arXiv subjects

O. Vallhagen

Publications and source records attributed to O. Vallhagen.

11 recordsLinked to original sources

arXiv2026

Runaway electron generation in ITER mitigated disruptions with improved physics models

We assess runaway-electron (RE) generation in ITER disruptions mitigated by shattered pellet injection (SPI) using improved physics modelling in the 1D disruption simulation framework Dream. To this end, we extend Dream with four ITER-relevant physics models: (i) a reduced model for RE scrape- off associated with the vertical plasma motion, (ii) a semi-analytical plasmoid- drift model for material deposition, (iii) an adaptive hyper-resistive transport model to suppress unphysical thin-current channels during the current quench (CQ), and (iv) an updated Compton RE generation seed calculated for the new ITER tungsten first-wall design. We simulate full-current 15 MA L-mode (H26, non-nuclear) and H-mode (DTHmode24, nuclear) scenarios, and an intermediate- current 7.5 MA H-mode non-nuclear case, from realistic ITER inputs. Complete avoidance of a multi-MA RE beam is found to require a long pre-thermal quench (TQ) duration to thermalize the hot-tail electrons, high deuterium assimilation with limited neon, and a representative seed current comparable to a single RE in ITER. As previously found with lower fidelity setups [Vallhagen et al, Nucl. Fusion 64 (2024)], these conditions are met by staggered or low-Ne injections in H26, but are typically violated in DT H-mode when nuclear seeds are present. In addition to analyzing the effect of the new models, we investigate the role of the current spike associated with the TQ and importance of radial transport of runaways in the CQ. After incorporating these additional physical effects into a comprehensive disruption model and analyzing their impact, we present a representative ITER DT H-mode SPI scenario which provides a theoretically viable route to tolerable RE currents in ITER fusion power operation.

physics.plasm-ph↗

arXiv2025

Simulation of Shattered Pellet Injections with Plasmoid Drifts in ASDEX Upgrade and ITER

Pellet injection is an important means to fuel and control discharges and mitigate disruptions in reactor-scale fusion devices. To accurately assess the efficiency of these applications, it is necessary to account for the drift of the ablated material toward the low-field side. In this study, we have implemented a semi-analytical model for ablation cloud drifts in the numerical disruption modelling tool DREAM. We show that this model is capable of reproducing the density evolution in shattered pellet injection (SPI) experiments in ASDEX Upgrade, for model parameters within the expected range. The model is then used to investigate the prospects for disruption mitigation by staggered SPIs in 15 MA DT H-mode ITER scenarios. We find that the drifts may decrease the assimilation of pure deuterium SPIs by about an order of magnitude, which may be important to consider when designing the disruption mitigation scheme in ITER. The ITER scenarios studied here generally result in similar multi-MA runaway electron (RE) currents, regardless of the drift assumptions, but the effect of the drift is larger in situations with a fast and early thermal quench. The RE current may also be more strongly affected by the drift losses when accounting for RE losses caused by the vertical plasma motion.

physics.plasm-ph↗

arXiv2025

The effect of plasmoid drifts on the pellet rocket effect in magnetic confinement fusion plasmas

We detail here a semi-analytical model for the pellet rocket effect, which describes the acceleration of pellets in a fusion plasma due to asymmetries in the heat flux reaching the pellet surface and the corresponding ablation rate. This effect was shown in experiments to significantly modify the pellet trajectory, and projections for reactor scale devices indicate that it may severely limit the effectiveness of pellet injection methods. We account for asymmetries stemming both from plasma parameter gradients and an asymmetric plasmoid shielding caused by the drift of the ionized pellet cloud. For high temperature, reactor relevant scenarios, we find a wide range of initial pellet sizes and speeds where the rocket effect severely limits the penetration depth of the pellet. In these cases, the plasma parameter profile variations dominate the rocket effect. We find that for small and fast pellets, where the rocket effect is less pronounced, plasmoid shielding induced asymmetries dominate.

physics.plasm-ph↗

arXiv2023

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 Alfvén 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.

