539 research outputs found
Interpretation of runaway electron synchrotron and bremsstrahlung images
The crescent spot shape observed in DIII-D runaway electron synchrotron
radiation images is shown to result from the high degree of anisotropy in the
emitted radiation, the finite spectral range of the camera and the distribution
of runaways. The finite spectral camera range is found to be particularly
important, as the radiation from the high-field side can be stronger by a
factor than the radiation from the low-field side in DIII-D. By
combining a kinetic model of the runaway dynamics with a synthetic synchrotron
diagnostic we see that physical processes not described by the kinetic model
(such as radial transport) are likely to be limiting the energy of the
runaways. We show that a population of runaways with lower dominant energies
and larger pitch-angles than those predicted by the kinetic model provide a
better match to the synchrotron measurements. Using a new synthetic
bremsstrahlung diagnostic we also simulate the view of the Gamma Ray Imager
(GRI) diagnostic used at DIII-D to resolve the spatial distribution of
runaway-generated bremsstrahlung.Comment: 21 pages, 11 figure
Experimental conditions to suppress edge localised modes by magnetic perturbations in the ASDEX Upgrade tokamak
Access conditions for full suppression of Edge Localised Modes (ELMs) by
Magnetic Perturbations (MP) in low density high confinement mode (H-mode)
plasmas are studied in the ASDEX Upgrade tokamak. The main empirical
requirements for full ELM suppression in our experiments are: 1. The poloidal
spectrum of the MP must be aligned for best plasma response from weakly stable
kink-modes, which amplify the perturbation, 2. The plasma edge density must be
below a critical value, ~m. The edge collisionality
is in the range (ions) and
(electrons). However, our data does not show that the edge collisionality is
the critical parameter that governs access to ELM suppression. 3. The pedestal
pressure must be kept sufficiently low to avoid destabilisation of small ELMs.
This requirement implies a systematic reduction of pedestal pressure of
typically 30\% compared to unmitigated ELMy H-mode in otherwise similar
plasmas. 4. The edge safety factor lies within a certain window.
Within the range probed so far, , one such window,
has been identified. Within the range of plasma rotation
encountered so far, no apparent threshold of plasma rotation for ELM
suppression is found. This includes cases with large cross field electron flow
in the entire pedestal region, for which two-fluid MHD models predict that the
resistive plasma response to the applied MP is shielded
Wide operational windows of edge-localized mode suppression by resonant magnetic perturbations in the DIII-D tokamak
Edge-Localized-Mode (ELM) suppression by resonant magnetic perturbations
(RMPs) generally occurs over very narrow ranges of the plasma current (or
magnetic safety factor q95) in the DIII-D tokamak. However, wide q95 ranges of
ELM suppression are needed for the safety and operational flexibility of ITER
and future reactors. In DIII-D ITER Similar Shape (ISS) plasmas with n=3 RMPs,
the range of q95 for ELM suppression is found to increase with decreasing
electron density. Nonlinear two-fluid MHD simulations reproduce the observed
q95 windows of ELM suppression and the dependence on plasma density, based on
the conditions for resonant field penetration at the top of the pedestal. When
the RMP amplitude is close to the threshold for resonant field penetration,
only narrow isolated magnetic islands form near the top of the pedestal,
leading to narrow q95 windows of ELM suppression. However, as the threshold for
field penetration decreases with decreasing density, resonant field penetration
can take place over a wider range of q95. For sufficiently low density
(penetration threshold) multiple magnetic islands form near the top of the
pedestal giving rise to continuous q95 windows of ELM suppression. The model
predicts that wide q95 windows of ELM suppression can be achieved at
substantially higher pedestal pressure in DIII-D by shifting to higher toroidal
mode number (n=4) RMPs
TokaMaker: An open-source time-dependent Grad-Shafranov tool for the design and modeling of axisymmetric fusion devices
In this paper, we present a new static and time-dependent MagnetoHydroDynamic
(MHD) equilibrium code, TokaMaker, for axisymmetric configurations of
magnetized plasmas, based on the well-known Grad-Shafranov equation. This code
utilizes finite element methods on an unstructured triangular grid to enable
capturing accurate machine geometry and simple mesh generation from
engineering-like descriptions of present and future devices. The new code is
designed for ease of use without sacrificing capability and speed through a
combination of Python, Fortran, and C/C++ components. A detailed description of
the numerical methods of the code, including a novel formulation of the
boundary conditions for free-boundary equilibria, and validation of the
implementation of those methods using both analytic test cases and cross-code
validation is shown. Results show expected convergence across tested polynomial
orders for analytic and cross-code test cases
Observation of a multimode plasma response and its relationship to density pumpout and edge-localized mode suppression
Density pumpout and edge-localized mode (ELM) suppression by applied n=2 magnetic fields in low-collisionality DIII-D plasmas are shown to be correlated with the magnitude of the plasma response driven on the high-field side (HFS) of the magnetic axis but not the low-field side (LFS) midplane. These distinct responses are a direct measurement of a multimodal magnetic plasma response, with each structure preferentially excited by a different n=2 applied spectrum and preferentially detected on the LFS or HFS. Ideal and resistive magneto-hydrodynamic (MHD) calculations find that the LFS measurement is primarily sensitive to the excitation of stable kink modes, while the HFS measurement is primarily sensitive to resonant currents (whether fully shielding or partially penetrated). The resonant currents are themselves strongly modified by kink excitation, with the optimal applied field pitch for pumpout and ELM suppression significantly differing from equilibrium field alignment.This material is based upon work supported by the U.S.
