FLUCTUATION STUDIES IN THE ALFVEN WAVE RANGE OF FREQUENCIES IN THE TOKAPOLE II TOKAMAK
(Poster (7W12) presented at the 29th Annual Meeting of the
Division of Plasma Physics of the American Physical Society
November 2-6, 1987, San Diego, CA)
PLP 1025
December 1987
M. A. LaPointe
R. N. Dexter E. J. Haines D. Kortbawi
S. C. Prager
J. C. Sprott
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.
Abstract (modified)
Fluctuation Studies in the Alfyen wave
range of frequencies in the Tokapole I I Tokamak* M. A. LaPointe, R. N. Dexter, E. j. Haines, D Kortbawi, S. C. Prager, j. C. Sprott, University of Wisconsin - Madison -- Magnetic fluctuation measurements in the frequency range of 200 kHz to 8 Mhz have been made. There exists some enhanced fluctuation levels betweell 1.0 and 1.3 MHz for the q < 1 discharges. Radial profiles of the transverse' fluctuation spectra are presented at jow and l1.igh q for the frequency range from 500 kHz to 2.0 Mhz. The q dependance of the fluctuation levels is also presented.
*Work supported by the NSF and the USDOE
1
Purpose
Initial magnetic measurements were taken to determine the
characteristics of the high frequency fluctuations in various plasma regimes. The questions to be addressed by this experiment consist of
.
I) Measure the turbulent fluctuations in the Alfven wave
range of frequencies (500kHz < f < 5 MHz)
2) Compare the fluctuation characteristics at high frequencies with the lo�' frequency results
3) Examine features of the spontaneous Alfven wave
spectrum
4) Compare results with calculations of the fluctuations due
to thermal noise.
? ..,
Tokapoie I I Parameters
Four node poloidai divertor Major radius
Minor radius
Toroidal field
Plasma current
Line averaged density
Electron temperaturie
Ion temperature
Discharge length
Base vacuum
, I 1(- ,.
50cm 8 - 10 em typical 2.5 - 5.5 kGauss 15 - 80 kAmps
2 - lOx 1012 cm-3
- 100 ev
- 20 ev
3 - 7+ msec
4 x 10-7 torr
�
.� CJl ::J C c ·
.- -0 cC
8�
o
""'r.'\�C:Jt.==:::::z���-- (/) ---o
�==��==���-t��-u,-----.- -0 0-
0·<1> �-lJ..:----
E ()
C\.I
_1
-1
I"�J ::r
The fluctuation spectrum at the higher frequencies is a continuation from the low frequency results
lCJU
10-1 ·r'. .... \-- .... ,"'
""-,.I\."."�,,, �-v,
, 'I I-� l----ri--ri-rrr
I
1 (] kHz t (I 30 CH( h z 1. 0 s hot s
� 10-2 '- , (/) .j') :J I'D
� 10<3
10-4
I\�I - -t
/�� v"") \r"'p�� fM, / \�f� � v Y I
a 2 t I) 8 r'�1 Hz 1 U s hot s
I
10 -'2 -L I I I I I 1 � 1-11 I , . I I I I I 11 � b I I
r. F t-' e (1 u e n C \J ( �1 H 7 ) , I
Calculations of the fluctuations due to the thermal noise in a ,.., .
plasma put an upper bound of BIB -10-7 on the fluctuation level (Z. Agim. poster 6S24, this conference)� Measured fluctuation
levels are larger those predicted by the thermal noise
calculations for all cases studied to date. '- . . ;
- -,�
�(
N
� Il Il
a t!1
The poloidal fluctuations at low q (-0.7) in the frequency range
of 500 kHz to 2 MHz have a radial profile that is peaked inside
the separatrix. The fluctuation level has a llf frequency
dependance.
10-2 '- , "
X= 70m
X = 8 om
X = 9 em X = 10 em
10-3 L 'IV 1�' � I� �.V:_ v Wvv X = 11 em
C V\-., V\fV III 'v \/\/\ X = 12 em t- � �\�I"\. /,.... , . \I 'I \ X = 13 em
10-4
10-5 0.5 1 1.5 2 2.5
Frequency (MHz)
R
The poloidal fluctuations at high q (-2) in the frequency range
of 500 kHz to 2 MHz show a relatively featureless radial profile.
The fluctuation level is constant across the profile. The
fluctuation level seems to have about a Ilf dependance at all
rad ial pos i tions.
'.
- "f�
9
0.00100
� 0.00050
� 11 12
� o
0.00010
0.00005
Radial scan of fluctuations in <q> = 2
'1 I'
0.5
x = 7 em 8 ... bots
X = 8 em 5 shots
X = 9 em 5 shots
X = 10 em 5 shots
X = 11 em. 6 eots
X = 12 em 5 shots
X = 13 em 5 shots
1 1.5 Frequency (MHz)
10
2
OJ
�
�mQ..
The total fluctuation level in the frequency range from 500 kHz to 2 MHz inside the sepG.I-atrix (x=7 cm from axis ) increases as
the q is decreased.
10-4
10-5
10-6
10-7
b-
tl
: Jl
$
<>
, r t � 0
� i
0 - ·t
1
<>
� 0 . 0<>
o� 0
�� �o 0 <>
�<>
80 <:i
2 3
Qa 11
At qa - 2 th.e toroidal fluctuation level is an order of magnitude
smaller than the poloidal fluctuations. The fluctuation level at qa - 0.7 inside the separatrix is a factor of 5 to 10 larger than
the poloidal Cluctuations at qa - 2. Outside the separatrix, the ,
Cluctuations at qa - 0.7 Call oCC to a level below that of the qa - 2.
" '") . '4'
Lr) o
o L
(-L
(/) C o rl t-> (0 :::) .\->
( ') '-'
() '-" f�)
� .-' --
r
� LL l I I
a
Ol\l0J II II II
I I
LO o �
'''.
181/8
I
o �
� - r--, �-r-....... (J
'-'
VJ ,.--i >< (0
� t:: - c)
L �. <...L -. C () r-i i-> ,.--i (j)
Q) () CL
---I ,.'0 ,.--i
U' (0
cr
Summary
The high frequency fluctuations show similar characteristics to
the low frequency fluctuations
� ,..., , Bp and Bt fluctuation levels for the high qa (-2) plasmas are
constant with radius.
,..., .
Bp fluctuations are about an order of magnitude greater than Bt fluctuations.
Bp fluctuation levels in low qa (-0.7) plasmas show profile that
is peaked toward the center.
Flucuation levels in the range 500 kHz to 2 MHz increase by three orders of magnitude as the effective edge q is lowered
from qa -3.5 to -0.7. .
All fluctuation levels measured are greater than those expected
from thermal noise.
14
Future Work
Continue investigations of the fluctuation spectra in the higher
frequency regimes at various q values
Investigate the radial fluctuation spectra at the various q values
in the higher frequency range
Start correlation studies in these frequency regimes
Begin driven wave studies in this frequency range
� ,t
1 !i