Collective instability-induced fast ion losses in NSTX

E. Fredrickson for the NSTX Team47th APS – DPP Meeting

Oct 24-28, 2005Denver, Co.

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAERIHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

ENEA, FrascatiCEA, Cadarache

IPP, Jülich

IPP, GarchingASCR, Czech Rep

U Quebec

College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin

Supported byOffice ofScience

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

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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:

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fci≈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 nfast

• Losses weakly correlated with mode amplitude

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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 012-2-1012Mirnov coilssoft x-ray

chordsNSTX 113523_0.301

• Bursts last for about 1 msec

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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

501001500“Edge”“Core”120 μs

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.

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Toroidal Alfvén Eigenmodes

Fast Ion Losses

Mode Structure

Simulations

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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

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• 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

Reflectometer (a.u.)0100200Frequency (kHz)Mirnov Coil (mG)201003210113544_0.2688sn = 1344555667

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1.01.5Major Radius (m)4321050.30.00.20.1Displacement (cm)NSTX 113544_0.2688_136.7kHzqDensity (1013 cm-3)MagneticAxis

Phaseπ0−π

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. 2004

Chirping TAE simulated with M3D

G.Y. Fu, PPPL

20-2108530t = 0.267200 μs

Time

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time010020030040050060070001-1

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.

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Compressional and Global Alfvén Eigenmodes (CAE/GAE)

Characteristics

GAE "hole-clumps"(Angelfish)

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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

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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

0123246810000.51.01.5Major Radius (m)q(R)Electron Density (1013)GAE116875

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) 213. 21

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.

€

Vb ||

Vb= [ωciω

+VtorVAlfvén

−1]VAlfvénVb

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