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The generation during disruptions of MA beams of runaway electrons (REs) poses a serious concern for future fusion devices like ITER. Two layers of defense have been proposed: (1) preventing RE generation to avoid the formation of RE currents (RE avoidance); (2) mitigating the runaway beam by injection of deuterium or high-Z impurities by Shattered Pellet Injection (SPI) in case its formation cannot be prevented (RE mitigation). In the past, the mitigation of REs by injection of high-Z species was analyzed by means of 0-D modeling [1]. It was found that impurity injection results in an increase of the critical electric field, ER, for runaway generation which, if the electric field drops below ER, leads to the dissipation of the runaway population. Here, the
dissipation of disruption generated runaway beams by injection of high-Z impurities is addressed by means of a 1D model. It is found that initially peaked runaway current density profiles, as those expected after the current quench phase of a disruption, result in a slower current dissipation, the effect increasing with the peaking of the current profile. Also, effects associated with a localised deposition of the impurities are considered. The runaway energy is dissipated in the deposition region, whereas energy is deposited and REs are formed outside due to the radial diffusion of the electric field. Hence, the dissipation efficiency decreases for non-uniform radial impurity deposition,
particularly for too inner or too outer injection, the most efficient dissipation occurring for radially uniform impurity deposition. The effect of a sudden W influx on the RE beam in ITER disruptions is also addressed.
[1] J.R. Martin-Solis et al., in Fusion Energy 2018 (Proc. 27th Int. Conf., Ahmedabad, 2018) (Vienna: IAEA) CD-ROM file TH/4-1.
*This work was done under financial support from Ministerio de Ciencia, Innovacion y Universidades (Spain), Project No.PID2022-137869OB-I00, and carried out under the coordinated research programme of the Disruption and Runaway Theory and Simulation group of the ITER Scientist Fellow Network. ITER is the Nuclear Facility INB no. 174. This paper explores physics processes during the plasma operation of the tokamak when disruptions take place; nevertheless the nuclear operator is not constrained by the results of this paper. The views and opinions expressed herein do not necessarily reflect those of ITER.