-1+-x-dx dx dx dx dx.

Solving Eq. (2.8) for dl/dx gives

di -d4 I (2.9) dx = dx (~+Lm+ :"x}

Substituting Eq. (2.9) into Eq. (2.6) gives

(2.10)

Equation (2.1 0) can be rewritten as 9

F =~~12 (2.11) L 2 dx .

Equation (2.11) is the most commonly used force equation in the design of rail guns.

Knowledge of the forces which act on the armature provides the information neces­ sary to calculate other parameters of interest, such as the acceleration, velocity and posi­ tion of the armature as a function of time. Equations for these parameters are easily derived from Newton's First Law and simple kinematics. Newton's First Law states that

2.Fext- =rna'- (2.12)

. where 2. -F ext represents all external forces on the launch package, ais the acceleration of the launch package and m represents the total mass being accelerated. The X. component of Eq. (2.12) implies that

~ Fextx ~=~-. (2.13) m

Acceleration, a, is defined such that

(2.14)

Velocity, v, is defined such that

dx v=-- dt. (2.15)

Equation (2.14) can be manipulated to give 10

t v(t) adt = f. dv. (2.16) l0 v(O) Equation (2.16) can be simplified to show that

v(t) =v(O) + f adt. (2.17)

Equation (2.15) can be manipulated to give

t f. x(t) vdt = dx. (2.18) l0 x(O) Equation (2.18) can be simplified to show that

x(t) = x(O) + f vdt. (2.19)

Substitution ofEq. (2.17) into Eq. (2.19) gives

x(t) =x(O) + llv(O) + f adt}t

x(t) =x(O)+v(O)t+ Llf adt}t. (2.20)

Equations (2.11 ), (2.13), (2.17), and (2.20) provide a model of the ballistics of rail guns

but cannot be solved without knowledge of the armature current in Eq. (2.11 ).

Modeling of the electrical circuit which delivers power to the rail gun is essential in determining the launch package accelerating force, FL, described in Eq. (2.11). The 11

railgun itself affects the operation of the power circuit because it has distributed resistances and inductances. In fact, the amount of rail gun resistance and inductance con­ nected to the power circuit increases with launch package position in a time-varying man­ ner dependent on the launch package velocity. As the launch package proceeds down the rails, the railgun current loop contains more rail length thus providing more resistance and more inductance. The time-varying resistance and inductance of the rail gun can be modeled as [3]

(2.21)

~(x) =~ 0 + L'x, (2.22) where RR(x) and ~(x) are the total gun resistance and total gun inductance as a function of x, respectively. The terms RRo and ~0 represent the initial railgun resistance and inductance, respectively, due primarily to circuit connections at the breech end of the gun. These terms will be neglected in the following analysis. The physical significance of

R' and L' are, respectively, the railgun bore resistance per unit length and the railgun bore inductance per unit length. That is

dR R' =___! (2.23) dx and ,=d4 L dx. (2.24)

A complete circuit model for the railgun is shown in Fig. 2.3 [3]. The equation for the breech voltage, V b' is given by 12

I u· 0!

- ~ ·--:::3 ~ u·- c :::3 b.O ·--~ ~

>SJ 13

di d4 vb =~dt + ldt+ IRR + v arc· (2.25)

The ~/dt term can be rewritten as

-=-- (2.26) dt dx dt.

Substituting Eq's. (2.15) and (2.24) into (2.26) gives

d~ ' -=Lvdt . (2.27)

Substituting Eq's. (2.27), (2.21), and (2.22) into Eq. (2.25) gives

, dl ' IR' V V b =L X dt + IL V + X + arc• (2.28)

Equation (2.28) can be used as the railgun load representation for any electrical power circuit used to drive the railgun, thus providing an equation for the railgun armature cur- rent.

The railgun power delivery circuit may take many forms. Pulse forming networks can be designed to deliver a constant current to the rail gun [8]. A much simpler approach is the use of an RLC circuit to supply power to the railgun. A circuit diagram of this sys­ tem is shown in Fig. 2.4. The capacitor, CP, represents an energy storage capacitor bank while ~ and LP represent the distributed resistance and inductance inherent in the capacitor bank and connections to the rail gun. The loop equation for this diagram is

1 di V (0) --ll ldt - L - - R I - V = 0 (2.29) p ~ 0 p dt ... 'p b • 14

.s:J >

Q.. ...J . bO ~ ...... ·-

a. u I I L------,~1---· ----- "-----/ ,...... 0...... , a. > 15

Substituting Eq. (2.28) into (2.29) and rearranging terms gives

-C1 l t Idt+ L P-ddi +~I+Lx-+ILv+I di ' p 0 t dt

IR'x + V arc- VP(O) = 0. (2.30)

This equation can be used to solve for the armature current Because this circuit is usually underdamped, a modification is sometimes made as depicted in Fig. 2.5. The crowbar switch is closed when peak current is reached. This eliminates the ringing of the armature current and greatly reduces ringing of the capacitor bank. There are two advantages to this type of system. One is that reduced ringing of the capacitor bank increases the life­ time of the bank. The other advantage is that by eliminating rail gun voltage reversal, rail erosion polarity effects may be studied. If the switch is closed at time t = lcbr, the resulting loop equations for t > 1cbr are

1 (t dl1 dl1 C, Jo 11dt + Lp1 dt + ~tl1 + Rcl1 + Lc dt - Rcl2-

dl2 Lc dt- V p(tcbr) =0 (2.31) and

(2.32) where Lc and Rc represent the distributed inductances and resistances of the crowbar switch, ~ 1 and ~ 1 represent the distributed inductances and resistances in the capacitor bank loop, and LP2 and RP2 represent the distributed inductances and resistances in the L R R Pl p2 p2

I I 12

t = teaR v p (Q) cP vb(

Fig. 2.5. Railgun With Crowbarred RLC Power Circuit

~ 0\ 17

connections between the crowbar switch and the railgun. Substitution of Eq. (2.28) into Eq. (2.32) gives

(2.33)

The initial conditions for Eq's. (2.31) and (2.33) would be the conditions existing in Eq.

(2.30) when the switch is closed. The solution ofEq's. (2.30), (2.31), and (2.33) will pro­ vide an equation for the armature current, I(t).

The complete model of the ballistics of the rail gun launch process includes equa­ tions for railgun armature current, like Eq's. (2.30), (2.31), and (2.33), along with Eq's.

(2.11 ), (2.13 ), (2.17) and (2.20). These equations must be solved simultaneously to provide a picture of the launch process. Several terms involved in these equations require further explanation. The L.Fexu term in Eq. (2.13) represents the vector sum of all forces acting on the launch package in the x direction. This includes not only FL(t), described by

Eq. (2.11), but also friction forces from projectile-bore contact as well as viscous drag forces on the projectile. These forces are usually small compared to FL(t) at low velocities and are sometimes neglected. The mass term, m, in Eq. (2.13) is, in general, not constant because of arc interaction with the bore, causing the effective mass to increase. The Varc term in Eq's. (2.30) and (2.33) also varies with time due to plasma dynamics in the arma­ ture. It is clear that the model represented by Eq' s. (2.30), (2. 31 ), (2. 3 3 ), (2.11 ), (2.13 ), 18

(2.17), and (2.20) can be made very complex by including these second order effects.

Computer models do exist, however, which include some of these effects in the analysis [9].

One aspect of the rail gun launch process which this model does not address is the forces induced on the railgun bore containment structure. Magnetic forces as well as ther­ mally induced pressure forces act to push the bore components apart. Exact values of these forces are hard to predict but bore pressures can be estimated using the equation [7]

(2.34) where h is the height of the projectile and w is the width of the projectile. It can be seen that Eq. (2.34) is simply FL of Eq. (2.11) divided by the cross-sectional area of the projec- tile. That is, P represents the magnetic pressure acting on the launch package. It can be shown [7) that the actual pressure acting on the rails is less than that acting on the launch package. Many designers, however, assume that the pressure acting on the projectile acts uniformly on all bore components. This results in a conservative design of the railgun bore containment structure.

One final aspect of the theory of rail gun operation which requires discussion is diagnostic methods available for measuring important railgun operating parameters.

Breech voltage, muzzle voltage, annature current, and projectile velocity are commonly measured quantities. Breech and muzzle voltage measurements can be accomplished with 19

standard voltage divider schemes. Armature current is often times measured with a

Rogowski coil. B dot probes or optical probes are generally used to measure in bore velocity. Rogowski coil and B dot probe operation will be discussed in some detail.

A simple diagram representing Rogowski coil operation is shown in Fig. 2.6. A lumped element circuit model of the Rogowski coil is shown in Fig. 2. 7. The induced voltage in the coil, V ind' can be written using Maxwell's equations as [10]

(2.35) where r is the major radius of the torus, N is the number of turns on the coil, A is the cross-sectional area of one turn of the coil, and I is the current to be measured. The indue- tance L of the coil is

(2.36) where I is the length of the coil (2m for this case). The resistance, R, in Fig. 2.7 is the oscilloscope input impedance or some lumped resistance placed across the coil. The loop equation for the circuit of Fig. 2. 7 is

dVout V out di --+-=-a-, (2.37) dt 't dt where

(2.38) 20

(.I')UDO..W

-0 ·-u

. b.O ·-u..

lt:Q 21

1- ::> D > r

G) "8- ~ ·--::s ~ -·-u ·-u0 ~ ·-V,) ~ _j 0 b.O 0 ~. r--: ("''. b.O ·-u..

