Observation of TAE activity in JET

Abstract

High-frequency magnetic activity with chrvacferisfics expected of tomidicityinduced AlfvCn eigenmodes CTAEs) has been observed in JET discharges heated by neutral beam injection and ion cyclotron resonance. heating. The mode frequency is found to scale in the expected way with the magnetic field and plasma densicy and the. frequency specua show the characteristic multimode TA@ ~Wchlre~ seen in other devices. A correlation bemeen fast-panicle loss levels and the mode amplitude is also suggested by the dafa . , Toroidicity-induced Alfvin eigenmodes (e.g. [l]) can be driven unstable by fast particles originating from neutral beam injection, RF heating or fusion reactions [f-5]. Their potential role in fusion reactors, where they may be destabilized by the super-Alfvinic a-population, is a subject of concern as they may lead to anomalously rapid losses of or-particles. Not only would this lead to a reduction in the efficiency of a-particle heating, but the expulsion of high-energy E'S from the plasma could result in significant damage to the first wall.Recent observations [6][7][8][9] have confirmed the excitation of such modes in additionally heated plasma and have shown that they are associated with losses of fast particles from the bulk plasma In this letter the evidence for the presence of TAE activity is reported in JET plasmas with neutral beam andor ion cyclotron resonance heating.In toroidal plasmas, poloidal mode coupling due to toroidicity leads to the existence of gaps in the shear Alfven continuum. TAE modes are discrete global eigenmodes

Observations of neutral beam and ICRF tail ion losses due to Alfven modes in TFTR

“…A similar behavior is seen in JET. [21] At the sawtooth crashes (seen in the third trace) the mode momentarily turns on again, and the losses increase correspondingly. This, it is postulated, is due to a rearrangement of the fast ion profile.…”

“…A similar linear dependence of fast ion loss on mode amplitude has been observed in JET as we11. [21] It is possible to cross-calibrate the 20" detector from the 90" detector under other quiescent discharge conditions where fusion product loss dominates. [5] Assuming the Scintillator response is linear with the fast ion power incident upon it, that calibration can be used for neutral beam losses as well.…”

“…Similar behaviors of mode onset and frequency scaling have also been observed during minority ICRF heating in JET and JT-6OU. [18][19][20][21] H-minority tail ion losses result from the TAE. These are most easily measured by the 45" and 60" probes which are in fixed locations, have apertures closer to the plasma than the 20" probe, and are better protected from the plasma heat flux than is the 20" probe.…”

“…This linearity is again in agreement with reported JET results. [21] Based upon the data from the go", 60°, and 45" ddtectors, the loss of tail ions increases toward the midplane. Unfortunately, the 20" detector is of a different geometry than the others, with its aperture 3 cm back from the probe tip, rather than 1 cm for the others.…”

“…[8, 9,16] Subsequent results have also come from PBX-M [17], JT-60U [18][19][20], and JET. [21] The results from JT-60U and JET concentrate predominantly on ICRF minority tail driven TAEs, although the JET work includes some on neutral beam driven modes. Recent work on JET has included external excitation of the TAE by an antenna in order to measure directly the damping of the mode.…”