EXPERIMENTS OF ACTIVE QSH 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 I p  80kA (standard current plasma) n e ≈10 19 m -3 T e ≈ eV  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 Active coils shell by Olofsson E.

EXTRAP T2R – algorithms At present, three algorithms are routinely used in EXTRAP T2R: Open Loop Intelligent Shell (close loop) IS Mode Control (close loop) MC Active coils plasma Sensor coils external helical magnetic perturbations Open Loop (OL) shell by Olofsson E. Active coils plasma Sensor coils external helical magnetic perturbations Close Loop (MC) shell Digital controller bcbc b 1,n  b c V c (t)  V 1,n(t) =-K n p [ref-b 1,n (t)] Fourier harmonics in real time input to active coils

DIAGNOSTICS Magnetic perturbations: b r  4 poloidal x 32 toroidal sensor saddle coils (m=1 connected) located inside the shell give the time integrated signals. TM RWM 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 N s >2 QSH N s <2 SH N s =1 Average duration  QSH ≈0.1ms Fraction of the discharge characterized by QSHs: The dominant mode is the first resonant. With the typical equilibrium: n DOM =-12 n=-12

vacuum OPEN LOOP EXPERIMENTS Active coils plasma Sensor coils external helical magnetic perturbations Open Loop (OL) shell Basic idea: Apply a static helical perturbation (n=-12) plasma V=1.4V V=1.2V V=1.0V V=0.8V V=0.6V V=0.4V V=0 Average between 5ms-end of discharge Study the plasma behaviour

OPEN LOOP EXPERIMENTS static A static helical field with n=-12 is applied at the plasma boundary. What happen to the corresponding rotating rotating TM? “Often”, the TM is larger than “usual” in the sense that more 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 b r -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? spontaneous Dominant TM Secondary TMs

CLOSE LOOP EXPERIMENTS (MC) Active coils plasma Sensor coils external helical magnetic perturbations Close Loop (Mode Control) shell Digital controller bcbc b 1,n  b c V c  V 1,n =-K p n b 1,n Fourier harmonics in real time Calculates input to active coils The mode suppression is controlled by k p -12 Each can be controlled separately. One mode free to grow All 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 modes suppressed …20 Gain reduction b r -12 increase -12

CLOSE LOOP EXPERIMENTS (MC) Using the optimal k p -12, F is scanned to test the dependence of P QSH. P QSH is clearly dependent on F This is mainly due to a change in 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 velocity Is the TM velocity affected by the external perturbation? Fitzpatrick model [NF 33, 1049 (1993)]