Transcript Document

THEORY SUMMARY
J. W. Van Dam
[email protected]
Please send me comments!
8th IAEA Technical Committee Meeting on
Energetic Particles in Magnetic Confinement Systems
San Diego, CA; 6-8 October 2003
OUTLINE
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Fast ion distribution
Fast particle effects on plasma equilibrium
Suprathermal electrons
Fishbones & internal kinks
Collective modes
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TAE
CAE, HAE
Alfvén cascades
Chirping modes
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Theory
Experiment
EPM modes
Fast ion transport
Alpha particles in current-hole plasmas
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FAST ION DISTRIBUTION
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Alpha profiles in ITER ELMy H-mode (Budny)
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Finds: [1] NNBI current affects alpha profile; [2] NNBI drive exceeds fast
alpha TAE drive; [3] Sawteeth affect the alpha density.
Fast particle losses during NSTX reconnection events (Yakovenko)
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Drops in neutron yield during reconnection events are due to fast particle
losses, not redistribution. Probable mechanism = magnetic drift stochasticity
Distribution and fields during ICRH and AE excitation (Hellsten)
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Idea to use ICRH fast ions to simulate alphas
JET antenna phasing (|| & anti-|| propagation) => different orbits (g emission)
Fast ion distribution details are important for AE stability/growth
ICRH causes decorrelation => affects nonlinear growth of AE modes
ICRH fast ion profile and confinement in LHD (Murakami)
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Calculates the velocity space distribution for ICRF-heated minority H ions
(up to 500 keV of energetic tail ions); also the power absorption and heat
deposition -- good experimental comparison
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Simulates transport of fast ions (especially, helically trapped particles)
FAST PARTICLE EFFECTS ON EQUILIBRIUM
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Plasma rotation from ICRF-heated fast particles (Eriksson)
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Possible theories: [1] Fast particles, [2] Neoclassical effects, [3] Accretion process
Investigates minority ion heating with co-current ICRF waves
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Fast ions absorb wave momentum, then transfer it to plasma (via collisions or radial current)
Experiments (JET) and simulations show that fast particles may be able to control the rotation
profile to some extent
Formation of internal transport barrier (Wong)
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Several experiments have observed toroidal co-current rotation with ICRH plasmas
with zero momentum injection
Proposes that sheared rotation and negative central shear (conducive to ITB
formation) can be created by having Alfvén eigenmode instabilities eject/redistribute
alpha particles out of central region toward edge => supported by DIII-D data
Estimates that in ITER, number of alphas for this mechanism is marginal
Ripple reduction with ferritic inserts (Shinohara)
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Experiments (JFT-2M) and simulations (3d OFMC) show improved orbit confinement
for NBI fast ions when ferritic inserts are used (in non-shear-reversed plasmas)
Speculates about use of large externally created local ripple for burn control
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SUPRATHERMAL ELECTRONS
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Runaway electrons and disruptions (Helander) (Andersson)
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Two approaches: [1] Monte Carlo simulations, [2] Reduced model
Findings:
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Most runaways generated by secondary/avalanche mechanism in JET and ITER
Typically 50% conversion of thermal current to runaways in JET (more in ITER)
Runaway density profile: [1] Easily corrugated => explain bursty x-ray emission?
[2] Peaked on axis (more than pre-disruption current) -- observed experimentally
Particle acceleration at magnetic X-points (McClements)
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Finds both discrete (w/wA = 0.8) and singular modes
X-points can accelerate fast particles
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FISHBONES & INTERNAL KINKS
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Near-threshold fishbone behavior (Breizman)
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Fishbone is good example of convective transport (single mode)
Two-step linear mode structure for finite-frequency fishbone
Weak fluid nonlinearity of q=1 layer destabilizes fishbone => bursting?
Fishbone stability with alphas (Fu)
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Estimates n=1 internal kink in ITER is not stabilized by alphas (contrary to
earlier Porcelli theory -- finite orbit width effect?)
