Task 53.RF/95 Plasma Neutraliser Development

Task 53.RF/95 Plasma Neutralizer Development

Authors : V.F.Arsent’ev, P.M.Kosarev, A.A.Skovoroda, V.A.Zhil’tsov,

V.S.Svischov, V.M.Kulygin, V.V.Platonov, V.P.Ukhov, V.F.Zubarev,

I.D.Karpushov,

E.Yu.Klimenko, S.A.Lelekhov, S.I.Novikov

1. Plasma Neutralizer for ITER NBI

2. PNX-1 experiment

3. PNX -2 experiment

4. Results of a plasma pumping experiment

5. PNX - U installation

6. Desing calculations of pumping system for PNX-U

7. Preliminary proposals on PNX-SU

8. Design conception of superconductive magnetic system of PN-ITER

9. Conclusion

1 2 1. Plasma neutraliser for ITER NBI

INTRODUCTION

Injectors of deuterium atom beams are developing now for ITER plasma heating and current drive are based on the idea of negative ion acceleration and further neutralisation. The reference design concept means use of a gas cell neutraliser. The maximum neutral fraction from the passage of a D- beam through a gas neutraliser is 60%. With a plasma neutraliser, the maximum neutral yield could exeed 80% ( Fig.1 ), and the replacement of the gas neutraliser by a plasma one is an attractive improvement of the ITER NBI system.

GENERAL REQUIREMENTS

Figure 1 mentioned above demonstrates a neutral yield from the passage of a D- beam through a gas medium with different ionisation degrees in depandance on a target thickness. These data give us a possibility to make a choice of the target thickness range and the range of the ionisation degree for ITER

NBI Plasma Neutraliser:

* The optimal plasma target thickness, nL, must be about 2.1015 cm-2 ;

* The target ionisation degree, f must be  0.3 .

As PN must be able to replace the gas neutraliser in the ITER NBI of the reference concept [ 1 ] its length, L , is defined and equal to 3 meters. So the necessary plasma density : n  7.1012 cm-3 .

To estimate a sufficient energy confinement time,   , for a plasma trap is necessary to arrange the plasma body of approximately 8 m3 we can use the rough expression :

-13 PPN [MW] = 1.6.10 nV/

3 -3 PPN - a power is needed for PN running , [ MW ]; n - plasma density, [ m ];  - energetic “ion cost”, [eV];  - confinement time, [ s]. V - plasma volume, [ m3 ].

The NBI power efficiency

 = Pin / Pm

Pin - neutral beam power, injected to the torus; Pm - total power to NBI from main.

PN could provide injected neutral beam power increasing by a factor 1.4 (80% of electron loss efficiency instead of 60%). So the total enhancing factor because of replacing the gas neutraliser by the plasma one could be :

 = 1.4 (1 - PPN /Pm )

Figure 2 demonstrates the dependance of  and Pm on PPN for the reference value of Pin = 12.5 MW

(per one NBI module). One can see that the “efficiency enchancement factor” will be of 1.34 - 1.3 , if P PN 

1 - 2 MW.

So, if we assume the parameters values as follows :

18 -3 3 n = 6.10 m , V = 8 m ,  = 100 ev , PPN = 1 MW

we can find out with the help of formula (1) that it is necessary to provide the confinement time of 10-3 s.

This confinement time could be provided with the magnetic confinement system which must be arranged in such a way in order that the increasing of initial ion beam divergence will not exceed 10 %.

PHYSICAL BACKGROUND

4 Previous experiments with Plasma Neutralisers.

Neutralization with plasma jets (methods of their production are different), which propagate within a magnetic channel [1-4] or in a vacuum [5], has been studied for a long time.

The main deficiencies of such PN are rather evident: high energy consumption, since the plasma confinement is bad; large deviations of beams because of a leading magnetic field. In Fig.1 one can see a PN [4] possessing of the best characteristics W~500 W/cm2 (W-power density consumed per surface unit for ion sources) in operation with hydrogen (for comparison W~2100 W/cm2

[3,Dimov] and see also [6]). In operation with heavy gases the power characteristics are thrice improved. One can see that this PN-system can be used at the limit of its abilities.

Let us note two deficiencies more for the PN shown in Fig.1:

- it is good for one source and bad for the whole injector (engineering advantages of one neutralizer for all the injector sources are evident);

- the whole plasma flux is neutralized within the source region (hydrogen flux into PN in Fig.1 is equal 5 l torr/s) and high power means for the gas evacuation are needed.

An interesting variety of the discharge in the crossed electric and magnetic fields, applicable to PN, was studied in [6]. However, high values of W~1600W/cm2 reduce the optimism of researchers.

Various schemes of magnetic plasma confinement are offered to improve the PN- parameters. The "magnetic wall" produced with the permanent magnets with alternating polarity

(another name of such a trap is a multicusp or a picket fence, proposed at the dawn of fusion studies [7]) seems to be the best modification.

The magnetic field is concentrated nearby the wall only and provides an extremely - small effect on deviation of the beam ions. Plasma can be injected into such a trap (arc plasma sources [8], hollow and plasma cathodes [4,9] are expected to be used for it) or be produced directly in the trap. The latter modification needs a smaller pumping [8] and was used in [10,11].

5 A PN from those studies is shown in Fig.2. It is characterized by the usage of RF-discharge

(frequency is a few MHz) and by the absence of a magnetic confinement along the beam direction.

A free plasma outflow to the end-face and its losses within the RF-antenna, submerged into plasma, provided a high power density, W~500 W/cm2 . Using Xe as a working gas, the plasma density 1014 cm-3 with the ionization degree equal 0.5 was obtained.

These results stimulated the PN-studies for ITER and the proposal shown in Fig.3,4 [9,12] was done. In these Figs. one can see the magnetic wall for the end faces is also designed. The main deficiency of this PN is the presence of an electrode in the plasma, which mainly provides its losses.

