Post on 28-Feb-2021
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.
PlaSMa Current (kA) 599 1 1 1
I
� I [ : : ��� A �VI . '\\
I \
J " ... \.
V p g
9 TiMe <Msec>
Poloidal Cap Voltage <Volts> 599 ---T-,---'--- T------r--- --, - r -,-
119 Y t s
LooP Voltage (Volts) 599
U 1 0 0 p
9 U 0 1 t s
r -599 1
<Mse�) 9 TiMe
I l
1 j
I l
29
1 29
6
Av Toroidal Field <Gauss> 5991--��TI��i��i������i��i�--'----'
< B t >
G a u s s ,
99�f--��--�;T� i�Me--7<Ms�e-
c�i�
) �--�--��2
dl
B t w
1
F 9
-1 9
Toroidal Field @ Wall <Gauss> I j I ! I ,
Field Reversal ParaMeter
TiMe (..sec) 2 lit
2
F �
1 U x � W �
e b
l e � s
I lit lit
T T e
Co�e Flux <Webe�s)
Ti_ <Msec>
Conductivity TeMPe�atu� (eU)
.., I
2 lit
2 lit
<A�b> 1
lit lit 29
AluMinuM Radiation l r--'--�--'---�-'r-�---r--'---�-,
AI.. U 1671 A
2 lit
7
� » < m r m z G) -I :t: ,....
J SIGNAL (normalized)
�--------------------------------I
o
, 0 01
3 C/J CD 0
N v.> CD
<
..... N
3 C/J CD o
en 0)
CD <
»0 �
0 01
-I
.....
OL-�� __________________________ �
8
() ---
� 0) � ---.J
»0
CJ 0 -0 -0 r m JJ
-0 JJ 0 11 -r m
F o
'Ii' .; l..l ... .. e ....... Reve�sal Pa:raMete:r
Pine}, Pa:raMete:r
9
2
1
F 9
Field Reve:rsal Pa:raMe t e :r "!i!.I'i�"" """""''''''''r'''''''''''''''''''''''·T''''''''''''''''''''''''''I'''''''''''''''''' ..... "''1'' .. ''''''''''''''' ........ 1'''' .. ''' ... " .... '''' ..... 1'''' ....... ... '''''''''''''1 ... ''''''''' ..... ''''''''"
I '---- ! I "Ei. - I \"" -J'!"t::!�1 "" I 1,1,""
�'"'� CO "'I .......... 1- I I"" ..... --.. ""I I \ .. I� i I"" ". u_ "" I
I " .... !-to I I .. �; i I"""""'" .. " .. ··,·,,,·,· .. ·,," , .. ·· .. ,, ...... ,, .. ,""',,·,, .. ,,·,·,,·,"' .. ''' . ... , . '' ..... '''''.''''''''''''' .... '''''''' '''' .• ''''.' ''' ' .. ''' ... ,�·.···"·" .. ,,,,·, .. ,,,,·:,,·· .. ·0 .. ··· .. ·,,,,· .. ·,···,,···· .. ··,,,,,,· . . · .. ··· .. ··"·" .. ""·· .. ·"·, .. ,,·,,,, .. ,,,,1 i . c. - I i ". LI -- I I '. L� i
I"" ,,-= COl'Ist .•.. , "I I", Th eor'';! �.... I "..I I "
. ""I t . I IIiH loti!. I I i • i
1 I I ' , , , I '. , I I -
t'If'"UUIIUIIUUHurrfuHIHunurUUUUUII:JUIHH'UUIUUUIUUIUlltu,urH'"Hlulluullllunulr,uItUU,uu,uul.nullUl:flH,HIHUIH,h.II,UU,If, .... "nn.�III.Hnlllllll(lHIUIlUIlIlL,IIIIIUIlIlHnnJlnlnIL:I'nnlluuIHla""n.!
9 Pinch Pa:raMete� 2
10
T e
e U
Conductivity TeMpe�atu�e (eU> 5 fA :=u,"uuuutl,aU':j'HIUrUuual<lItI'u:ur l:lllnu:.llnUU'CUIIII'HUHIIIUIItI:HIIlIUIII':tnnllfl,n:IlUIIIIIH'rn•H1H,urrll;fnUII"' j lflHH.IIUU'''''Hlntll! II.UIUIIUIIIUUIU'''Ur"IUHH,utHn""UUIIIIHIUU",.U:r""r:O':I ! '" I . , . • • I ! ! I I:.: . . .. I
I , ..
.. I I ....
I,,· I iUIi ! I
I", i I ....
I I .... !
-1-1 8
It. ct-
""I I .. .. \ I . ", 1 ! I "" ! i �
u�- 0 Ci 80 I 0 i
o .51 0 if:" co :-160 _q-. II ....
