TAE and EAE Damping in JET: Experimental Evidence and Theoretical Analysis

1. TAE and EAE damping on JETA.Fasoli, D.Testa, CRPP - EPFLC.Boswell, MITS.Sharapov, UKAEAand Contributors to JET-EFDA Workprogramme and Enhancements Burning Plasma Workshop and ITPA Meeting, Tarragona, Spain, July 2005

2. Toroidal and Elliptical Alfvén Eigenmodes • cylinder: Alfvén ‘continuum’ 2(r)=k||2(r)vA2(r) • small scales: strong damping • torus: Coupling of poloidal harmonics • gaps in continuum spectrum • weakly damped, global Toroidal Alfvén Eigenmodes (TAEs) Elliptical AEs (EAEs), ….

3. Passive • Modes observed if destabilised by fast particles (NBI,ICRH, fusion’s) • Active • In-vessel antennas drive low amplitude perturbations • Resonance in plasma response (e.g. on B-probes): global mode EAEs TAEs ICRH = fast ion source Active and passive AE spectroscopy on JET

4. n=1 TAE fTAE dB/dt (T/s) + time (s) fmeas dB gdamp AEs (TAEs, EAEs, NAEs, GAEs, kinetic TAEs and EAEs) only stable global modes in Alfvén range • Ex.: single n=1 TAE tracking using saddle coils • No ‘stable’ Alfvén Cascades seen to date

5. Lay-out of the talk • Recent results from last campaign using JET saddle coils (1994-2004, now dismantled, >3000 discharges) • TAE vs EAE damping at the edge • TAE edge damping reproduced by theory • Different scaling of EAE damping with plasma shape and edge shear? • AE damping in core: difficult to reproduce with present models • Outlook: the new AE antennas at JET (intermediate n’s)

6. TAE edge damping : experimental evidence • Shaping of cross-section increased magnetic shear increased mode conversion  strong damping • Quantitative agreement with gyro-kinetic code PENN • Consistent with observed PNBI threshold for TAE excitation

7. Consequences of strong edge damping • Damping in the plasma core can be studied with radially extended low-n AEs only for very low shaping (d95<0.35, k95 <1.5), i.e. in limiter plasmas • Strong dependence of damping on edge conditions and profiles • Example: difference in measured damping due to B-field reversal

8. TAE damping: effect of B-field direction • Damping of n=1 TAE about 2-3 times larger for reverse B-field (ion B-drift directed toward X-point, favorable for H-mode) • Comparison with ICRH-driven TAEs (n=3-10) Calculated fast ion drive at the onset of instability DRIVE(reverseB) > DRIVE(forwardB) difference decreases with n • Forward B more TAE unstable • Challenge for fluid/gyro-kinetic models • Role of plasma edge flows (ion B-drift direction)?

9. Consequences of strong edge damping • Damping in the plasma core can be studied with radially extended low-n AEs only for very low shaping (d95<0.35, k95 <1.5), i.e. in limiter plasmas • Strong dependence of damping on edge conditions and profiles • Extreme sensitivity on details of edge • Comparison with theory partly be inconclusive (small changes in edge profiles can be invoked), unless we look at • Scalings of measured damping • Comparison of TAE and EAE in similar conditions

10. 400 EAE 350 300 f (kHz) 250 TAE 200 150 3 TAE (%) 2 1 EAE 0 6 7 8 9 10 11 Time (s) TAE vs EAE damping • Nearly identical discharges • Ohmic, limited • =1.34, <>=0.004 • Constant ne, Te, Bt, Ip

11. 2 1.5 1 0.5 0 0 0.25 0.5 0.75 1 EAE and TAE calculated gap structures • Edge damping mechanisms for EAEs similar to TAEs? • Effect of edge magnetic shear and shaping

12. EAE Damping Rate, #61519 4 (%) 2 3 BT (T) 2 Ip (MA) 1 5 s95 0 0 5 10 15 Time (s) n=1 EAE dampings95 scan from ramping current and shape 3 < s95 < 4.5 scan done during a ramp in Ip and shape Scan in s95

13. 9 8 7 6 (%) 5 4 3 2 1 2 2.5 3 3.5 4 4.5 s95 n=1 EAE damping rate vs s95 • EAE damping small at high s95 • Similar results for elongation and triangularity • Opposite to the n=1 TAE trend at high elongation and triangularity • Hidden q0 dependence? • Effect of elongation on EAE gap width?

14. Summary and open questions • Low-n AE linear stability • Edge damping • Large , shape dependence, explained by theory for TAEs • Extreme sensitivity on edge conditions • Ex.: effect of B-field reversal on damping and stability • But EAE damping seems subject to a different scaling • Effect of q0, gap width dependence on elongation? • Core damping • Difficulty in reproducing measured  and scalings (see following talks) • Example of TAE damping dependence on q0

15. TAE damping (in the core?):  vs q0 • ~1500 measurement points for n=1 TAE damping • q0~0.76-1.6, 1.24<95<1.55; 0<95<0.25; 1.35<ne0(1019m-3)<4.2; 1.1<Te0(keV)<5.6; 2.5<q95<4.75 • Transition for q0~1 not reproduced by continuum  in CASTOR

16. saddle coils: n=0-2 fast ion driven modes: n=3-10 Outlook • JET saddle coil system limited to low n’s • Need to investigate most unstable n range for ITER: n3-15 • Identify systematic methods to compare experiments with theory in intermediate n range (many modes coexisting) • New AE active antenna on JET

17. + + + + ~1m + + + _ + + __ + _ + _ + __ + + + _ + New AE antenna spectrum in-vessel mounting • 45mm from LCFS, 18 turns; i ~ 20A, V~1000V, 10-500kHz • Coupling of n=5 calculated to be as n=2 with saddle coils • Local value of B/B can be larger

18. New AE antenna installed on JET Octant 8

20. New AE antenna installed on JET Octant 8

21. ‘wings’ to attach to poloidal limiter distance from LCFS ~45mm: need tiles open frame: no loop currents all frame parts are Inconel 625 plug&socket connector isolating hinges and supports, by-passed by straps of fixed R to balance halo currents 18-turns, inconel 718 wire, 4mm diameter, 4mm spacing Overview of new AE antenna design 2 antennas on Octant 4 and Octant 8

22. TAE =k||vA(r) TAE =k||vA (r) Linear mode stability -1-AE damping mechanisms • Direct ion, electron Landau • Mode conversion • Directly to shear AW (‘continuum damping’) or to kinetic AW: • large up to ~ 5-10 % • Tunneling to shear AW or kAW: ‘radiative damping’ • Collisional damping • el. coll/ (e/)1/2 e ~ ne / Te 3/2