-0z > 22

and

Rl a= 2mN· (2.39)

Depending on relative values of 't and the pulse shape of I, the output voltage, V out' may take the form of [ 10]

di v out--y-dt (2.40) or

V out= yl(t) ' (2.41) where yrepresents some constant. A coil with output voltage like that in Eq. (2.41) is called a self integrating coil. Coils with output voltages like that in Eq. (2.40) are called differentiating coils. The main variable which causes the Rogowski coil to act as a differ­ entiating or integrating coil is the pulse width of the current pulse being measured. For current pulse widths much smaller than t, the coil is self integrating. For current pulse widths much longer than 't, the coil will differentiate the signal. The output voltage of differentiating coils must be integrated in order to obtain a signal proportional to the cur­ rent being measured. A circuit diagram of a Rogowski coil with an RC integrator is shown in Fig. 2.8. For cases where Lis small and can be neglected, the node equation for

Voutcan be written as

V out - V ind dV out V out dV out + ci d + o + cs d =o , (2.42) Ri t ~~ t L R

v IND c, cs Rs V OUT

Fig. 2.8. Rogowski Coil Circuit Model With Integrator

N v.,) 24

where Ri and Ci represent the integrator resistance and capacitance and R5 and C5 repre­ sent the oscilloscope resistance and capacitance. Solution of Eq. (2.42) gives

(2.43) where

(2.44) and

(2.45)

These equations can be used to design the integrator so that the output voltage of the

Rogowski coil will be proportional to the measured current.

The B dot (B)• probes work much the same way as the differentiating Rogowski coil. The output voltage of the B dot probe takes the form of Eq. (2.40). Figure 2.9 shows one possible B dot probe configuration. As the armature passes the B dot probes, the large dB/dt induces a large voltage in the B dot loop which is monitored by the scope.

Because the large dB/dt occurs at the time that the armature passes, the induced voltage will be large at this time. Thus, with knowledge of the location of the probes, a discrete representation of x(t) can be obtained. The average velocity between B dot probes can be calculated by the expression 25

w t:Q D ~ 0..

c 0 ·--r: =bO ~c u0 0 .0 .....J .....J w Q.. ct <[ <[ - - D 0 ~ ~ u - V) 0 ~ X . 0\ ~ bi> ·-~ 26

~ (2.46) vave = ~t where & is the distance between probes, and ~t is the time between the peak v out of the two probes.

The equations developed in this section form the theoretical basis of rail gun opera­ tion. They will be used in the next section to design the railgun, the railgun power circuit, and the railgun diagnostics. CHAPTER III

DESIGN AND CONSTRUCfiON OF MAX II

The MAX II facility was designed and constructed for use in testing new rail and insulator materials which show promise as bore components for rail guns. Design require­ ments and specifications for the system are listed below.

(1) The system is to be capable of accelerating both free running arcs and projec­

tiles with plasma armatures.

(2) The peak rail linear current density is to be at least 300 kA/cm.

(3) Velocities in the range from 500-2,000 m/s are to be obtainable.

(4) The system must include diagnostics capable of in-bore velocity measurement,

armature current measurement, and muzzle velocity measurement.

(5) The railgun is to be no more than 8" in length.

(6) The railgun must adequately support brittle sidewall insulator material test spec­

Imens.

(7) No more than one day should be required to disassemble and then reassemble

the system (i.e., one day tum around between shots).

27 28

(8) The system should be able to accommodate the future addition of a preinjection

gun, and future operation in a repetitive mode.

The following discussion details the railgun system design, the electrical design and construction, the mechanical design and construction, and the design and construction of the diagnostics for the MAX IT facility.

Rail~un System Design

The railgun system design was accomplished using the model developed in chapter

II. A crowbarred RLC circuit was chosen for use as the rail gun power delivery circuit. A pulse forming network was considered, but this circuit was rejected because of the large energy dissipation requirements of the matching resistance. Equations (2.11), (2.13),

(2.17), (2.20), (2.30), (2.31), and (2.33) represent, then, the system model used in this design.

Certain design parameters were fixed by the design requirements and specifications listed previously. These included peak rail linear current density, the length of the gun, and muzzle velocity. The capacitance of the energy storage capacitor bank, CP' was also fixed by the availability of capacitors at 4.25 mF. These were, however, only a few of the design parameters which had to be determined.

The first design parameter which had to be decided upon was bore geometry. There are several different bore geometries currently used in railguns. These geometries can be 29

divided into two main classes, namely, round bores and square bores. A square bore geometry was chosen because it provides for easier fabrication of rails and insulators.

The square bore geometry, in tum, has several different possible rail and insulator config­ urations. The specific configuration chosen is depicted in Fig. 3.1. This bore geometry was specifically designed for use with brittle insulator materials [9]. This geometry is also very similar to that which was used in the CHECMATE gun at Maxwell Laborato­ ries, Inc. [11].

A peak bore pressure of 45 ksi was chosen for the MAX IT facility. This pressure is representative of present day railguns [9, 11] thus providing the rail and insulator test

specimens with a realistic exposure to the railgun bore environment.

The height, h, and width, w, of the bore had to be decided upon in conjunction with

bore pressure considerations, rail and insulator size considerations, as well as peak rail

linear current density considerations. Through an iterative process, the values h = w = 1

em were decided upon.

The inductance gradient, L', is entirely a geometry dependent quantity. The indue-

tance gradient for similar bore geometries [9] was calculated to be 0.4 J.LH/m and this

value was used in the design of the MAX II facility. Equation (2.34) can now be used to

calculate the peak armature current, IP.

1 ~ P =45ksi=310 264 OOOPa=-L'­ ' ' 2 h·w 30

INSULATORS

RAIL h

w

Fig. 3.1. Railgun Bore Configuration 31

~H 1 p =0.5 . 0.4 . 10 - . I;2 . . m O.Olm · O.Olm

Solution of this equation yields~=:: 400 kA. The peak value of rail linear current density can now be calculated as

1 ( eak) = ~ = 400kA . r p w 1cm

This value clearly satisfies the peak rail linear current density specification of a minimum of300kNcm.

With the aforementioned design parameters fixed, simplifications of the railgun launch model were considered. Because of the relatively low muzzle velocities required

(500-2,000 rn/s), the :LFext term in Eq. (2.13) was taken to include only the -J x -B force,

Fv That is, friction and viscous drag forces were neglected. Equation (2.28) was simpli­ fied by assuming that

I L'x =L'..! 2 and

I R'x =R'..!, 2

where 1 is the total length of the gun. The justification for this simplification was the fact 8

that, because of the short length of the gun, the total gun inductance, L'l8, and the total

gun resistance, R'/ , were much smaller than the LP and~ values of Eq's. (2.30}, (2.31), 8 32

and (2.33). An average value of the gun inductance and gun resistance was therefore taken. The V arc term was modeled as a resistance, R.rc· The value of Rare was taken to be

3 mn [12]. The L'v term in Eq. (2.28) has a maximum value, for Vmax = 2 km/s, of

L'v =0.4 · 10~H · 2000m =0.8 mn. m s

This value was found to be less than (at most, 15 per cent of) the total system resistance (~ + R' ~ + R.n,) and was therefore assumed negligible. With these simplifications, Eq. (2.28) can be rewritten as

v = L'lgdl + T(R' 18 + R J. (3.1) b 2 dt \ 2 arc

Substituting Eq. (3.1) into (2.29) gives

V (0)--1 it ldt-L --di R 1-LI ---1lg di ( R I -+Rlg J= 0 . (3.2) p c;, 0 p dt .. "p 2 dt 2 arc

Rearranging the terms in Eq. (3.2) gives

1 18 18 _!_ ( ldt+(L +L' )dl +(~+R' +R Jl-VP(O)=O. (3.3) c;,Jo P 2 dt 2 arc

Similar substitution into Eq. (2.32) gives

Rearranging terms in Eq. (3.4) gives

(3.5) 33

With these simplifications, Eq's. (2.30) and (2.33) take the form of Eq's. (3.3) and (3.5).

