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An overview of the present status of research toward the final design of the ITER disruption mitigation system (DMS) is given. The ITER DMS is based on massive injection of impurities, in order to radiate the plasma stored energy and mitigate the potentially damaging effects of disruptions. The design of this system will be extremely challenging due to many physics and engineering constraints such as limitations on port access and the amount and species of injected impurities. Additionally, many physics questions relevant to the design of the ITER disruption mitigation system remain unsolved such as the mechanisms for mixing and assimilation of injected impurities during the rapid shutdown and the mechanisms for the subsequent formation and dissipation of runaway electron current.

Demonstrating improved confinement of energetic ions is one of the key goals of the Wendelstein 7-X (W7-X) stellarator. In the past campaigns, measuring confined fast ions has proven to be challenging. Future deuterium campaigns would open up the option of using fusion-produced neutrons to indirectly observe confined fast ions. There are two neutron populations: 2.45 MeV neutrons from thermonuclear and beam-target fusion, and 14.1 MeV neutrons from DT reactions between tritium fusion products and bulk deuterium. The 14.1 MeV neutron signal can be measured using a scintillating fiber neutron detector, whereas the overall neutron rate is monitored by common radiation safety detectors, for instance fission chambers. The fusion rates are dependent on the slowing-down distribution of the deuterium and tritium ions, which in turn depend on the magnetic configuration via fast ion orbits. In this work, we investigate the effect of magnetic configuration on neutron production rates in W7-X. The neutral beam injection, beam and triton slowing-down distributions, and the fusion reactivity are simulated with the ASCOT suite of codes. The results indicate that the magnetic configuration has only a small effect on the production of 2.45 MeV neutrons from DD fusion and, particularly, on the 14.1 MeV neutron production rates. Despite triton losses of up to 50 %, the amount of 14.1 MeV neutrons produced might be sufficient for a time-resolved detection using a scintillating fiber detector, although only in high-performance discharges.

This letter presents a rigorous kinetic theory for relativistic runaway electrons in the near critical electric field in tokamaks. The theory provides a distribution function of the runaway electrons, reveals the presence of two different threshold electric fields and describes a mechanism for hysteresis in the runaway electron avalanche. Two different threshold electric fields characterize a minimal field required for sustainment of the existing runaway population and a higher field required for the avalanche onset. The near-threshold regime for runaway electrons determines the time scale of toroidal current decay during runaway mitigation in tokamaks.Introduction. -The importance of runaway electron production in plasma has been recognized more than half a century ago in a seminal work by Dreicer [1], followed by enlightening subsequent studies by Gurevich [2]. The initial non-relativistic results [1,2] have been generalized to the relativistic case by Connor and Hastie [3]. Similar to the previous work, [3] is based entirely on diffusive (small scattering angle) approximation for Coulomb collisions. The missing large-angle (knock-on) collisions are known to be weak compared to the small-angle collisions, but they can cause an avalanche-type growth of the runaway population, as pointed out in [4] and substantiated in [5,6]. In the absence of external magnetic field, the electric field can accelerate runaway electrons until they reach the pair-production energy range, but in magnetically confined plasmas the runaway energies are limited, rather, by synchrotron losses that accompany pitch-angle scattering. The significance of this mechanism was first shown in [7] and then emphasized in [8] and [9].The compelling need to mitigate runaway electrons or to control their behavior in ITER calls for additional attention to the above mentioned aspects of the runaway problem: relativistic energies of the runaways, avalanche mechanism of the runaway production, and the combined effect of pitch-angle scattering and synchrotron losses on the runaway distribution function (this effect was omitted in [6]). It is especially important to have an accurate theory for the near-threshold regime that represents long-term behavior of the runaways and is critical for the mitigation process. Even very strong initial inductive electric field is reasonably expected to drop down to the threshold-level values with the growth of the runaway population. The key questions in that regard are what is the threshold electric field and what is the growth rate of the avalanche when the electric field exceeds the threshold. The threshold electric field must at least overcome the collisional friction for ultra-relativistic electrons, which means that this field cannot be less than

Of all electrons, runaway electrons have long been recognized in the fusion community as a distinctive population. They now attract special attention as a part of ITER mission considerations. This review covers basic physics ingredients of the runaway phenomenon and the ongoing efforts (experimental and theoretical) aimed at runaway electron taming in the next generation tokamaks. We emphasize the prevailing physics themes of the last 20 years: the hot-tail mechanism of runaway production, runaway electron interaction with impurity ions, the role of synchrotron radiation in runaway kinetics, runaway electron transport in presence of magnetic fluctuations, micro-instabilities driven by runaway electrons in magnetized plasmas, and vertical stability of the plasma with runaway electrons. The review also discusses implications of the runaway phenomenon for ITER and the current strategy of runaway electron mitigation.