Primary experimental results of suppressing MHD instabilities in HT-7 by biased electrode Zhong Fangchuan, Luo Jiarong Shu Shuangbao College of Science,

Primary experimental results of suppressing MHD instabilities in HT-7 by

biased electrode

Zhong Fangchuan, Luo Jiarong Shu Shuangbao

College of Science, Donghua UniversityShanghai, 201620

HT-7 Data Meeting and Workshop, July 19-20

Introduction

Experimental Setup

Results MHD instabilities

Plasma rotation

Profile

Conclusion

Outline

Introduction

MHD instabilities is one of the main obstacles to realize the tokamak advance operation with high parameters.

MHD instabilities suppression and control is a basic and important topic of tokamak plasma research.

It has been proved that plasma rotation and

shear flow is benefit for MHD stabilities.

Locked mode and disruption

Introduction

A

crit

V

Device DIII-D JET JT-60U NSTX

12% 0 ． 61% 1% 46%

Minimum ration of plasma toroidal rotation to Alfven speed to keep the RWM stable in Tokamaks

Introduction

How to maintain or increase the plasma rotation?

NBI(most used one) RF Wave Biased Electrode

BvBvPen

E iri

r 1

Change Er Vary V 、 V .

Introduction

By using a biased electrode to add an externa

l electrical field on plasma. Through EB drif

t, the plasma edge rotation will be changed, an

d the MHD instabilities will be reduced.

Introduction

Device name

Coutry R/a(cm)

Bt(T) IP(kA) Ne(/m3) Te(eV)

SINP India 30/7.5 0.45 10-30 1-31019 ？ KT-5 China 32.5/8.5 0.5 10 1.0 1019 20

CASTOR Czech 40/8.5 1 15 1-21019 ？ ISTTOK Portugal 46/8.5 0.6 8 71018 260

TCABR Brazil 61/18 1.1 110 1-41019 600

TdeV Canada 87/27 1.5 210 ？ ？Phased-T USA 93/26 0.7-1.0 70-100 1.0 1019 400

CCT USA 150/40 0.3 50 21018 150

T-10 Russia 150/33 2.5 200-300

1-41019 ？

TEXTOR Germany 175/46 2.35 190 1-21019 1000-1500

Tokamaks which have conducted the biased electrode experiments

Device name

Biased polarity

Electrode size （ mm)

Distance to limiter (mm)

Bias voltage/cuurent

SINP negative 6 10 -20 300V/100A

KT-5 positive 30 4 10 150V/200A

CASTOR positive 50 >0 200V/40A

ISTTOK, bipolar 3 125 0 300V/400A

TCABR, positive 20 5 -20 600V/150A

TdeV( multi divertor 0 300V/100A

Phased-T positive 25 30 -30 500V/300A

CCT negative ? ? 1000V/40A

T-10 positive 154555 -20 450V/80-200A

TEXTOR bipolar 130 35 15

-60 900 V/100A

（ 1 ） Improve confinement （ LH)

Biased experiment in TEXTOR

* R.R. WEYNANTS, et. al, NUCLEAR FUSION, Vo1.32, NOS (1992)

* E.Y. WANG*, Xin WANG, D.A. DIEBOLD,et al, NUCLEAR FUSION, Vol. 35. No.4

BIASED H MODE EXPERIMENTS IN PHAEDRUS-T

Only Positive Biased trigger the LH Same results in T-10

In N biased, Ion current is too small to trigger LH ?

Biased experiment in ISSTK

Negative biased improve the confinement.Positive biased degrade the confinement.

* J A C Cabraly, et. Al, Plasma Phys. Control. Fusion 40 (1998) 1001–1019

Which is critical, Polarization ？Current ？

(2)Change rotation

Biased experiment in KT-5

Current injection Plasma biasing Hybrid biasing

Biased experiment in Tdev

Rotation experiment in HBT-EP ( 2009 APS meeting)

(3)Effect on MHD

I.C. Nascimento et. al, Nucl. Fusion 47 (2007) 1570–1576

Suppression and excitation of MHD activity at TCABR

Biased Electrode can

(1 ） Improve confinement （ LH)

(2) Change the plasma rotation

(3) Suppress or stimulate the MHD

instabilities

Introduction

But the action mechanism is still not clear.

