Experimental evidence on photo-assisted O$^-$ ion production from Al$_2$O$_3$ cathode in cesium sputter negative ion source

TL;DR

This study investigates how pulsed lasers influence negative ion production from an Al₂O₃ cathode in cesium sputter sources, revealing wavelength-dependent effects that could enhance ion beam currents for practical applications.

Contribution

It provides experimental evidence of photo-assisted negative ion production and clarifies the wavelength dependence, challenging the idea of a resonant process involving cesium's electronic states.

Findings

Wavelength-dependent photo-assisted effect observed

Beam current can more than double with laser assistance

Effect depends on cesium surface conditions

Abstract

The production of negative ions in cesium sputter ion sources is generally considered to be a pure surface process. It has been recently proposed that ion pair production could explain the higher-than-expected beam currents extracted from these ion sources, therefore opening the door for laser-assisted enhancement of the negative ion yield. We have tested this hypothesis by measuring the effect of various pulsed diode lasers on the O$^-$ beam current produced from Al$_2$O$_3$ cathode of a cesium sputter ion source. It is expected that the ion pair production of O$^-$ requires populating the 5d electronic states of neutral cesium, thus implying that the process should be provoked only with specific wavelengths. Our experimental results provide evidence for the existence of a wavelength-dependent photo-assisted effect but cast doubt on its alleged resonant nature as the prompt enhancement of beam current can be observed with laser wavelengths exceeding a threshold photon energy. The beam current transients observed during the laser pulses suggest that the magnitude and longevity of the beam current enhancement depends on the cesium balance on the cathode surface. We conclude that the photo-assisted negative ion production could be of practical importance as it can more than double the extracted beam current under certain operational settings of the ion source.