In [13] there is an information about the start-up of the experiments on the dense Xe- plasma production in a multicusp system with the usage of a microwave power (2.45 GHz,5 KW) under ECR condition. This direction seems to be the most promising for us, since such discharges do not have electrodes and the power is effectively deposited into electrons (only electrons ionize a gas) by the known laws of ECRH (in difference from the RF discharge, where the turbulent mechanism of electron heating is often realized [14]).

REFERENCES

[1] G.I.Dimov, G.V.Roslyakov. Nucl.Fus.15,551,1975

[2] G.I.Dimov, A.A.Ivanov,G.V.Rosljakov,Sov.JTF.47,1881,1977

[3] G.I.Dimov, A.A.Ivanov,G.V.Rosljakov,Sov.Plasma Phys.6,933,1980

[4] A.Hershcovitch, V.Kovaric, K.Prelec,3-th Eur. Workshop on Production

and Application of Light Negativ Ions,Amsterdam, 1988,p.217

[5] G.M.W.Kroesen, D.C.Schram, ibid. p.209

[6] K.Yoshikawa, Y.Yamamoto, H.Toku, N. Komoda, A plasma neutralizer

using magnetron discharge,IAEA Committee Meeting on NBI,Japan,1991

[7] J.L.Tuck, in Plasma Physics and Controlled Thermonuclear Fusion,

6 ed. by C.L.Longmire,(Pergamon, New York,1963),v.2,p.278

[8] P.M.Vallinga, D.C.Schram, H.J.Hopman,5-th Int. Simp.Prod. and Neutr.

Negativ Ions, Brookhaven,1990,p.729

[9] A.J.T.Holmes, L.M.Lea, K.P.Martel, M.F. Thornton, Study report on the

conceptual design of a NBI and a plasma neutraliser, Culham Lab.,1989

[10] J.R.Trow,K.G.Moses,4-th Int. Simp. Prod. and Neutr. Negativ Ions,

Brookhaven,1986,p.651

[11] K.J.Moses, J.R.Trow, J.C.Dooling, 5-th Int. Simp. Prod. and Neutr.

Negativ Ions, Brookhaven,1990,p.717

[12] A.J.T.Holmes, R.McAdams, Y.Takeiri, S.Wells, A.F.Newman, RF plasma

neutraliser and source developmen, ITER Specialist Meeting on NB

System, Moscow, 21-24 October,1991

[13] Y.Okumura, Negative ion beam development at JAERI, ibid.

[14] W.F.DiVergilio, H.Goede, V.V.Fosnight, Rev.Sci.Instrum.57,1254, 1986

Figure Captions

Fig.1 PN with a plasma stream [4].

Fig.2 PN with the RF discharge in the chamber with a magnetic wall

consisting of permanent magnets [10,11].

Fig.3 ITER -PN -design with an RF discharge [12].

Fig.4 ITER -PN -design with an RF discharge [12].

Atomic processes in PN.

We ought to take into consideration a lot of atomic processes which take place between energetic negative ions of the beam and different particles of the PN plasma. These are : electrons, positive ions of

7 different charges and neutral atoms. As we are interested in “near Mev” beam energy range, the most important processes are :

- a loss of one electron by a fast negative ion ( -1,0 - process cross section),

- a loss of one electron by a fast atom (0,1 ).

The other processes, as :

- a loss of two electrons by a fast negative ion (-1,1 ),

- a positive ion and gas atom charge exchange ( 1,0 ),

are not so important because of relatively small cross sections. However, -1,1 can be compared with 0,1 in the ion-ion collisions, but this can’t occur the noticible negative effect on the conversion efficiency (N) and on the optimal target thickness (nl). This can be found out from the next expressions could be easily derived using beam particle balance equations

N = k(k+p)-1 exp { -ln(k+p)/(k+p-1) } ,

-1 nl = <-1,0 > k ln(k+p)/(k+p-1) ,

<-1,0 > <-1,1>

k =   1, p =   1 .

< 0,1> < 0,1>

Here < > means the averaging over the target species

 i n   i ni

Fig. 2 demonstrates the efficiency (N) as a function of cross sections ratio (k). This ratio is almost

independant on energy because of the similar energy dependances of -1,0 and 0,1 and is fully determined by the target substance. The cross sections of interest for all the PN components are presented below.

Molecules and atoms

8 TABLE 1 Fast hydrogen ions (atoms) interaction with hydrogen molecules

Energy, MeV 0.25 0.5 0.75 1.0 -17 2 -1,0 , 10 cm 23 13 9.5 7.2 -17 2 -1,1 , 10 cm 1.5 0.7 0.48 0.35 -17 2 0,1 , 10 cm 6 3.2 2.2 1.7 -17 2 1,0 , 10 cm 0.07 0.03 0.00 0.00

TABLE 2 Data for gaseous targets (zero ionization) at H- energy of 0.2 MeV

Gas H H2 He N2 O2 Ne Ar Kr Xe K vap k 4 4 2.7 2 2.2 2.3 2.5 2.5 2.5 3 -17 2 -1,0 ,10 cm 16 26 15 75 90 35 69 88 90 133 15 -2 nlopt, 10 cm 11 7 11 1.9 1.6 4.2 2.2 1.7 1.7 1.2 N max 0.63 0.63 0.56 0.5 0.52 0.53 0.55 0.55 0.55 0.58

So, the optimal value of nl for the case of a gaseous target must be of 1016 cm2 for hydrogen and light gases and an order of value less of this in the heavy gases case. But the convertion efficiency is diminished in the last case and a tokamak plasma could be soild.

Electrons

-19 -2 -17 -2 The cross sections -1,0 ~1.7 10 cm and 0,1 ~5.2 10 ñm for hydrogen beam - electrons interaction at center of mass energy ~100 eV (0.2 MeV of H - energy corresponds to 110 eV of the center mass energy). It can be easily seen that the ratio k for the processes with electrons is essentially greater (k ~

30) than that for a gas target. The only electron component of the PN could provide a conversion efficiency of 0.89 at a low linear density nl of ~ 1015 cm-2 .

Single charged ions

The cross sections of interest for processes with ions participation don’t depend practically on ion species being close to those with electrons participation at correspon-dantly recalculated center mass

9 energies. So the ion component of the plasma simply doubles an effective charges density and reduces two times the necessary value of ne l .

