712 research outputs found
Device for dispersal of micrometer- and submicrometer-sized particles in vaccum
A simple, versatile device for dispersing micrometerâ and submicrometer-sized particles in vacuum is described. The source allows control of particle size (0.5 ÎŒmâ€lâ€200 ÎŒm) and particle flux density up to roughly 107 cmâ2âsâ1. Several types of microparticles were successfully dispersed
Low-frequency shear Alfv\'en waves at DIII-D: theoretical interpretation of experimental observations
The linear properties of the low-frequency shear Alfv\'en waves such as those
associated with the beta-induced Alfv\'en eigenmodes (BAEs) and the
low-frequency modes observed in reversed-magnetic-shear DIII-D discharges (W.
Heidbrink, et al 2021 Nucl. Fusion 61 066031) are theoretically investigated
and delineated based on the theoretical framework of the general fishbone-like
dispersion relation (GFLDR). By adopting representative experimental
equilibrium profiles, it is found that the low-frequency modes and BAEs are,
respectively, the reactive-type and dissipative-type unstable modes with
dominant Alfv\'enic polarization, thus the former being more precisely called
low-frequency Alfv\'en modes (LFAMs). More specifically, due to different
instability mechanisms, the maximal drive of BAEs occurs, in comparison to
LFAMs, when the minimum of the safety factor () deviates from a
rational number. Meanwhile, the BAE eigenfunction peaks at the radial position
of the maximum energetic particle pressure gradient, resulting in a large
deviation from the surface. Moreover, the ascending frequency
spectrum patterns of the experimentally observed BAEs and LFAMs can be
theoretically reproduced by varying and also be well interpreted
based on the GFLDR. The present analysis illustrates the solid predictive
capability of the GFLDR and its practical usefulness in enhancing the
interpretative capability of both experimental and numerical simulation
results
Forced Family Separation: U.S. crimes against Indigenous Peoples
Forced family separation under the U.S. Zero Tolerance policy is not only a crime against humanity, but also a crime against Indigenous Peoples, which includes the Maya. Rooted in white supremacist ideologies and settler colonialism, contemporary forced family separation continues historical violence inflicted upon Maya Peoples by the U.S. government. Submitted to the International Criminal Court, this amicus brief contends that the Court should investigate and hold the U.S. accountable for crimes against Indigenous Peoples under the U.S. Zero Tolerance policy. The amicus brief begins with an overview of Maya Peoples in present-day Guatemala and the meanings and practices of indigeneity. We then trace key historical periods that evidence the United Statesâ willful and systemic violence inflicted on Indigenous children and families over time. Periods include, among others, 1) removal of Indigenous children and placement in boarding schools; 2) U.S. support of military dictatorships during the 36-year armed conflict and genocide of Maya Peoples in Guatemala; 3) the intercountry adoption of children from Guatemala predominantly to U.S. families; and 4) ongoing punitive immigration policies that harm and in some instances kill Maya children. Perpetuating the U.S.âs long history of disappearing Indigenous people and culture, the Zero Tolerance policy specifically and U.S. immigration policy more generally is a patterned product of genocidal logics and state-inflicted harms by the United States designed to criminalize and terrorize families as a tool of deterrence
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Array of neutral particle analyzers at DIII-D
Local measurements of the fast-ion distribution in auxiliary-heated plasmas are key to understanding the behavior of energetic particles under a variety of conditions, such as beam-ion transport during Alfven instabilities and the acceleration of beam ions by fast waves. For the first time at DIII-D, line-averaged and local measurements of the energetic-particle density (for E = 5--75 keV) are possible using an array of four compact charge-exchange analyzers. The installation consists of three vertically-viewing analyzers with fixed sightlines, measuring particles with {chi} = 90{degree} (where {chi} is the angle between the particle`s velocity and the toroidal direction) and one horizontally-viewing analyzer with a variable sightline, measuring particles with 2{degree}{grave U} {chi} {acute U} 60{degree}. All the analyzers can make passive measurements while three detectors, with sightlines that intersect deuterium heating beams, can make active charge-exchange measurements
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Search for alpha-driven BAE modes in TFTR
A search for alpha-driven beta-induced Alfven eigenmodes (BAE modes) was conducted in low current (1.0--1.6 MA) TFTR supershots. Stable high-beta deuterium-tritium (DT) discharges were obtained with B{rho} = 2.4 and central alpha beta of 0.1%. Instabilities between 75--200 kHz were observed by magnetic probes in many DT discharges, but the activity was also present in deuterium-deuterium (DD) comparison discharges, indicating that these modes are not destabilized (principally) by the alpha-particle population. Losses of fusion products are also similar in the two sets of discharges
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 Ï > 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
Nonlinear dynamics and transport driven by energetic particle instabilities using a gyro-Landau closure model
Energetic particle (EP) destabilized Alfvén eigenmode (AE) instabilities are simulated for a DIII-D experimental case with a pulsed neutral beam using a gyro-Landau moments model which introduces EP phase-mixing effects through closure relations. This provides a computationally efficient reduced model which is applied here in the nonlinear regime over timescales that would be difficult to address with more complete models. The long timescale nonlinear evolution and related collective transport losses are examined including the effects of zonal flow/current generation, nonlinear energy cascades, and EP profile flattening. The model predicts frequencies and mode structures that are consistent with experimental observations. These calculations address issues that have not been considered in previous modelling: The EP critical gradient profile evolution in the presence of zonal flows/currents, and the dynamical nature of the saturated state. A strong level of intermittency is present in the predicted instability-driven transport; this is connected to the zonal flow growth and decay cycles and nonlinear energy transfers. Simulation of intermittent AE-enhanced EP transport will be an important issue for the protection of plasma facing components in the next generation of fusion devices.This material is based upon work supported by the US Department
of Energy, Office of Science using the DIII-D National
Fusion Facility, a DOE Office of Science user facility, under
Awards DE-AC05-00OR22725, DE-FC02-04ER54698,
and the US DOE SciDAC ISEP Center. Support is also
acknowledged from project 2019-T1/AMB-13648 founded
by the Comunidad de Madrid and Comunidad de Madrid
(Spain)—multiannual agreement with UC3M Excelencia
para el Profesorado Universitario EPUC3M14 Fifth
regional research plan 2016-2020. This research used
resources of the National Energy Research Scientific Computing
Center (NERSC), a US Department of Energy Office
of Science User Facility located at Lawrence Berkeley
National Laboratory, operated under Contract No. DE-AC02-
05CH11231. We would like to thank Matt Beidler of Oak
Ridge National Laboratory for helpful suggestions on this
manuscript
Sheared-flow induced confinement transition in a linear magnetized plasma
A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential (ÎŽn/n~eÎŽÏ/kTe~0.5) are observed at the plasma edge, accompanied by a large density gradient (Ln=âŁâŁâlnnâŁâŁâ1~2cm) and shearing rate (Îł~300kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (Vbias) on the obstacle and the axial magnetic field (Bz) strength. In cases with low Vbias and large Bz, improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by EĂB drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller Bz, large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m=1 and large radial correlation length dominates the outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge
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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