PLP 1022

THE DESIGN OF THE MST REVERSED FIELD PINCH

(Poster (5T14) presented at the 29th Annual Meeting of the Division of Plasma Physics of the American Physical Society

November 2-6, 1987, San Diego, CA)

A.F. Almagri S. Assadi

J. Beckstead G. Chartas R.N. Dexter

D.J. Den Hartog Y.L. Ho

D.W. Kerst T.W. Lovell S.C. Prager J.S. Sarff

W. Shen C. Sprag ins J.C. Sprott

November 1987

'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.

flhlcl .��m d 1tJh1.cI ]MIST mW(Ilrr_(lmI JF1fi(lfi� JP.>m@JDlO

AF. ALMAGRI, s. ASSADI, 1. BECKSTEAD, G. CHARTAS, R.N. DEXTER, DJ DEN HARTOG, Y.L. HO, D.W. KERST, T.W. LOVELL, S.C. PRAGER, IS. SARFF,

W. SHEN, C. SPRAG INS and lC. SPROTT

The MST (Madison Symmetric Torus) experiment w111 soon begin operation.

The vacuum vessel is 5 cm thick aluminum (R= 1.5 m and a=52 em). Most RFP devices employ a metallic liner to protect voltage gaps from plasma. MST is unconventional in that the vacuum vessel serves the function of liner, conducting shell and toroidal and equilibrium field coils. One implication of this des�n Is that insulated $Zaps are exposed to plasma, thus a combination of low

VOltage startup and Special gap prOtection is required to prevent ardng. To

this end we have designed a gap protection scheme that Is successfully tested

in the prescence of plasma up to 300 volts. By employing large current feed

flanges and using the vacuum vessel as the equilibrium field coils, field errors

are reduced to small levels. In addition, two primary systems are used for

the poloidal field to reduce field errors to extremely small levels. Also all

portholes are limited to no greater than 3.2 em in diameter. The first experiments w111 investigate shell proXimity requirements using a movable

limiter.

* Work supported by U.S.D.O.E.

1

INTRODUCTION

The Madison Symmetric Torus (MST) is a reve�� field pinch device (Ro= 150 em I a=52 em) designed for the dual purpose of

exploring the baSic physics of the RFP configuration and functioning

as a key support experiment for ZT -H and RFX . The wealth of

experience accumUlated from past RFP devices and from past

wisconsin machines has been incorporated into the MST design, which is unconventional in several ways. The dominant physics

goal is to study the role of the boundary condition on RFP stability and confinement.

2

SCIENTIFC GOALS

I ) Boundary Cond i tion Stud ies

To examine the effects of RFP boundary conditions on

stabUit Y I fluctuations and confinement. The first experiment will be

to vary the width of the vacuum region between plasma and the

conducting wall using a movable toroida111miter .

2) q Scaling Studies

A future objective is to track stabillt y and confinement as the plasma configuration is varied from RFP to tokamak.

MACHINE DESCRIPTION

MST is designed with special attention payed to the

minimization of magnetic field errors due to shell gaps I iron core

winding and portholes. Ease of disassembly was alSO required for alteration of the boundary. The resulting machine design contains

several unconventional features :

1 ) A thick (Scm) aluminum vacuum vessel that functions as

Bt, field winding and conducting shell.

2) A gap protection scheme that protects the gap in the

presence of plasma up to at least 300 volts 3) Flanged Bt and Bp gaps to reduce field errors

4) A continuity winding and guard core transformer to

inhibit current from flowing on the outSide surface of vacuum vessel

5) Small pumping and diagnostic portholes to reduce field

errors

6) Three primary Winding systems : One primary Winding (magnetizing) to reduce the iron core

stray flux. The second primary winding (polotdal field) to

reduce the radial field in the Bp gap. The third winding

(d.c.bias) to reverse bias the iron core to obtain a 2 volt-sec

flux swing.

