SECOND RESULTS FROM THE WISCONSIN NON-CIRCULAR RFP

J.C. Sprott

PLP 978

May 1986

Plasma Studies

University of Wisconsin

These PLP Reports are informal and preliminary and as

such may contain errors not yet eliminated. They are

for private circulation only and are not to be further

transmitted without consent of the authors and major

professor.

SECOND RESULTS FROM THE WISCONSIN NON-CIRCULAR RFP

by J.C. Sprott

Included here are copies of the posters presented at the 1986 IEEE

International Conference on Plasma Science in Saskatoon, Canada, May 19-21,

1986. (Ref: IEEE Catalog No. 86CH2317-6, page 79, 1986). The data

represent more recent and non-overlapping results than those presented in

PLP 969. The major new results are the extension of the 200 kA/8 msec

discharges to 300 kA/10 msec and estimates of the plasma density,

temperature, and confinement time. The changes were brought about by

operation at higher poloidal bank voltages (4400 volts vs 3500 volts), core

biasing, correction of some field errors at the poloidal gap, improved

vacuum conditions, and better control of the gas puffing.

Some improvements in plasma parameters have been obtained, but there is

still no evidence of a quiet period when the field reverses, and the

resistivity is still 5-10 times the ZT-40 value at the same plasma current.

Reduction of some known large field errors at the poloidal gap did not lower

the resistivity. The most likely cause is the influx of impurities as

evidenced by a rising resistivity and density, and a rising level of oxygen

and aluminum radiation during the pulse. The discharges continue to show

slow improvement with surface cleanliness and are easily spoiled by vacuum

accidents.

Interferometer measurements indicate a typical line- averaged density of

5 x 1012 cm-3, which is consistent with the fill pressure of -0.2 mtorr

(gauge). Measurements of the oxygen line radiation, Doppler broadening of

-2-

carbon III, and the neutral charge exchange spectrum suggest a peak

temperature of Te - Ti - 100 eV, which implies Zeff - 5. For a plasma

current of 300 kA, a loop voltage of 60 volts and a volume of 8.6 m3, these

numbers correspond to an energy confinement time of

This estimate may be low because it comes from data at the time of peak

current which is near the end of the discharge after a significant impuri ty

influx has occurred.

Near-term plans call for installation of stainless, toroidal limiters

in May '86, installation of divertor rings in July '86, and installation of

the Thomson scattering system in September '86. The installation of the new

MST vacuum vessel is scheduled for April '87, wi th first RFP plasmas

expected in October of '87.

ABSTRACT

By removing the internal rings from the Levitated Octupole vacuum

vessel, a large, non-circular RFP was produced. The major radius is 1.39 m,

and the cross section is about 1 m2. The device is unconventional in that

the vacuum vessel, which consists of 5-cm thick aluminum with a single

poloidal and toroidal gap, serves as the vacuum liner, conducting shell, and

poloidal and toroidal field coils. A toroidal field of up to about 1 kG can

be produced, and the poloidal field is driven by a 600 kJ capacitor bank

through a 2-volt-second iron core. Discharges are initiated with �200 volts

per turn using self-reversal of the toroidal field in order to prevent

arcing of the poloidal gap which is exposed to the plasma. The gap is

protected with a 20-cm wide strip of ceramic.

The best RFP discharges have a peak current of -200 kA and a duration

of - 10 msec. The toroidal field reverses when the current reaches - 100 kA,

making this one of the lowest current density RFP's in existence. The

current ramps up to the final value over � 10 resistive diffusion times and

terminates only because the volt-second limit of the iron core is reached.

The F-8 trajectory lies sli ghtly to the right of the A=constant theory as do

all other RFP devices. Discharges have been produced with 8 up to 2.5 and F

as low as -0.8.

A feature of the device is that it is capable of producing discharges

with plasma current of � 100 kA and � 10 msec duration over a wide range of

safety factor from the q> 1 tokamak limit to the deeply-reversed, RFP limit.

The highest current discharges (-300 kA ) are obtained at q-0.5.

3

The plasma noncircularity ( indented at the midplane ) provides an

opportunity to gain experimental information on whether fluctuations in an

RFP are current-driven or pressure-driven instabilities generated by

unfavorable poloidal curvature. To this end, we are measuring the edge

magnetic fluctuations in the separate good and bad curvature regions on a

given magnetic surface. Results will be presented for both reversed and

non-reversed discharges ( at various q values ) .

The resistivity of the RFP discharges is lower than non- reversed

discharges but a factor of 10 higher than other RFP's with the same current.

The resistivity correlates strongly with vacuum conditions, indicating a

need for more agressive cleaning and impurit y control. The time-dependence

of the plasma electrical parameters agrees with a simple electrical circuit

model in which the plasma resistivity is given

const/lp where the constant is typically 50- 100 volts

surface cleanliness.

for all times by Rp =

and depends on the

Plans call for improving the cleanliness of the machine, improving the

electrical circuits and measuring the density and temperature. Over the

longer term, new internal rings will be installed to attempt RFP operation

with a magnetic limiter ( or poloidal divertor ) . Thereafter, the vacuum

vessel will be replaced with a new circular vessel with R = 1.5 m and a =

0.52 m. The new device, called MST, is scheduled for completion in late

1987.

4

-2-

5) The plasma resistance drops, and the conductivity temperature

rises when the plasma enters the reversed-field state. However, there

is no noticeable quiet period, and the resistivity is still an order of

magnitude higher than in ZT-40. The conductivity temperature strongly

correlates with vacuum conditions, indicating a need for more

aggressive discharge cleaning. Removal pumping rate per unit wall

surface area is quite low (-1000�/sec/40m2).

6) Numerical circuit modeling using the experimentally observed

F-6 curve and a plasma resistance of Rp = 65/I� at all times produces a

remarkably accurate prediction of the plasma electrical waveforms. If

the plasma resistivity can be lowered to the ZT-40 value, small

improvements in the electrical circuits should allow 500 kA/20 msec RFP

discharges.

7) Temperature and density (and hence confinement time) have not

yet been measured. The conductivity temperature never exceeds 20 eV,

but shows improvement with surface cleanliness. Density is apparently

low as evidenced by the optimum H2 fill pressure of -0.1 millitorr.

The ability to start up at low pressure is greatly enhanced by the use

of -50 watts of 2450 MHz ECRH preionization.

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NEAR-TERM PLANS:

7/86

9/86

9/86

4/87

4/87

:1.0/87

:1.:1./87

17