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