By assuming that the crowbar switch is ideal (Rc = Lc = 0), and that LP2 = ~ and RP2 = RP (i.e., the crowbar switch is connected very close to the capacitor bank), Eq. (3.5) can be rewritten as

(3.6)

By assuming that Rc = Lc = 0, the need for Eq. (2.31) is eliminated. The simplified model

of the railgun launch process can now be written as

(3.7)

(3.8)

v=v(O)+ fadt (3.9)

x = x(O) + v(O)t + lll' adt}t (3.10)

18 18 _!_ (tidt+(L +L' )di +(~+R' +R JI-VP(O)=O (3.11) C., Jo P 2 dt 2 arc

(3.12) G'~ + Lp): + ( ~ + R'~ + R~} = 0'

where Eq. (3.11) governs I until the crowbar switch is closed and Eq. (3.12) governs I

after the switch is closed. The circuit diagram represented by Eq's. (3.11) and (3.12) is 34

shown in Fig. 3.2. Equations (3.7) - (3.12) form a set of uncoupled linear integra­ differential equations which can be easily solved. The equations for I(t), v(t), and x(t) derived from the model are

I(t) = e-Ql[A sin(rodt)] , forO< t < tcbr, (3.13) where

(3.14)

(3.15)

(3.15)

VP(O) (3.17) A=~, ---1-r ~ cp 4

and

(3.18)

I(t) =~eH'''t), for tcbr < t < oo, (3.19)

where (3.20) I

Lt

t = tcbr v p (0) Cp Rt

Fig. 3.2. Circuit Diagram For Simplified Model

V.) Vl 36

4 t=- (3.21) ~'

(3.22) and

(3.23)

Cfi ~ (e-2at - 1)il ~ , 1c or 0 <_ t - < tcbr. (3.24) a

(3.25)

2 x(t)=x(O)+v(O)t+-L' ( A J{ ( -2a J[(-2asin(rodt)- 2 m 4a2 + 4rod 2 4 a 2 + 4 rod2

rode-2m[ -2a sin(2rodt)- 2rod cos(2rodt)] + 2ro~ 4a2 +4ro~

2 2 (J)d 2at (J)d +-(e- -1) +-t} , for 0 < t < tcbr. (3.26) 2a2 a 37

for tcbr < t < oo. (3.27)

A computer program was written, implementing Eq's. (3.13)- (3.27). The input

parameters included L', m, v(O), VP(O), CP, L1, and~. As discussed earlier, L' and CP were fixed. The initial projectile velocity was taken as 0 (i.e., projectile starts from rest).

The maximum allowable voltage on the capacitor bank limited V P(O) to a maximum of 11 k V. Assuming the use of Lexan projectiles from ( 1 em x 1 em x 1 em) to ( 1 em x 1 em x

2 em) in size, the mass of the projectile was limited to the 1.2 - 2.4 g range. The loop resistance, ~' represents the stray resistance in the power circuit and the railgun. The loop inductance, L., represents the stray inductance of the power circuit and the rail gun and any lumped inductance required to obtain the desired pulse shape. The stray indue- tance and resistance of the power circuit, as will be discussed in the next section, were taken to be approximately 472 nH and 5.4 mn, respectively.

The computer program was run for several different combinations of the input parameters. The purpose was to insure that the railgun system could meet the peak cur­ rent requirements (lp = 400 kA), the velocity requirements (500-2000 rn/s}, and that the current pulse width would be such that the projectile would exit the gun as the current pulse went to 0 in order to prevent excessive arc burning at the muzzle of the gun. 38

Sample outputs from the computer program are shown in Fig's. (3.3), (3.4), and

(3.5). The ends of the traces correspond to the exit of the projectile from the 8" (20.31 em) long gun. The input data for these sample outputs were as follows:

L' 0.4~ = m

m=2.4g

v(O) =0

Vp(O) =5.8kV

CP=4.25mF

4=472nH

~=5.4mil.

These figures proved that the rail gun system was capable of meeting the peak current, velocity, and pulse width criteria discussed previously. Higher predicted velocities can be obtained by reducing the projectile mass. The price paid for this, however, is increased arc burning at the muzzle end of the gun due to the larger currents at the time of projectile exit. It was also shown that no lumped inductance was required for the railgun system. 400 ----

350

300 rl ~ L...l 1- 250 z w ~ ~ :J u 200 w ~ ~ ~ 150 ::';! ~ c{ 100

50

0 ~--~--~--~--~--~------~--~ 0 40 80 120 160 200 240 280

TIME [MICRO-SEC.]

Fig. 3.3. Sample Model Armature Current

w \0 1

0.9

0.8

0.7 r-1., 0.6 E ".::i. L-J 0.5 ~ u 0 w_J 0.4 > 0.3

0.2

0.1

0 r ------.- ., .-- -.-· 1 0 40 80 120 160 200 240 280 TIME [MICRO-SEC.]

Fig. 3.4. Sample Model Projectile Velocity

~ 0 21 ------·-·------~ ---- -· ------20 19 18 17 16 15 14 13 12 r-1 E 11 0 L-J 10 X 9 8 7 6 5 4 3 2 1 0 0 40 80 120 160 200 240 280 TIME [MICRO-SEC.]

Fig. 3.5. Sample Model Projectile Displacement

~ ...... 42

Electrical Design and Construction

The basic elements of the electrical power system are the power supply, the capaci­ tor bank, the crowbar switch, the railgun, and the connections between the elements.

With the basic rail gun parameters defined in the previous section, a detailed design of the electrical system was undertaken.

The power supply used for charging the capacitor bank is a 2 A- 10 kV constant current supply. This power supply uses a three-phase monocyclic network to obtain con­ stant current. The constant current supply is preferable to a constant voltage supply because of better efficiency and shorter charging time. The equation for the charging time of the capacitor bank can be written as

(3.28) where tm is the charging time and I, is the supply current. For example, with V p(O) = 5800 V, CP = 4.25 mF, and Is= 2 A, the charging time, tch, is 12.325 s.

The capacitor bank consists of five Maxwell 50 kJ capacitors, model #32316. The measured capacitance, C, of the five capacitors is approximately 850 JlF per can. The measured internal inductance of each capacitor, Leap' is approximately 40 nH per can. The parallel combination of these capacitors yields an equivalent total capacitance, C.,, of 4.25 mF in series with an 8 nH inductor. The capacitors are rated at 11 kV with currents 43

limited to 150 k.A per can. In order to insure that these two ratings were not exceeded during operation, a fault protection scheme was developed.

Fuses placed in series with the capacitors are often used to protect against over­ current operation. An inexpensive fuse can consist of a number of coaxial cables con­ nected in parallel. The cables act not only as an inductive and resistive current limiter, but also as a fuse in that the cables will blow apart, because of magnetic forces, at a certain current. RG - 8 cables have been known to blow apart at 25 k.A [ 13]. Therefore, four RG

- 8 cables were used to connect each capacitor to the crowbar switch in order to prevent over-current operation.

Over-voltage protection of the capacitor bank was achieved by placing spark gaps across the capacitor bank which were designed to break down at voltages lower than the rated voltage. Two gaps, one set to break at 9.5 kV and one set to break at 10.5 kV were placed across the capacitor bank with water resistors placed in series with each gap. The water resistors act to limit current in the gaps to acceptable levels.

A dump relay in series with the water resistors was also placed across the capacitor bank (i.e., in parallel with the spark gaps) so that the bank energy could be dumped if desired. Grounding sticks, one connected as a short, the other connected in series with a water resistor, were also provided as a backup dump system for the bank. 44

Figure 3.6 shows the capacitor bank with the RG - 8 cables connected and the

grounding sticks in place. Figure 3.7 shows the water resistors, the dump relay, and the over-voltage protection spark gaps.

As stated previously, four RG- 8 coaxial cables were used to connect each capaci­

tor to the crowbar switch. The resistance of the cables was found to be approximately

3 rn.{lfft and the inductance of the cables was found to be approximately 80 nH/ft. With 4

of these cables in parallel, each approximately 6' in length, the total inductance, Lcabt' and resistance, Rab1, between each capacitor and the crowbar switch was calculated to be 120

nH and 4.5 m.Q, respectively.

A size E ignitron was used as the crowbar switch. The NL - 1057 ignitron is made

by Richardson Electronics. The ignitron [14] consists of an anode, a mercury pool cath­

ode, and an ignitor pin in contact with the mercury pool cathode (see Fig. 3.8). When a voltage is applied between the ignitor pin and the cathode, mercury vapor is generated

and a mercury glow discharge forms between the ignitor pin and the mercury pool cath­

ode. While this condition exists, the presence of a positive voltage between anode and

cathode of approximately 100 V will cause ionized mercury vapor to fill the tube and

allow conduction to occur.

The ignitron is fired by pulsing the ignitor pin with approximately 1 - 3 kV when a

positive voltage(= 100V) exists between anode and cathode. In order to sense the voltage

between anode and cathode, a Tektronix P- 6015 lOOOX voltage divider was used. The 45

...... c ·-u ~ 0.. uco; (...... , 0 ~ .....~ u 0: '1:3 ('f) u:on

. .. • SPARK GAP

Fig. 3.7. Picture Of Capacitor Bank Fault Protection

~ 0\ 47

1------ANODE TERMINAL

11 n I

r- -U

1------ANODE SEAL

It------METAL ENVELOPE

ANODE ------+t----l

/ ~--+----- luNITOR \ .., MERCURY POOL------++-- I I - - ( I l I Ill I I Ill I I I I I &r------luNITOR TERMINAL CATHODE TERMINAL ------/

Fig. 3.8. Elementary Ignitron 48

signal from this voltage probe was input into a comparator circuit (see Fig. 3.9). The ref­ erence voltage of the comparator circuit is adjustable, allowing the user to vary the igni­ tron anode-cathode voltage at which the ignitron is to be fired. The slope of the signal at

which to fire the ignitron can also be set by the user ( + or -). When the prescribed

anode-cathode voltage and slope of the signal exists, the comparator circuit supplies a

trigger signal to the pulse generator. A schematic for the pulse generator is shown in Fig.

