Effect of Partially Screened Nuclei on Fast-Electron Dynamics

Abstract

We analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms, where the screening effect of bound electrons must be included. We derive analytical expressions for the deflection and slowing-down frequencies, and show that they are increased significantly compared to the results obtained with complete screening, already at sub-relativistic electron energies. Furthermore, we show that the modifications to the deflection and slowing down frequencies are of equal importance in describing the runaway current evolution. Our results greatly affect fastelectron dynamics and have important implications, e.g. for the efficacy of mitigation strategies for runaway electrons in tokamak devices, and energy loss during relativistic breakdown in atmospheric discharges.Introduction.-Fast electrons, having speeds well above the thermal speed of the bulk plasma population, are ubiquitous in space and laboratory plasmas. An important process leading to such high-energy electrons is the runaway mechanism. Runaway electrons can be produced in the presence of an accelerating electric field if it exceeds the critical value E c = n e e 3 ln Λ 0 /4πǫ

Experimental and synthetic measurements of polarized synchrotron emission from runaway electrons in Alcator C-Mod

“…Histograms of measured θ pol and f pol are shown in figures 2a and 2b, respectively. The distributions of θ pol are well-localized for most channels, and an interesting The discharge numbers are 2, 3,6,7,14,16,17,19,20,[23][24][25][26]30, and 34 for experiment #1140403; 15 and 18-22 for experiment #1151002; and 20, 21, and 23-27 for experiment #1160824. spatial trend is observed: At the innermost radius (channel 1, r tan /a = −0.4), θ pol ≈ 0 • indicates horizontally-polarized synchrotron light. ¶ For |r tan /a| ≤ 0.23, i.e.…”

Impact of a minority relativistic electron tail interacting with a thermal plasma containing high-atomic-number impurities

“…The impact of including relativistic ICS effects ( ) compared to using ICS without the Møller/generalized Breit interaction ( ) or simply assuming a thermal plasma (•) is apparent. the partial screening effect, which has a direct dependence on the ion charge state distribution, according to Hesslow et al [21,36]. The physical importance is that enhanced pitch angle scattering would limit the energy of the O-point of the runaway vortex, which is responsible for a lower runaway energy but broader pitch distribution [37,38].…”

“…For each ion species in a specific charge state, which is denoted by subscript α, there is a unique mean excitation energy I α and bound electron density n eα . In fact, the standard treatment in runaway modeling is to incorporate the runaway slowing down due to excitation and ionization via a friction in the Fokker-Planck collision operator using the Bethe formula [20,21]. The effect of ion charge state distribution (CSD) on runaway and thermal bulk plasma evolu-tion is through a collisional-radiative (CR) model that deals with only the background Maxwellian population at given temperature.…”

“…The effect of ion charge state distribution (CSD) on runaway and thermal bulk plasma evolu-tion is through a collisional-radiative (CR) model that deals with only the background Maxwellian population at given temperature. In a dilute plasma such as those in tokamak disruption, the aforementioned decoupled treatment of background thermal electrons and runaways [20,21] has a straightforward prediction for the radiative power loss as the bulk electrons are cooled to below a few eV. Namely, the background electron density drops precipitously due to cold thermal electron recombination with ions.…”

A fluid-kinetic framework for self-consistent runaway-electron simulations

“…with the deterministic trajectory obeying the ordinary differential equations dx/ds = v and dp/ds = µ. The integral (12) can be computed efficiently to high accuracy using Gauss-Hermite quadrature rules as demonstrated previously 28 . One should note that for each point z the values (X ∆s , P ∆s ) are needed only once regardless of how many intervals ∆s are used for the recursive computation.…”