The constraint of plasma power balance on runaway avoidance

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 $\{n_α\}$ with $α$ labeling the atomic species, the power balance sets the plasma temperature, ion charge state distribution $\{n_α^i\}$ with $i$ the charge number, and through the electron temperature $T_e$ and ion charge state distribution $\{n_α^i\},$ 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.