3.1 0. The amplitude of the voltage pulse from the pulse generator is adjustable also. The

output of the pulse generator is connected to a pulse transformer for isolation purposes.

The output of the pulse transformer is connected between the trigger pin and the cathode.

A block diagram for this circuit is shown in Fig. 3.11. The Tektronix voltage probe and

pulse transformer are shown in Fig. 3.12. The comparator circuit (trigger generator) and

pulse generator are shown in Fig. 3.13.

The ignitron tube was configured into a coaxial geometry in order to maintain uni­

form fields within the tube and to limit the inductance of the tube stand. A schematic

representation of the ignitron tube configuration is shown in Fig. 3.14. A picture of the

actual ignitron tube setup is shown in Fig. 3.15. The calculated resistance of the ignitron

tube and stand, Rc, is 1.3 mil. The inductance of the ignitron tube and stand, Lc, was cal­

culated to be approximately 190 nH.

Twenty parallel RG - 8 cables, each approximately 5' in length, were used to con­

nect the crowbar switch to the rail gun connections. Using the resistance and inductance SHi&J. COUP! NG U!l1II.. DlGG.EI TERI'!IINADON llEAn.I WJ.Illl[ l!illll;_rr:ACIDC 611K 4.1K +5V .!llfI 1 ls~~~~ +UV UK 2200 I 5001 +5V

611K +UV ,.! ~5V

+5V

lRil&EB MQ.D£ WEI L.E.!£1.

Fig. 3.9. Ignitron Trigger Generator Schematic

~ \0 POWER H 115vac LINE nd. FILTER 60Hz N LINE

M3ER CALl B. >~ • I AC +I t -· ' I 20meg HY HIGH VOLTAGE POWER SUPPLY 2w ADJUST someg SKV@ Sma PULSE

+ • jAC PC board Jmh~ bnc @6KV 1 5 s; 6l75451p I.8.lY.a LN 41 18 6.21<: mhv mhv PULSE 1 : 1 IN l ( I.Q heat-sink) l ..n. !GN!TQR I 150 rN:1 ~ ' 1~ 7 aoslout W~~J.--,i2~a + ICOm. 33uf mh~ I ( [ ~~ - sov 1 t~~~~ I T~LQI

1N4001

Fig. 3.1 0. lgnitron Pulse Generator Schematic VI 0 ~

PULSE TRANSFORMER PULSE GENERATOR

IGNITOR - OUT IN ----- OUT IN • -~ I CATHODE II I VOLTAGE PROBE TRIGGER GENERATOR I I d) CATHODE • - IN OUT f-- - -= IN OUT =-

ANODE - Fig. 3.11. lgnitron Trigger Circuit Block Diagram -U'l PULSE TRANSFORMER

:~i~

Fig. 3.12. Picture Of Voltage Probe And Pulse Transformer For Ignitron Trigger

'Jl IJ r·--·---·W·······

"'f'.. ~ .... ~-~

,t ''"~ ' ~. -:!f · ":: ~ ' TRIGGER GENERATOR

PULSE GENERATOR

,. "'~ ~ '~ ~ ! i

I' (

Pig. 3.13. Picture Of Trigger Generator And Pulse Generator

'J• 'J.J /ANODE

~L--

IGNITRIJI STAND

A A

IGNJTRON

CABLES TO CAP II ~cAnmDt CABLES TO RAJLGUN BANK I ~ CONNECTIONS f------1 . • - 1------~ SECTION A - A

Vl Fig. 3.14. Ignitron Tube Configuration ~ r-...... c 0 ...... 1-. ·c- -0..0 56

values stated previously for these cables, the total resistance, R:ab2, and inductance, Lcab2, between the crowbar switch and the railgun connections were calculated to be approxi­ mately 0.75 mn and 20 nH, respectively.

The connection between the coaxial cables and the rail gun (rail gun connections) consists of parallel plate bus bars. A schematic representation of the parallel plate con­ nection is shown in Fig. 3.16. With h1 = w1 = 7.62 em (3") and h2 = w2 = 3 em (1.182"), the inductance per unit length of this connection was assumed to be approximately

0.5 J.!H/m because of its similarity to the square bore rail gun geometry. The reason for selecting the 7.62 em plate separation was to facilitate the future addition of a preinjec­ tion gun which must fit in between the plates. The total length of the parallel plate con­ nection is approximately 0.762 m (30"). The total inductance of this connection, Lgc' was calculated to be approximately 380 nH. The total resistance of the connection, Rae, was calculated to be 0.5 rn.Q. Figure 3.17 shows the parallel plate connection between the coaxial cables and the railgun.

The final electrical component in the system is the railgun itself. As discussed pre­ viously, the rail gun was characterized by a lumped inductor in series with a lumped resistor. The inductance of the gun, Lg, was calculated to be 40 nH (L'/g/2). The resistance of the gun, Rg, was calculated to be 0.25 rn.Q (R'/g/2). The armature resistance, Rare, was assumed to be 3.0rn.Q (as mentioned previously). I RG - B CABLES ~-2 ·-,-- r il

TOP VIEV

,...., r- RG - B CABLES

rr . I RAIL C[]NN[CTI~ POINTS hi illh2 J

SIDE vu:v

Fig. 3.16. Schematic Of Parallel Plate Railgun Connections

Vl -.J Fig. 3.17. Picture Of Para1lel Plate Rail gun Connections

'Jl X 59

An equivalent circuit diagram for the railgun power system is shown in Fig. 3.18.

Computer (PSPICE [15]) simulation of this circuit for cases when the crowbar switch is not fired is shown in Fig's. 3.19- 3.22. Figure 3.19 shows the simulated capacitor bank voltage as a function of time for a charge voltage, VP(O), of 4.2 kV while Fig. 3.20 shows the simulated armature current for this same VP(O). Figures 3.21 and 3.22 show the simu­ lated capacitor bank voltage and armature current respectively for VP(O) = 5.8 kV. Fig­ ures 3.23 - 3.26 show PSPICE simulation of the circuit for cases when the crowbar switch is frred at an anode-cathode voltage of 300 V. Figures 3.23 and 3.24 show the simulated capacitor bank voltage and armature current, respectively, for Vp(O) = 4.2kV.

Figures 3.25 and 3.26 show the simulated capacitor bank voltage and armature current, respectively, for Vp(O) = 5.8kV. The effects of using the crowbar switch are evident in Fig's. 3.19- 3.26. Not only does the crowbar switch eliminate ringing of the armature current, it also reduces the ringing of the capacitor bank voltage. The difference between the armature current depicted in Fig. 3.26 and that predicted in Fig. 3.3 is due mainly to the assumption of an ideal crowbar switch (i.e., assuming Lc = 0 and Rc = 0).

Mechanical Desi~n and Construction

The railgun bore containment snucture design was based on a design proposed by

Sparta Inc. for a laboratory railgun [9}. As mentioned previously, the peak railgun bore pressure was fixed at 45 ksi. The components of the rail gun bore containment structure L COAX CABLES l ...J -1 COAX CABLES 2 GUN CONNECTJDNS I,.-----., ,.-----., ,.-----., ,.-----., ,.-----., r------, r------, I I I I I I I I I I I I ll c ab2 -- L cob! I Rae -l;- I I I I CATHODEl.------.J l.------.J I lultll lcobll lcoltll ----, I I I I r- --, I I I I I I I I I I s c I I I I I I I I I R z R I I A I I I I 0 1 11 E I I I._ 11c•1tll_, ._ nbll_, Rultal v ______.__ 1...--'---' B I I~ . . E A I I u r----, r----, r----, I 1 R I IN I I I I I I ~ I II I I I I I I I N S L ____ J I Lupl Lupl I L c-..1 Lupl I I I V A I I I I I I I I T I R I I I I I I I I R T M I I I I I I I I NO He A I II I I' ' T I I I I I u R E v.,.. ( I r r c! I c! : r c! I c! L-J~D£~ 1 L ____ J L----J L ____ J L----J L----J MAXVELL 50kJ CAPACITORS

Fig. 3.18. Railgun System Equivalent Circuit ~ s~-----

I I 4

r--1 3 ~ L...J w (.!) ~ 2 -' 0 > a:: 0 1- 1 u a.< ~ 0

-1 I I I -2 - . ------. ---. I ~ 0 200 400

llME [MICRO-SEC.]

Fig. 3.19. Simulated Capacitor Bank Voltage For Vp(O) = 4.2 kV (Uncrowbarred)

...... 0\. 300 ------

250

200 ,...... , ~ L...J 150 .... z w 0:: 100 0:: ::J u w 50 0:: ::J I 1- I ~ :::! 0 ------ct:: ~ ·::::__~

-50

-100

-150 ----. 1 0 200 400

TIME [MICRO-SEC.]

Fig. 3.20. Simulated Armature Current For Vp(O) = 4.2 kV (Uncrowbarred)

0'\ N 6 -r------·--

5

4

,...... , ~ L...J 3 w (!)

