Contribution of KIT to LHD

Topics from collaboration research

on MHD phenomena in LHD

S. Masamune, K.Y. Watanabe1), S. Sakakibara1), Y. Takemura,KIT group, LHD experimental group 1),2)

Kyoto Institute of Technology1) National Institiute for Fusion Science

2) Kyoto University

Motivation

• Comparative studies on MHD phenomena associated with current-driven and pressure-driven global instabilities in high-beta plasmas in current-free LHD and Ohmically-heated RFP

• Accumulation of data set for pressure-driven global MHD instabilities for active control of low-order MHD modes in LHD plasmas

Mag. WellHill

0.5

1.0

1.5

0 0.2 0.4 0.6 0.8 1

vac~1%~2%~3%

Rotaional transform

<> (%)

Low order rational surfacem<=3

Magnetic hill exists in the finite beta gradients region=>MHD instabilities (interchange/ pressure driven) would appear in high beta regime.

Characteristics MHD equilibrium related to stabilityCharacteristics MHD equilibrium related to stability

Ap=6.2, p~(1-2)(1-8)(%)dia >

m/n=3/2

m/n=2/1, 3/2,1/1(,2/2,3/3), 3/4,2/3

3/4

1/1(,2/2,3/3)

2/1

2/3

Study on MHD instabilities in LHD I

Objectives: - Avoidance of confinement degradation caused by global MHD instabilities in Heliotron plasmas- Development of control scheme of the MHD instabilities

# Understanding the excitation mechanism of the mode excitation Identification of the condition of mode appearance and mechanism of the mode axand mechanism of the excitatio => Experimental identification of the conditions for mode appearance and comparison with predictions from linear and nonlinear theories

# Quantitative estimate of the effects of MHD instabilities on plasma confinement and understanding the mechanism of confinement degradation=> Experimental identification of the relationship between confinement degradation and MHD modes, including the relationship between the mode structure and edge magnetic fluctuations. Comparison of the experimental results with predictions by nonlinear theories.

# Development of active control methods of MHD instabilities to avoid confinement degradation=> Use of the resonant magnetic field, control of plasma flow, etc.

# Core resonant MHD instabilities　⇒ - causing local flattening of the pressure profile. - stable in high-beta region.# MHD instabilities resonant in the peripheral region ･ rotating modes

　⇒ - dominant modes in high-beta region. - appearing when DI 0.2, D≾ R>0 - magnetic fluctuation amplitudes increase with increase in beta and magnetic Reynolds number・ non-rotating modes　⇒ - appearing in low magnetic shear configuration with

relatively low beta - appearing when DI>0.2, DR>0 - accompanying generation of magnetic island, with confinement degradation (decrease in beta by ~50%)

MHD instabilities in LHD II magnetic fluctuation behavior

Magneti

c R

eynold

s N

um

ber,

S

<>%

rotating

(%)0.01

10-3

10-4

10-5

1.2 1.0 0.8 0.6

a-0

non-rotating

(%)0.06

0.04

0.02

0.0

What have been known for pressure-driven modes

・ Effect of rotating MHD modes on plasma confinement・ Growth (decay) and saturation of non-rotating MHD modes

New Findings:

Effects of rotating MHD modes on confinement I

Analysis of internal mode structure and change in Te profile associated with apperance of the modes have relealed the effects of MHD modes on plasma confinement.Experiments have been performed under marginal stability condition to rotating MHD modes resonant in the peripheral region where <b>~1%.

1.8s

5%

Coh

in b

1.9s

f~1.8kHz

line integral

No phase change (or inversion) => No magnetic island => linear ideal/resistive interchange characteristic

instability disappeared at t=1.9s

2

exp~ resr

rr

fitting =>

/ap~5%

Major radial profile in SXR emissionat t=1.8s

plasma displacement

Effects of rotating MHD modes on confinement II

0

200

400

600

800

1000

1200

3000 3500 4000 4500

te@92000_1800-1900

t=1.8st=1.83t=1.87st=1.9

B

1.8s

5%

1.9s

Normalized difference from Te at t=1.9s (just after the disappearance of fluctuation ) -0.4

-0.2

0

0.2

0.4

3000 3500 4000 4500

ts@93624_1_96-2_13_ref1_96

GGGGGG

1.8s1.831.87

0

0.2

0.4

-1 -0.5 0 0.5 1

ts@92000_1_8-1_9_ref1_9-norm_r1.8S;c5=(C3-C4)/ C41.83S;c5=(C3-C4)/ C41.87S;c5=(C3-C4)/ C4

Te

Te/Te

04

exp

04H

ISS

E

ISS

1.9s

Global confinement properties have been improved by 10% when the mode having normalized width of 5% with edge magnetic fluctuation level of 0.01% disappeared.