Multicharged ions

Unessential cross sections encreasing occurs when effective ion charge (q) increases.The only cross

section 0,1 is presently studied rather well. The dependence on ion species is rather weak, so we can use an approximation as follows :

-16 0,1 = 4.6q 10 (32q / E)[1-exp{-E / 32q } ,

where E is the energy in KeV / nucleon. One can see that the dependence is   q2 when the energy is high enough and q is rather small (to cancel the exp) . At low energies and high q the dependence is   q. In the

 intermediate range we can assume   q , where 1 <  < 2. The only measurement of -1,0 for this case was performed for Ar ions with q  8 [ ]. The data obtained are represented in the next Table.

TABLE 3

q 1 2 3 4 5 6 7 8 --15 2 -1,0,10 cm 4.5 12 20 28 40 50 63 75 --15 2 -1,1, 10 cm 0.25 0.35 0.6 1.2 2.5 3 --15 2  0,1, 10 cm 0.25 0.65 1.1 1.5 2.1 2.5 3.2 4 k 18 19 18 19 19 20 20 19 --15 2 *-1,0,10 cm 5 14 33 53 73

One can see that the ratio k weakly depends on the ion charge number q. I it was previously noted that k

weakly depends on the energy as well. The approximative formula for -1,0 can be assumed as follows

-14 *-1,0 = q 10 (32q / E ) [1-exp{-E/32q}]

10 and the comparison of the last and the second rows of the Table demonstrates a good agreement between the approximation and the experimental data.

We can do the conclusion : the multicharged ions can’t influence on conversion efficiency N, but can reduce 2-3 times the necessary value of nl.

Plasma as a whole

All the plasma components exist in the system. The densities of atoms and molecules are not rigidly connected with the electron density, but the charged components densities are bound with the quasineutrality condition :

ne =  qi ni

One can see that the density of q times chararged ions mast be q times lower than the electron density if the plasma consists of q times charged ions and electrons only.

Fig. demonstrates the dependence of efficiency (N) and optimal linear density (nl) on an ionization degree of a hydrogen plasma. The parameter which defines the different curves is H- energy. One can conclude that the plasma neutraliser is effective at a rather high ionization degree (  0.4 ).

Magnetic system for Plasma Neutraliser

To arrange a plasma body with so high ionization degree without spending more energy than we could save due to using of PN instead of GN, a good confinement magnetic system must be developed.

Besides a high confinement time this system must provide an acceptable magnetic field influence on negative ion motion before their conversion into neutral atoms.

We assume that the best magnetic system to meet the demands listed above could be constructed on the base of multicasp. We’ll try to validate this assumption below.

11 Plasma confinement in a multicasp.

Plasma confinement in multicasp systems is studied rather well [ ]. The main ideas can be given as follows.

The plasma particle flux upon the wall (I) in such a system is determined by the expression well known from the probe theory

1/2 I = Ss 0.6 ne { qTe / Mi }

where instead of a probe surface area is used an effective area of magnetic slits :

Ss = N L1 

N - number of slits, L1 - a length of one slit,  - an effective width of a slit .

A plasma particle confinement time (  ) can be defined as follows :

 = 

I where V is a plasma volume. Lot of theoretical and experimental studies were performed to determine an effective slit width  . We assume the following expression is rather acceptable :

 = 4 ( e I )

where e, I - are larmor radii of electrons and ions correspondently for a slit magnetic field B s. This expression was checked experimentally only at small plasma volumes.

The above relationships give us a possibility to obtain a scaling low for multicusp confinement time

12 2 1/4 V / L1 [cm ] Bs [G] A

 [sec]  3  10-8 

3/4 1/4 N Te Ti [eV]

This formula shows clearly that the confinement time has a very weak dependence on Ti , ion mass number

(A) and plasma density, no dependence on ion charge number and more srongly depend on magnetic field , geometry and electron temperatute.

Magnetic field influence on the ion motion.

The use of a strong magnetic field in the PN for improving the plasma confinement provides a deviation of the H- /D- ion motion from a rectilinear one. The requirement to the smallness of deviation is very stringent and essentially defines the scheme of magnetic confinement. We approach the necessity of plasma confinement by a "magnetic wall" (multicusp), where, actual, there is no magnetic field in the main volume of the trap and the field is concentrated at the walls.

Let us consider the flat (xz-surface) "magnetic wall" which is produced with straight conductors carrying alternately directed currents along z axis. The distance between the conductors is l. An analysis of particle motion in such a field is simple as z-component of generalized momentum is conserved

P = mv - eA /c = const .

Under injection of particles from the region of magnetic field absence at x with the velocity v along y-axis, one has P =0. After simple transformations one obtains the deviation within the trap

V lx x  V 2 2 where the larmor ion radius,  , is determined at the point with a maximal magnetic field on the trajectory. For the slit size l~6 cm,x ~2 cm the deviation ~5 10-3 will be provided at the maximal

13 field ~ 1kG. Thus multicusp magnetic field can provide small deviations at rather strong magnetic fields in slits.

The computer code was developed for calculations of spatial distribution of magnetic field components and field lines for a system of PN superconducting coils. This code provides also calculations of trajectories of beam particles passing through PN and neutral beam phase space diagram taking into account a probability of D- ions stripping when some assumptions is made about plasma and cold gas distribution in the PN volume and in the entrance and exit windows region.

Fig.1 shows an example of magnetic field lines in the PN volume for a system of coils, shown in Fig.2. Fig.3 gives a behaviour of magnetic field components on PN side: By(y) in the magnetic slit plane with 1 T strength in a slit and about 6 Gs near the beam edge and Bz(y) in the plane between the slits when current in the coils is Ic = 83 kA. Distribution of transverse component of magnetic field By(z) along the centreline of PN window for ion beam passing is represented in

Fig.4. Current in a set of coils that produce magnetic field in the region of PN windows is reduced to obtain maximum field less than 0.1 T (to reduce a growth of neutral beam angle in vertical direction).