~_J 2 0 > 0:: 0 1- u < a. 0 ~

-1

-2

-3 ---,------. 0 200 400 TIME [MICRO-SEC.]

Fig. 3.21. Simulated Capacitor Bank Voltage For Vp(O) = 5.8 kV (Uncrowbarred)

w~ 400 -r----

300

....., ~ L..J 200 1- zw n:: n:: ::J u 100 w n:: :::> ~ ~ n:: 0 c(

-100

-200 --1------,------,------0 200 400

TIME [MICRO-SEC.]

Fig. 3.22. Simulated Armature Current For Vp(O) = 5.8 kV (Uncrowbarred)

~ 5 ~- -~--~---~--~------~

4 I I I

,....., 3 ~ L...l w u ~ 2 _J 0 > ~ g 1 a.~ 5 o 1------~---T--

-1

-2 ----,------~-~-,------,--- 1 0 200 400 I 600

TIME [MICRO-SEC.]

Fig. 3.23. Simulated Capacitor Bank Voltage For Vp(O) = 4.2 kV (Crowbarred)

0\ Vt 300 ----- 280 260 240 220 ,...... , ~ 200 L...J 1- 180 z w lk: 160 0:: :J u 140 w 0:: 120 :J ~ 100 ~ 0:: 80 < 60 40 20 0 -20 -.------. T- 0 200 I 400 600

TIME [MICRO-SEC.]

Fig. 3.24. Simulated Armature Current For Vp(O) = 4.2 kV (Crowbarred)

0\ 0\ 6--r------

5

4

r-1 ~ L-J ,W 3 C) ~ ~ 0 > 2 n: 0 t: 0 <( a. <( 0 0

-1

-2 -t-·------.,.------~- ----~------r-----~------1 0 200 400 600

TIME [MICRO-SEC.]

Fig. 3.25. Simulated Capacitor Bank Voltage For Vp(O) = 5.8 kV (Crowbarred)

0\ .....,J 400 --~~ ------I

350 -I I \ I I 300 -I I \ I r-, ' ~ ~ 250 I 1- l z I w I \ a::ll:: 200 :Ju w a:: 150 :J ~ a::~ 100 4:

50

of "~ ::::...... __ ~ I

-50 --,- 0 200 400 600

TIME [MICRO-SEC.]

Fig. 3.26. Simulated Armature Current For Vp(O) = 5.8 kV (Crowbarred)

0\ 00 69

were designed to withstand a static pressure of 45 ksi using simple beam equations. Fig­ ure 3.27 is a drawing of the muzzle end of the rail gun. This rail gun was specifically designed and constructed to support brittle insulator samples adequately and to provide for easy assembly/disassembly when changing out bore components.

The outer containment structure was constructed of AISI 4140 steel. The backup insulators were originally designed to be ceramic (Coors AD- 94). Unfortunately, these

ceramic pieces were never obtained and G - 10 backup insulators were used in their

place. This greatly reduced the stiffness of the bore and had adverse effects during firing.

Four 1.5" diameter, SAE grade 8 bolts were used to compress the bore containment struc­

ture and to hold it together during firing. Figure 3.28 shows a side view picture of the

rail gun.

Figure 3.29 is the shop drawing used for the fabrication of the rail and insulator

samples. The close tolerances indicated in Fig. 3.29 reflect requirements for all bore com­

ponents in order to maintain a closely toleranced bore dimension of 1 em x 1 em. This

closely toleranced bore dimension will be necessary when adding a preinjection gun so

that bore alignment between the two guns can be achieved.

The breech end of the gun was sealed by a Vi ton gasket held in place by a breech

flange made of G - 10 (see Fig. 3.30). The exposed muzzle end of the rail gun bore was

also protected from the muzzle blast by a Viton gasket and a nylon muzzle flange (see

Fig. 3.31 ). 70

-- ) r -- 1 I I I I I I I I I I I I . I I I I I I I I I I I I I I I I I I I I I I I I I I I BACKl.P I I I I I INSULATOR I I I I I I I I I I I I I I 1 J I I I - I lN~Ll.ATIJR ) RAll I I - I SAHPL£1 I I SAr[ "''-.._ ~ I I ..,. / I I I I I I I I AlSI Jl I I ~ ~ ~ BACkUP I ~l~D I !.ct. I 12.1 STEEL lN~UI..A [R lNSLl.:ATDRI I I - I I I I - I I r l 't I I I I I I I J BACICl.P ' I lNSULATOR I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I l ld ~ '-" I 12.182"

Fig. 3.27. Drawing Of Muzzle End View Of Rail gun 71

' r• • j lilt i' { ~ ~ A

_[ 0.3940'±0.0010' ... - fL a.oooo·±o.ooso· J

SIDE VIEW' A

~ 1-- 1.1220'±0.0010' 0.0300'± 0.0010' ] t T ~ ~±0.10' I 1·--....J ---o-.3-9-40-.±-o-.oo_1_o.-J~- -..f 1-- 0.3940'±0.0010'

VIE'W A - A

Fig. 3.29. Rail And Insulator Sample Drawing

-.....1 N 6.

G - 10 BREECH FLANGE

Fig. 3.30. Picture Of Breech Flange

""-.] 'J.J 74

0 e.o c ~ -u.. 0 <; N -N ~ ;";(~· =' :E '- 0 0 z~t&J '""=' 9Ne, u ~z - >--~:3 ·0..- Z::=E~ -('f) ('f)

bl) ·u..- 75

Magnetic pressure forces had to be considered when designing other MAX II com­ ponents such as the parallel plate bus bars and the ignitron stand. Again, all parts were designed to withstand the largest expected pressure applied in a static manner. Figure

3.32 shows the G- 10 clamps and the ANSI B7 stud bolts used to hold the parallel plate bus bars in place.

Dia~ostics Desi~ and Construction

The Rogowski coil used for measuring armature current is pictured in place on the parallel plate bus bars in Fig. 3.33. The number of turns, N, on this coil is 20, the cross

5 2 sectional area of each turn, A, is approximately 2.045 x 10" m , and the major radius of the torus, r, is approximately 0.073 m. The relative permeability of the coil, ~ is 1 (i.e., air core). Equation (2.35) can be used to calculate the induced voltage in the coil, V md, as

5 v. = _ ~Adl =-400md0-9 · 20 · 2.045x10- dl =-1. 12x10-9dl [VJ. ind 2m dt 2 · 1t · 0.073 dt dt

The inductance of the coil, L, can be calculated, using Eq. (2.36), as

2 2 _ ~ A _ 400x10-9 ·1t · 20 • 2.045x1o-s _ H L- - -22 .41 n . 2m 2 · 1t · 0.073

The time constant of the coil, 't, cannot be made much smaller than approximately 45 ns

(corresponding toR= 0.5 Q- see Eq. (2.38)). Because the pulse width of the expected

armature current (= 300- 500 J..LS) is larger than any conceivable coil time constant, this coil would act as a differentiating coil (see Eq. (2.40)) in this situation. An RC integrator was therefore used with the coil in order to obtain a signal proportional to the current ,.

7 < ~

Fig. 3.32. Picture Of Parallel Plate Support Structure

-..J 0\ 77

- u0

~ v; 3 0 t;J) 0 ~ '- 0 0 """~ --u ·-p...

~ r"· ~

t;J) ·-LJ.. 78

being measured. Equations (2.43), (2.44), and (2.45) were used to design the integrator.

, By selecting Ri much less than ~' Ci much greater than C5 and RiCi much greater than the pulse width of the armature current, a viable integrator was obtained. The final ele­ ment values decided upon for the integrator circuit are Ri = 1200 Q, ~ = 1 M Q, Ci = 8 Jl.F', and Cs = 47 pF. The viability of the integrator was demonstrated using the PSPICE simulation depicted in Fig. 3.34. The sensitivity, S, of the coil and integrator was calcu­ lated using the relation

where

and

This equation was obtained from an analysis of Eq's. (2.43)-(2.45). The calculated value of S for this case is 8,571,000 AN. Figure 3.35 shows the simulated armature current and

Fig. 3.36 shows the simulated measured armature current. The two waveforms are identi- cal.

The B dot probes used to measure in-bore velocity are pictured in Fig's. 3.37 and

3.38. The probes are mounted on metal inserts which fit into the steel bore containment structure (see Fig. 3.39). This type of probe mounting allows the use of up to 16 different Lt Rt

I v p (Q) cp I 1.12 nH vind

RAILGUN CIRCUIT

R.I

VSCOPE 1!1 c.I Rs cs I MEASURED vind S x VSCOPE

INTEGRATOR CIRCUIT CIRCUIT USED TO

Fig. 3.34. Integrator Simulation Circuit

...,J \0 500 ------. - .. - .. ------. - -- - -

400

,....., 300 ~ L.-1 .... z w 0:: 200 0:: ::J u w 0:: 100 ::J ~ ~ 0:: <( 0 _,_ ------· - -· .

-100

-200 ---.-- ·------·- 0 200 400

TIME [MICRO-SEC.]

