APEC Youth Scientist Journal Vol.6 / No.2

DESIGN AND CONTROL OPTIMIZATION ON ELECTROMAGNETIC PULSED POWER ACCELERATOR SYSTEMS FOR HIGH SPEED TRANSPORTATIONS

∗ Min Hyuk PARK 1

1 Yeoido High School, 37 Gukjegeumyung-ro 7-gil, Yeongdeungpo-gu, Seoul, KOREA

ABSTRACT

There has been increasingly large demand for next generation high speed transport system to compliment air travel and for aerospace vehicle launchers for uses such as low orbit space transports. In this study, characteristics of two pulsed power accelerator designs commonly referred as Rail Gun and Gauss Gun were tested with the custom designed experimental set up. Velocity and various side effects during the operation of the accelerator, such as “suck-back” effect and rail erosion were noted and were used to achieve minimum set of controls defined as controlled acceleration, deceleration and deflection respectively. The experimental system with four sets of acceleration motors were able to achieve acceleration with measured energy efficiency peaking at 3 percent. It was concluded that the concept of transport system utilizing electromagnetic pulsed power system can be viable with relatively simple requirements while allowing unique designs without its form factor limited to conventional strictly linear path.

Keywords: Rail Gun, Gauss Gun, pulsed power, electromagnetism, accelerator

∗ Correspondence to : Park Min Hyuk ( [email protected] )

- 258 - APEC Youth Scientist Journal Vol.6 / No.2

1. INTRODUCTION

Recently, high speed next generation transportation system proposals have been gathering significant attention. Global transportation needs are on steady increase due to spread of globalization. Increase of global trade is also demanding reliable and high speed transportation infrastructure that can push more cargo and people promptly across the globe, complimenting existing air transport system. Also, aerospace application for high speed accelerators as potential launchers for aircrafts or even space-crafts is also being discussed. Such system, if deployed, can move energy requirement for take-off out of the vehicle, increasing cargo capability and lengthening potential engine lifespan. However, next generation transport systems are also subject to various sustainability requirements of 21st century. Analysts warn that existing sources for fossil fuel, which is primary source of power for civilizations today, will run out, making fossil fuel increasingly uneconomical option as power source. Given that transport is responsible for more than 40 percent of global fossil fuel usage, it is likely that it would be very first victim of fuel shortage. Also, even though man’s degree of contribution to climate change and global warming is debatable, it is widely accepted that global warming is in fact on-going process threatening environmental sustainability of today’s world. Thus, aforementioned systems must be able to be powered by renewable, environmentally sustainable and large scale energy sources. There are some renewable energy sources being actively developed that fits above requirements. Wide scale solar and wind farms are of particular interest, as they are relatively less location specific and has seen significant generation efficiency improvements in the last decade. However, they are not without problems. These types of renewable energy suffer from characteristics of low energy density, or amount of energy generated per area. It requires large amount of space to generate same amount of power generated by a small combustion engine. This not only means large land usage but also high transmission loss as a result due to resistance of electrical wires. This poses practical limit to amount of energy supplied to central grid by low density renewable energy sources and suggests radical changes to how massive cities operate should they adopt this route. Pulsed power on a decentralized grid is the solution that solves this problem while not violating universal energy conservation rule. Systems incorporating pulsed power would store constant but low density power released it tentatively as required, increasing usability of

- 259 - APEC Youth Scientist Journal Vol.6 / No.2 it to power power-hungry applications by pulsing stored energy to the load. An excellent example that can benefit this kind of power management in a localized grid is large scale transportation infrastructure such as trains. A train system, with rails covered in solar cells for example, can store low amount of power slowly yet steadily and release it when train approaches. High peak power achieved from the discharge can be utilized to overcome minimum power requirements for locomotion such as friction and do work on the system. This paper aims to investigate feasibility of low density power sources powering high speed transports by analyzing properties of electric acceleration motors that utilize pulsed power. Basic physics and engineering concepts involved in the construction will be discussed in brief detail. By constructing smaller scale prototype utilizing concepts mentioned above and testing its performance and operational characteristics, a full transport system with basic controls involving acceleration, deceleration and change of direction can be designed. This study is significant as this will help validate viability of currently proposed next generation transportation systems using low density renewable power sources and potentially allow components of renewable energy modules to be trickled down overtime, making transition to them much more economical to general public. Also, as transport needs ranging from intercontinental cargo to outer space will keep going up, it would be economically beneficial to have a transportation infrastructure that does not rely on expensive fuel. It is hoped that through this study, further innovations on pulsed power systems and its transport application can be discovered , bridging our era with the era of nuclear fusion.