Fig.5 shows on example of calculated neutral beam phase space diagrams in horizontal (y,y') and vertical (x,x') directions at the exit of PN. Each D- ion beam passing through the window is represented in horizontal plane with 7 large particles, equally spaced along the window width, their current decreases along z-axis as I = I0 exp(-w), where w = òdw = òn(z)sdz. Each point on phase

0 diagram represents a fast D that arises when dI/I0 = 1/400. Calculations show that if initial 1 MeV

D- beam has 5 mrad angle, the neutral beam average (rms) angle at the neutralizer exit will increase to about 5.5 mrad in horizontal direction and up to ~14 mrad in vertical one. Thus the end walls

14 magnetic system leads to increas of beam divergence within allowable limits in horizontal direction.

The increase of vertical angle can be reduced by decreasing of transversal field and providing of sharp plasma and cold gas gradients in the windows region.

Figure capture

Fig.1. Magnetic field lines in yz-plane

Fig.2. Side and end wall magnet coils with gas boxes

Fig.3. Variation of magnetic field components in PN middle in y-direction; By(y) in the magnetic slit plane and Bz(y) in the plane between the slits.

Fig.4. Variation of magnetic field component By on the PN end wall opening in beam direction

Fig.5. a- calculated neutral beam phase space diagrams in horisontal yy’ direction ; b- in vertical xx’ direction after PN

2. PNX-1 Experiment

The experiment on PN-1 device (see Fig.1) has shown, that by stationary work in 70l

11 -3 multicusp chamber the plasma density 5 10 cm =7ncut-off was produced at low absorbed power

3 density 0.02W/cm . Thus at Te=4-10eV the ratio Te/=10 was obtained at the ionization degree 0.1.

The original surface type discharge was realized, when the microwaves were exited in plasma wavequides, ab, formed by plasma surface and metallic chamber wall (see Fig.2). The plasma was heated on periphery in conditions of upper hybrid plasma resonance. The gas ionization takes place on periphery. The detail investigation of this discharge can be found in work [1]. PN-1 was also used as a base for investigation of a superpermeability of Nb tubes in a real microwave discharge conditions. The tube with a thickness of 0.1 mm was checked for a pumping of hydrogen . Plasma

15 flow of 0.25A was fallen on a tube in steady state conditions and up to 33 % of the flow penetrated inside the tube. In this

experiment conditions the gas pressure inside the tube 1000 time exceeded the pressure in the discharge chamber.

16 Fig.4. PN-1 device

17 Fig.2. Formation of plasma waveguide in multipole mirror PN-1 [5].

REFERENCE

[1] V.A.Zhil’tsov,P.M.Kosarev,A.A.Skovoroda.Plasma Phys.Report,22,No.3, 1996,pp.246-258

18 3. PNX-2 Experiment

PNX-2 device is a modification of PNX-1. Compared to PNX-1 was set up the new

elements (Fig.1):-

-klystron amplifier 3 kW, 7 GHz, steady state operation,

-transmission line which includes ferrite circulator, duplexing assembly, divider, vacuum

tight ceramic window, waveguide lines 15x35 mm,

-calorimetric measurements transmitted and reflected microwave power

A plasma is produced under absorption microwave power from two generators : (1)3 kW, 7

GHz and (2) 5 kW, 2.45 Ghz. Experiments on PNX-1 device ( the first generator only) are shown that at plasma density above critical most of the microwave power reflects. Only about 1 kW from 5 kW is absorbed under plasma resonance near the wall (surface microwave discharge [1]). It should be pointed out that plasma is produced easily because there is the electron cyclotron resonance( ECR) on frequency 2.45 GHz. There is no ECR on 7 GHz but for this frequency the critical density is much more then for frequency 2.45 GHz.. The microwave emission from 7 GHz generator inputs near liner between magnets (Fig.1,2) for excitation of the plasma surface discharge

The main purposes of PNX-2 experiment are:

-study of microwave power absorption physics,

-investigation of possibility to increase plasma density under plasma resonance heating.

PNX-2 parameters are pointed in Table 1.

19 Table 1.

Plasma diameter, m 0,4

Plasma length, m 0.6

Plasma volume,m3 0,075

Microwave generators frequency and power:

(1) 2.45 GHz, 5 kW

(2) 7 GHz, 2 kW

Resonant magnetic field, T:

(1) 0.0875

(2) 0.25

Critical density, m-3:

(1) 7.5.1016

(2) 6.1.1017

Microwave generators mode of operation, steady state

Magnetic field in slits, T 0.3

Max.magnetic field between slits, T 0.022

The scheme of experiment is shown on Fig.1 . A plasma is produced under absorption microwave power from two generators : (1)1 kW, 7 GHz and (2) 5 kW, 2.45 GHz. Experiments on

PNX-1 device ( the second generator only) have shown that at plasma density above critical most of the microwave power reflects. Only about 1.5 kW from 5 kW is absorbed under plasma resonance near the wall (surface microwave discharge [1]). It should be pointed out that plasma is produced

20 easily because there is the electron cyclotron resonance( ECR) on frequency 2.45 GHz. There is no

ECR on 7 GHz but for this frequency the critical density is much more then for frequency 2.45

7 GHz generator consists of a signal generator and two-stage klystron amplifier. A magnetron is a 2.45 GHz generator.

The most impotent plasma neutralizer parameter is plasma particle confinement time. A turn off 2.45 GHz generator tends to change of the plasma structure and behaviour. A life time measurement is rendered possible by a small independent modulation plasma density. A 7 GHz generator was used for this purpose.

The next diagnostics are used:

 8 mm interferometer for nl measurement,

 H optical measurements.

About 1 kW at 7 GHz was entered into vacuum camera wherein previously was produced a plasma at 2.45 GHz with a density 1011 cm-3.Approximately 0.5 kW at 7 GHz was absorbed by plasma.

Two generation regime of 7 GHz klystron was used at our experiments: steady state operation and modulation regime. Some results on modulation regime are given below.