physics.plasm-ph↗

arXiv2022

Runaway dynamics in reactor-scale spherical tokamak disruptions

Understanding generation and mitigation of runaway electrons in disruptions is important for the safe operation of future tokamaks. In this paper we investigate runaway dynamics in reactor-scale spherical tokamaks. We study both the severity of runaway generation during unmitigated disruptions, as well as the effect that typical mitigation schemes based on massive material injection have on runaway production. The study is conducted using the numerical framework DREAM (Disruption Runaway Electron Analysis Model). We find that, in many cases, mitigation strategies are necessary to prevent the runaway current from reaching multi-megaampere levels. Our results indicate that with a suitably chosen deuterium-neon mixture for mitigation, it is possible to achieve a tolerable runaway current and ohmic current evolution. With such parameters, however, the majority of the thermal energy loss happens through radial transport rather than radiation, which poses a risk of unacceptable localised heat loads.

physics.plasm-ph↗

arXiv2022

Effect of Two-Stage Shattered Pellet Injection on Tokamak Disruptions

An effective disruption mitigation system in a tokamak reactor should limit the exposure of the wall to localized heat losses and to the impact of high current runaway electron beams, and avoid excessive forces on the structure. We evaluate with respect to these aspects a two-stage deuterium-neon shattered pellet injection in an ITER-like plasma, using simulations with the DREAM framework [M. Hoppe et al (2021) Comp. Phys. Commun. 268, 108098]. To minimize the obtained runaway currents an optimal range of injected deuterium quantities is found. This range is sensitive to the opacity of the plasma to Lyman radiation, which affects the ionization degree of deuterium, and thus avalanche runaway generation. The two-stage injection scheme, where dilution cooling is produced by deuterium before a radiative thermal quench caused by neon, reduces both the hot-tail seed and the localized transported heat load on the wall. However, during nuclear operation, additional runaway seed sources from the activated wall and tritium make it difficult to reach tolerably low runaway currents.

physics.plasm-ph↗

arXiv2021

Modelling of runaway electron dynamics during argon-induced disruptions in ASDEX Upgrade and JET

Disruptions in tokamak plasmas may lead to the generation of runaway electrons that have the potential to damage plasma-facing components. Improved understanding of the runaway generation process requires interpretative modelling of experiments. In this work we simulate eight discharges in the ASDEX Upgrade and JET tokamaks, where argon gas was injected to trigger the disruption. We use a fluid modelling framework with the capability to model the generation of runaway electrons through the hot-tail, Dreicer and avalanche mechanisms, as well as runaway electron losses. Using experimentally based initial values of plasma current and electron temperature and density, we can reproduce the plasma current evolution using realistic assumptions about temperature evolution and assimilation of the injected argon in the plasma. The assumptions and results are similar for the modelled discharges in ASDEX Upgrade and JET, indicating that the implemented models are applicable to machines of varying size, which is important for the modelling of future, larger machines. For the modelled discharges in ASDEX Upgrade, where the initial temperature was comparatively high, we had to assume that a large fraction of the hot-tail runaway electrons were lost in order to reproduce the measured current evolution.

physics.plasm-ph↗

arXiv2020

Effects of magnetic perturbations and radiation on the runaway avalanche

The electron runaway phenomenon in plasmas depends sensitively on the momentum-space dynamics. However, efficient simulation of the global evolution of systems involving runaway electrons typically requires a reduced fluid description. This is needed for example in the design of essential runaway mitigation methods for tokamaks. In this paper, we present a method to include the effect of momentum-dependent spatial transport in the runaway avalanche growth rate. We quantify the reduction of the growth rate in the presence of electron diffusion in stochastic magnetic fields and show that the spatial transport can raise the effective critical electric field. Using a perturbative approach we derive a set of equations that allows treatment of the effect of spatial transport on runaway dynamics in the presence of radial variation in plasma parameters. This is then used to demonstrate the effect of spatial transport in current quench simulations for ITER-like plasmas with massive material injection. We find that in scenarios with sufficiently slow current quench, due to moderate impurity and deuterium injection, the presence of magnetic perturbations reduces the final runaway current considerably. Perturbations localized at the edge are not effective in suppressing the runaways, unless the runaway generation is off-axis, in which case they may lead to formation of strong current sheets at the interface of the confined and perturbed regions.

physics.plasm-ph↗

arXiv2020

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.

physics.plasm-ph↗

arXiv2020

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.

physics.plasm-ph↗

arXiv2020

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.

physics.plasm-ph↗