2. LITERATURE SEARCH

2.1. Overview on pulsed power Pulsed power, which is the main theoretical basis of this paper, is the concept of accumulating and storing energy over a long period of time, with means such as high capacity electrical capacitors, and releasing it in very short period of time, often to subject load to high instantaneous power, which in physics is denoted as energy per unit time. While it does not violate energy conservation of a particular system, it allows practical increase of power subjected to the load by decreasing time in the following power equation. P = E/T First developed during world war 2 to be used in radars, pulsed power’s uses are widely recognized in modern engineering in construction of particle accelerators, high

- 260 - APEC Youth Scientist Journal Vol.6 / No.2 density magnetic field research facilities and weapon systems such as Rail Guns and Coil Guns. While its purpose is fundamentally different, many low energy electrical circuits use electrical pulses for power purposes as well in a form of “Pulse Width Modulation” (PWM), which allows such circuit to control power delivered to the load without resorting to complex analog control circuit. Pulsed power is achieved by storing steady stream of energy within electrostatic fields, magnetic fields or storing it as mechanical energy (rotational or gravitational energy) or chemical energy and releasing it quickly when desired. Storage of such energy can be realized by capacitors, inductors, flywheels or object at high height and chemicals respectively. While it can be released in various ways, it is most commonly released in a form of electrical pulse, which will be main focus in this paper. Such electrical pulse will consist of sharp spike of current applied to the load, which will have a peak point normally referred to as peak power. In case of electrical current, Ohm’s law can be used to determine peak current to the load in a given circuit and thus, peak power applied to the load. The following equation illustrates Ohm’s law.

=

Figure1: Example of simulated graph of electrical pulse discharge. Red curve is voltage stored in capacitors and blue is amount of current applied to the load, which in this case is a solenoid.

Two key factors that define pulse is pulse length and peak power. They are of great interest to engineers and scientists working on pulsed power to optimize the system. There are various ways to tweak these afore mentioned two variables. The two variables are generally considered inversely proportional when energy available is fixed as lower peak power allows more gradual and lengthy discharge. One can adjust the two variables by either compressing the pulse during the discharge cycle through magnetic flux compression or by

- 261 - APEC Youth Scientist Journal Vol.6 / No.2 fundamentally adjusting energy source to provide higher peak power or through more gradual release over longer period of time. To explain briefly, magnetic flux compression uses one- way destructive process to physically compress a solenoid, increasing magnetic density or flux and thus induced current. This paper, however, will focus on the latter of two options for designing pulsed power systems. In case of electrical circuits and capacitors, the speed at which capacitors discharge can be controlled by voltage in which capacitors are storing electrical energy relative to desired load. Capacitors will continue to discharge its energy, which is proportional to capacitance of the capacitor, until either its stored electrical charge is depleted or the circuit gets broken. However, this is assuming that intended load for the circuit is a simple perfect resister. Depending on the characteristics of the intended load, there can be changes to discharge pattern. Sharp change in current in wires causes magnetic field around them and in case of solenoid, it can be cause of impedance, or apparent resistance, as change in magnetic field induces negative voltage current across the poles. Depending on inductance of the solenoid, it can even cause negative voltage current by storing part of the energy as magnetic field and releasing it, causing them to be detrimental to capacitors, requiring special path with diodes for this current to flow back and decay to protect polarized circuit components. This will be explained more in detail in the section c.

2.2. Capacitor characteristics and arrangement options Capacitors are fundamental energy storage unit for electrical pulsed power system that stores electrical energy in electrical field in between parallel electrical plates. Ideally, capacitors used in pulsed power systems should have no limit on working voltage, no leakage of current, no resistance across its poles and have no temperature or age sensitive characteristics that changes over time. Also, they should preferably have safe failure modes, near infinite lifespan and be very economical to manufacture. However, this is not the case and careful engineering must be done to meet every situation’s needs. Capacitors energy storage can be illustrated with following two equations. 1 E = ^(2) 2 (Energy stored in a capacitor)

Q = Cb[1 − e(/C )] (Equation dictating charging phase of a capacitor)