4

MST Device

5

PROJECTED PLASMA PARAMETERS

The prOjected plasma parameters for MST are determined by

a combination of electrical drcuit mode1ing� based on the mcdified

bessel function model� and assumptions about the scaling of plasma

parameters with plasma current and machine SiZe. A range of

possibilities that bracket the actual case have been examined. We

are faily confident of achieving 400 kA discharges reversed for 1 0

msec (pessimistic) � since these were achieved in the noncircular

RFP tests. However the best we can expect is to obtain 1.0 MA

discharges reversed for 40 msec (optimistiC). Table 11 1 gives

plasma parameters for both pessimistic and optimistic limits.

TABLE 11 1 Parameter Pessimistic Optimistic

Major radiUS 1.50m 150m Minor radius 0.52 m 0.52 m

Plasma current 400.0 kA 1.0 MA Loop voltage 75.0 V 4.0 V Average toroidal field 1000.0 G 2500.0 G Toroidal field at wall -300.0 G -150.0 G Poloidal flux swing 1.8 V-s 1.6 V-s Electron temperature 100.0 eV 1000.0 eV Average plasma density 3xl019 m-3 4xl019 m-3

Poloidal P 10� 207b Confinement time 0.4 ms 40.0 ms

Discharge duration 10.0 ms 40.0 tr.lS

I ..

< II �

� I "'

'''' '1 t "

.. -' ..• -----.. ............

.. �",,-

,." ----._------

---------

� Hag .. . tl'Zjnq Cur ...... 4: (J(n)

-----.--r--,.--...,--.---,--...,.---,--,-- J i .. .. ,

I'

/ / /1

• .-=.,Tr:-=0"' .. '"'0:r=l..:cl:.:.=rl ......:G::;.�p:......;U�O:.:l:..t.::.;.�g:.:.::.--T-(c.:U�C�lrt;:;·�)...-_-,

� UUI---------------_____ ,,�·� _______ --__ ---

! �

Field Reversal Par�Meter

29"A-r __ .-_-.c=-o-=-=r..;.r-"'F..;I:,U;=)("- ... ( :.. ... :;;W;ob=-r-) _-,�_.,-_-,

Typical wavef')rms for a pessimistic, 400-kA, lO-ms discharge in MST.

p

l"'�'U'-r_-r-=-P..:.I.�_"_M-� Cu ........ t (kA)

...... ---_.--. �,#' ----

--------

<OM .. .. .. .. �----.

Poloidal Cap Vultage <Volts)

:l-� VCit 1------'-�--""""===========I y • •

:t.

I M .. •

�'-� �-'" u .. I-----------��================� 1 •

\ -2H-�--���lnl·�M.��(�=s=.o�I)r-��I�"

Typioal waveforms fo� an optimistio, l-MA, 40-ms disoharge 1n MST.

8

TOROIDAL FIELD SYSTEM

The MST toroidal field system circumvents some of the

problems associated with discrete toroidal field coil systems such as

field ripples I loss of diagnostic access and disassembly. However I

using the vacuum vessel as a one turn toroidal field winding

requires the use of a step-up current transformer( 48: 1). The

poloidal current is fed to the shell from the transformer via a long

axisymmetric flange system to symmetrize the shell current. The

flange system consists of a pair of annular horizontal disks Which

connect to a long coaxial cylindrical section . The cylinders are

connected at four locations to the transformer secondary (see fig •

1 ).

The calculated toroidal field ripple is about 0.05% of the

polOidal field with the dominant component being m=O I n=4 I and

the resultant magnetic island width of about 1 em. This design

allows virtually unlimited diagnostic access and easy maintenance

of the vacuum vessel.

9

POLOIDAL FIELD SYSTEM

The poloidal field system that produces the toroidal current

and pololdal magnetic field consists of three major components I

poloidal winding system, poloidal flange, and the continuity winding

( see fig 1/1 2).