Fig. 3.35. Simulated Armature Current For Integrator Test

00 0 soo~---

400

,....., 300 ~ L-.1 1- z w cr 200 0:: ::::> (.) Q w 100 0:: :J Vl ~ ~ 0 -- _, - ·-- --

-100

-200 ~~~------~------~------~ T 0 200 400 TIME [MICRO-SEC.]

Fig. 3.36. Simulated Measured Armature Current For Integrator Test

00 ...... ~') ( -

-,....

-,...~ ,...- <-

-"'

...... 0 0 co !.-.. 0 ~ ...... ~ u ·0..- r- ~. ~

eJ) ·~- u= t: ::::: u

< = ·v.:-

v.: ~ ..0 2 0... 84

( -"-- I I I -"---I I I I I I I I I I I I I I I I l I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I ~ B DOT PROBE I I I I I I I I I I I 1 r I I I I l J I I I I I I I fETAL iINSEfT I I I I I I [!a m; I I I I ~ I I I I I I I I I I I I I I I I I I r J l l I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

~ '-...... >10 I

Fig. 3.39. B Dot Probe Mounting Detail 85

probes at one time. The probes consist of 2-turn coils wrapped on rubber grommets. A

1 Mil oscilloscope plug-in was used with these probes. The two pictures shown on Fig's.

3.37 and 3.38 show the probes oriented in 2 different directions. The probes shown in

Fig. 3.37 sense both rail currents and armature currents while the probes shown in Fig.

3.38 sense only armature current [7].

The final type of diagnostic used on the MAX IT is a muzzle velocity measuring scheme using the break-wire method. The break-wire is shown schematically in Fig. 3.40

[7]. The electrical circuit used for the setup is shown in Fig. 3.41 [7] and a picture of the system is shown in Fig. 3.42. The output of the circuit is 2.2 V until the frrst wire is bro­ ken, at which time the voltage falls to 1.1 V. When the second wire is broken, the output voltage falls to 0. The distance between the wires divided by the time it takes the output voltage to fall from 1.1 V to 0 V gives a measure of the muzzle velocity of the projectile. 86

1~------' ,'' ~ I I I I I I I -;..., I

"'~ III ~ ''e> I ~ I v> ~.> ~ ~ I ~ ,..----- \. 87

c 0 Lf)

C\J -# # w w ~ ...... ~ ~ ~ ..... ·-=' J~ (..) ~ c c ·-~ I .::t. .::t. ~cu - - ~ ~. -~ M bil ·-~

> C\J C\J ,....,...

I ~ ~ ~ c::J (.o...., 0 ~ ...... ::::l u ·-0.. N -.:t M 0.0 ·-u.. CHAPTER IV

MAX II OPERATION

The MAX II facility was designed to require no more than 1 day between shots.

The major tasks which must be performed to prepare for a shot are described in the fol­ lowing discussion.

When changing out the rail and insulator samples, the gun components should be cleaned with methanol. The gun pans should then be put in place. H brittle insulator sam­ ples such as ceramics are used, it is recommended that the contact surfaces between the rails and insulators be coated with a thin layer (0.001 ")of Dow Coming 3120 RTV

silicone rubber before being placed in the gun in order to prevent point loading of the insulators. The railgun bore must then be prestressed so that the plasma armature will not escape through the insulator-rail contact surface. This prestressing is accomplished by loading the 1.5 " bolts with stud tensioners. The four hydraulic nut stud tensioners (Pil­

gram Moorside LTD. type LRL) are shown in place in Fig. 4.1. Each bolt can be loaded independently using the hydraulic pump system (H. Lorimer Corp. pan #80-220-00)

shown in Fig. 4.2. The pump uses a differential area piston to induce hydraulic pressure.

The hydraulic pressure generated is equal to 400 times the air pressure which drives the pump. This air pressure is in tum controlled by a regulator on the inlet of the pump. The hydraulic area of each hydraulic nut is 3.019 square in. Therefore, the load induced in each bolt, Fb, is equal to 3.019 times the pump pressure in psi. The total prestressing load is, then, 4Fb. A recommended total prestressing load for the railgun bore is 141,840 lbs.

89 90 91

·­-::; c-::.... ;:;.-.. "":I: \.o-< 0 ~...... ::; u 0: ~ -.::t .....~ u.. 92

This is equal to the force exerted on the rails by the 45,000 psi bore pressure. Care should be taken to make sure that the finished bore is square for the entire length of the bore and that the finished bore dimensions are acceptable. The sidewall insulators are somewhat oversized in an effort to compensate for compression of the G - 10. However, if the verti­ cal and horizontal bore dimensions differ by too much, shimming of the sidewall insula­ tors with mylar tape is recommended. Once the bore is prestressed, the muzzle flange and breech flange are then bolted on.

After the railgun is assembled, the final vertical and horizontal bore dimensions

should be obtained. The projectile can then be fabricated. The shop drawing for a Lexan

projectile is shown in Fig. 4.3. The finished bore dimensions for which this projectile was

fabricated were 0.392" x 0.390". It can therefore be seen that the projectile is oversized,

allowing for a press fit. The drilled and tapped hole allows for removal of the bullet if

desired. A fuse must be placed on the back end of the projectile as shown in Fig. 4.4

before inserting the projectile into the bore. The length of the fuse determines the voltage

at which the railgun will fire and can be calculated approximately by the relation

VP(O) ~ GAP[in] · 50, oo{ ~]. (4.1)

where GAP is equal to the final vertical bore dimension minus the fuse length. The fuse

material used was 0.005" thick copper tape. Once the fuse is properly fixed to the projec­

tile, the projectile is inserted into the gun. It is recommended that the projectile not be

inserted all the way to the breech flange. This is done to reduce the thermally induced Cl

I 0.5600"±0.0020. I 0.3950'±0.0010'1WI-- DRILL AND TAP .]975" ---.I DEEP FOR 6-32 THD. f r- 0.395 J•±o.ooto· -+---Cl L_ l I _,_:J ~ 1-- 0.2500'±0.0020' 0.2500"±0.0020" ~ r- TOP VIE\J END VIE\J

0.0200'±0.0020' -fi-

0.3750.±0.0020f • - -f 0.0100"±0.0020"

SIDE VIE\J

Fig. 4.3. Projectile Drawing

\0 I..J.,) 94

u~ cd -0.. c: -0.. ·-...... I-. C/1 ~ Vl u..:l ...... c...... ~ ~ ...... ·--u ~ ·~ L... 0.. ~ f 0 ~ L...... :l u I ·-0.. I ~ 'o:::t i 0.0 '; ·-u.. 95

pressure on the breech flange by providing a larger volume in which the plasma can expand. A distance of 0.5" is recommended between the breech of the gun and the fuse end of the projectile upon insertion.

The projectiles were fired into the catch tank shown in Fig. 4.5. Three phone books of approximately 1.5" thickness were placed in the back of the tank in order to "catch" the projectile. The break-wire system was then placed in front of the books. A distance of approximately 24" is recommended between the break-wire and the muzzle of the railgun in order to limit the noise induced in the break-wire circuit.

The final major tasks to preform before firing the MAX II is the connection of all diagnostics cables to the oscilloscopes and to adjust the ignitron trigger/pulser settings.

An cathode-anode voltage setting of -300 V was used along with a negative slope setting.

The pulse generator voltage was set at 3 kV. 96 CHAPTER V

EXPERIMENTAL RESULTS

The MAX II facility has successfully been flred a total of 10 times. All shots except the tenth were frred without using the crowbar switch because the ignitron trigger genera­ tor had not yet been obtained. The frrst 3 shots were low energy, structural test shots using copper rails and G - 10 insulators with no projectile (free running arc). The fourth shot was a free running arc shot using copper rails and G - 10 insulators at full rated energy and current. The flfth shot was the frrst projectile shot, again using copper rails and G - 10 insulators. The sixth and seventh shots were projectile shots using copper rails and Mycalex insulators. The eighth, ninth, and tenth shots were projectile shots using molybdenum rails and G- 9 insulators. Rogowski coil and B dot data were taken with a

Nicolet 4094A oscilloscope, while the break-wire data was taken with a Tektronix 7834 oscilloscope. Data from the eighth shot was lost due to oscilloscope trigger failure. Data from the fourth, flfth, sixth, seventh, ninth and tenth shots are presented here.

The fourth shot was the last free running arc shot made on the MAX II. The capaci­ tor bank charge voltage, VP(O), was 6 kV. The capacitor bank voltage trace is shown in

Fig. 5.1. The armature current trace is shown in Fig. 5.2. The total mass, m, is small for free running arc shots. This leads to higher velocities which causes the arc to arrive at the muzzle end of the gun sooner than for projectile shots. Therefore, bore damage on this shot was mainly at the muzzle end due to excessive arc burning there.

97 6 -...... ,...------

5

4 ,..., ~ L-J 3 w C)

~_J 2 0 > n: 0 1 t- u <( D.. <( 0 u

-1

-2

-3 i ---,------,~ ------, .. ------,------..,.--- 0 100 200 300 400 TIME [MICRO-SEC.]

