1,388 research outputs found
Quasi-linear analysis of the extraordinary electron wave destabilized by runaway electrons
Runaway electrons with strongly anisotropic distributions present in
post-disruption tokamak plasmas can destabilize the extraordinary electron
(EXEL) wave. The present work investigates the dynamics of the quasi-linear
evolution of the EXEL instability for a range of different plasma parameters
using a model runaway distribution function valid for highly relativistic
runaway electron beams produced primarily by the avalanche process. Simulations
show a rapid pitch-angle scattering of the runaway electrons in the high energy
tail on the time scale. Due to the wave-particle
interaction, a modification to the synchrotron radiation spectrum emitted by
the runaway electron population is foreseen, exposing a possible experimental
detection method for such an interaction
First dedicated observations of runaway electrons in the COMPASS tokamak
Runaway electrons present an important part of the present efforts in nuclear fusion research with respect to the potential damage of the in-vessel components. The COMPASS tokamak a suitable tool for the studies of runaway electrons, due to its relatively low vacuum safety constraints, high experimental flexibility and the possibility of reaching the H-mode D-shaped plasmas. In this work, results from the first experimental COMPASS campaign dedicated to runaway electrons are presented and discussed in preliminary way. In particular, the first observation of synchrotron radiation and rather interesting raw magnetic data are shown
On the inward drift of runaway electrons during the plateau phase of runaway current
The well observed inward drift of current carrying runaway electrons during
runaway plateau regime after disruption is studied by considering the phase
space dynamic of runaways in a large aspect ratio toroidal system. We consider
the case where the toroidal field is unperturbed and the toroidal symmetry of
the system is preserved. The balance between the change in canonical angular
momentum and the input of mechanical angular momentum in such system requires
runaways to drift horizontally in configuration space for any given change in
momentum space. The dynamic of this drift can be obtained by taking the
variation of canonical angular momentum. It is then found that runaway
electrons will always drift inward as long as they are decelerating. This drift
motion is essentially non-linear, since the current is carried by runaways
themselves, and any runaway drift relative to the magnetic axis will cause
further displacement of the axis itself. A simplified analytical model is
constructed to describe such inward drift both in ideal wall case and no wall
case, and the runaway current center displacement as a function of parallel
momentum variation is obtained. The time scale of such displacement is
estimated by considering effective radiation drag, which shows reasonable
agreement with observed displacement time scale. This indicates that the phase
space dynamic studied here plays a major role in the horizontal displacement of
runaway electrons during plateau regime.Comment: 25 pages, 9 figures, submitted to Physics of Plasma
Runaway electrons and ITER
The potential for damage, the magnitude of the extrapolation, and the importance of the atypical—incidents that occur once in a thousand shots—make theory and simulation essential for ensuring that relativistic runaway electrons will not prevent ITER from achieving its mission. Most of the theoretical literature on electron runaway assumes magnetic surfaces exist. ITER planning for the avoidance of halo and runaway currents is focused on massive-gas or shattered-pellet injection of impurities. In simulations of experiments, such injections lead to a rapid large-scale magnetic-surface breakup. Surface breakup, which is a magnetic reconnection, can occur on a quasi-ideal Alfvénic time scale when the resistance is sufficiently small. Nevertheless, the removal of the bulk of the poloidal flux, as in halo-current mitigation, is on a resistive time scale. The acceleration of electrons to relativistic energies requires the confinement of some tubes of magnetic flux within the plasma and a resistive time scale. The interpretation of experiments on existing tokamaks and their extrapolation to ITER should carefully distinguish confined versus unconfined magnetic field lines and quasi-ideal versus resistive evolution. The separation of quasi-ideal from resistive evolution is extremely challenging numerically, but is greatly simplified by constraints of Maxwell's equations, and in particular those associated with magnetic helicity. The physics of electron runaway along confined magnetic field lines is clarified by relations among the poloidal flux change required for an e-fold in the number of electrons, the energy distribution of the relativistic electrons, and the number of relativistic electron strikes that can be expected in a single disruption event
SOFT: A synthetic synchrotron diagnostic for runaway electrons
Improved understanding of the dynamics of runaway electrons can be obtained
by measurement and interpretation of their synchrotron radiation emission.
Models for synchrotron radiation emitted by relativistic electrons are well
established, but the question of how various geometric effects -- such as
magnetic field inhomogeneity and camera placement -- influence the synchrotron
measurements and their interpretation remains open. In this paper we address
this issue by simulating synchrotron images and spectra using the new synthetic
synchrotron diagnostic tool SOFT (Synchrotron-detecting Orbit Following
Toolkit). We identify the key parameters influencing the synchrotron radiation
spot and present scans in those parameters. Using a runaway electron
distribution function obtained by Fokker-Planck simulations for parameters from
an Alcator C-Mod discharge, we demonstrate that the corresponding synchrotron
image is well-reproduced by SOFT simulations, and we explain how it can be
understood in terms of the parameter scans. Geometric effects are shown to
significantly influence the synchrotron spectrum, and we show that inherent
inconsistencies in a simple emission model (i.e. not modeling detection) can
lead to incorrect interpretation of the images.Comment: 24 pages, 12 figure
Experimental investigation of kinetic instabilities driven by runaway electrons in the EXL-50 spherical torus
In this study, the first observation of high-frequency instabilities driven
by runaway electrons has been reported in the EXL-50 spherical torus using a
high-frequency magnetic pickup coil. The central frequency of these
instabilities is found to be exponentially dependent on the plasma density,
similar to the dispersion relation of the whistler wave. The instability
frequency displays chirping characteristics consistent with the Berk-Breizman
model of beam instability. Theoretically, the excitation threshold of the
instability driven by runaway electrons is related to the ratio of the runaway
electron density to the background plasma density, and such a relationship is
first demonstrated experimentally in this study. The instability can be
stabilized by increasing the plasma density, consistent with the wave-particle
resonance mechanism. This investigation demonstrates the controlled excitation
of chirping instabilities in a tokamak plasma and reveals new features of these
instabilities, thereby advancing the understanding of the mechanisms for
controlling and mitigating runaway electrons
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