The influence of a shield on intense ion beam transportation

Abstract

This article presents the results of a study on transportation of a pulse ion beam of gigawatt power. This beam is formed by a self-magnetically insulated diode with an explosive-emission cathode. The experiments have been performed using the TEMP-4M pulsed ion accelerator configured in double-pulse formation mode with the first negative pulse (300-500 ns, 100-150 kV), followed by the second positive pulse (150 ns, 250-300 kV). To increase the effectiveness of ion beam focusing, a metal shield is installed on a grounded electrode. Investigations are performed using a strip focusing diode, a cone diode, and a spiral diode with metal shields of different constructions. We observed that the beam diameter at the focus decreases from 60 mm (without shield) to 40-42 mm (with a shield), which leads to an increase in the energy density by a factor of 1.5-2 being 4-5 J/cm 2 at the focus. We analyzed different mechanisms for ion trajectories deviation from an ideal one: Coulomb repulsion due to incomplete space charge neutralization, influence of electromagnetic fields, etc. It is found that for a strip focusing diode the concentration of low-energy electrons accompanying the ion beam exceeds the concentration of ions by 1.3-1.5 times. The use of a metal shield improves the transportation properties of the ion beam by keeping neutralizing electrons within the beam volume which ensures its space charge neutralization during the transport.

Experimental evidence of energetic neutrals production in an ion diode

“…To increase focusing efficiency and prevent the loss neutralizing electrons from the beam during transport to the target, we used a metal shield installed on a grounded electrode of the diode [14].…”

Melting of a thin aluminum film on a dielectric substrate under a high-power ion beam

“…This irradiation is based on the radial distribution of the surface energy density over the cross section of the beam generated by a focusing diode: the energy density decreases almost linearly from the cen tral part of the beam toward its periphery [6]. Figure 2 shows an AFM image of the initial surface of a 1.9 μm thick aluminum film.…”

The effect of energy stabilization of an ion beam formed by a self-magnetically insulated diode

“…To improve the beam focusing, a continuous 1 mm thick stainless steel shield is mounted on the grounded electrode of the diode [11]. Figure 2 shows a photo graph of the strip focusing diode and a typical distribu tion of the beam energy density in the transverse cross section.…”

“…6). To efficiently focus the HPIB, it is required that the metal shield be closed [11]. However, when the beginning and end of the diode with self magnetic insulation are connected to each other, the electron component of the total diode current considerably increases, and the diode changes to the short circuit ing mode [14].…”