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Injecting shattered pellets is the critical concept of the envisaged ITER disruption mitigation system (DMS). Rapid deposition of large amounts of material should presumably result in controlled cooling of the entire plasma. A considerable transfer of thermal energy from the electrons of the background plasma to the ions accompanies a localized material injection due to the ambipolar expansion along the magnetic field line of the cold and dense plasmoid produced by the ablated pellet. Radiation initially plays the dominant role in the energy balance of a composite plasmoid containing high-Z impurities. A competition between the ambipolar expansion and the radiative losses defines the Thermal Quench scenario, including the amount of pre-quench thermal energy radiated on a short collisional timescale—possibly detrimental for the plasma-facing components. The present work quantifies plasmoid energy balance for disruption mitigation parameters. For pure hydrogen injection, up to 90% of the pre-pellet electron thermal energy may go to the newly injected ions. We also demonstrate that a moderate high-Z impurity content within the plasmoid can reduce highly localized radiation at the beginning of the expansion. The thermal energy will then dissipate on the much longer ion collisional timescale, which would be attractive for ITER DMS.

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 (RE) taming in the next generation tokamaks. We emphasize the prevailing physics themes of the last 20 years: the hot-tail mechanism of runaway production, RE interaction with impurity ions, the role of synchrotron radiation in runaway kinetics, RE transport in presence of magnetic fluctuations, micro-instabilities driven by REs in magnetized plasmas, and vertical stability of the plasma with REs. The review also discusses implications of the runaway phenomenon for ITER and the current strategy of RE mitigation. [ABSTRACT FROM AUTHOR]

This work provides a systematic description of electron kinetics during impurity dominated thermal quenches. A Fokker–Planck equation for the hot electrons and a power balance equation for the bulk plasma are solved self-consistently, with impurity radiation as the dominant energy loss mechanism. We find that runaway production is facilitated by heavy injection of impurities up to prompt conversion of the total current into a sub-MeV runaway current. We also find that runaway formation is less efficient in plasmas with high pre-quench temperatures and predict significant radial variation of the runaway seed in such plasmas. [ABSTRACT FROM AUTHOR]

Runaway electrons (REs) generated during disruption events in tokamaks have to be mitigated to minimize the detrimental impact from their massive losses to the wall, especially in ITER. RE-driven micro-instabilities, such as whistlers and magnetized plasma waves, can cause enhanced RE scattering and thereby alleviate the mitigation problem. This work presents a newly developed ray-tracing code COIN which enables stability analysis of runaway-driven waves in a tokamak. The code uses a standard ray-tracing procedure to calculate a wave-packet trajectory in a realistic plasma equilibrium and integrates the runaway kinetic drive and collisional damping of the wave. This approach captures convective aspects of wave amplification, such as the evolution of the wave vector due to plasma non-uniformity and internal reflection of the wave from the plasma boundary. An illustrative ray-tracing calculation of the instability threshold is presented for ITER-like parameters. [ABSTRACT FROM AUTHOR]