  In this Letter we investigate factors that influence the effective critical
electric field for runaway-electron generation in plasmas. We present numerical
solutions of the kinetic equation and discuss the implications for the
threshold electric field. We show that the effective electric field necessary
for significant runaway-electron formation often is higher than previously
calculated due to both (1) extremely strong dependence of primary generation on
temperature, and (2) synchrotron radiation losses. We also address the
effective critical field in the context of a transition from runaway growth to
decay. We find agreement with recent experiments, but show that the observation
of an elevated effective critical field can mainly be attributed to changes in
the momentum-space distribution of runaways, and only to a lesser extent to a
de facto change in the critical field.
