76 research outputs found
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
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 Alfve´nic activity including toroidicity-induced Alfve´n eigenmodes and
reversed shear Alfve´n 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. ´
Reversed-shear profiles are created by injecting beam power during the plasma
current ramp. Beam ion losses stemming from Alfven eigenmode activity ´
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 ´
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
Alfven eigenmode stability and fast ion loss in DIII-D and ITER reversed magnetic shear plasmas
Neutral beam injection into reversed-magnetic shear DIII-D plasmas produces a variety of Alfvenic activity including ´
toroidicity-induced Alfven eigenmodes (TAEs) and reversed shear Alfv ´ en eigenmodes (RSAEs). With measured ´
equilibrium profiles as inputs, the ideal MHD code NOVA is used to calculate eigenmodes of these plasmas. The
postprocessor code NOVA-K is then used to perturbatively calculate the actual stability of the modes, including
finite orbit width and finite Larmor radius effects, and reasonable agreement with the spectrum of observed modes
is found. Using experimentally measured mode amplitudes, fast ion orbit following simulations have been carried
out in the presence of the NOVA calculated eigenmodes and are found to reproduce the dominant energy, pitch
and temporal evolution of the losses measured using a large bandwidth scintillator diagnostic. The same analysis
techniques applied to a DT 8 MA ITER steady-state plasma scenario with reversed-magnetic shear and both beam
ion and alpha populations show Alfven eigenmode instability. Both RSAEs and TAEs are found to be unstable ´
with maximum growth rates occurring for toroidal mode number n = 6 and the majority of the drive coming from
fast ions injected by the 1 MeV negative ion beams. AE instability due to beam ion drive is confirmed by the non-perturbative code TAEFL. Initial fast ion orbit following simulations using the unstable modes with a range of amplitudes (δB/B = 10−5–10−3) have been carried out and show negligible fast ion loss. The lack of fast ion loss is a result of loss boundaries being limited to large radii and significantly removed from the actual modes themselves.US Department of Energy DE-FC02-04ER54698, DE-AC02-09CH11466, SC-G903402, DE-AC05-00OR22725, DE-FG03-97ER5441
Beam ion losses due to energetic particle geodesic acoustic modes
We report the first experimental observations of fast-ion loss in a tokamak due to energetic particle driven geodesic acoustic modes (EGAMs). A fast-ion loss detector installed on the DIII-D tokamak observes bursts of beam ion losses coherent with the EGAM frequency. The EGAM activity results in a significant loss of beam ions, comparable to the first orbit losses. The pitch angles and energies of the measured fast-ion losses agree with predictions from a full orbit simulation code SPIRAL, which includes scattering and slowing-down.U.S. Department of Energy DE-FC02-04ER 54698, SC-G903402, DE-AC02-09CH1146
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
Electron cyclotron heating can drastically alter reversed shear Alfven eigenmode activity in DIII-D through finite pressure effects
A recent DIII-D experiment investigating the impact of electron cyclotron heating (ECH)
on neutral beam driven reversed shear Alfvén eigenmode (RSAE) activity is presented. The
experiment includes variations of ECH injection location and timing, current ramp rate, beam
injection geometry (on/off-axis), and neutral beam power. Essentially all variations carried out
in this experiment were observed to change the impact of ECH on AE activity significantly.
