Runaway electron generation in ITER mitigated disruptions with improved physics models

Abstract

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.

Figures (14)

• Figure 1: Initial plasma profiles for the scenarios considered. Solid blue lines correspond to the intermediate current scenario (DD7.5), dashed magenta lines to H26, and dotted yellow lines to DTHmode24. Shown are (a) electron temperature, (b) electron density, and (c) ohmic current density as functions of the radial coordinate.
• Figure 2: Fitted Compton energy spectra for different ITER first wall materials. (a) In black the original spectrum from Martin-Solis2017, in red the new Be spectrum, and in green the new W spectrum. (b) In black, the raw data from the new radiation transport simulations for Be, in red the new fit to this data, and in blue the old fit by Martìn-Solìs et al. (c) In black, the raw data from the new radiation transport simulations for W, and in red the new analytical fit to this spectrum.
• Figure 3: Correlations between the representative runaway current, $I_{\mathrm{RE}}^{\mathrm{repr}}$, and key quantities in the baseline SPI simulations for the L-mode H26 scenario. Panel (a) shows $I_{\mathrm{RE}}^{\mathrm{repr}}$ versus the number of assimilated protium atoms, $N_{\mathrm{H,assim}}$, while panel (b) shows $I_{\mathrm{RE}}^{\mathrm{repr}}$ versus the number of assimilated neon atoms, $N_{\mathrm{Ne,assim}}$. Panel (c) presents $I_{\mathrm{RE}}^{\mathrm{repr}}$ as a function of the representative runaway seed current, $I_{\mathrm{seed}}^{\mathrm{repr}}$, and panel (d) shows it versus the pre-TQ duration $t_{\mathrm{pre\text{-}TQ}}$. Each point represents a simulation and colours distinguishing injection schemes are indicated in the top legend.
• Figure 4: Correlations between the representative final runaway-electron current, $I_{\mathrm{RE}}^{\mathrm{repr}}$, and key quantities in the baseline SPI simulations for H-mode DTHmode24 plasmas. Panel (a) shows $I_{\mathrm{RE}}^{\mathrm{repr}}$ versus the number of assimilated protium atoms, $N_{\mathrm{H,assim}}$, panel (b) shows $I_{\mathrm{RE}}^{\mathrm{repr}}$ versus the number of assimilated neon atoms, $N_{\mathrm{Ne,assim}}$. Panels (c,d) present $I_{\mathrm{RE}}^{\mathrm{repr}}$ as a function of the representative seed current, $I_{\mathrm{seed}}^{\mathrm{repr}}$ and the pre-TQ duration $t_{\mathrm{pre\text{-}TQ}}$ respectively. Each point represents a simulation and colours distinguishing injection schemes are indicated in the top legend.
• Figure 5: Correlations between the representative final runaway-electron current, $I_{\mathrm{RE}}^{\mathrm{repr}}$, and key quantities in the baseline SPI simulations for H-mode DTHmode24 plasmas without nuclear seeds are shown. Panel (a) shows $I_{\mathrm{RE}}^{\mathrm{repr}}$ versus the number of assimilated protium atoms, $N_{\mathrm{H,assim}}$, panel (b) shows $I_{\mathrm{RE}}^{\mathrm{repr}}$ versus the number of assimilated neon atoms, $N_{\mathrm{Ne,assim}}$. Panels (c,d) present $I_{\mathrm{RE}}^{\mathrm{repr}}$ as a function of the representative seed current, $I_{\mathrm{seed}}^{\mathrm{repr}}$ and the pre-TQ duration $t_{\mathrm{pre\text{-}TQ}}$ respectively. Each point represents a simulation and colours distinguishing injection schemes are indicated in the top legend.