- 262 - APEC Youth Scientist Journal Vol.6 / No.2

Capacitors store electrical energy as charge. They are charged by being exposed to high voltage current across its poles. High voltage current, whether it is half-wave rectified Alternating Current(AC), pulsing current or Direct Current (DC) can be used to charge capacitors, which capacitors will store in electrical field. Often, it’s necessary to charge capacitors to higher voltage than the available power source without violating energy conservation law. This is generally achieved by AC- AC conversion, DC-AC-AC-DC conversion or DC-DC conversion. The first and second uses transformer and half-wave rectifier for main process while second uses boost converter circuit utilizing large inductor. For this paper, boost converter circuit was used, due to its ability to work with DC voltages. There are several types of capacitors available with current technology, each type with its strength in specific applications. However, they do share basic characteristics of retaining charge between its plates. Two types of capacitors are mainly in use for the explicit purpose of storing power. Electrolytic capacitors are polarized capacitors that uses electrolyte as one of the plates in a cylinder like form factor to achieve great capacitance. They are relatively cheap and features much higher capacitance compared to other types of capacitors but, they are polarized and have sensitive working environment. Contrary to electrolytic capacitors, large film capacitors, on the other hand, which uses insulating film as the dielectric, feature much higher working voltage while being non-polarized but tend to have much lower capacitance compared to electrolytic capacitors. Two are generally considered for different applications, and one must consider pulse characteristics which load will be subjected to. It should be noted that exposing capacitors beyond their certified working environment, in factors such as temperature and working voltage, can cause issues such as electrical breakdown and thus leaking current, overheat or cause destructive failure of the capacitor. To address needs of better capacitors compared to a single one, capacitors can be arranged in series or parallel. By connecting capacitors in parallel, the entire capacitor bank can act as giant capacitor with same working voltage but having sum of all of the capacitor’s capacitance. Also, by connecting capacitors in series, the entire capacitor bank can be charged past its working voltage as individual capacitors can “divide” high voltage across other capacitors. This allows higher voltage capacitor banks but, as energy storable in a capacitor is uniform, capacitance must be decreased accordingly to accommodate for higher voltage capability. Also, it is recommended to have balancing resister across each capacitors in series for even voltage distribution, as failure or manufacturing differences on individual

- 263 - APEC Youth Scientist Journal Vol.6 / No.2 capacitors can put high stress on other capacitors in the bank. Capacitance and voltage characteristics are illustrated in equations below. For practical uses, both arrangements are normally combined to address the specific needs. Below are resultant voltage and capacitance from the combinations.

otageinseries: = 1 + 2 … n 1 1 1 1 Capacitanceinseries: = + + ⋯ C 1 2 Capacitanceinparallel: C = C1 + C2 … Cn

2.3. Acceleration motors powered by pulsed power In order to convert electrical energy stored in capacitors into kinetic energy with pulsed power, there are largely two options. The two types of accelerators act as a load on the circuit and while it does not work with constant power source, they work well when exposed to a pulsed power source and are relatively efficient in converting electrical energy applied to kinetic energy of the projectile. It should be noted, however that they both have unique characteristics of their own. The first is by utilizing a multi-turn solenoid, or coil, as accelerator in a design known as Gauss Gun. According to Ampere’s circuital law, magnetic field around a wire causes magnetic field around the wire. The magnetic field produced is proportional to current applied to the wire. By wounding the wire into a solenoid, and applying pulse current, high gradient of change of current will produce intense magnetic field in the solenoid for a short period of time, around 40ms for a 400v 2800uf capacitor. This would in return accelerate ferromagnetic projectile such as iron at high speed. Factors that are known to affect its performance are: voltage applied and thus peak current, pulse length, relative starting position of the projectile to center of the solenoid, density of wires of the coil and material of the projectile and barrel, which has to be ferromagnetic and nonmagnetic respectively. Solenoids utilized in pulsed power system have interesting properties when exposed to pulsed current. As amount of current decrease after peak power is achieved, due to faraday’s law of induction, the solenoid will try to compensate for decreasing magnetic field strength with reverse current and thus reverse direction magnetic field of its own. This causes “suck-back” effect in which unwanted magnetic field attracts the projectile backwards, reducing efficiency of the system at certain conditions. Depending on energy applied to the solenoid and location of the projectile, it can even pull the projectile backwards. This is

