Trajectories of nonthermal electrons in the hot spot of a vacuum spark

The trajectories of high-energy electrons in the inner part of a micropinch (hot spot) are calculated. It is shown that the collisionless electrons (b orn, for example, in the tail of Maxwellian distribution) drift to the pinc h axis in the current-induced magnetic field and the inductive electric fie ld to form axisymmetric orbits with radial oscillations and a shift along t he z-axis to the anode (modified run-away). On average, the time when the e lectron moves chiefly across the z-axis exceeds the period of its movement along the discharge axis, hence the probability of exciting the ions is hig her when the electrons have a relatively larger perpendicular velocity comp onent. This can explain the experimental observations of polarization of x- ray lines emitted from hot spots in a vacuum spark. Modified run-away can a lso play a crucial role in the energy balance of the micropinch plasma. (C) 2000 American Institute of Physics. [S1070-664X(00)02608-2].