346 research outputs found
Influence of massive material injection on avalanche runaway generation during tokamak disruptions
In high-current tokamak devices such as ITER, a runaway avalanche can cause a
large amplification of a seed electron population. We show that disruption
mitigation by impurity injection may significantly increase the runaway
avalanche growth rate in such devices. This effect originates from the
increased number of target electrons available for the avalanche process in
weakly ionized plasmas, which is only partially compensated by the increased
friction force on fast electrons. We derive an expression for the avalanche
growth rate in partially ionized plasmas and investigate the effects of
impurity injection on the avalanche multiplication factor and on the final
runaway current for ITER-like parameters. For impurity densities relevant for
disruption mitigation, the maximum amplification of a runaway seed can be
increased by tens of orders of magnitude compared to previous predictions. This
motivates careful studies to determine the required densities and impurity
species to obtain tolerable current quench parameters, as well as more detailed
modeling of the runaway dynamics including transport effects.Comment: 6 pages, 2 figure
Effect of impurities on the transition between minority ion and mode conversion ICRH heating in (3He)-H tokamak plasmas
Hydrogen majority plasmas will be used in the initial non-activated phase of
ITER operation. Optimizing ion cyclotron resonance heating (ICRH) in such
scenarios will help in achieving H-mode in these plasmas. Past JET experiments
with the carbon wall revealed a significant impact of intrinsic impurities on
the ICRH performance in (3He)-H plasmas relevant for the full-field initial
ITER phase. High plasma contamination with carbon impurities resulted in the
appearance of a supplementary mode conversion layer and significant reduction
in the transition concentration of 3He minority ions, defined as the
concentration at which the change from minority heating to mode conversion
regime occurs. In view of the installation of the new ITER-like wall at JET, it
is important to evaluate the effect of Be and W impurities on ICRH scenarios in
(3He)-H plasmas. In this paper, an approximate analytical expression for the
transition concentration of 3He minority ions is derived as a function of
plasma and ICRH parameters, and accounting for typical impurity species at JET.
The accompanying 1D wave modeling supports the analytical results and suggests
a potential experimental method to reduce 3He level needed to achieve a
specific heating regime by puffing a small amount of 4He ions additionally to
(3He)-H plasma.Comment: 23 pages, 9 figure
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
Mode Conversion of Waves in the Ion-Cyclotron Frequency Range in Magnetospheric Plasmas
Waves in the ion-cyclotron range of frequencies with linear polarization detected by satellites can be
useful for estimating the heavy ion concentrations in planetary magnetospheres. These waves are
considered to be driven by mode conversion (MC) of the fast magnetosonic waves at the ion-ion hybrid
resonances. In this Letter, we derive analytical expressions for the MC efficiency and tunneling of waves
through the MC layer. We evaluate the particular parallel wave numbers for which MC is efficient for
arbitrary heavy ion/proton ratios and discuss the interpretation of the experimental observations
Tunneling and mode conversion of fast magnetosonic waves in the magnetospheres of Earth and Mercury
Narrow-band linearly polarized waves, having a resonant structure and a peak
frequency between the local cyclotron frequency of protons and heavy ions, have
been detected in the magnetospheres of Earth and of Mercury. Some of these wave
events have been suggested to be driven by linear mode conversion (MC) of the
fast magnetosonic waves at the ion-ion hybrid (IIH) resonances. Since the
resonant IIH frequency is linked to the plasma composition, solving the inverse
problem allows one to infer the concentration of the heavy ions from the
measured frequency spectra. In this paper, we identify the conditions when the
MC efficiency is maximized in the magnetospheric plasmas and discuss how this
can be applied for estimating the heavy ion concentration in the magnetospheres
of Earth and Mercury.Comment: 13 pages, 5 figure
Effective Governance of Global Financial Markets:An Evolutionary Plan for Reform
Runaway electrons, which are generated in a plasma where the induced electric field exceeds a certain critical value, can reach very high energies in the MeV range. For such energetic electrons, radiative losses will contribute significantly to the momentum space dynamics. Under certain conditions, due to radiative momentum losses, a non-monotonic feature - a âbump' - can form in the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Here, we study the conditions for the existence of the bump. We derive an analytical threshold condition for bump appearance and give an approximate expression for the minimum energy at which the bump can appear. Numerical calculations are performed to support the analytical derivation
DREAM: a fluid-kinetic framework for tokamak disruption runaway electron simulations
Avoidance of the harmful effects of runaway electrons (REs) in
plasma-terminating disruptions is pivotal in the design of safety systems for
magnetic fusion devices. Here, we describe a computationally efficient
numerical tool, that allows for self-consistent simulations of plasma cooling
and associated RE dynamics during disruptions. It solves flux-surface averaged
transport equations for the plasma density, temperature and poloidal flux,
using a bounce-averaged kinetic equation to self-consistently provide the
electron current, heat, density and RE evolution, as well as the electron
distribution function. As an example, we consider disruption scenarios with
material injection and compare the electron dynamics resolved with different
levels of complexity, from fully kinetic to fluid modes.Comment: 32 pages, 11 figure
Runaway dynamics in the DT phase of ITER operations in the presence of massive material injection
A runaway avalanche can result in a conversion of the initial plasma current
into a relativistic electron beam in high current tokamak disruptions. We
investigate the effect of massive material injection of deuterium-noble gas
mixtures on the coupled dynamics of runaway generation, resistive diffusion of
the electric field, and temperature evolution during disruptions in the DT
phase of ITER operations. We explore the dynamics over a wide range of injected
concentrations and find substantial runaway currents, unless the current quench
time is intolerably long. The reason is that the cooling associated with the
injected material leads to high induced electric fields that, in combination
with a significant recombination of hydrogen isotopes, leads to a large
avalanche generation. Balancing Ohmic heating and radiation losses provides
qualitative insights into the dynamics, however, an accurate modeling of the
temperature evolution based on energy balance appears crucial for quantitative
predictions.Comment: 24 pages, 8 figure
Impurity transport in Alcator C-Mod in the presence of poloidal density variation induced by ion cyclotron resonance heating
Impurity particle transport in an ion cyclotron resonance heated Alcator
C-Mod discharge is studied with local gyrokinetic simulations and a theoretical
model including the effect of poloidal asymmetries and elongation. In spite of
the strong minority temperature anisotropy in the deep core region, the
poloidal asymmetries are found to have a negligible effect on the turbulent
impurity transport due to low magnetic shear in this region, in agreement with
the experimental observations. According to the theoretical model, in outer
core regions poloidal asymmetries may contribute to the reduction of the
impurity peaking, but uncertainties in atomic physics processes prevent
quantitative comparison with experiments.Comment: 32 pages, 12 figure
Drift of ablated material after pellet injection in a tokamak
Pellet injection is used for fuelling and controlling discharges in tokamaks,
and it is foreseen in ITER. During pellet injection, a movement of the ablated
material towards the low-field side (or outward major radius direction) occurs
because of the inhomogeneity of the magnetic field. Due to the complexity of
the theoretical models, computer codes developed to simulate the cross-field
drift are computationally expensive. Here, we present a one-dimensional
semi-analytical model for the radial displacement of ablated material after
pellet injection, taking into account both the Alfv\'en and ohmic currents
which short-circuit the charge separation creating the drift. The model is
suitable for rapid calculation of the radial drift displacement, and can be
useful for e.g. modelling of disruption mitigation via pellet injection.Comment: 22 pages, 4 figures. Submitted to Journal of Plasma Physic
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