The oscillograms of 7 GHz power modulation, interferometer and H intensity signals are shown on Fig.3. The same oscillograms during one modulation period are presented on Fig 3. A modulation frequency is 500 Hz. It is necessary to note, that only a varying part of signals from modulation is pointed on oscillograms Fig.3,4. A constant with time part of signals due to absorption of 2.45 GHz microwave power is far more.

21 A maximum ratio of varying part to constant part of signals (modulation coefficient) is 0.3 under absorbed power 1.5 kW at 2.45 GHz and 0.5 kW at 7 GHz. This coefficient is depended linearly on introduced 7 GHz power. A power is absorbed over plasma volume uniformly. It will be recalled that 2.45 GHz power is absorbed on plasma surface under plasma resonance.

As will be seen from Fig.4 the decay time of interferometer and H intensity signals is equal

~1 ms and it closely to theoretical value.

Reference

[1] V.A.Zhil’tsov,P.M.Kosarev,A.A.Skovoroda.Plasma Phys.Report,22,No.3, 1996,pp.246-258

SIGNATURES.

Fig.1. PNX-2 device scheme:

1- magnetron 2.45 Hz, 5 kW, 2- circulator, 3- load,

4- continuous attenuator, 5- matching transformer,

6- vacuum tight ceramic window, 7- vacuum chamber,

8- copper liner, 9- Nd-Fe-B permanent magnets,

10- microwave lead-in 2.45 GHz, 11- lead-in 7 GHz,

12- turbomolecular pump, 13- vacuum tight ceramic window,

14- duplexer, 15- load, 16- directional coupler, 17- circulator,

18- load, 19-klystron amplifier 7 GHz, 3 kW, 20-klystron preamplifier 7 GHz, 20

21- signal generator 7 GHz. 20 mW, 22- waveguide 15x35 mm,

- calorimetric measuremrnts

Fig.2. The location of the microwave power input.

1- waveguide 15x35 mm, 2- Nd-Fe-B permanent magnets, 3- liner, 4- magnetic field line.

An arrow points the microwave electric field direction in the waveguide.

22 Fig.3. Oscillograms of 7 GHz power modulation, 8 mm interferometer and H intensity signals

Fig.4. The same oscillograms during one modulation period.

23 4. Results of a plasma pumping experiment

A measurement of the pumping efficiency of Nb tubes placed in the multipole trap slits had been presented in detail in the preceding report (Phase 1 Before 30 June 1996). The measurements show that the use of Nb tubes with rather large thickness (0,1-0,2 mm) under conditions of low pressure (10-4 torr) microwave discharge allows to receive a pumping rate up to 0.3 of falling plasma flow in steady state conditions. A gas compression factor exceeds 103 without loss of a pumping rate.

These experiments show the severity of the problem. A physics of hydrogen superpermeability in Nb is unknown. A plasma flows had a strongly inhomogeneous density and temperature profiles under our experimental condition. A temperature distribution on tube surface under the action of inhomogeneous plasma is unknown.

In order to investigate superpermeability phenomenon and the possibility to increase a pumping rate has been constructed a new installation (Fig,1). The main parts of this installation are: distributed ECR plasma source, vacuum chamber, Nb probe .

Plasma source parameters are pointed in Table 1.

Table 1.

Plasma flow diameter,m 0.2

Plasma flow inhomogenity 0.05

Max. plasma flow density, mA/cm2 10

Electron temperature,eV 5-10

Working gas H2

Gas pressure,torr (1-4).10-4

24 A volume of the stainless steel vacuum chamber is ~60 liters and it is exhausted by turbomolecular pump with a capacity 103 l/s

Nb probe is a tube with independent evacuation. A plane Nb foil is secured vacuum tightly on its forward end. A thickness of the foils are 0.02-0.2 mm. The foil temperature can be set up to

700oC. The foil is insulated from source and it is fed voltage in the range 100 V.

A dependance of a hydrogen permeation on reciprocal temperature is shon on Fig.2. A foinl was heated by electron flux from a plasma due to positive foil potential and by light from 40 W electric lamp, which is spaced 40 cm apart from the outer foil wall. A heating power by the lamp much below than an electron heating, but it has an impotent bearing on permeability.

SIGNATURES.

Fig.1. A scheme of installation for Nb foils permeability invtstigation.

1- magnrtron 3 kW, 2.45 Ghz , 2- waveguide 45x90 mm,

3- microwave divider, 4- distributed ECR hlasma source,

5- plasma, 6- plane Nb foil, 7- vacuum chamber,

8- mass analyzer, 9- vacuum gage, 10- valve, 11- vacuum chamber,

12- vacuum pump.

Fig.2. The dependance Nb foil permeation on the reciprocal foil temperature.

-4 H2 pressure 2.8.10 torr, foil thickness 0.2 mm, foil potential =60 V, electron current 1

5. PNX-U installation.

INTRODUCTION.

The basic purposes of PNX-U experiment are:

25 -a search for scaling low in multipole magnetic trap at a high density (1018-1019 m3) low temperature (~10 eV) high power ionization (~0.2) plasma in a large volume (~1 m 3) under microwave discharge.

-an investigation of low pressure microwave discharge physics for a plasma creating,

-a solution to numerous technological problems like a niobium plasma pumping and vacuum conditions, the input of a high microwave power into a plasma, design problems.

The fundamental PNX-U parameters for hydrogenous plasma are given in Table 1.

Table 1

Plasma length, m 2.5

Plasma diameter, m 0.6

Plasma volume, m3 0.7

Max. magnetic field in slits, T 0.5

Max. magnetic field between slits, T 0.1

Microwave generator frequency, GHz 7

Microwave generator power, kW 50

Resonant magnetic field, T 0.25

Critical plasma density, m-3 6.1017

Line plasma density, m-2 1.5.1018

Total slits length, m 26

Energy confinement time, s 0.006

Ionization power, 0.2

VACUUM SYSTEM .

The general view of the vacuum chamber and high-vacuum pumps is shown on Fig.1. The chamber sizes and pumps capacity is presented in Table 2. All vacuum connections are sealed by rubber gaskets.

