The constraint of plasma power balance on runaway avoidance

TL;DR

This paper discusses how the plasma power balance, involving heating and cooling processes, critically constrains the conditions needed to prevent runaway electrons during rapid tokamak plasma termination.

Contribution

It reveals the fundamental constraint of plasma power balance on runaway electron avoidance, linking atomic composition, charge states, and electric fields in post-thermal-quench plasmas.

Findings

Power balance determines plasma temperature and charge states.

Threshold electric field for runaway growth is influenced by atomic mixture.

Power balance constraints limit runaway electron mitigation strategies.

Abstract

In a post-thermal-quench plasma, mitigated or unmitigated, the plasma power balance is mostly between collisional or Ohmic heating and plasma radiative cooling. In a plasma of atomic mixture $\{n_\alpha\}$ with $\alpha$ labeling the atomic species, the power balance sets the plasma temperature, ion charge state distribution $\{n_\alpha^i\}$ with $i$ the charge number, and through the electron temperature $T_e$ and ion charge state distribution $\{n_\alpha^i\},$ the parallel electric field $E_\parallel.$ Since the threshold electric field for runaway avalanche growth $E_{av}$ is also set by the atomic mixture, ion charge state distribution and its derived quantity, the electron density $n_e,$ the plasma power balance between Ohmic heating and radiative cooling imposes a stringent constraint on the plasma regime for avoiding and minimizing runaways when a fusion-grade tokamak plasma is rapidly terminated.