- 264 - APEC Youth Scientist Journal Vol.6 / No.2 interesting area to be research on and can have potential applications in operating accelerators using pulsed power. The second is by utilizing dual rail holopolar motor design or “Rail Gun” design. This design conducts electricity from anode to cathode, which each rail is connected to, through conductive ferromagnetic projectile. While the projectile is conducting the electricity, strong magnetic and electric field appears around the rail and Lorentz force will propel the projectile forwards. This method is simple yet able to conduct large amount of electrical energy. However, due to high amount of localized ohmic heating between the rail and the projectile, slight “Rail erosion” can occur in which thin layer of surface of the rail melt and “erode” with the projectile. This is the reason this design cannot accelerate projectile from standstill and must be given high starting velocity to spread out heating across the rail. Due to its simplistic design and induction not being as problematic as gauss Gun design however, it can generally operate with higher amount of energy. Factors that are known to affect its performance are: voltage applied and thus peak current, instantaneous contact status of the projectile and the rail, material of the projectile which has to be ferromagnetic and rail erosion affecting how the projectile contacts and exits the barrel.

2.4. Considerations for pulsed power systems There are two properties of pulse current that has to be taken note of in engineering. The first is high joule heating. According to Joule’s first law, the amount of heat is proportional to current squared multiplied by resistance of the circuit. As pulsed power optimizes the circuit to have high instantaneous current, often measured in Kilo-Joules to Mega-Joules, the instantaneous amount of heat generated can have problems if not properly accounted for. While most parts of the system can take and dissipate heat over time, contact points with uneven surfaces or thin wires would be first to be overwhelmed by heat and melt due to “skin effect” explained below. Special switch and wires are needed to solve this problem. Also, due to high “frequency” of the pulse, referring to inverse of pulse power length in seconds in this case, materials exposed to pulsed power tend to only conduct current on the “skin” of the conductive material. This shift of current density creates effective “skin depth” in which the current flows. As such, effects of joule heating can be amplified if the system does not feature large surface area for conductive parts and electrical contact parts. Every point of system has to be engineered to withstand this instantaneous load, such as by using heavy duty wire rope.

- 265 - APEC Youth Scientist Journal Vol.6 / No.2

3. Materials and methods

Based on existing military and experimental designs and data from literature search, a modular multi-stage electromagnetic accelerator using Gauss Gun and Rail Gun design was constructed and secured onto the mainframe for the ease of adaptability. It was designed to propel 1cm*1cm*2cm iron projectile, which is ferromagnetic and thus suitable to be used in electromagnetic accelerator. For purpose of this research, minimum set of controls needed for transportation system will be defined as acceleration, deceleration and (angle of). General performance and efficiency of the accelerator will be measured through speed measurement. Aforementioned special characteristics of the acceleration motors will be investigated. Finally, the model will be tested if it can achieve stated sets of controls.

Figure2: Initial layout and assembled experimental set-up excluding 4th stage optional coil for direction control experiments

The design involves four accelerating motor modules, first a solenoid for Gauss Gun and two separate rails. At the end lies mounting spot for second solenoid for additional experiments. Acrylic was used as mainframe due to its transparency and thus its ease of being used for laser based speed measuring and machining with basic tools. Also it is non- conducting, which is vital for the safety of the experiment. Earlier internal tests revealed that acrylic can stand 200°C heat for 5 minutes without much deformation, which in theory should be enough for the experiment. However, for higher energy experiments, stronger materials such as fiber glass should be considered. To hold modules in place, holes were drilled onto mainframe and were secured by stainless steel bolts and nuts. They offer high structural strength, relatively similar thermal expansion and conductivity with copper and most importantly, are non-magnetic, which

- 266 - APEC Youth Scientist Journal Vol.6 / No.2 means that it would not interfere with the operation of the accelerator. To allow buffer to vibrations though, 10mm wooden plates were placed between the mainframe and copper bar module. The first module is powered by parallel grid of small capacitors hooked up to solenoid to provide initial acceleration. This allows accelerator to “break the projectile free” from static friction and allows projectile to smoothly enter the system. Due to unavailability of precision manufacturing equipment such as 3D printers in the experiment, however, custom friction fit adapter involving wooden rods supported by acrylic pieces was used to “inject” the projectile in. This way, injector piece can be separately engineered outside the system but, because this uses friction fit to achieve it, much energy will be lost during the injection process. Thus, it was tested outside the system before being put in for further experiments. However, this can be easily avoided by precision manufacturing tools such as 3D printers to create accurate models. Similarly, fourth stage coil was hooked up to 400V 7800uf capacitor bank accompanied by laser gate circuit which triggers the coil. It was wound more in an attempt to better utilize inductance of the solenoid to achieve the objective. Also, it has no wooden rod guides, allowing about 5mm clearance from diagonal radius of the projectile.