Fishbone nonlinear saturation (Todo)
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Mode with n=1 saturates, while n=0, 2 continue to grow
Linear gyrokinetic calculation (Lauber)
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Benchmarked on internal kink
Nonperturbative simulation of n=1 JET observations (Gorelenkov)
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Reproduces LF ideal and HF resonant modes (but no 2-step profile -- zero
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orbit width?)
COLLECTIVE MODES – TAE
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JET antenna coil observations (Testa)
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TAE in NCSX (Fu)
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Simulation finds unstable TAE mode (n=?)
3d geometrical effects reduce its growth rate
Discrete TAE in high-beta second-stable tokamak (Hu)
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Complex dependence of TAE damping rate on bN, PNBI => decreases, then increases
Damping rate (n=1) independent of r*I, not linear as predicted for radiative damping
Error field distorts B topology and kills q=2 TAE => control of hot pressure gradient?
Existence of mode is due to potential well created by high plasma pressure
gradient (continuum damping is weak)
Could be excited by fast particles at bounce + precession mixed resonance
Multi-ion species kinetic/fluid hybrid model (Cheng)
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Will include:
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Gyroviscosity (along with pressure tensor)
Hall term in Ohm’s law (nfast/ne not assumed negligible)
Thermal particle kinetic effects
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COLLECTIVE MODES – CAE, HAE
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Sub-cyclotron NBI-driven instabilities in NSTX (Belova)
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Most unstable mode is n=4/m=2 GAE (nm<0) with w/wci = 0.3: large k||,
large compressional component dB||/dBperp ~ 1/3
Nonlinear saturation level dB/B = 10-3 ~ 10-4
Helical AEs in compact stellarators (Spong)
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Calculates shear Alfven continuum spectrum for W7-AS, QPS, and NCSX:
find that TAE, HAE, and MAE modes are possible
W7-AS observes nonlinear bursting with fast ion loss and Te drop
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ALFVEN CASCADE MODES
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Cascade observations in JET and interpretation (Sharapov) (Breizman)
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“Grand cascade” (many simultaneous n-modes) occurrence is coincident with ITB
formation (when qmin passes through integer value) => diagnostic to monitor qmin
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Cascade mysteries (with ICRH fast ions):
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Transition to TAE as q0 decreases -- calculated theoretically
No cascade modes at low frequency -- effect of continuum damping (calculated)?
Sinusoidal frequency rolling -- ?
Cascades also occur when toroidicity effect exceeds that of fast particles (e.g., lowdensity alphas in TFTR)
“Reversed-shear Alfven eigenmode” observations in JT-60U (Shinohara)
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For NNB injection into reversed shear plasma, observe n=1 mode located at qmin, with
strong up and down frequency chirping
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Proposes creating ITB by application of main heating shortly before a Grand Cascade is
known to occur
Earlier such observations were with ICRH fast ions
Cascade observations in C-Mod (Snipes)
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Also observe EAE modes in H-mode plasmas (but why rotating in w*e direction?)
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CHIRPING MODES – THEORY
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Non-adiabatic description of phase-space hole/clump (Berk)
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Simulation of frequency sweeping (Pinches)
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Predicted frequency bifurcation seen experimentally (Gryaznevich/MAST)
Frequency sweeping undergoes non-adiabatic instability, which can cause sideband
generation
Two cases:
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JET parameters: Only chirps downward (due to choice of numerical distribution function-experiments see both up/down sweeping modes)
MAST parameters: Up/down sweeping, saturates at dB/B = 10-4
Use as nonlinear diagnostic to infer dB/B from chirping rate
Nonlinear hole/clump formation (Vann)
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Simulation of full single-mode (n=1) nonlinear dynamics => Finds 4 solutions
(damped, steady-state, periodic, chaotic) for various parameter regimes
Simulation of NBI-driven mode in weakly reversed-shear NSTX (Fu)
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Calculates unstable n=2 mode (TAE?)
Mode structure moves out radially as frequency chirps
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CHIRPING MODES – EXPERIMENT
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Fast ion instabilities in NSTX (Fredrickson)
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Observation of strongly bursting fast ion loss due to multiple TAE modes
(2<n<6): 40% drop in bfast
New “fishbone”-like mode also causes fast ion loss (up to 50%)
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n up to 5 (or more); often q(0)>1 and m>1; bounce-precession resonance;
possibly ballooning character; strongly chirping when fast ion loss is large
Coupled to TAEs and CAEs? Correlated with H-mode transition?