Fig. 5.1. Capacitor Bank Voltage Trace For Fourth Shot

\() 00 500 .------~

400

,....., 300 ~ L....l ..._ z w 0:: 200 0:: ::J 0 w 0:: 100 ::J ~ ~ 0:: c( 0

-100

-200 ~------~------~----~~----~------~------~------~----~ 0 100 200 300 400

TIME [MICRO-SEC.]

Fig. 5.2. Armature Current Trace For Fourth Shot

\0 \0 100

The fifth shot was the frrst shot where a projectile was accelerated. Two B dot probes positioned to sense both rail and armature currents were monitored on this shot.

The Rogowski coil was also monitored. The first probe, B dot probe #1, was located

0.75" from the muzzle end of the gun while B dot probe #2 was located 0.25" from the end of the gun. The armature current for this shot is shown in Fig. 5.3. A composite fig­ ure showing each of the two B dot probes and the armature current is shown in Fig. 5.4.

There is very little, if any, time shift between the two B dot probe signals. Excessive arc burning at the muzzle end of the gun was also observed. The projectile used for this shot was not a press fit projectile as described in the previous chapter. It was instead a loose fitting (== 0.005" clearance) Lexan projectile. These facts were used to hypothesize that armature blowby had occurred during this shot. This hypothesis was reinforced by evi­ dence obtained in the ninth and tenth shots using the press fit projectiles. Evidence of plasma leakage through the rail-insulator contact surface as well as excessive rail erosion

(much more than for free running arc shots) were also noted at the breech end.

The sixth and seventh shots were made in order to verify the ability of the MAX II facility to support brittle insulator materials adequately. Mycalex was used instead of ceramic because it has approximately the same mechanical properties as ceramic but is relatively inexpensive. Loose fitting projectiles were used for each of these shots. V eloc­ ity measurements on the first shot were lost due to oscilloscope trigger failure. A terminal velocity measurement on the second shot using the break-wire system is shown in Fig.

. . l m/ ( 4" 0.1016m) Th 5.5. This figure shows a muzzle velocity of approximate y 1270 s 80 ~ = 80 ~ • e 400 -~ ------~------~-- - --

300

.---. 200 ~ L...l .... z w rr 100 rr :J 0 w rr 0 :J ~ rr::::E < -100

-200

-300 -4------r--- ~ ------. 0 200 400 600

TIME [MICRO-SEC.]

Fig. 5.3. Armature Current Trace For Fifth Shot ,.... '

...... 0...... 1.4

1.2 ~B DOT 1 r-1 6 w 1 C) ~ J 0.8 0 > 1- 0.6 0 0 m. 0.4 r-1 <( ~ 0.2 L..J 1- z w 0 0:: 0:: :J -0.2 0 w 0:: -0.4 :J ~ ~ -0.6 0:: <( -0.8

-1 ------,------,------,-- -· ------.--- 0 200 400 600 TIME [MICRO-SEC.]

Fig. 5.4. Composite Armature Current And B Dot Traces For Fifth Shot

...... 0 N 103 104

peak armature currents for these two shots were in the 300-400 kA range. Although the erosion damage to the Mycalex was severe, no structural damage was incurred. The breech section (first 2.5") of the copper rails also showed excessive erosion damage.

The ninth shot was made using the press fit Lexan projectile ( 1.5 g) described in

Chapter IV. The initial capacitor bank voltage, VP(O), was approximately 5.2 kV. The armature current trace is shown in Fig. 5.6. Three B dot probes positioned to sense both rail and armature currents were monitored for this shot. The first probe, B dot probe #1, was located 1.5" down bore (from the fuse end of the projectile), #2 was located 3.5" down bore, and #3 was located 6.5" down bore. A composite figure showing the armature current and the three B dot traces is shown in Fig. 5. 7. A definite time shift between the B dot probe signals can be see in Fig. 5. 7. It is difficult if not impossible, however, to infer velocities from these data because there are too many peaks. Several of the peaks can probably be attributed to the dl/dt of the current as it goes through zero. The break-wire signal for this shot is shown in Fig. 5.8. The muzzle velocity inferred from Fig. 5.8 was approximately 1450 m/s. Erosion damage was severe at the breech end of the gun and evidence of plasma leakage was observed there. Arc burning at the muzzle end of the gun was comparatively mild for this shot. This fact along with the time shift evidenced in the

B dot probe signals showed that no armature "blowby" had occurred. This was attributed to the use of the press fit projectile.

The tenth shot was the first and only shot which made use of the crowbar switch.

The crowbar was set to frre at an anode-cathode voltage of 300V. A 1.49 g Lexan press 400 -r------~------~-----~------l

300 I

1 200 L-J ..._ z w 100 ~ ~ :J 0 I w I 0:: 0 I --- :J -~l ~ r \ I ~ 0:: < -100

-200

-300 - ~I ~. 0 200 400 600 TIME [MICRO-SEC.]

Fig. 5.6. Armature Current Trace For Ninth Shot

...... 0 Vl 1 ~ ·------·-··------0.8 ,....,

L..J> 0.6 w (j 0.4 ~ :.J 0.2 0 > f- 0 0 a -0.2 ,....,m. c( -0.4 ~ L..J -0.6 zf- w -0.8 0::: 0::: :::> -1 0 w -1.2 0::: :::> ~ -1.4 ~ 0::: c( -1.6 -1.8

-2 --~---, ------, ------y------.., 0 200 400 600

TIME [MICRO-SEC.]

Fig. 5.7. Composite Armature Current And B Dot Traces For Ninth Shot

~ 0 0\ 107

z 108

fit projectile was used for this shot. The initial capacitor bank voltage, VP(O), was 4.2 kV.

The armature current trace is shown in Fig. 5.9. Three B dot probes positioned to sense armature current only were monitored for this shot. The first probe, B dot probe #1, was located 1.5" down bore, #2 was located 6" down bore, and #3 was located 7" down bore.

The output signals from B dot probe #1, #2, and #3 are shown in Fig's. 5.10, 5.11, and

5.12, respectively. A composite figure showing the armature current and the three B dot probe traces is shown in Fig. 5.13. The time shift between the B dot probe signals is apparent. Figure 5.10 suggests that the armature passed B dot probe #1 at approximately t=90 J..l.S. This suggests an average velocity in the frrst 0.0381 m (1.5") of approximately

423 m/s. Figure 5.11 suggests that the armature passed B dot probe #2 at approximately t=268 J..l.S. This suggests an average velocity between x = 0.0381 m (1.5") and x = 0.1524 m (6") of approximately 642 m/s. The time at which the armature reaches B dot probe #3 is somewhat more difficult to determine. The zero crossing forB dot probe #3 is at t=278

J..l.S (see Fig. 5.12). This would suggest an average velocity between B dot probe #2 and B dot probe #3 of approximately 2540 m/s. This velocity is much higher than expected (i.e., from model runs). The signal from B dot probe #3 is very small in the area of armature passage and could easily be distorted by noise. The B dot probe trace in Fig. 5.12 could also be interpreted to show that the armature passed the probe at t=300 J..l.S. This would give a more realistic average velocity between B dot probes #2 and #3 of approximately

794 m/s. This also agrees with the fact that the muzzle velocity (break-wire) measure­ ment was missed (i.e., broke second wire after the end of the oscilloscope trace) because the velocity was too low (see Fig. 5.14). These low velocity measurements reflect the fact 300 ------, 280 260 240 220 ,...... , ~ 200 L-.1 f-z 180 w a:: 160 a:: :J 0 140 w a:: 120 :J ~ 100 ~ 0': 80 <{ 60 40 20 0 -20 0 200 400 TIME [MICRO-SEC.)

Fig. 5.9. Armature Current Trace For Tenth Shot

...... 0 \0 0.6

0.5

0.4

0.3

0.2

,....., 0.1 L-1> w 0 C) -0.1 ~:..J 0 > -0.2 f- 0 Q -0.3 m -0.4 -0.5 -0.6

-0.7

-0.8

-0.9 ---.- I 0 200 400 TIME [MICRO-SEC.]

Fig. 5.10. B Dot Probe #1 Trace For Tenth Shot

-0 0.09 0.08 0.07 0.06 0.05 ,..., 0.04 > ~ 0.03 w 0 0.02 :.J~ 0 0.01 > 1- 0 0 0 -0.01 m -0.02 -0.03 -0.04 -0.05 I -0.06 -0.07 ·---,---- r-· ··-· 0 200 400 TIME [MICRO-SEC.]

Fig. 5.11. B Dot Probe #2 Trace For Tenth Shot

...... 0.06

0.05

0.04

0.03

0.02 ~ 0.01 w (!) 0

:.J~ 0 -0.01 > .... -0.02 0 c m -0.03 -0.04

-0.05

-0.06

-0.07

-0.08 ------~------.------,------,------.,.....------~ 0 200 400

TIME [MICRO-SEC.]

Fig. 5.12. B Dot Probe #3 Trace For Tenth Shot - -t-J 0.6 ~-

0.5 ,...... , 0.4 L-J> w u 0.3 ~ J 0.2 0 > .... 0.1 0 a 0 m ,...... ,. < -0.1 ~ L-J .... -0.2 z w -0.3 0:: 0:: :::J -0.4 0 w -0.5 0:: :::J ~ -0.6 ~ 0:: < -0.7 -0.8

-0.9 -r -- --~ -- -.--- -~ ---,------,~ 0 200 1 400 TIME [MICRO-SEC.]