Figure3: first stage coil and fourth stage coil. First was wound 20times * 20 layers while fourth was wound 60 times * 20 layers.

Secondly, two modules of capacitors, each consisting of 3*4700uf 450 WV electrolytic capacitors in parallel configuration with a 20K Ohm bleeder resister, were connected to two bars of pure oxygen-free copper bars. The copper bars were 9mm * 30mm * 200mm. The size was to allow them to have good contact with the projectile and to make use of large thermal capacity to limit thermal expansion of the copper bars. The copper bars for each module were separated by approximately 5cm from each other, separated by non- conducting acrylic adapter, to isolate two systems electrically. These modules were designed to accelerate the projectile with forward Lorentz force produced from the discharge when projectile comes in contact with two copper bars. It is hoped that by placing them one after the other, it can create smooth rail in where energy for most of forward acceleration can be

- 267 - APEC Youth Scientist Journal Vol.6 / No.2 discharged on, as solenoids has much more diminishing returns when exposed to stronger pulse.

Figure 4: 2nd stage and 3rd stage rail structure

Figure 5: Main capacitor bank. Two right-most banks supply power to 2nd and 3rd stage, while left-most bank supplies power to 4th stage. 1st stage capacitors are not in picture.

It is important to note that skin effect and localized joule heating problem was addressed by having the capacitor bank connected wide aluminum plates were used for connecting poles of the capacitors and having neon rope wires in the discharge route of the capacitors.

Figure 6: Schematic for charger .12v input to VCC, connect desirable capacitor at Cx

To charge the circuit from low density energy sources, Direct Current (DC) to DC conversion circuit was used. The circuit shown above is a close to manufacturer reference

- 268 - APEC Youth Scientist Journal Vol.6 / No.2 boost converter circuit utilizing a voltage controller MCP34043, 2000V rated Insulated-gate bipolar transistor(IGBT) and a large 3.5 A rated inductor. In principle, by using voltage controller to repeatedly turn IGBT on and off, Inductor would be subjected to pulsating current. When ‘on’ cycle is initiated, the current would flow through the inductor and part of the energy received will be deposited in the form of magnetic field. However when ‘off’ cycle is initiated, current will no longer flow through the inductor and will release its stored electrical energy as higher voltage pulse. By directing this high voltage pulse with fast- switching diode and sending it to storage capacitor, effective increase of voltage can be achieved. The power draw for the constructed circuit was observed to be about 10W, with effective efficiency of approximately 40 to 50 percent. The circuit was chosen as specification matches commercially available small scale solar panels.

Figure7: Schematic and circuit picture for thyristor control circuit

To control Gauss Gun circuit, SCR (Silicon controlled rectifier) or thyristor was used as solid-stage switch. Above voltage divider circuit was designed to give ON signal to SCR by causing voltage fluctuation across N and P semiconductors to induce avalanche effect. The effect allows current to flow then even though it normally acts as backwards biased diode. The choice of solid state switch is important in Gauss Gun design as charge and discharge cycle must be physically isolated from each other without dealing with contacts welding from joule heating by using a normal mechanical switch. Also, it allows microcontrollers to easily control high power circuits, which would be necessary in later experiments.

- 269 - APEC Youth Scientist Journal Vol.6 / No.2

Figure 8: Schematic and microcontroller for laser light gate system

Figure 8 illustrates how speed measurement circuit works. By creating a voltage divider with a photo-resistor and a fixed 100K Ohm resistor, it is possible to read light received on the photo-resistor based on voltage fluctuation in the readings. So, by shining laser beam to the photo-resister, the microcontroller can detect and process sudden drop in voltage when the projectile passes through and break the beam. This kind of non-contact detection was deemed crucial to minimize loss of kinetic energy during the experiment.

Figure9: Final layout of the experiment rig. The view is on the operator’s end. The four part experiments conducted with the accelerator would be focused on learning how the accelerator operates when important variables are changed and if basic controls can be achieved by applying the observed data. Each experiment was conducted by charging capacitors as needed to full working voltage and by operating each of the accelerator modules as needed. After each test, the average speed of the projectile exiting the system from standstill was measured by the aforementioned laser gate system and 300fps video camera. Motion tracking software was used in post-processing video from the camera. Results from both sources were cross referenced with each other to be averaged as one result.

