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 (λ) and times (τ). The ratio of λ to τ agrees well with the speed of the avalanche, which ranges from 2.61 × 10 8 m s ‐1 to 2.72 × 10 8 m s ‐1 . Thus, using the speed of light, c , for the ratio of λ to τ can cause a 10% error when estimating λ from τ. A 10% error in λ 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 0.987 c and higher. Using a propagation speed of 0.89 c causes a dramatic change in the predicted beaming pattern of electromagnetic radiation caused by RRB in these models.