137 research outputs found
Measurement of a 2D fast-ion velocity distribution function by tomographic inversion of fast-ion D-alpha spectra
We present the first measurement of a local fast-ion 2D velocity distribution function f (v , vâ„). To this end, we heated a plasma
in ASDEX Upgrade by neutral beam injection and measured spectra of fast-ion Dα (FIDA) light from the plasma centre in three
views simultaneously. The measured spectra agree very well with synthetic spectra calculated from a TRANSP/NUBEAM
simulation. Based on the measured FIDA spectra alone, we infer f (v , vâ„) by tomographic inversion. Salient features of
our measurement of f (v , vâ„) agree reasonably well with the simulation: the measured as well as the simulated f (v , vâ„) are lopsided towards negative velocities parallel to the magnetic field, and they have similar shapes. Further, the peaks in the simulation of f (v , vâ„) at full and half injection energies of the neutral beam also appear in the measurement at similar velocity-space locations. We expect that we can measure spectra in up to seven views simultaneously in the next ASDEX Upgrade campaign which would further improve measurements of f (v , vâ„) by tomographic inversion
Transport of energetic ions due to sawteeth, Alfven eigenmodes and microturbulence
Utilizing an array of new diagnostics and simulation/modelling techniques, recent DIII-D experiments have
elucidated a variety of energetic ion transport behaviour in the presence of instabilities ranging from large-scale
sawteeth to fine spatial scale microturbulence. Important new insights include sawteeth, such as those of the ITER baseline scenario, causing major redistribution of the energetic ion population; high levels of transport induced by low-amplitude Alfven eigenmodes can be caused by the integrated effect of a large number of simultaneous modes; ÂŽ and microturbulence can contribute to the removal of alpha ash while having little effect on fusion alphas. This paper provides an overview of recent and upcoming results from the DIII-D Energetic Particles research programme.US Department of Energy SC-G903402, DE-FC02-04ER54698, DE-FG02-89ER53296, DE-FG02-08ER54999, DE-AC05-00OR22725, DE-AC02-09CH11466, DE-FG03-08ER54984, DE-FG02-07ER5491
Characterization of off-axis fishbones
Repetitive bursting instabilities with strong frequency chirping occur in highbeta, beam-heated plasmas with safety factor q > 1 in the DIII-D tokamak.
Although the mode structures differ, in many ways, the off-axis fishbones
are similar to the q = 1 fishbones first observed on the Poloidal Divertor
Experiment (PDX). The modes are driven by energetic trapped ions at the fastion precession frequency. During a burst, the frequency changes most rapidly
as the mode reaches its maximum amplitude. Larger amplitude bursts have
larger growth rates and frequency chirps. Unlike PDX fishbones, the decay
phase is highly variable and is usually shorter than the growth phase. Also,
the waveform is highly distorted by higher harmonics during the latter portion
of a burst. The radial mode structure alters its shape during the burst. Like
PDX fishbones, the modes expel trapped ions in a âbeaconâ with a definite
phase relationship relative to the mode. Seven types of loss detectors measure
the beacon. The losses scale linearly with mode amplitude. The neutron rate
changes most rapidly at maximum mode amplitude but, depending on the loss
diagnostic, the losses often peak a few cycles later. The non-ambipolar fast-ion
losses cause a sudden change in toroidal rotation frequency across the entire
plasma. In addition to an overall drop, the neutron signal oscillates in response
to the wave. Unlike the beacon of lost particles, which maintains a fixed phase
relative to the mode, the phase of the neutron oscillations steadily increases
throughout the burst, with the greatest phase slippage occurring in the highly
nonlinear phase near maximum mode amplitudeUS Department of Energy SC-G903402, DE-FC02-04ER54698, DE-FG02-07ER5491
Scintillator-based diagnostic for fast ion loss measurements on DIII-D
A new scintillator-based fast ion loss detector has been installed on DIII-D with the time response
100 kHz needed to study energetic ion losses induced by Alfvén eigenmodes and other MHD
instabilities. Based on the design used on ASDEX Upgrade, the diagnostic measures the pitch angle
and gyroradius of ion losses based on the position of the ions striking the two-dimensional
scintillator. For fast time response measurements, a beam splitter and fiberoptics couple a portion of the scintillator light to a photomultiplier. Reverse orbit following techniques trace the lost ions to their possible origin within the plasma. Initial DIII-D results showing prompt losses and energetic ion loss due to MHD instabilities are discussed. © 2010 American Institute of Physics.U.S. Department of Energy DE-FC02-04ER54698, SC-G903402, DE-FG03-94ER5427
Measurements and modeling of Alfven eigenmode induced fast ion transport and loss in DIII-D and ASDEX Upgrade
Neutral beam injection into reversed magnetic shear DIII-D and ASDEX Upgrade plasmas
produces a variety of AlfveÂŽnic activity including toroidicity-induced AlfveÂŽn eigenmodes and
reversed shear AlfveÂŽn eigenmodes (RSAEs). These modes are studied during the discharge current
ramp phase when incomplete current penetration results in a high central safety factor and
increased drive due to multiple higher order resonances. Scans of injected 80 keV neutral beam
power on DIII-D showed a transition from classical to AE dominated fast ion transport and, as
previously found, discharges with strong AE activity exhibit a deficit in neutron emission relative
to classical predictions. By keeping beam power constant and delaying injection during the current
ramp, AE activity was reduced or eliminated and a significant improvement in fast ion confinement
observed. Similarly, experiments in ASDEX Upgrade using early 60 keV neutral beam injection
drove multiple unstable RSAEs. Periods of strong RSAE activity are accompanied by a large (peak
dSn=Sn 60%) neutron deficit. Losses of beam ions modulated at AE frequencies were observed
using large bandwidth energy and pitch resolving fast ion loss scintillator detectors and clearly
identify their role in the process. Modeling of DIII-D loss measurements using guiding center
following codes to track particles in the presence of ideal magnetohydrodynamic (MHD)
calculated AE structures (validated by comparison to experiment) is able to reproduce the
dominant energy, pitch, and temporal evolution of these losses. While loss of both co and counter
current fast ions occurs, simulations show that the dominant loss mechanism observed is the mode induced transition of counter-passing fast ions to lost trapped orbits. Modeling also reproduces a coherent signature of AE induced losses and it was found that these coherent losses scale proportionally with the amplitude; an additional incoherent contribution scales quadratically with the mode amplitude. VC 2011 American Institute of Physics.US Department of Energy DE-FC02-04ER54698, SC-G903402, DE-AC02-99CH11466, DE-FG03-97ER54415, DE-FG02-89ER53296, DE-FG02-08ER5499
Convective beam ion losses due to Alfven eigenmodes in DIII-D reversed-shear plasmas
Coherent losses of neutral beam ions are observed at frequencies corresponding
to toroidal and reversed-shear Alfven eigenmodes (RSAEs) in DIII-D. ÂŽ
Reversed-shear profiles are created by injecting beam power during the plasma
current ramp. Beam ion losses stemming from Alfven eigenmode activity ÂŽ
contribute to flattening of the energetic ion density profile in such discharges.
