TRITA-EPP-7U-06 BREAKDOWN AND PLASMA FORMATION IN A ...

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TRITA-EPP-7U-06

BREAKDOWN AND PLASMA FORMATION

IN A ROTATING PLASMA DEVICE

by

B. Bonnevier and

A. H. Sillesen

Stockholm and Risd in ^arch 1974.

Department of Plasma Physics and Fusion Research

Royal Institute of Technology

S- 100 1*4, Stockholm 7j, Sweden

Danish AEC, Risd,

^ Roskilde, Denmark

*$?!• >

'---*•

Abstract

Breakdown and formation of a plasma with high ion

temperature have been studied in the Puffatron at Risd.

The conditions for plasma formation is understandable along

the same lines as breakdown in an ordinary crossed field

discharge, Penning, PIG or magnetron discharge where ions

are not magnetically confined, as in the present experiment.

The growth of ionization occurs much faster than expected

from ionization by thermal electrons. This has earlier been

seen in some rotating plasma experiments. It is suggested

that work along the present lines can result in a time inde-

pendant arc discharge, where ions have thermonuclear energies.

1.

S 1. Introduction

Different forms of crossed field discharges, that is dis-

charges with strong electric fields transverse to magnetic fields,

have been considered in the past. Here we will discuss such dis-

charges with time-independent magnetic fields or whose

time-dependance is unimportant for the main characteristics of the

discharge. Two types have been considered to a great extent by

different authors.

In the first type the electron motion, but not the ion motion,

is modified strongly by the magnetic field. Here we find work on

Penning discharges, P.I.G. (Philips ionization gauge) discharges

and coaxial diode or magnetron discharges. For the present dis-

cussion it is of special intrest that some of these discharges

can be obtained at neutral gas pressures from about 10 to 1 Pas-

cal. It is this great range of possible operating pressure which

has made these discharges so applicable in ionization pressure

gauges.

In the second type of discharge mentioned above, both electron

and ion motion are modified strongly. These discharges aie often

called rotating plasmas. A thorough review was given by Lehnert

(1971). These discharges give us one of the most efficient methods

known to produce a fully ionized plasma. In some of these dis-

21 -3charges a dense, n = 10 m , but fairly low temperature,

5 oT - 10 K, plasma is obtained. In other discharges, like

the present one, ions can directly gain energy from the

electric field and swiftly should be able to obtain the

10 keV energy around which nuclear reactions begin to take

place. Thus we have a direct conversion of high voltage direct

current energy into plasma ion energy. High voltage crossed field

discharges is an experimental field almost totally neglected,

however» but with present day technique it should be fairly easily

accessible. In crossed field discharges considerable mass motion

velocities are obtained. Therefore, the centrifugal force might

give an extra, important, possibility for plasma confinement, added to

ordinary mirror confinement. This was for instance mentioned by

Bonnevier and Lehnert (1959), who pointed out, that 70 keV deuterons

could conceivably be contained by the centrifugal force created from

crossed electric and magnetic fields. Thus the low energy ions in a

mirror machine, with rotating plasma,are trapped. This is important,

because low energy ions swiftly get scattered out in an ordinary

mirror machine, due to the large Coulomb crossection. This gives a

considerable energy loss. This is further discussed by Bonnevier(197H).

Another advantage of rotating plasmas is that impurities

are centrifuged out, as discussed by Bonnevier (1966, 1970).

There is much scattered evidence of crossed field discharges.

In order to get a more unified approach it seems logical to examine

the ionization processes, the breakdown and the plasma build up in

crossed field geometries. Also as our interest lies in high vol-

tage effects, we started with the present configuration of the

Puffatron at Risd, where anode voltages up to 50 kV can be applied.