.. ,,1,: I-I 0 1::r.:.O - (I -" iJ - -
1.... 0° � ctl .... ! I i
a I' . I . ! . I I I ! I III::J IIIUUUU'UUllluuu .. t nr:UUUHUU.HIH'Ul f HlllnU'Ullllluune:nL lllln"rll"rr'UltlHIU i lllllll:IIUIIIIIHIIII.uJ " uu,rr,.u,u,uuuun l""'IIIIIIUUlurUH'U' IIIIIUUlllUlluaUlllu l,UHtr'UII,,,,,,,,uuu, f uIUHIU'UCIIUHIIHI., 1 9 Pinch Pa�aMete� 2
11
I p
k A
PlaSMa Cu��ent (kA) 5 99 ..... " " "' .. ,,!' " ...... ''''' .... ''''' .. 'T'''' ... '''' ..... , ... '''''' j''" ....... . '''' ... ''''' .. T''''''''''' ... "''''''' ... 1''''' .... ''''''' .... '''' ... 1'''' ... '' ... '' .... " .. ''''' 1' ........ .. , .. '''''' ..... '' 1'''''''''' .. '''''''' .... "'1'''' .. '''' .. '''''''' ..... ''1
I I ... , ! f
I
I ""\
1 i
I "'" i
I " "I o I ""I
• .-. t • i
�I .,-
I
11_' ! I 0 .-. 0 Ii ""\
(" .-._
0 - .-._ - - aI' :
II 00 I I I I 0 (I I o I
I"'..
_ I) t) ""I
ILl I o ""I
r 0° I JUt: I'll!
I I� I i " I! _ I:. -
u ut
I OW I .. ! , 01 I
I I ! I i ! i ! ! I ! .:IIIHHUIHltllIUUlllnluHIH"UUI,UHlnUIIHIIlUHIHIUHuuan"u u,rU'JlUf:'UHIIHlrlu JuunIHIIrIUIH:r'IIIII:l anul,uIIHUlllflIfIUIIUtlHlu,HHI""uuu:r i"ulllllluulrrluHlllullluruuHllul,JrllulluluHHaluuutor:,uu,u f
9 Pinch Pa�aMete� 2
12
u p g
l.1 o I t s
Poloidal Gap Uoltage (Uolts) ? � .r::. u,u,uHHr:Hpu,u"II'II •• unu:rO:"I'lIufulI,ul:u,nrnulJ(,,,uUUUII'u,tlnrrnUIUH'"I1HUUH'HH'UHfuIHHunUIIIUUIHHlli ,·UIUUUIIUHH'UUIHIHIIU:"IHUlHIHUU·I> IUIHU.UIIlI:lHIUur'''J'H .. IIHU .... UU'''U'II.
IE.- iUlKJi i I ! j ! j _ ! I j ! I Ii U I f - !
I"" ""I I c· 0 I I
,
I'
"
''
0.-.0 ""
!' ,<:1 CI.-. 0
I"" Q - ""I
I
0 ,_, CI 0 (I " .. II j,n. IJ
I oo o�c€ 0
,..I I' 0 000
0 c. I
I .. · (I 6-' 0 ,,,,I
.,11
'
.
ID
Q:.
om I I) I j"" 0 I) ,,,,I
I 0 : '.f''' .9 0 1i':1 _
,,,,I o (I - I_I !
I ! rn, 00 Ii c�
""I i - I I
., I -
9 11I111I,rrIlUI:IHHllllluI.UIIU'HUIIU:UIUIf,uL"UH'HUUI'lIluuu,J,"uH,rr,ullllllllnu,J ,"nnuuuu,HnufluJuurnuIHHIIHIfHIIII,I ,u"" •• IIII,."UIIIII.lli .. :nJlUUIIUllIlUluuJ '"'U'IIUUHIU'UIIUuL ,ltuuuU:rUUUIII:uJ 9 Pinch Pa�aMete� 2
13
u p g
u o 1 t s
Poloidal Gap Uoltage (Uolts) 2 99 "" .. "" .... 'T'''' .. " ... """"''''''T''''''''''""" .. "' .. '"l'''''''"' ...... ", .. " .. "!'''''''' ...... "'''" .. "''!'" .... ,, .. ""''''''''''''j'''''''''''''''' " ........ '!" ............. " .......... i""' ... "" .... ,," .. " .. '1' ......... " .. ""' .. ",, .. \ ! I I I I"" .. '" I I I I i i ! i , I ,.. .. ON LY .... I I RFP SHOT"S (I I t.. 0 1 i 0
0
""I ""I I
I lIuf !
I I i.. I) ""I
I'" V PG = "l' 0 (R../ a..2) Ip 3/90 0 't. 0 "" I I.... FroW\ 2;- - '1-0 M ""I I I i I � I 9 !.Ul1aUIIU,.II'UfuuutIlUIUHI.rI,r:rHI:J,uJauIHU"rlUIUnrrllllll.rrn.1I1UIIIUIlUUfll,J."UIIHIfHHUlrlHIUI,L ... rr.u,uu'UUU:tUI,IUIUUI'UHIIIUIUIIHIl 11I111f,fllllnulllu,uni'""H,nu:,u"uu,uuI,u,,,,,,uuuurrulI,lu'
9 PlasMa Cu��ent (kA) 599
14
• 0,+8 F
&.f.lf. k V
t.5 F
280 V
POLOIOAL FIEL.D CI�CU/T
TOROIDAL FIELD CIRCUIT
15
I 6: I
2. 0: I
COUPLe.D
VIp..
F (9)
1
.16
NEAR-TERM PLANS:
7/86
9/86
9/86
4/87
4/87
:1.0/87
:1.:1./87
17