26 Table 2

Vacuum chamber length, m 4.0

Vacuum chamber diameter, m 1.2

Vacuum chamber volume, m3 4.5

Chamber material, stainless steel

Working pressure, Pa 5.10-2- 10-4

Working gases, H2, D2, Ar, Xe

Max. total pumping rate by hydrogen, m3/s 10

A chamber ig exhausted by two pumps to high (10-4 Pa) vacuum. A capacity of turbomolecular pump is 10 m3/s, and diffusion pump 7 m3/s. But gates and stub tubes reduce the total capacity to 10 m3/s.

A gas bleeds throuth a leak in cetnter of magnetic system.

A chamber isn’n bakeable. A chamber degasing occurs by argon discharge cleaning under argon pressure about 1 Pa. A discharge voltage upto 600 V, a total discharge current upto 10 A. A cleaning area is about 38 m2 (chamber and coil cases) and therefore current density ib about 25.10-6

A/cm2. A subsequent cleaning is achieved by microwave and radio frequency discharges. A gas analysis is made by quadrupole mass spectrometer

The most passage diameter is 1200 mm (magnetic system flange). Passage flange diameters from 32 mm to 750 mm are used for microwave inputs and diagnostic ports.

The drawings of the system are in attachtment.

MAGNETIC SYSTEM.

The multipole magnetic field is produced by water-cooled copper coils placed on the side and two ends cylindrical surface. The magnetic field line scheme is shown on Fig.2. Side magnetic system consists of 22 circular coils with rectangular cross section. The direction of current are the

27 same in hair of coils and opposite in neighbour pair of coils. For each end there are three circular coils and one elongated coil. The current structure scheme for end magnetic system is pictured on

Fig.3. Some magnetic system parameters are pointed in Table 3.

The magnetic system is assembled in conjunction with the end vacuum flange outside the vacuum chamber (Fig.4) and then it is moved into the chamber. The position of the magnetic system inside vacuum chamber is shown on Fig.5. Each coil integral with current lead is placed into vacuum-tight envelope (Fig.6). The current leads are brought out to atmosphere through

28 tubes. A vacuum sealing is provided by wringing out of each tube by rubber gasket (Fig.7,8).

The pushing apart forces between coils is balanced out by eight clamping up studs.

The end magnetic systems are mounted independent of the side system. The view on a large scale of end coils and their mechanical connection to side system is illustrated in Fig.9.

There are one circular and one elongated slits on each end. The view of end magnetic system is shown on Fig.10 The view and dimensions of elongated coil are shown on Fig.11 Two windows are situated at each end (Fig.3,10) which simulate windows for negative ion injection. The magnetic field lines direct transversely to windows.Two enlarge circular gaps (47.4 mm) between pair of coils for microwave waveguide are on side magnetic system (Fig.12).

Cooling of the coils is provided with distillated water. Water flow in each coil is independent (they are connected to water main in parallel) and the connections are fulfilled with insulating pipes. The distillated water goes to heat exchanger where heat removal is provided with

“technology” water supply. Full heat power which must be removed is 3 MW.

Coil resistance is 0.15 Ohm, maximal voltage drop is 570 V. Insulation of the coils was tested at voltage of 2.5 kV.

Table 3

Side slits number, 11

Side coil number 22

28 Slit size between side copper coils, mm 30

Slit size in view, mm 21

Distance between side slits, mm 206

Turns number in side coil, 16

Bar size of side coils, mm 15x15

Water cooled duct diameter, mm 11

End slits number, 4

End coil number 6

Bar size of end coils, mm 12x12

Water cooled duct size, mm 7.4x7.4

Max. bar current, kA 3.8

Max. power dissipated by magnetic system, MW 2.2

Water consumption, m3/min 1

The drawings of the magnetic system are attached.

MICROWAVE SYSTEM.

PNX-U microwave system must provide 50 kW of microwave power at a frequency of 7 Ghz. The scheme of the system is shown on Fig. 13. A microwave power from local generator amplifies by three stage klystron power amplifier and transmits into a plasma through the vacuum tight ceramic window. An output microwave power of each stage is pointed on Fig.13. The stages are decoupled between them by ferrite circulators. A duplexer after third klystron serves to measure output klystron power and reflected from plasma microwave power by calorimetric method. Two microwave power inputs are developed, fabricated and installed at the system. The first one is tangential . In this case the microwave power is launched into a slit with width of 45 mm (see

29 Fig.12) tangentially to inner coil diameter into a channel of ring shape which is opend at plasma side. (Fig.14). The second variant of microwave input is the wavegyuide with opend edge wich is directed through the end coil elongated aperture (Fig.10) in parallel direction to axis of installation.

The waveguides with crossection size of 15x35 mm are used. All the waveguides are water cooled and filled ( from output powerful clystron to ceramic vacuum tite window ) with air of 3 atm pressure to increase a possible transmitted microwave power without breakdowns. Power losses in the waveguide are about 0.1 db/m .

SIGNATURES

Fig.1. PNX-U vacuum chamber.

1- turbomolecular pump, 2- diffusion pump

Fig.2. The magnetic field structure.

1- side magnetic system, 2- end magnetic system, 3- plasma, 4- slits

Fig.3. The current structure for end magnetic system.

1- end coils,2- side coil,3- windows for beams

Fig.4. The magnetic system assembling.

Fig.5. The location of the magnetic system in vacuum chamber.

Fig.6. The side coil in the vacuum tight envelope.

Fig.7. A location of the current leads.

Fig.8. A sealing of the current leads tube.

Fig.9. The end coils.

Fig.10. The end magnetic system.

Fig.11. The elongated end coil.

Fig12. The gap for a waveguide pass.

Fig.13. PNX-U microwave system.

30 6. Desing calculations of pumping system for PNX-U

The plasma neutralizer (PN) is a large volume multipole magnetic trap for a cold plasma. A plasma confinement is obtained by magnetic wall- the system of coils with opposite direction of currents. A plasma is produced by electrodeless microwave discharge under electron cyclotron resonance condition. The PN vacuum system maintains the gas circulation in neutralizer chamber without a big gas load from PN.