§ 2. Apparatus

The puffatron at Risd is shown in Fig. 1. The vacuum vessel

of the device has a diameter of 0.15 m and a length of 2.5 m. A

part of this vessel also serves as the outer electrode. The inner

electrode is a stainless steel rod, placed along the center line

of the apparatus. An axial magnetic field with a mirror ratio of

1.5 and a magnetic field strength of up to 1.6 Tesla in the cen-

tral parts of the machine is used, and a condenser bank of luF with

voltage up to 50 kV has been applied to the electrodes. In

normal operation the apparatus is pumped to a low base pressure, «

by oil free pumps. When magnetic field is already applied, gas,

from a small chamber of 50 mm , is injected through a fast valve

in the center of the machine, and finally the voltage between the

is switej?**} on,

3.

In operation with V = 30 kV, B = 1.5 Tesla and a pressure p = 60 kP

of H2> in the small chamber behind the fast valve, a plasma with

20 —3m

20 —3a density of ^U^IO m , ion energy around 500 eV and estimated

electron energy of 20eV is produced. The density has been measured

by a He-Ne laser interferometer and the ion energy is obtained

from charge-exchange neutrals. The plasma flows along the magnetic

field lines and interacts with the walls after about 5ys. The

breakdown and plasma formation thus are very efficient in a device

of this type. This will be discussed further in the present paper.

Improvement of the confinement will be discussed elsewhere. A

description of the puffatron and some earlier results obtained in

this apparatus have been published by 0ster (1969).

i

5 3. Experiments

Here we wish to report on the discharge characteristics

obtained in the breakdown and plasma formation stages. Voltage

and current characteristics of the discharge were obtained. Further,

a logarithmic amplifier made it possible to analyse the current

in greater detail, in the way earlier reported by Rasmussen et al.

(1969). In Fig. 2 is shown that distinct types of discharge

characteristics were obtained for different combinations of ap-

plied voltage and magnetic field strength. The measure-

ments reported in this paper are made with positive inner elec-

trode. At low magnetic field or low applied voltage,

different types of low current discharges were obtained,

an example of which is shown in Fig.2a. In these discharges the

current rise was slow, up to a current plateau of some amperes.

In the other region of high voltage and high magnetic field

strength the current rises to several kiloamperes, producing a

dense plasma with high ion temperatures, Fig. 2,b,c. the current

rise in this region was in many cases rather slow at currents less

than ten amperes but increased at higher currents by a factor

of ten.

The border V^ = f(B*,p) between the high current and the low

current regimes is conveniently devided in two parts, a low vol-

tage branch and a low magnetic field branch, Fig.3.

In the low voltage branch, the voltage V, increases steeply

when the pressure decreases: P = 150kP gives V, = 5kV,

p = 50kP gives Vfc = 25-3OkV and p = 40kP gives V^ higher than

U5 kV. These values are almost independent of the magnetic

field strength.

The low magnetic field branch depends slightly on the voltage.2

Experiments seem consistent with V. = g(p)«B. , where g(p) depends

on pressure only. For pressure p = 50kP, g(p) is very near to the

value obtained for proton cutoff. For protons leaving the anode,

calculations give the Hull cut-off condition

V = qB2(r2-r2)/8mr2.

With proton charge q and mass m, an anode radius r =0.025 and

3 2r=0.075 we get V=53»10 *B , where V is in volt and B in tesla. At

30 kV and varying pressures we get for the magnetic field B at the

border the following values: p=60 kP gives B=0.30 Tesla, p=50

gives B=0.52 and p=H5 gives B=0.80. The Hull cutoff at 30 kV is at

B=0.75 Tesla. Thus the exact position of the branches varies with

pressure.

The two branches of the Vb=f(Bb,p) curve meet at a certain

point. For psSOkP, Vj»25 kV, Bb~0.5 Tesla is the common point.

6

This corresponds to a velocity E/B of 10 m/s. In the measurements

with neutral detector referred to earlier, it was shown that the

most probable velocity of the neutrals corresponds to 2x(E/B),

what is also equal to the maximum velocity of a particle in cycloi-

5.

within the whole high current regime. Near to the low magnetic

field branch, the ions will therefore get an energy near to the

applied anode voltage.