This is the first observation of convective beam ion losses due to RSAEs.
The energies and pitch angles of lost ions are measured and found to exist
within a well-defined region of phase space. Loss flux signals decrease in
time as current penetrates and Alfven eigenmode activity becomes more core ÂŽ
localized. Preliminary Monte Carlo simulations of energetic ion interactions
with measured mode structures show the dominant loss mechanism is a
transition from a counter-passing orbit to a trapped orbit that is lost to the
wall.US Department of Energy DE-AC05-06ER23100, SC-G903402, DE-FC02-04ER5469
Fast ion profile stiffness due to the resonance overlap of multiple Alfvén eigenmodes
Fast ion pressure profiles flattened by multiple Alfvén eigenmodes (AEs) are investigated for various neutral beam deposition powers in a multi-phase simulation, which is a combination of classical simulation and hybrid simulation for energetic particles interacting with a magnetohydrodynamic fluid. Monotonic degradation of fast ion confinement and fast ion profile stiffness is found with increasing beam deposition power. The confinement degradation and profile stiffness are caused by a sudden increase in fast ion transport flux brought about by AEs for fast ion pressure gradients above a critical value. The critical pressure gradient and the corresponding beam deposition power depend on the radial location. The fast ion pressure gradient stays moderately above the critical value, and the profiles of the fast ion pressure and fast ion transport flux spread radially outward from the inner region, where the beam is injected. It is found that the square root of the MHD fluctuation energy is proportional to the beam deposition power. Analysis of the time evolutions of the fast ion energy flux profiles reveals that intermittent avalanches take place with contributions from the multiple eigenmodes. Surface of section plots demonstrate that the resonance overlap of multiple eigenmodes accounts for the sudden increase in fast ion transport with increasing beam power. The critical gradient and critical beam power for the profile stiffness are substantially higher than the marginal stability threshold
Fast-ion D-alpha measurements at ASDEX Upgrade
A fast-ion D-alpha (FIDA) diagnostic has been developed for the fully tungsten
coated ASDEX Upgrade (AUG) tokamak using 25 toroidally viewing lines
of sight and featuring a temporal resolution of 10 ms. The diagnosticâs
toroidal geometry determines a well-defined region in velocity space which
significantly overlaps with the typical fast-ion distribution in AUG plasmas.
Background subtraction without beam modulation is possible because relevant
parts of the FIDA spectra are free from impurity line contamination. Thus,
the temporal evolution of the confined fast-ion distribution function can be
monitored continuously. FIDA profiles during on- and off-axis neutral beam
injection (NBI) heating are presented which show changes in the radial
fast-ion distribution with the different NBI geometries. Good agreement
has been obtained between measured and simulated FIDA radial profiles
in MHD-quiescent plasmas using fast-ion distribution functions provided by
TRANSP. In addition, a large fast-ion redistribution with a drop of about 50%
in the central fast-ion population has been observed in the presence of a q = 2
sawtooth-like crash, demonstrating the capabilities of the diagnostic
Scrape-off layer ion acceleration during fast wave injection in the DIII-D tokamak
Fast wave injection is employed on the DIII-D tokamak as a current drive and electron heating method. Bursts of
energetic ions with energy Eo > 20 keV are observed immediately following fast wave injection in experiments
featuring the 8th ion cyclotron harmonic near the antenna. Using the energy and pitch angle of the energetic ion
burst as measured by a fast-ion loss detector, it is possible to trace the origin of these ions to a particular antenna. The ion trajectories exist entirely within the scrape-off layer. These observations are consistent with the presence of parametric decay instabilities near the antenna strap. It is suggested that the phase space capabilities of the loss detector diagnostic can improve studies of wave injection coupling and efficiency in tokamaks by directly measuring
the effects of parametric decay thresholds.US Department of Energy SC-G903402, DE-FG03-97ER4415, DE-FG02-89ER53296, DE-FG02-08ER549
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