An end wall magnetic system,a plasma structure, gas boxes (GB) position, injected negative

D- beams and magnetic field lines are shown on Fig.1. Each GB includes Nb foil pump for a plasma flow evacuation. However plfsma is not absorbed totally and there is a gas counterflows into a plasma which determine a gas pressure and the gas flow throughout windows for beam injection.

A gas flow from GB are concedered in this report. The adopted plasma parameters are pointed in Table 1.

A scheme of slit and magnetic line for PNX-U instalation is pointed on Fig.2. A plasma flow density distribution across slit is assumed in the form:

2 2 J(z)=J0 exp(-z /a )

A parameter a value is assumed 1.2 cm. It is adoped also that plasma flow converts fully into molecules and desorbsinto a plasma volume again.

A calculation was fulfiled by 3-D Monte-Carlo method. A gas pressure along the slit axis and gas flow directivity diagram are shown on Fig 3,4..

At the present time are fulfiled a calculations with regards to ionization of gas flow by plasma.

Table 1.

31 Plasma radius, cm 30

Plasma volume, cm3 6.8.105

Plasma density, cm-3 1012

Confiinemrnt time, ms 1

Number of slits, 15

Slit width, cm 2

Area of the one slit, cm2 380

Ion flow throgh one slit, ion/s 4.2.1019

SIGNATURES.

Fig.1. Placement of gas boxes.

Fig.2. Scheme of gas box and magnetic field line in slit.

Fig.3. H2 and D2 pressure distribution along GB axis.

A distance is measured from the bottom of GB..

Fig.4. Exit from GB gas flow directivity diagram..

32 7. Preliminary proposals on PNX-SU

The essential feature required of any ITER systems is their experimental checking in actual practice. Thus to measure plasma neutralizer parameters it is necessary to create a special installation where can fulfil the main requirements ITER plasma neutralizer (PN-ITER) in conditions with existing JAERI facility.

The main requirements to PN-ITER are:

-line density of optimal target nl1-2.1015 cm-2,

-ionization degree 0.5,

-beam divergence in passing through PN 5 mrad,

-a small power consumption, that is an adequate confinement time.

An adopted PN-ITER parameters are pointed in Table 1.

The preliminary results of calculations and design of minimum size neutralizer for experimrntal testing show that it size is close to PNX-U but to provide a high magnetic field value.

Therefore the magnetic field coils are superconducting. A taken for design PNX-SU parameters are pointed in Table 2.

Table 1. PN ITER parameters.

Line density,cm-2 2.1015

Neutralizer lengh,m 3

Plasma volume,m3 8

Plasma density,cm-3 7.1012

Electron temperature,eV 5

Confinement time,ms 1

Negative ion energy,MeV 1.3

Working gas D2

33 Ionization degree 0.5

Magnetic field in slits,T 1

Microwave generator frequency,GHz 28

Microwave power,MW 1

Neutralisation efficiency 0.8

Magnetic system type superconducting

Microwave generator type gyrotrons

Table 2. PNX-SU parameters.

Line density,cm-2 1015

Neutralizer lengh,m 3

Plasma volume,m3 1

Plasma density,cm-3 5.1012

Electron temperature,eV 5

Confinement time,ms 1

Negative ion energy,MeV 0.3-0.5

Working gas D2

Ionization degree 0.3

Magnetic field in slits,T 1

Microwave generator frequency,GHz 28

Microwave power,MW 0.1

Neutralisation efficiency 0.7

Magnetic system type superconducting

Microwave generator type gyrotrons

34 8. Design conception of superconductive magnetic system of PN-

Introduction

This proposal demonstrates an opportunity to use a multipole superconducting (SC) magnet for plasma confinement in a plasma neutralizer (PN). We made our estimations for a winding described in [1], quite realising that a lot of efforts is ahead of us to perfect it. The magnet consist of 30 rectangular side coils and 11 face coils at every end. The main aim was to clear particularities of this rather uncommon SC magnet and to use them to optimise the design.

We would like to note the following particularities:

1. The requirements to accuracy and stability of the magnet geometry are sharp and rather close tolerances should be held.

2. A cryostat should provide unobstructed passages for escaping particles moving along field lines in magnetic plugs.

3. A clearances between coils in the plug areas are very small. The difference in thermal contractions of the magnet and the cryostat causes anxiety.

4. The energy stored in the magnet is rather small in spite of large dimensions of the winding.

5. Operating pressure of the neutralizer is very low (10-4 torr). It can be used as insulating vacuum between room temperature and 80 K shells.

A design described below takes into account every one of the particularities. Fig.1 and 4 show the general views of the magnet and cryostat.

SC winding and structure

Table 1 shows the main parameters of the magnet. The superconducting material is NbTi

Table 1

Number of coils 52

35 Overall Inductance H 0.14 Operating current kA 2 Stored energy MJ 0.28 Max. field at winding T 1.9 Conductor cross section mm2 2´3.5 Critical current at 2 T, 4.2K kA 4.5 Cu/SC ratio 3:1 Overall length of the conductor km 10.2 Overall cold mass t 2.5 Operating temperature K 4.5

We chose a united mechanical structure (Fig1) to provide rigidity of the magnet and to reduce labour consumption and heat input. The coils (Fig.2) are attached to rectangular frames (1) joined with jibs (2) arranged on the structure outside. The electromagnetic forces are not very high in the magnet. By way of example repulsive force applied to a side coil disposed at the edge of the magnet is as low as 53.3 kN. It is a reason to use a structure similar to honeycomb sandwich made from thin

(1 mm) stainless steel sheet.

Every side coil (3) consists of two doublepancake with 4´10 turns. Every pancake is adhesive bonded to stainless steel sheet (4) at both faces to provide high stiffness of the winding and to exclude even probability of any turn movement.

The end coils (5) (Fig.1,2,3) are wound on multiple partitions (6) joining two structure sheets (7) and providing the end structure stiffness despite of a lot of through slots (8) for escaping particles.

