Abstract
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 deuterium–tritium 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 modelling of the temperature evolution based on energy balance appears crucial for quantitative predictions.
Highlights
One of the critical areas of research supporting the successful operation of ITER and other large-current tokamaks is the development of techniques to mitigate the high thermal and magnetic energies released in plasma-terminating disruptions (Boozer 2015)
Even in the absence of the hot-tail seed, our results indicate that if losses due to magnetic perturbations do not occur during a large fraction of the current quench (CQ), impurity injection leads to high runaway currents in the deuterium–tritium (DT) phase of ITER operation, even if it is combined with deuterium injection
When partially ionized impurities are introduced into the plasma, there are two competing effects which affect the avalanche growth rate: (1) additional target electrons become available for avalanche multiplication (represented by the prefactor nteot in the growth rate formula (2.8)), which leads to an increasing growth rate; and (2) the critical runaway momentum p increases due to the enhancement of the collisional slowing-down and pitch-angle scattering processes, which leads to a decreasing growth rate
Summary
One of the critical areas of research supporting the successful operation of ITER and other large-current tokamaks is the development of techniques to mitigate the high thermal and magnetic energies released in plasma-terminating disruptions (Boozer 2015). During the CQ phase of disruptions in reactor-scale devices, such as ITER, large RE currents are expected to form (Boozer 2015; Breizman et al 2019) These energetic electrons are of particular concern, as they may give rise to localized power deposition and cause melting of plasma-facing components. Even in the absence of the hot-tail seed, our results indicate that if losses due to magnetic perturbations do not occur during a large fraction of the CQ, impurity injection leads to high runaway currents in the deuterium–tritium (DT) phase of ITER operation, even if it is combined with deuterium injection. The reason is that the cooling associated with a large amount of injected material results in low temperatures leading to recombination and corresponding high value of the total-to-free electron density ratio, which in turn enhances the avalanche growth rate. A consequence of these results is that successful runaway mitigation during the non-nuclear phase of ITER operation may not provide a sufficient validation of the ITER disruption mitigation strategy, since the presence of radioactive sources of superthermal electrons during the DT phase changes the dynamics
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