Propagation speed of runaway electron avalanches

Simulations of relativistic runaway breakdown (RRB) are performed as functions of both time and space, resulting in explicit calculations of e-folding lengths (lambda) and times (tau). The ratio of lambda to tau agrees well with the speed of the avalanche, which ranges from 2.61 x 10(8) m s(-1) to 2.72 x 10(8) m s(-1). Thus, using the speed of light, c, for the ratio of lambda to tau can cause a 10% error when estimating lambda from tau. A 10% error in lambda will cause a factor of three error in the predicted number of runaway electrons for every ten estimated e- foldings. In addition, previous models that predict peak radiated electric fields from RRB have used avalanche speeds of 0.987c and higher. Using a propagation speed of 0.89c causes a dramatic change in the predicted beaming pattern of electromagnetic radiation caused by RRB in these models.