Ion emission from solid surfaces induced by intense electron beam impact

Abstract

Ions or ionized neutrals released from solid surfaces by electron beam impact can be accelerated and trapped in the beam potential causing beam disruption. Experiments have been performed on the DARHT-I accelerator (1.7 kA, 19.8 MeV, 60 ns) to study this phenomenon. The beam, focused to a range of diameters, was transmitted through thin targets made of various materials. The time evolution of the beam radial profile was measured downstream of the target. For low current density, the downstream-beam radial profile was time invariant as expected for a pure electron beam. At higher current density, the downstream beam was clearly disrupted during the pulse followed by a large-amplitude transverse centroid instability. Two-dimensional calculations using the Lsp particle-in-cell code show that if the space-charge-limiting ion current is allowed to flow after the target surface temperature increases by about 400 K, the main features of the experimental observations are replicated. Three-dimensional Lsp calculations show growth of the ion hose instability at a frequency close to that observed in the experiments.

Initial electron-beam results from the DARHT-II linear induction accelerator

“…This causes ion focusing or disruption of the beam [11], either of which invalidates the imaging data. The threshold for these effects is over 300 C [12]. We estimated the temperature of the target by integrating time resolved reconstructions of the beam current-density profile to produce a map of charge deposition, and then estimated the peak temperature using the stopping power and specific heat of the SiO target.…”

Quantifying spot size reduction of a 1.8 kA electron beam for flash radiography

“…If we ignore beam-target interactions 21,22 for the moment, then the three main contributing factors governing the spot size r at the final focus are emittance, beam transport mismatches, and spherical aberration. The scaling of r can be determined analytically by considering the root sum of the squares for each contributing factor and finding the optimum spot size in terms of the convergence angle a.…”

Neutral desorption from intense electron beam impact on solid surfaces

“…In a recent publication, results of an experimental ͑using the DARHT first axis beam; 19.8 MeV, 1.7 kA, and 60 ns flattop pulse͒ and theoretical study to understand the extent of this phenomenon were reported. 7 There, the beam, focused to a range of diameters, was transmitted through thin targets made of various materials, and the time evolution of the beam radial profile was measured downstream of the target. A number of metal targets along with graphite were used.…”