Analysis of Avalanche Runaway Generation after Disruptions with Low-<i>Z </i>and Noble Gas Species

Abstract

The effects of impurities on runaway electron generation are studied using a zero-dimensional disruption simulation code. For describing collisions between fast electrons and partially stripped ions, a charge-resolved expression of the Coulomb logarithm is employed. Numerical analysis of the avalanche growth rate using the adjoint Fokker-Planck method is compared with two existing semi-analytic models, showing (i) the convergence of the growth rate to strong electric field limit of the Rosenbluth-Putvinski (R-P) model and (ii) the cancellation of the effect of second-order collisional diffusion for intermediate electric fields. Using the developed current quench (CQ) simulations, the parametric study is performed with the aid of the power balance analysis, which characterizes the onset of strong avalanche amplification in the presence of low-Z and noble gas species. Thermal quench (TQ) simulations are also developed for self-consistent evaluation of hot-tail seed electrons. The deposition timescale of impurity neutrals is shown to have significant impacts on hot-tail seeds, depending nonmonotonically on the pre-TQ temperature and the injected impurity density.

Modelling of ohmic startup and runaway electron formation in support of JT-60SA initial operation

“…In this study, the time evolution of the plasma parameters in the startup phase of JT-60SA is numerically investigated using the 0D simulation code INDEX-S. This 0D code was originally developed to study the plasma current quench and RE generation during disruptions [35]. For the present analyses, we have implemented the additional models outlined below.…”

“…To date, RE generation during startup has long been studied [18,19,29], and different RE models have recently been incorporated into the 0D plasma burn-through codes, such as SCENPLINT/TRANSMAK [7,[30][31][32], STREAM [33], and DYON [34]. In a similar approach, the possibility of RE discharges occurring at JT-60SA is studied numerically using the RE generation model [35]. The simulation shows that startup REs are generated during the temperature rise immediately after the burn-through under relatively low-density conditions-depending on the prefill gas pressure and the recycling process.…”

Study of runaway electron dynamics at the ASDEX Upgrade tokamak during impurity injection using fast hard x-ray spectrometry

“…The experimental observations were investigated with numerical calculations performed to estimate RE energy dynamics using a simple 0-D code PREDICT [41,43,44]. The code employs the relativistic test particle equations that govern RE energy dynamics in momentum space, including collisional and synchrotron-radiation induced losses [44][45][46][47]. It also considers different RE-generation mechanisms such as the secondary avalanche and primary RE-generation mechanisms, namely Dreicer, hot-tail, tritium decay and Compton scattering (from γ-rays emitted from activated walls) [46].…”

“…It also considers different RE-generation mechanisms such as the secondary avalanche and primary RE-generation mechanisms, namely Dreicer, hot-tail, tritium decay and Compton scattering (from γ-rays emitted from activated walls) [46]. In the present simulation work, the following test particle equations utilized where the effect of impurities are taken into account considering collisions of REs with free and bound electrons and scattering from the full and partially-shielded nuclear charge [46,47]:…”

“…m e is the mass of an electron, ε 0 is the permittivity of free space. The parameters α e (γ)and Z coll (γ) are correctional factors that take into account the collisions of REs with free and bound electrons as well as scattering from full and partially-shielded nuclear charge as calculated and described in [47]. In these simulations, all temporal evolution of plasma parameters has been taken as an input and the only free parameter is a temporal evolution of injected argon atoms density profile.…”