Energetic particle-driven modes in MAST (Gryaznevich)
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Behavior in three distinct regimes:
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Low-beta regime: n=1 up/down chirping modes
Medium-beta regime: “humpbacked fishbones”
High-beta regime (Next Step ST): cyclotron-range fast particle driven instabilities,
but no TAEs
No observation of fast ion loss in MAST (smaller population?)
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EPM MODES
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Bursting modes in JT-60U (Todo)
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Further measurements (by Shinohara et al.) of TAE frequency mode for NNB
injection in low-shear plasma, now with neutron emissivity diagnostic, to
study fast ion profile
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Observe two types of modes: fast frequency sweeping (FFS) and abrupt largeamplitude events (ALE)
FFS modes have little effect on neutron emissivity
ALE modes reduce fast ion population by 20% in r/a<0.4 central region => 14%
are redistributed to outer region and 6% lost to outside
Numerical simulation finds an n=1 EPM that is “nonlocal”-- suggested by
location (not on continuum), frequency (TAE-ish), and profile dependence
(on fast ion pressure)
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Frequency chirps both up and down, as in the experiment, if fast ion classical
distribution in the simulation is reduced (due to loss or redistribution)
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FAST ION TRANSPORT
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Diffusive transport with multiple modes (Breizman/Todo)
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Simulations show intermittent loss of fast ions.
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Co-injected NBI ions are confined better than counter-injected.
Phase-space resonances overlap at mode saturation.
Transition to strong transport in burning plasma (Zonca)
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Onset of fast ion avalanche transport is close to EPM linear stability
threshold--above which, EPM can propagate radially with convective
amplification. “Relay-runner” model captures nonlinear transport dynamics.
EPM-induced alpha particle transport (Vlad)
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Considers ITER-FEAT, FIRE, and IGNITOR for monotonic and reversed
shear: only ITER is unstable wrt EPMs (n=2 worst)
These unstable modes broaden alpha profile, first convectively (via
avalanche) and then diffusively
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ALPHAS IN CURRENT HOLE PLASMA
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Alpha confinement in current-hole fusion plasma (Tobita)
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Alpha loss can be serious for large current hole: Loss jumps from 2.0% (at
rhole = 0.4) to 12.6% (at when rhole > 0.5) => unacceptable for heat load on
first wall
Solution for current hole loss is low aspect ratio: (1) trapped particles are
less sensitive to TF ripple; (2) TF ripple drops along R
VECTOR device (A=2, superconducting): Even for wide current hole (rhole =
0.6), can confine alphas (loss as low as 2%)
Current hole effect on fast ion confinement in JET (Yavorski)
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Orbit of alpha is lost when current hole radius equal to or greater than 0.6
First orbit loss is enhanced by current hole from 8-25% for monotonic profile
(no hole), to 20-50% with current hole (of radius 0.6)
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PERSONAL IMPRESSIONS
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Signs of health:
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Research:
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Researchers:
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Significant number of young, talented scientists at this meeting
Facilities: Variety of confinement configurations that are currently
studying fast particle physics (as represented at this meeting)
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New, intriguing experimental findings
On-target theoretical explanations
Impressive numerical simulations
Tokamaks (JET, C-Mod, JT-60, JFT-2M, DIII-D)
Spherical tori (NSTX, START, MAST)
Helical/Stellarators (LHD, CHS, QPS, NCSX)
Others: linear (LAPD)
Anticipation:
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Short-term:
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New fast particle diagnostics
Tritium campaign (JET) & planned experiments on various machines
Long-term:
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ITER
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POSTSCRIPT
“Pope of Fast Particle Physics”
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Contributed seminal research
(e.g., alpha excitation of shear
Alfven waves, TAE continuum
damping, etc., etc.)
Trained students in this area
Chief Scientist for ITER
Participated in previous Fast
Particle Tech Comm Meetings
“Yearning for Burning”: influential
advocate for ignition physics
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