A group of the coils (9) (Fig.2) presented to an ions source is removable to provide an access into the PN. As an option, we consider a removable door (10)(Fig.3) holding only five coils immediately adjacent to ion beams inlet ports. In either case, there is no problem to fix the removable coils because their interaction with the remainder is well below their weight. The magnet rests on four fiberglass supports (11) (Figs.4)

Energising and Protection

All the coils are energised in series. The magnet inductance is extremely low and no problem should arise with both energising and protection processes.

36 The latter may be provided by traditional method of current dumping by means of an external circuit breaking. However the stored energy is so small in comparison with rather high LHe consumption, that the magnet may be made self protected and the energy will dissipated inside. The persistent current mode may be provided.

Cryostat

The cryostat has no own room temperature shell. It uses one of the whole neutralizer(12) (Figs 4). A vessel (13) (Fig.1,2,3,4.) containing the magnet is cooled with LN and its temperature is about 80K.

The vessel is vacuum tight to provide insulating vacuum better than 10-6 torr around the winding. It consists of two shells (13.1 and 13.2) one inside other. Inner one (13.1) is a rectangular shell of the

PN operational chamber. A cylindrical external one (13.2) isolates the magnet from room temperature external shell (12) (Fig.4) of a whole neutralizer. An insulating vacuum of 10-4 torr between room temperature and 80K shells is not sufficient and we intend to reduce LN consumption by covering both external surface of 80K shell as well as internal surface of room temperature one with multilayer insulation. The inner surface of 80 K shell is covered with 30 mm thick super insulation mats (

The shell containing operational area has a corrugated wall to provide exits (15) (Fig.2) to vacuum receptacles (16) (Figs.2) for particles escaping from operational area. The receptacles are disposed behind plugs around the circumference of the operational chamber. They seems to be rather delicate devices and provision is made to replace a spoiled one without disassembling of 80K vessel. Four straight receptacles (16) (Fig.1,2) are admitted into every corrugate through branch pipes (17)

(Figs.1,4) joining operational chamber (13.1) with the cylindrical shell (13.2). The pipes are made also for the 80K vessel stiffness. Though there is vacuum inside as well outside this vessel in operational conditions, it can withstand against atmospheric pressure in every space that can arise at accident or during adjusting. The corrugated wall is covered with 10 mm layer of super insulation.

Table 2

37 Heat leaks to 80 K shell Radiation W 150 Residual gas W 200 Supports W 17

LN overall consumption will be about 1200 kg per week. A thermosyphon provides the coolant circulation along tubes attached to shells.

Table 3

Heat leaks to 4.5K mass Radiation W 15 Residual gas W 0.2 Supports W 1.5 Current joints W 0.2 Neutron and nuclear radiation W 18 Heat leaks to feeder and transfer lines Through vacuum insulation W 0.5 Current leads l/hour 6.5

The end parts of the vessel couldn’t be done corrugated for reason of restricted space between ion beams inlet ports. The channels for escaping particles are made as discontinuous slots (8) (Figs 3) between coils with opposite currents.

It isn’t clear yet how much heat should be absorbed by the chamber walls from the plasma. A water cooled shield (18) (Figs.1,2) is designed to absorb the main part of this energy.

The coils and structure are cooled with LHe flowing in tubes attached to pancakes and elements of structure. There are about 500 parallel channels for LHe that flows from lower collector (19) to upper one(20) (Figs.4). Removable door has its’ own collectors.

A very important point is coincidental precooling of the magnet structure and its’ shell from room temperature down to 80 K. Only in this case the difference in thermal contractions will be negligible.

Cryogenics

38 An ultimate option of cryogenics should agree with ITER common cryogenic conception. Here we consider an option of self-contained cryogenics. The heat leaks to 80 K shell and to 4.5 K cold mass are pointed in Tables 2,3. A thermosyphon makes LHe to flow along the channels. A 1000 l cryogenic vessel used as a feeder is mounted at a roof of the PN box. It provides autonomous operation for about a week after every filling procedure. The operation may be continuos if 60 W refrigerator would be located adjacent to the feeder. A pair of current leads are inserted into the feeder.

Assembling

*An assembling procedure is universally accepted for the most part and needs no especial description on this stage. The only feature needed a mention is an assembling of the side coils and corrugated wall of operational chamber (Fig.2). The separate frame (1) carrying a pair of coils(3) and every corrugations are mounted one after the other, every frame to be bolted to premounted one with jigs (2), and every corrugation to be welded (21) to premounted corrugation. The mat (22) of super insulation is protected from a damage at welding with an air gap (23) and a metal backing

(24). In an emergency, one of the flanged welds should be broken down. All the interframe joints, both electrical and cryogenic ones, are mounted after completion of this stage of assembling on the outside of the structure.

Conclusion

From the aforesaid it follows that a SC magnet may be successfully used with PN. There is a need to carry out a detailed calculation of a neutron fluxes and their related the heat leaks.

SIGNATURES

Fig.1. PN-ITER. Side view in section.

Fig.2. Side magnetic coils and Nb boxes structure . View in section.

39 Fig.3. The edge magnetic system structure.

Fig.4. The PN-ITER edge view.

40 9. Conclusion

1. The conception of Plasma Neutraliser for ITER NBI was presented. The conception is based on multicasp magnetic system for plasma confinement and microvawe plasma creation system.

2. The results of all calculations and preliminary model experiments were carried out during the step of the work was completed are in acceptable concordance with the concept. The plasma confinement scaling low was confirmed in relatively small size system (PNX-1,2), the mechanism of microvawe plasma generation was confirmed as well.

3. The next step in PN modelling was prepared. PNX-U installation was designed, constructed and commissioned. This already gave us usful engineering experience. PNX-U experiments, if successful, will give us a possibility of ten time increasing of nl . These experiments will provide data base to design full scale PN .

4. Beam-plasma testings to be fully adequate ought to be done with nl higher than that “planned” for PNX-U. The nl increasing could be provided with magnetic field increasing and superconducting magnetic system becomes necessary. The parameters of suggested system (PNX-

SU) are defined.

5. A sketch concept of PN-ITER being developed have demonstrated that it seems to be possible to realize adequate technical decisions with existing means in the case of necessary plasma parameters attainment.