§ U. Discussion

a. Current characteristics

In earlier experiments on a rotating plasma device at

Amsterdam, Rasmussen, Barbian and Kistemaker (1969), it was repor-

ted that in 50 per cent of the shots there was an exponential in-

-1 2crease in discharge currents from 10 A to a few times 10 A. Thecurrent rise could be accounted for by putting the discharge cur-

rent derivative -rr - irz s nn <av>. . . From simple theory theat dt n loniz

entity <ov>. . for ionization by thermal electrons was derivedloniz J

as a function of E/B. Good agreement between theory and experiment

was reported.

On the other hand, it has been pointed out by Lehnert (1966)

that the growth of ionization in the rotating plasma experiments

in Stockholm is almost two orders of magnitude faster than expec-

ted from ionization by thermal electrons. A fast growth of ioniza-

tion in a straight crossed field discharge has also been reported

by Eninger (1966) and Axnäs (1972). The fast ionization in these

experiments is related to the critical velocity phenomenon first

described by Alfven, By some processes electrons gain energy from

the relative motion of ions and neutrals. This enhances the ioni-

zation probability drastically. For reviews seé Danielsson (1973)

and Sherman (1973). In the present experiments, breakdown near the

low voltage branch can follow thermal ionizatioi» theory to about

10A. At higher currents a much faster ionization is going on. It

^

is thus interesting "»-hat in the same shot we get the two behaviours.

The fast current rise in our experiment could be due to processes

ot importance for the critical velocity phenomenon. An increase in

the current rise would also occur if the electric field deviates

strongly from the electric vacuum field due to sheath effects.

In the present experiments, ionization of neutrals by fast ions

can not be neglected.

b. Border between low and high current regime

The border curve V, = f(B. ,p) presented earlier is very similar

to the curves presented for breakdown in a cylindrical magnetron

as presented by Redhead (1958) and Schuurman (1966). In their case,

only electrons are contained in the magnetic field and ion gyro

radii are much greater than dimensions of the apparatus. Ions

created in the gas will fall towards the cathode. There electrons

are produced by y-processes. The electrons drift into the discharge

volume, with reduced mobility due to the transverse magnetic field,

towards the anode. The theory of the discharge is thus equivalent

to the ordinary Townsend discharge. The Townsend breakdown criterion

can be written

-1) = 1 or

In (1 + i) = /adr

Here o is the first Townsend coefficient defined in the

usual way as the number of ionizing collisions performed by an

electron per unit displacement in the direction of the electric

field. At low pressures, the electron will move in cycloid orbit2^ffigwith diameter D * ̂ffig and hence a

be taken between the inner and outer electrode.

1/V * §§». The integral shall2mE

7.

As the coefficient is placed in the argument of the

logarithmic function, very large changes in the value of y

are required to change the breakdown condition.

With a in the form obtained above, we see, that one

discharge condition is that there is a sufficient number of

gyroradii between the electrodes. This gives the upper branch

of the breakdown curve. A second requirement is that the electron,

in some part of its cycloidal orbit, must have energy enough for

ionization, or else gain this energy through elastic collisions.

But then a number of steps with length of order D has to be taken.

This lowers a. Thus the requirement is reasonably given >>y

V. < E-D = 2 - • £-3 e B2

This corresponds to the lower branch of the breakdown curve for

an infinitely long cylinder. It has been obtained experimentally

by other authors but very often electron emission from end plates

give a lower branch with constant voltage. For the condition tog

be fulfilled E/B>10 is needed for electrons.

The essential characteristics of the breakdown when ions

are not contained can thus be described by two processes. One

iB the ionization of neutral gas by electrons, the second is

electronemission from the negative electrode due to ion impact.

When the magnetic field gets greater the ion orbits begin to

be curved. This alters in the first instance the y-processes.

- • v*. -

8.

The Y-process will be more efficient when ions hit the electrode

with a high tangential velocity. A further increase of the

magnetic field will make the ions contained. Then y-processes by

charge-exchange neutrals should be dominant. Further at high ion

energies the ionization probability of the neutral gas by ion

impact is not negligible. We will not judge at present which of

these processes are the most important ones.

Acknowledgement. We wish to thank Mr. Knud^Vilhelm Weissberfc

foT the construction of the logarithmic amplifier, and Mr, John

Petersen for his helpful assistance with the experiments,

9.