In some cases, RSAEs were observed to be enhanced with ECH near the off-axis minimum
in magnetic safety factor (qmin), in contrast to the original DIII-D experiments where the
modes were absent when ECH was deposited near qmin. It is found that during intervals
when the geodesic acoustic mode (GAM) frequency at qmin is elevated and the calculated
RSAE minimum frequency, including contributions from thermal plasma gradients, is very
near or above the nominal TAE frequency (fTAE), RSAE activity is not observed or RSAEs
with a much reduced frequency sweep range are found. This condition is primarily brought
about by ECH modification of the local electron temperature (Te) which can raise both the
local Te at qmin as well as its gradient. A q-evolution model that incorporates this reduction
in RSAE frequency sweep range is in agreement with the observed spectra and appears to
capture the relative balance of TAE or RSAE-like modes throughout the current ramp phase
of over 38 DIII-D discharges. Detailed ideal MHD calculations using the NOVA code show
both modification of plasma pressure and pressure gradient at qmin play an important role
in modifying the RSAE activity. Analysis of the ECH injection near the qmin case where no
frequency sweeping RSAEs are observed shows the typical RSAE is no longer an eigenmode
of the system. What remains is an eigenmode with poloidal harmonic content reminiscent of
the standard RSAE, but absent of the typical frequency sweeping behavior. The remaining
eigenmode is also often strongly coupled to gap TAEs. Analysis with the non-perturbative
gyro fluid code TAEFL confirms this change in RSAE activity and also shows a large drop in
the resultant mode growth rates.RCUK Energy Programme EP/I50104
Fast ion transport during applied 3D magnetic perturbations on DIII-D
Measurements show fast ion losses correlated with applied three-dimensional (3D) fields in
a variety of plasmas ranging from L-mode to resonant magnetic perturbation (RMP) edge
localized mode (ELM) suppressed H-mode discharges. In DIII-D L-mode discharges with a
slowly rotating n = 2 magnetic perturbation, scintillator detector loss signals synchronized
with the applied fields are observed to decay within one poloidal transit time after beam turnoff indicating they arise predominantly from prompt loss orbits. Full orbit following using
M3D-C1 calculations of the perturbed fields and kinetic profiles reproduce many features of
the measured losses and points to the importance of the applied 3D field phase with respect
to the beam injection location in determining the overall impact on prompt beam ion loss.
Modeling of these results includes a self-consistent calculation of the 3D perturbed beam ion
birth profiles and scrape-off-layer ionization, a factor found to be essential to reproducing the
experimental measurements. Extension of the simulations to full slowing down timescales,
including fueling and the effects of drag and pitch angle scattering, show the applied n = 3
RMPs in ELM suppressed H-mode plasmas can induce a significant loss of energetic
particles from the core. With the applied n = 3 fields, up to 8.4% of the injected beam power
is predicted to be lost, compared to 2.7% with axisymmetric fields only. These fast ions,
originating from minor radii ρ > 0.7, are predicted to be primarily passing particles lost to
the divertor region, consistent with wide field-of-view infrared periscope measurements of
wall heating in n = 3 RMP ELM suppressed plasmas. Edge fast ion Dα (FIDA) measurements
also confirm a large change in edge fast ion profile due to the n = 3 fields, where the effect
was isolated by using short 50ms RMP-off periods during which ELM suppression was
maintained yet the fast ion profile was allowed to recover. The role of resonances between
fast ion drift motion and the applied 3D fields in the context of selectively targeting regions
of fast ion phase space is also discussed
Modulation of prompt fast-ion loss by applied n=2 fields in the DIII-D tokamak
Energy and pitch angle resolved measurements of escaping neutral beam ions (E approximate to 80 keV) have been made during DIII-D L-mode discharges with applied, slowly rotating, n = 2 magnetic perturbations. Data from separate scintillator detectors (FILDs) near and well below the plasma midplane show fast-ion losses correlated with the internal coil (I-coil) fields. The dominant fast-ion loss signals are observed to decay within one poloidal transit time after beam turn-off indicating they are primarily prompt loss orbits. Also, during application of the rotating I-coil fields, outboard midplane edge density and bremsstrahlung emission profiles exhibit a radial displacement of up to delta R approximate to 1 cm. Beam deposition and full orbit modeling of these losses using M3D-C1 calculations of the perturbed kinetic profiles and fields reproduce many features of the measured losses. In particular, the predicted phase of the modulated loss signal with respect to the I-coil currents is in close agreement with FILD measurements as is the relative amplitudes of the modulated losses for the co and counter-current beam used in the experiment. These simulations show modifications to the beam ion birth profile and subsequent prompt loss due to changes in the edge density; however, the dominant factor causing modulation of the losses to the fast-ion loss detectors is the perturbed magnetic field (delta B/B approximate to 10(-3) in the plasma). Calculations indicate total prompt loss to the DIII-D wall can increase with application of the n = 2 perturbation by up to 7% for co-current injected beams and 3% for counter-current injected beams depending on phase of the perturbation relative to the injected beam.US Department of Energy DE-FC02-04ER54698, SC-G903402, DEAC02- 09CH11466, DE-FG02-04ER54761, DE-FG02- 05ER5480
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