Department of Energy, Office of Science, Office of Fusion
Energy Sciences, using the DIII-D National Fusion Facility,
a DOE Office of Science user facility, under Awards No. DE-FC02-04ER54698, No. DE-AC02-09CH11466,
No. DE-FG02-04ER54761, No. DE-AC05-06OR23100,
No. DE-SC0001961, and No. DE-AC05-00OR22725.
S. R. H. was supported by AINSE and ANSTO
Robust avoidance of edge-localized modes alongside gradient formation in the negative triangularity tokamak edge
In a series of high performance diverted discharges on DIII-D, we demonstrate
that strong negative triangularity (NT) shaping robustly suppresses all
edge-localized mode (ELM) activity over a wide range of plasma conditions:
m, MW and
T, corresponding to
. The full dataset is consistent with the
theoretical prediction that magnetic shear in the NT edge inhibits access to
ELMing H-mode regimes; all experimental pressure profiles are found to be at or
below the infinite- ballooning stability limit. Importantly, we also report
enhanced edge pressure gradients at strong NT that are significantly steeper
than in traditional ELM-free L-mode plasmas and provide significant promise for
NT reactor integration.Comment: 5 pages, 5 figure
Spatiotemporal Evolution of Runaway Electron Momentum Distributions in Tokamaks
Novel spatial, temporal, and energetically resolved measurements of bremsstrahlung hard-x-ray (HXR) emission from runaway electron (RE) populations in tokamaks reveal nonmonotonic RE distribution functions whose properties depend on the interplay of electric field acceleration with collisional and synchrotron damping. Measurements are consistent with theoretical predictions of momentum-space attractors that accumulate runaway electrons. RE distribution functions are measured to shift to a higher energy when the synchrotron force is reduced by decreasing the toroidal magnetic field strength. Increasing the collisional damping by increasing the electron density (at a fixed magnetic and electric field) reduces the energy of the nonmonotonic feature and reduces the HXR growth rate at all energies. Higher-energy HXR growth rates extrapolate to zero at the expected threshold electric field for RE sustainment, while low-energy REs are anomalously lost. The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy.United States. Department of Energy (DE-FC02-04ER54698)United States. Department of Energy (DE-FG02-07ER54917)United States. Department of Energy (DE-AC05-00OR22725)United States. Department of Energy (DE-FC02-99ER54512)United States. Department of Energy (DE-SC0016268
On the minimum transport required to passively suppress runaway electrons in SPARC disruptions
In [V.A. Izzo et al 2022 Nucl. Fusion 62 096029], state-of-the-art modeling
of thermal and current quench (CQ) MHD coupled with a self-consistent evolution
of runaway electron (RE) generation and transport showed that a
non-axisymmetric (n = 1) in-vessel coil could passively prevent RE beam
formation during disruptions in SPARC, a compact high-field tokamak projected
to achieve a fusion gain Q > 2 in DT plasmas. However, such suppression
requires finite transport of REs within magnetic islands and re-healed flux
surfaces; conservatively assuming zero transport in these regions leads to an
upper bound of RE current ~1 MA compared to ~8.7 MA of pre-disruption plasma
current. Further investigation finds that core-localized electrons, within r/a
< 0.3 and with kinetic energies 0.2-15 MeV, contribute most to the RE plateau
formation. Yet only a relatively small amount of transport, i.e. a diffusion
coefficient ~18 , is needed in the core to fully mitigate these
REs. Properly accounting for (i) the CQ electric field's effect on RE transport
in islands and (ii) the contribution of significant RE currents to disruption
MHD may help achieve this
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