EXPERIMENTS OF ACTIVE QSHEXPERIMENTS OF ACTIVE QSHCONTROL IN EXTRAP-T2R CONTROL IN EXTRAP-T2R

L. Frassinetti, P.R. Brunsell, E.K.J Olofsson and J.R. Drake

• INTRODUCTION

(1) The device and the feedback system

(2) Diagnostics for MHD modes

(3) Spontaneous QSH in EXTRAP T2R

• EXPERIMENTAL RESULTS

(1) Open loop experiments

(2) Close loop experiments

• CONCLUSIONS

OUTLINE

EXTRAP T2R – the device

• R=1.24m• a=0.18m

• Ip 80kA (standard current plasma)

• ne≈1019m-3

•Te ≈200-400eV

• pulse≈20ms (w/o FeedBack)• pulse≈up to 90ms (with IS)

EXTRAP T2R – the feedback

• shell≈6ms

• 4 poloidal x 32 toroidal sensor saddle coils (m=1 connected) located inside the shell

• 4 poloidal x 32 toroidal active saddle coils (m=1 connected) located outside the shell

• Digital controller

Sensor coils

Activecoils

shell

byOlofsson E.

EXTRAP T2R – algorithmsAt present, three algorithms are routinely used in EXTRAP T2R:

• Open Loop• Intelligent Shell (close loop) IS• Mode Control (close loop) MC

Active coils

plasmaSensor

coils

external helical magnetic

perturbations

Open Loop (OL)Open Loop (OL)

shell

byOlofsson E.

Active coils

plasmaSensor

coils

external helical magnetic

perturbations

Close Loop (MC)Close Loop (MC)

shell

Digitalcontroller

bc

b1,n bcVc(t) V1,n(t)=-Knp[ref-b1,n(t)]

Fourier harmonics in real time

inputto active coils

DIAGNOSTICS

Magnetic perturbations:

br 4 poloidal x 32 toroidal sensor saddle coils (m=1 connected) located inside the shell give the time integrated signals.

TM RWM-12 -13-14

Raw signals

b 4 poloidal x 64 toroidal local sensors (m=1 connected) located inside the shell give the time derivative of the signals.

SPONTANEOUS QSHs IN EXTRAP T2R

QSH example in EXTRAP T2R

MH Ns>2QSH Ns<2 SH Ns=1

Average duration QSH≈0.1ms

Fraction of the discharge characterized by QSHs:

1%QSHQSH

pulse

P

The dominant mode is the first resonant. With the typical equilibrium: nDOM=-12n=-12

vacuum

OPEN LOOP EXPERIMENTS

Active coils

plasmaSensor

coils

external helical magnetic

perturbations

Open Loop (OL)Open Loop (OL)

shell

Basic idea:

• Apply a static helical perturbation (n=-12)

plasmaV=1.4VV=1.2VV=1.0VV=0.8VV=0.6VV=0.4VV=0

Average between5ms-end of discharge

• Study the plasma behaviour

OPEN LOOP EXPERIMENTS

A staticstatic helical field with n=-12 is applied at the plasma boundary.

What happen to the corresponding rotatingrotating TM?

“Often”, the TM is larger than “usual”

in the sense thatmore and longer QSHs

are generated

But the TM still rotates!

spontaneous QSH

induced QSH

OPEN LOOP EXPERIMENTS

More QSHs when the external helical perturbation is used

But when br-12 is too large,

even secondary modes increase

120 QSHs are considered

Amplitude of dominant TM during QSHs

Is the amplitude of the rotating TM during QSH affected

by the external helical perturbation?

spon

tane

ous

QSHQSH

pulse

P

Dominant TM

Secondary TMs

CLOSE LOOP EXPERIMENTS (MC)

Active coils

plasmaSensor

coils

external helical magnetic

perturbations

Close Loop (Mode Control)Close Loop (Mode Control)

shell

Digitalcontroller

bc

b1,n bcVc V1,n=-Kpn b1,n

Fourier harmonics in real time

Calculates inputto active coils

The mode suppression is controlled by kp

-12 Each can be controlled separately.

One mode free to growAll other modes suppressed

n=-12

Secondary modes

CLOSE LOOP EXPERIMENTS (MC)

Amplitude of dominant TM during QSHs

210 QSHs are considered

Are QSH and rotating TMs affected?

The gain of mode n=-12 is reduced

All m

odes suppressed

…20

Gain reduction

br-12 increase

-12

CLOSE LOOP EXPERIMENTS (MC)

Using the optimal kp-12, F is scanned

to test the dependence of PQSH.

PQSH is clearly dependent on F

This is mainly due to a changein the position of the resonant radius.

(1,-11)

(1,-12)

(1,-13)

(1,-14)

(1,-15)

Dependence of resonant radii with F

MH std

CONCLUSIONS

(1) A static helical perturbation can affect the corresponding rotating TM

- Higher QSH probability- The TM velocity is affected- But NOT the TM mode amplitude during the QSH

(2) With the MC better results are obtained

- QSH probability higher than in Open Loop- QSH are more “pure” (i.e. lower secondary modes)- The amplitude of the rotating TM is affected

QSH OL

MH MC

QSH MC

OPEN LOOP EXPERIMENTS

Is the TM velocityvelocity affected by the external perturbation?

Fitzpatrick model [NF 33, 1049 (1993)]