“…Experimental investigations of a nanosecond surfacedischarge (NSD), i.e., SD formed by high-voltage impulses of nanosecond duration [8,9,11,12], indicated that under Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/actaastro certain conditions, a NSD plasma is an intensive source of soft X-rays via generation of "runaway" electrons. The "runaway" effect occurs if the tangential component of the electric field intensity in a growing NSD plasma exceeds the critical value (Е cr , see Table 1), which can be found from the energy balance of an electron moving in the electric field [10].…”