Electron runaway across a magnetic field in a collisional high-atomic-number plasma

Summary form only given, as follows. Nonthermal X-ray spectra observed in high-atomic-number Z-pinch plasmas indicate that electrons with energies greatly in excess of the plasma temperature are present. A favorite mechanism for the production of these nonthermal electrons is acceleration in inductive electric fields produced by localized collapse of plasma into pinch spots. One problem with this acceleration mechanism is the presence of intense azimuthal magnetic fields embedded in the plasma which impede the runaway of electrons along the electric field. In this work, a fluid model for nonthermal electron flow in dense, high-atomic-number plasmas is employed to determine how collisions affect their energy gain in crossed electric and magnetic fields. The simple scaling laws derived from this model are compared with IPROP particle-in-cell simulations of the same plasma environment. Once a threshold electric field is exceeded, a large number of scattering collisions across the magnetic field and along the electric field can occur in an energy-loss time and much larger energy gains are possible than in hydrogenic plasmas. Results indicate that these collision processes are marginally able to explain observed Gamble II nonthermal X-ray spectra for electric fields derived from radiatively collapsed pinch spots. Electric fields produced in LASNEX simulations of Saturn nonthermal tungsten-wire experiments are an order-of-magnitude below those required by our results to explain nonthermal electron production.