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This presentation discusses the damping of Toroidal Alfvén Eigenmodes (TAEs) and Elliptical Alfvén Eigenmodes (EAEs) in JET. The talk includes experimental evidence of TAE and EAE damping at the edge, the reproduction of TAE edge damping by theory, and the different scaling of EAE damping with plasma shape and edge shear. It also covers the challenges of reproducing AE damping in the plasma core and provides an outlook on the new AE antennas at JET.
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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
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), ….
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
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
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)
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
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
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)?
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
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
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
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
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?
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
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
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: n3-15 • Identify systematic methods to compare experiments with theory in intermediate n range (many modes coexisting) • New AE active antenna on JET
+ + + + ~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
‘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
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