117 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 partially-screened nuclei on fast-electron dynamics
We analyze the dynamics of fast electrons in plasmas containing partially
ionized impurity atoms, where the screening effect of bound electrons must be
included. We derive analytical expressions for the deflection and slowing-down
frequencies, and show that they are increased significantly compared to the
results obtained with complete screening, already at sub-relativistic electron
energies. Furthermore, we show that the modifications to the deflection and
slowing down frequencies are of equal importance in describing the runaway
current evolution. Our results greatly affect fast-electron dynamics and have
important implications, e.g. for the efficacy of mitigation strategies for
runaway electrons in tokamak devices, and energy loss during relativistic
breakdown in atmospheric discharges.Comment: 6 pages, 3 figures, fixed minor typo
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
Effect of plasma elongation on current dynamics during tokamak disruptions
Plasma terminating disruptions in tokamaks may result in relativistic runaway
electron beams with potentially serious consequences for future devices with
large plasma currents. In this paper we investigate the effect of plasma
elongation on the coupled dynamics of runaway generation and resistive
diffusion of the electric field. We find that elongated plasmas are less likely
to produce large runaway currents, partly due to the lower induced electric
fields associated with larger plasmas, and partly due to direct shaping
effects, which mainly lead to a reduction in the runaway avalanche gain.Comment: 11 pages, 3 figure
Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade
Massive material injection has been proposed as a way to mitigate the
formation of a beam of relativistic runaway electrons that may result from a
disruption in tokamak plasmas. In this paper we analyse runaway generation
observed in eleven ASDEX Upgrade discharges where disruption was triggered
using massive gas injection. We present numerical simulations in scenarios
characteristic of on-axis plasma conditions, constrained by experimental
observations, using a description of the runaway dynamics with self-consistent
electric field and temperature evolution in two-dimensional momentum space and
zero-dimensional real space. We describe the evolution of the electron
distribution function during the disruption, and show that the runaway seed
generation is dominated by hot-tail in all of the simulated discharges. We
reproduce the observed dependence of the current dissipation rate on the amount
of injected argon during the runaway plateau phase. Our simulations also
indicate that above a threshold amount of injected argon, the current density
after the current quench depends strongly on the argon densities. This trend is
not observed in the experiments, which suggests that effects not captured by 0D
kinetic modeling -- such as runaway seed transport -- are also important.Comment: 17 pages, 15 figures, published in Journal of Plasma Physics (Invited
Contributions from the 18th European Fusion Theory Conference
Spatiotemporal analysis of the runaway distribution function from synchrotron images in an ASDEX Upgrade disruption
Synchrotron radiation images from runaway electrons (REs) in an ASDEX Upgrade discharge disrupted by argon injection are analysed using the synchrotron diagnostic tool Soft and coupled fluid-kinetic simulations. We show that the evolution of the runaway distribution is well described by an initial hot-tail seed population, which is accelerated to energies between 25-50 MeV during the current quench, together with an avalanche runaway tail which has an exponentially decreasing energy spectrum. We find that, although the avalanche component carries the vast majority of the current, it is the high-energy seed remnant that dominates synchrotron emission. With insights from the fluid-kinetic simulations, an analytic model for the evolution of the runaway seed component is developed and used to reconstruct the radial density profile of the RE beam. The analysis shows that the observed change of the synchrotron pattern from circular to crescent shape is caused by a rapid redistribution of the radial profile of the runaway density
LightOn Optical Processing Unit: Scaling-up AI and HPC with a Non von Neumann co-processor
We introduce LightOn's Optical Processing Unit (OPU), the first photonic AI
accelerator chip available on the market for at-scale Non von Neumann
computations, reaching 1500 TeraOPS. It relies on a combination of free-space
optics with off-the-shelf components, together with a software API allowing a
seamless integration within Python-based processing pipelines. We discuss a
variety of use cases and hybrid network architectures, with the OPU used in
combination of CPU/GPU, and draw a pathway towards "optical advantage".Comment: Proceedings IEEE Hot Chips 33, 202
Recommended from our members
Einheit von Park und Architektur : der Park am IG Hochhaus
Cerebellar granule cells, which constitute half the brain's neurons, supply Purkinje cells with contextual information necessary for motor learning, but how they encode this information is unknown. Here we show, using two-photon microscopy to track neural activity over multiple days of cerebellum-dependent eyeblink conditioning in mice, that granule cell populations acquire a dense representation of the anticipatory eyelid movement. Initially, granule cells responded to neutral visual and somatosensory stimuli as well as periorbital airpuffs used for training. As learning progressed, two-thirds of monitored granule cells acquired a conditional response whose timing matched or preceded the learned eyelid movements. Granule cell activity covaried trial by trial to form a redundant code. Many granule cells were also active during movements of nearby body structures. Thus, a predictive signal about the upcoming movement is widely available at the input stage of the cerebellar cortex, a
Electrophysiological Characterization of The Cerebellum in the Arterially Perfused Hindbrain and Upper Body of The Rat
In the present study, a non-pulsatile arterially perfused hindbrain and upper body rat preparation is described which is an extension of the brainstem preparation reported by Potts et al., (Brain Res Bull 53(1):59–67), 1. The modified in situ preparation allows study of cerebellar function whilst preserving the integrity of many of its interconnections with the brainstem, upper spinal cord and the peripheral nervous system of the head and forelimbs. Evoked mossy fibre, climbing fibre and parallel fibre field potentials and EMG activity elicited in forelimb biceps muscle by interpositus stimulation provided evidence that both cerebellar inputs and outputs remain operational in this preparation. Similarly, the spontaneous and evoked single unit activity of Purkinje cells, putative Golgi cells, molecular interneurones and cerebellar nuclear neurones was similar to activity patterns reported in vivo. The advantages of the preparation include the ability to record, without the complications of anaesthesia, stabile single unit activity for extended periods (3 h or more), from regions of the rat cerebellum that are difficult to access in vivo. The preparation should therefore be a useful adjunct to in vitro and in vivo studies of neural circuits underlying cerebellar contributions to movement control and motor learning
- …