References

' -n+l

Axnäs, I., Royal Inst. Technology, Stockholm,

Rep. TRITA-ÉPP-72-31, 1972.

Bonnevier, B., Lehnert, B., Ark. Fys. 16, 231-236, 1960.

Bonnevier, B., Ark. Fys. £3, 255-270, 1966.

Bonnevier, B. , Plasma Physics 13., 763-774, 1971.

Bonnevier, B., Report TRITA-EPP-74-10, Royal Institute of

Technology, Stockholm 1974.

Danielsson, L., Astrophysics and Space Science

24 > , 1973.

Eninger, J., Proc. VII. Intern Conf. on Phenomena in Ionized

Gases, 1, 520, Beograd 1966.

Lehnert, B., Physics Fluids $, 774-779, 1966.

Lehnert, B., Nuclear Fusion 11, 485-533, 1971.

Rasmussen, C.E., Barbian, E.P., Kistemaker, J.,

Plasma Physics ljL, 183-195, 1969.

Redhead, P.A., Canad. J. Phys. 3J5, 255-270, 1958.

Schuurman, W., Investigation of a low pressure Penning Discharge

Rijnhuizen Report 66-28, 1966.

Sherman, J., Astrophysics and Space Science 2£, 487-510, 1973.

Sillesen, A., 1972, to be published.

0ster, F.f Risd Report No 191, 1969.

10.

Captions to figures

Fig. 1. The puffatron device. The length of the vacuum vessel is

2.5 m. The magnetic field is in the axial direction and has a

mirror ratio of 1.5.

Fig. 2. Discharge current in linear and logarithmic display at

different magnetic field strengths and condenser bank charged

to HO kV and a pressure p=50 kP. Curve with upper partto the left is proportional

to the current. Increasing current gives a downward trace,

with sensitivity 10 A/div. Curve with lower trace to the left

is proportional to the logarithm of the current, above 1 ampere

Time scale is 5 usec/div.

a) Magnetic field strength B = 0.65 Tesla

b) B = 0.70 Tesla,

c) B = 1.20 Tesla.

The points a»b,c are indicated on fig. 3.

Fig. 3. The two distinct discharge regions experimentally

obtained, for a pressure in the small chamber, behind the

puffvalve of 50 kP(kilopascal). The cutoff condition for

2protons according to the formula V = 53 B is indicated. The

points a,b,c refer to the oscillograms of Fig. 2.

Pulse line

Slots

Spring arrangementHammer coil

Makrolon rodInsulator

Gas inlet

Pyrex insulator

Outer electrode (diameter 15cm)Inner electrode (diameter 23cm)

Diamagnetic loop

Thickening (diameter 5cro)

Valve openings

Coil

Extension of the inner electrode(diameter 1 cm)

nriii£orp- Sublimationion pump

pumps

k.it

Fig.1

Ill

Fig. 2

cx

CURRENT

DISCHARGE

CURRENT

DISCHARGE

Cutoff conditionfor protons

0.5 1.0 B [Testa]Fig. 3

TRITA-EPP-74-06

Royal Institute of Technology, Department of Plasma Physics

and Fusion Research, Stockholm, Sweden

BREAKDOWN AND PLASMA FORMATION IN A ROTATING PLASMA DEVICE

B. Bonnevier and A.H. Sillesen, March 1974, 10 p. in English

Breakdown and formation of a plasma with high ion temperature

have been studied in the Puffatron at Risd. The conditions for

plasma formation is understandable along the same lines as

breakdown in an ordinary crossed field discharge, Penning,

PIG or magnetron discharge where ions are not magnetically

confined, as in the present experiment. The growth of ioni-

zation occurs much faster than expected from ionization by

thermal electrons. This has earlier been seen in some rotating

plasma experiments. It is suggested that work along the

present lines can result in a time independant arc discharge,

where ions have thermonuclear energies.

Key words Crossed field discharge, Magnetron discharge,

Rotating plasma, High voltage effects, Breakdown, Critical

velocity phenomenon, Plasma formation, Plasma heating,

Thermonuclear fusion.