Collective instability-induced fast ion losses in NSTX E. Fredrickson for the NSTX Team 47 th APS – DPP Meeting Oct 24-28, 2005 Denver, Co. Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAERI Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin Supported by Office of Science

NSTX routinely operates with a large, super-Alfvénic, fast ion population Low field, moderate density, and 60 to 100 kV beam injection energy make NSTX an excellent platform for ITER-relevant fast ion-induced instability studies Large  * implies easily measured fast ion losses/transport 2

Fast Ion Modes dominate MHD spectrum 1.Compressional and Global Alfvén Eigenmodes (CAE and GAE) –Natural plasma resonance –CAE parallel  B,  E is transverse –GAE mixed transverse/parallel  B 2.Toroidal Alfvén Eigenmodes (TAE) –Natural plasma resonance –Strong drive pushes off-resonance –Shear wave, lower frequency 3.Energetic Particle Modes (EPM) –Mode defined by fast ion parameters (fishbone) –Frequency chirping common –Include non-fishbones, n > 1 Fast Ion Modes can be sorted into three categories: 3 f ci ≈3MHz

Fast ions heat plasma, drive current Transport/confinement affected by instabilities Beam (fast ion) driven current drive important NSTX ITER heated with super-Alfvénic fusion  's Outline –Energetic Particle Modes (EPMs) –Toroidal Alfvén Eigenmodes (TAE) –High frequency modes (CAE/GAE) 4

EPM induced losses pervasive In above example, different EPMs closely spaced (in time) cause much different loss Mode structure plays big role in interaction with fast ions Neutrons primarily from beam- target; neutrons indicate n fast Losses weakly correlated with mode amplitude 5

Frequency chirp of EPMs matches range of fast-ion bounce frequencies Precession frequency often low or negative due to low A, high  Many different mode-fast ion resonances are possible 6

Fast ions affected over all energies Strongest modulation is seen for lowest energies; below the "half" energy. Neutron drops of 10% suggest high energy ions also lost. Broad range of energy interaction consistent with bounce-resonances S. Medley, PPPL 7

EPMs core localized, kink-like Seen in all operational regimes Example here is n=1, but higher n's are also common Reflectometer and SX cameras measure the internal structure No phase inversion; not an island Bursts last for about 1 msec 8

EPMs localized to low shear Inverted SX emission profile and EFIT equilibrium, used to "invert" soft x-ray data. Simulate fast ion losses with mode amplitude/structure N. Crocker, S. Kubota, W. Peebles, UCLAD. Stutman, K. Tritz, JHU 9

EPM can evolve to a continuous mode Mode structure same as EPM bursts, but –Amplitude larger than preceding EPMs. Decays fast after beams turn off Similar behavior seen for fishbones in conventional tokamaks. 10

Toroidal Alfvén Eigenmodes Fast Ion Losses Mode Structure Simulations 11

TAE-induced losses scale with amplitude TAE in absence of EPM are rare; modes work together No scaling with , up to  tor ≈25% Similar to scaling seen on TFTR for ICRF and NBI induced TAE modes TFTR modes more core localized; similar losses, weaker Mirnov fluctuations 12

TAE typically chirp and burst 13 Series of small chirps, multiple modes, then big burst with many more modes Neutron drops correlated with big bursts Sequence repeats Often EPM triggered by big bursts Often sequence of small bursts/chirps

Reflectometer shows "sea of TAE" Many modes suggest strongly non-linear problem Mirnov signal good measure relative mode amplitude Single TAE amplitude is of the order  n/n ≈ 1% N. Crocker, S. Kubota, W. Peebles, UCLA 14

TAE activity seen up to highest  Unlike MAST, where TAE activity is not present at highest  's* High beam voltage in NSTX means high , high density plasmas still have significant fast ion populations (  fast /  beam ≈ 30%). *Gryaznevich,Sharapov, PPCF 46 (2004) S15-S29 15

M3D Nonlinear hybrid simulations of beam-driven modes in NSTX shows a bursting n=2 TAE as the mode moves out radially G.Y. Fu et al., IAEA Fusion Energy conference Chirping TAE simulated with M3D G.Y. Fu, PPPL Time 16

Multiple TAE/EPM simulates "sea of Alfvén modes" expected in ITER EPM and TAE responsible for most fast ion loss events –Distinction between TAE and EPM somewhat artificial Continuum of mode behavior from chirping/bursting EPMS to quasi-coherent TAE-like. 17

Compressional and Global Alfvén Eigenmodes (CAE/GAE) Characteristics GAE "hole-clumps" (Angelfish) 18

Modes identified by their polarization Polarization measured by orthogonal Mirnov coils TAE/EPM/MHD have transverse (shear) polarization GAE, a shear wave, also couples to the compressional polarization CAE have magnetic fluctuations aligned with pitch of magnetic field, compressional 19

GAE mode amplitude  n/n ≈ 10 –4 Measurement made in low shear region where GAE expected to be localized Polarization measurement suggests GAE, not CAE Reflectometer spectrum shows same peaks as Mirnov coil S. Kubota, N. Crocker, W. Peebles, UCLA 20

New observation: GAE "Hole-clump" behavior GAE bursts chirp both up and down during early NBI on NSTX. Red curve is single parameter fit to frequency evolution using Berk, Breizman, Petviashvili model of hole- clump pair creation* GAE drive through Doppler-shifted ion cyclotron resonance - –Hole-clump-like behavior shows long correlation time for interaction of mode with fast ion population *H.L. Berk, B.N. Breizman, N.V. Petviashvili, Phys. Lett. A 234 (1997)

NSTX is well diagnosed testbed for fast ion instability studies NSTX fast ion loss events typically occur with multiple modes ("sea of TAE", as predicted for ITER) Lower frequency, strongly chirping (EPM) modes correlated with most fast ion loss events Fast ion losses from EPM/TAE are not serious in NSTX, but –Next step is to document effect on heating profile and on beam driven currents –Fast ion losses may be important for first-wall issues in next-step devices

Range of frequency chirp agrees well with fast ion "bump-on-tail" The red line indicates fast ions initially resonant (Doppler- shifted cyclotron) with the mode. For k || fixed, the fast ions resonant at the minimum/maximum of frequency chirp are indicated by the blue lines. 23