Fig. 5.13. Composite Armature Current And B Dot Traces For Tenth Shot

~

~ v.> 1 1--t

0,....

,...... _2 115

that the fuse strip on the projectile broke at a lower voltage than anticipated, causing lower currents than expected (i.e., 260 kA peak versus 350 kA peak). Erosion damage was, again, severe at the breech end of the gun caused by low velocities and high currents in this region. Arc burning at the muzzle end of the gun was almost nonexistent implying that the arc had extinguished before reaching the end of the gun. This, again, can be attributed to the low currents experienced in this shot.

Bore component erosion was much greater when projectiles were accelerated. This is to be expected because of the lower velocities due to the increased mass of the launch package. The first three free running arc shots were made without cleaning the bore com­ ponents. However, the gun had to be disassembled and the bore components cleaned

and/or remachined after each projectile shot. This was because the insulators were either

too conductive or would no longer hold off enough voltage for a shot. Data were taken on

the ninth shot in order to quantify the bore erosion, bore resistance, and bore self break­

down voltage before and after the shot. The mass of the bore component samples before

the ninth shot were measured to be as follows:

Left G- 9 Insulator: 73.45 g

Right G - 9 Insulator: 73.30 g

Top Molybdenum Rail: 413.49 g

Btm. Molybdenum Rail: 413.82 g. 116

The bore self breakdown voltage before the shot was found to be 11.5 k V. The bore resistance measured at the railgun connections using a Keithley Electrometer was found to be 130 Mn. The bore resistance after the shot was found to be only 12 M!l and the bore self breakdown voltage was too low to be measured because the insulators were too conductive. The mass of the bore component samples after the shot were measured to be as follows:

Left G- 9 Insulator: 73.36 g

Right G- 9 Insulator: 73.20 g

Top Molybdenum Rail: 413.11 g

Btm. Molybdenum Rail: 413.53 g.

The majority of the erosion problems were caused by rail melting in the breech section

(first 2.5") of the gun. Some of the melted rail material plated out on the insulators, reducing bore resistance and bore self breakdown voltage.

The results presented here from the testing of the MAX II facility show that it satis­ fies the design requirements listed at the beginning of Chapter III. The future addition of ceramic backup insulators and a preinjection gun would greatly improve the operation of the facility, however. The preinjection gun would decrease erosion at the breech end of the gun by decreasing the residence time of the arc there. It would also lessen the ten­ dency for plasma leakage through the rail-insulator contact surface by decreasing the time 117

that the thermally induced arc pressure is applied to this section of the gun. Finally, it

would allow greater control of the firing voltage (initial capacitor bank voltage). The

bank could be charged to the desired voltage at which time the gun could be fired by

injecting a bullet with a fuse strip which covers the entire back of the projectile.

The problem of plasma leakage through the rail-insulator contact surface can be

blamed mainly on the G- 10 backup insulators. The elastic modulus of G- 10 insulators

is only approximately 5 per cent of that of Coors AD - 94. This means that the G - 10 is

20 times more flexible than the ceramic which suggests that most of the preload goes into

compressing the backup insulators instead of preloading the rail-insulator contact surface.

The ceramic backup insulators would therefore reduce the tendency for plasma leakage.

The ceramic backup insulators would also provide better support for the brittle insulator

samples. CHAPTER VI

CONCLUSIONS

The MAX II facility has demonstrated that it satisfies the design requirements and specifications established for the system. The model developed proved to be a good approximation of the launch process and was helpful in determining required circuit ele­ ment values such as the inductance of the circuit. The final circuit design was modeled with PSPICE in order to gain a more exact simulation of the system currents and voltages. Each component was carefully designed to withstand the large magnetic forces induced by the high currents in the system. Testing of the system verified the structural integrity of the system as well as the models used to simulate the system. The largest peak current observed thus far in testing of the MAX II facility is approximately 420 kA

(fourth shot). The highest measured muzzle velocity obtained to date is 1450 m/s with a

1.5 g projectile (ninth shot).

The system was designed for relatively quick turn around between shots. The aver­ age time needed to assemble the gun was no more than 3 hours. The system could easily be frred once a day even if the bore materials had to be replaced after each shot.

This facility was designed and constructed for use in testing promising railgun bore materials. The small size of the rail gun allows for the relatively inexpensive testing of materials. A set of molybdenum rails can be obtained for around $800. A set of G - 10 or

G - 9 insulators can be obtained for around $200, while a set of ceramic insulators can be

118 119

obtained for around $1,200. Thus, the total cost of a complete set of bore components is approximately $1,000- $2,000. This cost is much lower than that of larger railgun sys­ tems. This facility is therefore a cost-effective system in which promising materials can be screened before being tested in a larger rail gun system.

Two further improvements to the system are recommended, however. As discussed in the previous chapter, the replacement of the G- 10 backup insulators with Coors AD-

94 ceramic will reduce the tendency for plasma leakage through the rail-insulator contact

surface by greatly increasing the stiffness of the bore. The addition of a preinjection gun

will also provide advantages. These include lower erosion in the breech of the gun, con­

trol over the initial capacitor bank voltage, and reduction of plasma leakage through the

rail-insulator contact surface.

As stated in Chapter I, the materials problem in railgun technology must be solved

before railgun operation can become practical. The MAX II facility should prove to be a

helpful tool in the search for new rail and insulator materials which can improve the oper­

ation of rail guns. LIST OF REFERENCES

[1] Farrar, C.L and Leeming, D.W. Military Ballistics: A Basic Manual. Royal Mili­ tary College of Science, UK. Brassey's Defense Publishers, 1983.

[2] Parker, J.V. Why Plasma Armature Railguns Don't Work. Los Alamos National Laboratory, Los Alamos, New Mexico. IEEE Transactions on Magnetics, Vol. 25, #1, pages 418-424, Jan. 1989.

[3] Honig, E.M. Railgun Circuit Considerations. Los Alamos National Laboratory, Los Alamos, New Mexico. Los Alamos National Laboratory Report LA-UR-86- 890, presented at the workshop on High Current Opening Switches, Tamarron Co., Mar. 10-14, 1986

[4] Donaldson, A.L., Kristiansen, M., and Engel, T.G. High Current Discharge Effects on Electrodes and Insulators. Texas Tech University, Lubbock Texas. Presented at the 19th ICPIG Conference, Belgrade, Yugoslavia, 1989.

[5} Engel, T.G., Kristiansen, M., Smith, B.D., and Marx, J.N. High Performance Insu­ lator Materials for High Current Switching Applications. Texas Tech University, Lubbock Texas. Presented at the 7th IEEE Pulsed Power Conference, Monterey, Ca., 1989.

[6] Rosenwasser, S.N. and Stevenson, R.D. Selection and Evaluation of Insulator Materials for High Performance Railgun Bores. Sparta Inc., Del Mar, Ca. IEEE Transactions on Magnetics, Vol. MAG-22, #6, pages 1722-1729, Nov. 1986.

[7] Parker J.V. A Personal View of Plasma Armature Rail guns: Their Prospects and Problems. Los Alamos National Laboratory, Los Alamos, New Mexico. Railgun Seminar, unpublished, 1989.

[8] Nunnally, W.C. Pulse Forming Networks. Center for Energy Conversion Research, The University of Texas of Arlington, Arlington Texas. Presented at the Pulse Power Short Course, Texas Tech University, 1989, unpublished. [9] Vrable, D.L., Rosenwasser, S.N., and Cheverton, K.J. A Laboratory Rail gun for Terminal Ballistics and Arc Armature Research Studies. Sparta Inc., Del Mar, Ca. U.S. Army Ballistics Research Laboratory, Contract Report BRL-CR-572.

[10] Krompholz, H. Current and Voltage Measurements. Texas Tech Univers~ty, ~ub­ bock, Texas. Presented at the Pulse Power Short Course, Texas Tech University, 1989, unpublished.

120 121

[11] Holland, M.M., Wilkinson, G.M., Krickhun, A.P., and Dethlefson, R. Six Me~a­ joule Rail Gun Test Facility. Maxwell Laboratories, Inc. San Diego Ca. IEEE Transactions on Magnetics, Vol. MAG-22, #6, pages 1521-1526, Nov. 1986.

[12] Engel, T.G., Donaldson, A.L., and Kristiansen, M. The Pulsed Dischar~e Arc Resistance and Its Functional Behavior. Pulsed Power Laboratory, Texas Tech University, Lubbock Texas. IEEE Transactions on Plasma Science, Vol. 17, #2, pages 323-329, Apr. 1989.

[13] Huber, S.H. and Amundson, E.P. Protective Characteristics of Current Limitin~ Capacitor Fuses. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-94, #6, pages 225-232, Nov./Dec. 1975.

[14] I~itrons - Capacitor Dischar~e and Crowbar Service. General Electric, Tube Products Department, Schenectady, New York. Information Brochure, 1974.

[15] Microsim Corporation. PSPICE. Irvine, Ca. Jan. 1989.