- 270 - APEC Youth Scientist Journal Vol.6 / No.2

4. EXPERIMENTS AND RESULTS

It should be noted that the dimensions of the iron projectile used is 1cm*1cm*2cm with mass approximately weighing at 13.6g. The first stage Gauss Gun, powered by 2800uf electrolythic capacitor bank charged at 390 volts, has theorectical energy of 212.94J, as calculated in equation in the literature search.

Displacement of the projectile

60mm 45mm 30mm 15mm 0mm -15mm -30mm -45mm -60mm

-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

-60mm -45mm -30mm -15mm 0mm 15mm 30mm 45mm 60mm 1st at 200V/m 0 0 0.2 0.6 0.1 -0.2 -0.1 0 0 1st at 300V/m 0 0.01 0.6 0.8 0.5 0.08 -0.16 0 0 1st at 400v/m 0.03 0.6 1.2 0.9 0.3 0.04 -0.2 -0.1 0

Figure 10: Displacement of the projectile based on distance from the center of the solenoid to center of the projectile

To test how the relative position from center of the solenoid roughly affects performance of the Gauss Gun, the first experiment measured displacement of the projectile on an even wooden surface with varying voltages. It was noted that inductive “kick-back” or tendency for solenoid to store pulsed energy and releasing it in reverse direction when magnetic field dies down tends to dominate as the voltage in capacitors, or amount of energy available decreases, and optimal position for best forward displacement tends to go backwards in the perspective of the solenoid.

- 271 - APEC Youth Scientist Journal Vol.6 / No.2

Table2: 1st stage performance in energy efficiency Attempt 1 Attempt2 Attempt 3 Average 1st stage 20cm/102m 20cm/114ms 20cm/108ms 20cm/108ms only s(12.9J) (10.4J) (11.4J) (11.6J)

For the second part of the experiment, effeiency of the modules were tested. Testing first stage coil individually at full charge outside the system reveals that it has efficiency of around 5.45 percent in converting electrical energy into kinetic energy in its optimal configuration.

Table 3: performance characteristics of the accelerator modules in a system Test set 1 Test set 2 Test set 3 Average Test1: 45cm/465ms 45cm/376ms 45cm/40ms 4.09J 1st stage (3.18J) (4.88J) (4.20J) only Test2: 1st 45cm/144ms 45cm/135ms 45cm/131ms 37.0J stage + 2 nd (33.3J) (37.7J) (39.9J) stage Test3: 1st 45cm/116ms 45cm/104ms 45cm/96ms 63.0J stage+ 2 nd (51.1J) (63.0J) (75.4J) stage + 3 rd stage

Efficiency of the modules

3.5

3 2.668241101 2.55249591 y = 0.326x + 1.7603 2.5 2.016219738 2

1.5

1

0.5

0 1st stage 2nd stage 3rd stage

TS1 TS2 TS3 Average 선형 (Average)

Figure 11: Efficiency of the modules.

- 272 - APEC Youth Scientist Journal Vol.6 / No.2

Knowing that 2nd stage and 3rd stage capacitors has theorectical energy of 1128J, efficiency was calculated based on kinetic energy from speed measurement against total amount of energy available in each case as follows. Note that thin layer of conductive grease were applied to the system before each tests for better contact. Aside from efficiency drop of first stage due to friction fit adapter mechenism, it appears that higher speed improves efficiency of the modules, as there was increase in overall efficiency when operating third stage module.

Figure 12: Rail Gun section under operation

Figure 13: Rail erosion comparision under stereoscope. From top left to bottom right. Control, scratches from rail contact, 2nd stage at 200V no grease, 2nd stage at 400v, 2nd stage at 400V with conductive grease

- 273 - APEC Youth Scientist Journal Vol.6 / No.2

For the third part of the experiment, just like Gauss Gun, key factor affecting the performance for Rail Gun portion was observed for possible trends. To investigate rail erosion phenomenon for Rail Gun portion, projectile after being fired in the accelerator was observed under the 40 X stereoscope. It is apparent that upon contact with high voltage rails, massive amount of current heats up the contacts and causes local area to melt and effectively mix as the projectile exists the accelerator. Traces of copper can be seen on the eroded projectile as well as protruding parts of iron. Increasing voltage and thus current across the rail appears to increase the damage significantly. By adding conductive grease to “spread” contact area from center to surroundings (4th picture), the erosion is much more spread out but damage depth appears to be much less. To experiment on controls, the 3rd experiment involved 4th stage module being installed at the end of 3rd module with dedicated laser trigger system controlled by a micro controller and a capacitor bank, which was 400WV , 3900uf *2. It had theoretical energy capacity of 624J. All stages were fired at full power during this section of the experiment and the average value from above tests were used to give timing values for microcontroller to trigger accounting for delay for SCR triggering (100ms) with data from the manufacturer’s specification sheet. To boost the projectile (acceleration), the trigger was activated -30mm from the center of the solenoid, while to decelerate the trigger was activated 30mm from the center of the solenoid. Muzzle velocity from T S 3 4th Stage ( 3 test averages ) Acceleration 45cm/96ms(75.4J) 20cm/36ms(104.7J) Deceleration 45cm/96ms(75.4J) 20cm/57ms(41.7J)