1) Poloidal Winding System

The poloidal field windings consist of three separate windings, the d.c. bias winding, the magnetizing winding, and the poloidal field winding.

A) Bias Winding

The bias winding is used to reverse bias the iron core to

accomplish a 2.0 volt -second flux swing . this 40 tum winding wt1l

carry 24 kA-tums for 3 to 10 seconds. Since the biasing field is a d.c. field, it is very critical that the winding be distributed around

the iron core properly to minimize flux leakage into the plasma

volume. This Winding distribution has been empirically determined

and the stray field within the plasma region was reduced to a value

less than 1.0 gauss everywhere (see fig 1/1 3).

B) Magnetizing Winding

The magnetiZing winding is used to carry the magnetiZing

current that is required for a finite permeabil1t y transformer. This

40 tum Winding is distributed around the core so as to minimize

the leakage flux in the plasma regton. The desired distribution was

empirically determined for a magnetizing current of 400 kA -turns. The residual field was reduced to less than 5.0 gauss at the shell poloidal gap and will only arise late in the discharge as the

core saturates . this winding consists of aluminum straps wrapped

around the iron core and designed to carry a magnetiZing current

up to about 0.5 MA and a stand-off voltage of 20.0 kV (see fig - 4).

C) Poloidal Field Winding

The poloidal field winding is used to force the surface

(plasma image) current in the poloidal flange to flow in the radial

direction as it crosses the poloidal gap. this is accomplished by

threading these windings through holes in the poloidal flange. The

total amp-turns in these Windings must equal the plasma current.

To accompliSh that I a set of current transformers is used. The poloidal distribution of the toroidal current denstt y in the wall will

be measured prior to placement of this winding.

11

2) Poloidal Flange The flange is used to reduce the field errors that result from

the imperfect matching of the poloidal field winding distribution to that of the surface current distribution in the shell walt due to using a finite number of turns (40). this "flange effect" arises because the poloidal component of the surface current, resulting

from imperfect matching, will not be localiZed at the gap but will be distributed over the entire flange. As a result, the radial field will be weaker at the plasma edge .

3) Continuity Winding The return current on the exterior surface of the shell woUld

have a djfferent distribution from that on the inner surface of the

shell (plasma image current). Hence, if this current is allowed to

flow on the exterior surface, a poloidal component of the current

woUld exist at the gap to connect the two distributions I thereby

prooucing a large m= 1 magnetic field at the gap(see fig II 5).

To avoid this field error I a continuity winding is used to inhibit retur-n currents frOftl flowing on the exterior surface of tbe shell. A field error of smaller magnitude woUld still exist if the

poloidal distribution of current in the continuity winding does not match the distribution of shell wall current. To eliminate these

errors the poloidal field winding , which is distributed to match the

wall current distribution, is used to impose the right current

12

SUmtllary

To facilitate the boundary condition studies, MST was

designed to be relatively free of magnetic field errors so that

the magnetic integrity of the boundary layer and the core

region is unquestionable. Field error minimization was a driving

criterion for design of the toroidal and poloidal nlagnetic field

systems I and the vacuum pumping systent Field errors are

minimized by

1 ) Using the conducting vacuum vessel as the Bt winding

2) Using flanges for Bt and Bp gaps

3) Using continuity winding

4) Using separate primary system for the magnetizing

current

5) Using small porthols properly placed} for daignostic and

vacuum pumping

13

AXIS�

B T FEE 0 --==:::::--_

CYLINDERS

ONE OF FOUR BT FEED COLUMNS

BT TRANSFORMER

150 CM

Figure 1 : Schematic illustration of torOidal field system 14

TYPICAL MOVEABLE LIMITER

51 CM R

PUMPING HOLES

PUMPING DUCT

".-.

U't

Poloidal Field Winding

Continuity Winding

Biasing

Winqing

Magnetizing

Winding

Poloidal Flange

Image

Guard Core

Transformer

Vacuum Tank

FigUre 2: Schematic illustration of poloidal field system

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