Collisional avalanche exponentiation of runaway electrons in electrified plasmas

Abstract

In contrast to earlier expectations, it is estimated that generation of runaway electrons from close collisions of existing runaways with cold plasma electrons can be significant even for small electric fields, whenever runaways can gain energies of about 20 MeV or more. In that case, the runaway population will grow exponentially with the energy spectrum showing an exponential decrease towards higher energies. Energy gains of the required magnitude may occur in large Tokamak devices as well as in cosmic-ray generation.

Runaway generation during disruptions in JET and TEXTOR

“…Above the threshold almost all disruptions are free of high energetic runaways. The avalanche process becomes relevant if the runaways can achieve energies above 10 − 20 MeV [34]. The absence of these, shows that the avalanche is successfully suppressed by RMP, which is consistent with the reduction by a factor 3 in runaway current.…”

Suppression of Runaway Electrons by Resonant Magnetic Perturbations in TEXTOR Disruptions

“…Thus, for perturbation fields above this threshold, runaways are lost before they reach energies above 25 MeV. The avalanche rate becomes relevant if the runaways can achieve energies above about 10 -20 MeV [12]. The absence of these high energetic electrons indicates the suppression of the avalanche.…”

Energetics of runaway electrons during tokamak disruptions

“…This so-called avalanche process is discussed in detail in Refs. [1][2][3][4][5][6][7][8]. In the next stage of the disruption the runaway plasma usually drifts toward the wall, where it induces eddy currents, particularly when the plasma starts touching the wall and the current is being scraped off.…”

“…In large tokamaks, such as ITER, most runaway electrons are produced in avalanches caused by collisions at close range between existing runaways and thermal electrons [5][6][7], in which case the average Lorentz factor becomes γ ∼ 2 ln Λ [2]. After a disruption in ITER we thus expect most of the "free" energy to reside in the poloidal magnetic field, W m /W k ≫ 1.…”