611 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
Assessment of vertical stability for negative triangularity pilot plants
Negative triangularity (NT) tokamak configurations may be more susceptible to
magneto-hydrodynamic instability, posing challenges for recent reactor designs
centered around their favorable properties, such as improved confinement and
operation free of edge-localized modes. In this work, we assess the vertical
stability of plasmas with NT shaping and develop potential reactor solutions.
When coupled with a conformal wall, NT equilibria are confirmed to be less
vertically stable than equivalent positive triangularity (PT) configurations.
Unlike PT, their vertical stability is degraded at higher poloidal beta.
Furthermore, improvements in vertical stability at low aspect ratio do not
translate to the NT geometry. NT equilibria are stabilized in PT vacuum vessels
due to the increased proximity of the plasma and the wall on the outboard side,
but this scenario is found to be undesirable due to reduced vertical gaps which
give less spatial margin for control recovery. Instead, we demonstrate that
informed positioning of passively conducting plates can lead to improved
vertical stability in NT configurations on par with stability metrics expected
in PT scenarios. An optimal setup for passive plates in highly elongated NT
devices is presented, where plates on the outboard side of the device reduce
vertical instability growth rates to 16% of their baseline value. For lower
target elongations, integration of passive stabilizers with divertor concepts
can lead to significant improvements in vertical stability. Plates on the
inboard side of the device are also uniquely enabled in NT geometries,
providing opportunity for spatial separation of vertical stability coils and
passive stabilizers
The role of edge resonant magnetic perturbations in edge-localized-mode suppression and density pump-out in low-collisionality DIII-D plasmas
The density dependence of edge-localized-mode suppression and pump-out by resonant magnetic perturbations in the DIII-D tokamak editors-pick
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
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