Figure14: 4th stage control test results

- 274 - APEC Youth Scientist Journal Vol.6 / No.2

5. DISCUSSION

From the tests, the average value for energy efficiency ended up being around 5 percent for Gauss Gun portion and 3 percent from Rail Gun portion. This finding matches results from literature search. By improving upon precision manufacturing and doing away with tight friction fit design however, efficiency is expected to go up by another percent or so on all accelerator modules. As we observed higher efficiency at higher speeds, based on the trending we can expect to see higher efficiencies of about a percent at more massive scale. This is possibly due to lessening effects of solenoid inductance and rail erosion at higher speeds. While this is not particularly impressive, it should be noted that due to “engine” being outside of the accelerator, much higher energy can be put into the lighter projectile. Wire materials, voltages and innovations on stronger electromagnets such as super conductors have potential to improve efficiency further though and should be a topic of future study. Two unique properties of Rail Gun and Gauss Gun acceleration systems, rail erosion and coil inductance, were tested to be documented as well. Conventionally both factors are considered as performance reducing factors of each of the systems. Literature search revealed that if both factors are not considered in engineering process, it can to either reduce lifespan of the system or waste energy, reducing performance. This paper however investigated if both can be used as part of more flexible control strategy in a full scale accelerator system, even in as simple as abort scenario. It turns out however, while utilizing inductance in Gauss Gun design showed significant acceleration, deceleration and deflection when placed at an angle, rail erosion happens without distinct pattern and only affects the projectile at relatively low speeds compared to instantaneous current. Maybe shorter pulses of higher voltage can give more reliable control to rail erosion to give as much reliability to act like a brake or steering wheel by controlling relative friction between the rail and the vehicle. Right now, the best apparent thing to just minimizing effects of rail erosion appears to be increasing effective instantaneous contact between rail and the projectile, by means such as applying conductive grease. It is also recommended to coat the projectile with same material as rail to prevent rail and projectile from “chipping away” each other’s metal which would affect performance in the long run. Finally, utilizing what was learnt from Gauss Gun’s properties due to inductance and by changing relative position of the projectile compared to solenoid’s center, it was possible to achieve acceleration, deceleration and deflection with computerized control system

- 275 - APEC Youth Scientist Journal Vol.6 / No.2 without resorting to traditional linear one-way design. Also, the design, proven working with the experiment, does not use complex relay or control system, allowing designs to be more streamlined and optimization-friendly. This also opens innovative rail design options for pulsed power electromagnetic accelerators vehicle transports and as launchers, as it allows complex curved tracks, with relatively large radius given small deflection as with shift of center of solenoid. The only specialty equipment needed would be computer with positioning and speed data can control the accelerator. Proposed reference design incorporating basic sets of controls and optimized efficiency is illustrated below. Note that from experimental result, it is estimated to reach peak efficiency at higher speeds.

Legend: Rail design: copper rails separated by junctions of non- conducting rails Cuboid at the center: ferromagnetic vehicle Front cylinder- smaller shorter solenoid Back cylinder – bigger longer solenoid Rectangles on the right- solar panels Cylinders on the left: capacitors

Figure 15: Possible design proposal for large scale electromagnetic pulsed power transport infrastructure.

6. CONCLUSION

Electromagnetic pulsed power accelerators are solution that can take low density energy sources such as solar energy while potentially addressing modern needs of high speed transportations complimenting air travel. It can also be used in variety of other important applications such as launchers for aerospace applications. By studying two types of acceleration motors, Gauss Gun and Rail Gun, in a single system, it was concluded that

- 276 - APEC Youth Scientist Journal Vol.6 / No.2 utilizing both kinds of accelerators, basic reliable control for pulsed power transport system can be achieved. This validates existing efforts to bring previous limited use from linear one- way weapon systems to be used in civilian transport applications. Should someone decide to further develop the concept of pulsed power accelerators into more mature design, findings from this paper would prove to be useful.