10-4

10-3

10-2

10-1

1.74 1.745 1.75

sxmagfluc@92000tm1.82fmi1.70fma2.10ch24lm0.7div1024nf11vwataplus

magfluc0_mp(x1e-5) magfluc512_mp(x1e-5)

(b~

11)2/B

0 (%)

time (s)

x100(8ms)

=>1.7msx100(2.6ms)

=>0.6ms

Growth and saturation of the rotating MHD modes磁

場揺

動の

相関

sx15, 14, 13, 12, 11sx18, 17, 16

SXR fluctuation profileline integrated

(t=1.8s)

- No positional dependence in the initial grow phase.-Slower grow at further location from resonant surface in the second stage.

Two-step growth of rotating MHD modes Initial phase: growth time ~0.6ms Second phase: growth time ~1.7ms x1~x10 of linear growth time

10-3

10-2

10-1

1.74 1.745 1.75

sxmagfluc@92000tm1.82fmi1.70fma2.10ch24lm0.7div1024nf11vwatapl_15-11-2

sx15sx14sx13sx12sx11

time (s)

x100(2.6ms)

=>0.6ms

(A.U.)

10-3

10-2

10-1

1.74 1.745 1.75

sxmagfluc@92000tm1.82fmi1.70fma2.10ch24lm0.7div1024nf11vwatapl_sx15-18-2

sx15sx16sx17sx18

time (s)

x100(2.6ms)

=>0.6ms(A.U.)

0

1

2

3

0

10

20

30

40

1 2 3

wp@93651-2

C B E

ne(1019m-3)

<dia

>(%)

Ip/B (kA/T)

#93651

time (s)

Growth (decay) and saturation of the non-rotating MHD modes

0.0

1.0

2.0

2.8 3.2 3.6 4 4.4

0.84s0.87s0.90s

0.93s0.97s

R(m)

#55397

ts_real_prof0298_thomson_i5_s10@55397t836_2

0.0

0.5

1.0

0.0 0.2 0.4 0.6 0.8 1.0

B_6_55397t836_vr_1028

Ideal

S=106

(m/n=1/1)

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1

ts_real_prof0298_thomson_i5_s10@55397t836_xi-e

Q te_fit O

Te/|dT

e/d|

#55397(t=0.84s)t~30ms

PredictionLinear growth rate (FAR3D) ~1/(100s)

Non-rotating MHD instability; often observed in low shear, high hill configurations# growth time of tild-b :50~100ms(similar to decay time)# linear theory prediction (g):~100s

inconsistent

No localized rapid decay phenomena?

Prior to global decay, intermittent small-scale Te0 decay is observed with time scale of linear theory prediction.

relation with linear theory?relation with global decay?

~100s

1 2 3

ifil0302@93651dt10st300et3400-2

br11/Bt(1e-4) br11/Bt(%)_mod;c17=100*(c16-0.87/2*c5)

0.000.050.100.15

br11

/B0(%)

time (s)

0

1

2

3

0.00

0.10

0.20

0.30

0.40

1.9 2 2.1 2.2

wp@93651+br11-zoom1

C B Ebr11/Bt(%)_mod;c17=100*(c16-0.87/2*c5)

ne(1019m-3)

<dia

>(%)

br11

/B0(%)

#93651

time (s)

growth time; 50~100ms

Further study

Decay profile in Te: similar to structure of linear ideal interchange

Summary I

1-(1) Rotating MHD mode (m=1/n=1) has caused confinement degradation. Quantitative estimates have shown that he mode having mode width of 5% with edge magnetic fluctuation of 0.01% has caused 10% degradation of global confinement.1-(2) The rotating MHD mode (m=1/n=1) grows with two steps. The growth time in the initial phase is consistent with linear theory prediction, while the growth time in the second phase is order-of-magnitude longer than linear theory prediction.

Future work:- Data accumulation on relationship between mode widths and edge magnetic fluctuation amplitudes, on mode widths and confinement degradation. - Nonlinear saturation mechanism of the mode through detailed comparison with linear and nonlinear theories of ideal and resistive interchange modes.

Summary II

2. Growth (or decay) time of non-rotating MHD mode (m=1/n=1) is 50-100 ms, two- or three-order-of-magnitude longer than the linear theory prediction. Prior to the global decay of beta, intermittent small-scale decay in electron temperature occurs with time scale consistent with linear theory prediction.

Future work:- Internal mode structure of the intermittent temperature decay.- Relation between the intermittent phenomena and succeeding global decay-Nonlinear saturation mechanism of the mode through comparison with theories.