Retractable electrode

Bias Power

plasma

Voltage 0 ～ 650V,

Current 0 ～ 300A

Experimental Setup

Experimental Setup

Three type of electrodes

Movable limiter 15050 40

Mushroom shape 60

Movable poloidal limiter 56010010

Results Effect on MHD

Typical time evolution of discharge with/without biased(Signals from top to bottom are : plasma current IP, line integrated density ne, SX-ray emision, H, Mirnov and biased current)

0 20000 40000 600000

30

60

90

120

Am

p. (

a.u

.)

Frequency (Hz)

113684 w/O biased 113692 with biased

Effect on MHD

It is a time delay for the MHD is suppressed and burst out again after the biased voltage on and off

~25ms

Time delay for the effect on

~60ms

Time delay for the effect decay away after bias voltage is off

Suppression effect depend on biased current .

Mushroom electrode

Suppression effect depend on biased current .

Small limiter

Suppression effect depend on biased current . Large limiter

Electrode typesurface area

(cm2)

Minimum action curre

nt(A)

current density(A/cm2 ）

mushroon 28.27431 14 0.49514913

small limiter 150 45 0.3

large limimter 622.03482 80 0.128610164

Minimum biased current density for the MHD suppression for different electrode

MHD suppression effect is strongly depended on the biased current, there is a minimum biased current need for the action !!!

Results Effect on rotation

The effect on toroidal rotation M=ln(Is+/Is-)/K

-4

-2

0

2

4

Mir

no

ve

shot 13692

0.0

0.3

0.6

0.9

1.2

Ma

ch

nu

mb

er

0.00 0.25 0.50 0.75 1.00

0

70

140

Bia

se

d c

urr

en

t

Time ( s)

0.3 0.6 0.9 1.20.0

0.3

0.6

0.9

1.2M

ach

Nu

mb

er

Time (s))

0A 80A 100A 110A 140A 160A 185A

Biasing on

Effect of biasing current on toroidal rotation

0.3 0.6 0.9 1.20.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2M

ach

Nu

mb

er

time (s)

275 Ibiase=0 275 Ibias=70A 278 Ibias=70A 281 Ibias=70A

Biasing on

Toroidal rotation behavior in different radius

The phase difference from two floating potential signals measured by Langmuir probes nearby along the poloidal field line can be used to calculate the poloidal rotation speed*.

Poloidal field line

Z

Suppose the cross phase of two probes signals is (t), then:

tt )(Where t is wave travel time between two position,

)(/ tVzt

*Rev Sci. Instru. Vol 70(1) , 874,1999

So V(t) can be calculated

Effect on poloidal rotation

Effect on ploidal rotation

When biasing, the phase difference is decrease, imply the poloidal rotation is increased

Shot 112252Shot 112254

0.30 0.45 0.60 0.750.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x1040.30 0.45 0.60 0.75

0

100

200

300

400

500

600

V

(m/s

)

Time(s)

shot 113252 w/o biasing shot 113254 with biasing biasing current

Time evolution of poloidal rotation with/without biasing

0.30 0.45 0.60 0.750.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

V(

m/s

)

Time (s)

275w/o Biasing 273 Ibias=70A 275 Ibias=70A 280 Ibias=70A

Biasing on

Poloidal rotation in different radius

Effect on profile

-20 -10 0 10 200

20

40

60S

X E

mis

sion

(a.

u.)

r (cm)

520 560 600 620 640 680 700 720 740 760 780

Time evolution of SX emission profile

-20 -10 0 10 200.0

0.2

0.4

0.6

0.8

1.0S

X E

mis

sion

(a.

u.)

r (cm)

520 ms 600 680 700 720 740 760 780

Time evolution of normalzied SX emission profile

Conclusion

Biasing electrode can suppress MHD instabilities effectively.

Suppression effect is strong depended on the biased current

Edge plasma rotation is increased greatly by biasing voltage

Plasma profile is changed by biasing electrode.

Further study is need.

Acknowledgements

The supports and helps from ASIPP are great appreciated.

Great thank Dr. Zhao Hailin and Dr. Kong Defeng from USTC for provide the Langmuir probe data.

The research is supported by Ministry of Science and Technology with grant No. 2008CB717807 and 2009GB107006.

Thank you for your attention