7. ACKNOWLEDGEMENTS

I would like to thank Natural Science Department of Yeoido High School, South Korea for providing me with laboratory space and apparatus to conduct the study and Mr. Song Su Ahn for his supervision to ensure safety during dangerous parts of the experiments.

8. REFERENCES

[1] James A.Rabchuk (2003), The gauss Rifle and Magnetic Energy, The Physics Teacher, pg. 158- 161 [2] Takao Namihira(2007), Electron Temperature and Electron Density of Underwater Pulsed Discharge Plasma Produced by Solid-State Pulsed-Power Generator, IEEE Transactions on plasma science, 2007 pg. 1-2 [3] Gennady A. Mesyats(2008), Pulsed Power, Kluwer Academic, pg. 1-25 [4] Gennady A. Mesyats (2004), The RADAN Series of Compact Pulsed Power Generators and Their Applications, Proceedings of the IEEE Vol 92, pg. 1-3 [5] Hidenori Akiyama(2007), Industrial Applications of Pulsed Power Technology, IEEE Transactions on Dielectrics and Electrical Insulation Vol 14, pg. 1-5 [6] Sitzman (2006), Design, Construction and Testing of an Inductive Pulsed-Power Supply for a small railgun, Institute for Advanced Technology, The University of Texas at Austin, pg. 1-5 [7] Ian R. McNab (2003), Launch to Space with an Electromagnetic Railgun, IEEE Transactions on Magnetics Vol 39, Pg. 1-5 [8] Malcolm Buttram (2003), Some Future Directions for Repetitive Pulsed Power, IEEE Transactions on Plasma Science Vol 30, Pg 1-4 [9] M.S. Aubuchon (2005), Results from SANDIA national Laboratories/ Lockheed

- 277 - APEC Youth Scientist Journal Vol.6 / No.2

Martin Electromagnetic Missile Launcher (EMML) , Sandia National Laboratories, pg 1-4 [10] Paul R. Berning (1999), A coilgun Based Plate Launch System , IEEE Transactions on Magnetics, pg 1-3 [11] David Griffiths, Introduction to Electrodynamics, 1st ed. (Prentice Hall, Englewood Cliffs, NJ, 1981), Vol. I, Chap. 7, pp. 270–271. [12] Hugh D. Young (2007), Sears and Zemansky’s College Physics 8 th edition, pg 595- 599, pg 680-684

- 278 - APEC Youth Scientist Journal Vol.6 / No.2

9. APPENDIX (Optional)

9.1.Sequence diagram for light gate

*Note: Sensor output is 0-1023 and it normally outputs 950. Function micros () outputs amount of time past in microseconds.

Start

Loop

Yes If sensor 1 Check =1;

>= 920 Time1= micros();

No No

If sensor 2 Yes If Check Yes >= 920 ==1 Time2=micros();

No Print (Time2-Time1)

LED On

Delay 1s

- 279 - APEC Youth Scientist Journal Vol.6 / No.2

9.2. Safety procedures involved in safe operation of capacitor based electromagnetic accelerator

Due to the nature of the experiment, high voltage in high energy, it deemed necessary to dedicate a section for safety considerations of pulsed power experiments. Due to nature of high energy available should abnormality strike, it can result in catastrophic failure scenarios, including but not limited to, explosion, most likely at the capacitor or the accelerator which acts as “load” in the circuit, electrocution of the operator, flying debris from structural failure and accidents while leaving the setup unattended. As such, proper safety gear must be worn at all times and operators must adhere to safety measures while dealing with charged capacitors. Proper abort scenarios must be considered prior to dealing with these set ups. For this experiment, bleeder resistor, or resistor permanently mounted onto the capacitors, are placed as final abort scenario. While not involved in main operation of the capacitors, it acts as load which slowly drains capacitors of energy. It should be noted that without external set-up capacitors might retain lethal charges for days. The resistance and load capability of the resister must be calculated properly as resisters can fail should high enough voltage and thus high enough load is place onto them. Also, it is highly recommended to secure wire connections to hard surfaces to prevent disconnections and rogue unconnected wires. Measuring equipment must be able to reliably and safely measure voltages across capacitors at all times to give the operator clear idea of the danger involved in the operation. Stand-by measuring equipment must be ready at all times to replace failed equipment should a failure arise. For reliability of the measuring system, sensitive electrical components should be electrically shielded too, as fluctuations in magnetic field can cause noise to nearby electrical equipment.

- 280 -