Operating a cesium sputter source in a pulsed mode

Abstract

A scheme is presented for pulsing of a cesium sputter negative ion source by periodically switching on and off the high voltage driving the sputtering process. We demonstrate how the pulsed ion beam can be used in combination with a pulsed laser (6 ns pulse length) that has a 10 Hz repetition rate to study the photodetachment process, where a negative ion is neutralized due to the absorption of a photon. In such experiments, where the ion beam is used only for a small fraction of the time, we show that the pulsed mode operation can increase the lifetime of a cathode by two orders of magnitude as compared with DC operation. We also investigate how the peak ion current compares with the ion current obtained when the source is run in a DC mode. We find that the peak current in the pulsed mode is strongly dependent on the ion species. In some cases, we observed a strong enhancement, whereas others showed only a moderate enhancement, or even a decrease, in the peak current. We conclude that the pulsed mode operation can be of great value when the negative ion to be investigated requires cathodes that have short lifetimes, expensive materials, or those with relatively small ion beam yields, in the latter case limited to elements with large enhancement factors.

Photo-assisted Cl-, Br- and I- production in caesium sputter ion source

“…It has been recently demonstrated that the negative ion beam current from the SNICS source can be enhanced with cathode voltage pulsing [22,23]. For example, in [22] it was reported that the C − and C − 4 beam currents can be enhanced up to 400% by pulsing the cathode voltage in Cs-depleted condition, although the same beam current was obtained with continuous cathode voltage at higher Cs feed rate (oven temperature).…”

Experimental evidence on photo-assisted O− ion production from Al2O3 cathode in cesium sputter negative ion source

“…The role of Cs coverage is best illustrated by the experiments with the high power laser using varying pulse repetition rates and pulse lengths, demonstrating that with sufficient laser power and inappropriate Cs coverage, the long-term effect of the laser can be adverse. It has been observed 18 that operating the SNICS source in pulsed mode can sometimes (under certain operating conditions) lead to enhanced beam currents although systematic trends covering various ion species were not found. It is plausible that, similar to photo-assisted negative ion production with the laser exposure, the performance of the SNICS ion source in pulsed mode is sensitive to variations of the cathode Cs coverage.…”

“…To our knowledge, the possible role of photon absorption in the electron capture process has not hitherto been studied, whereas Blahins et al have recently used a laser to study photodetachment with a cesium sputter source. 18…”

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

“…Finally, it could be argued that photon absorption might increase the negative ion yield by allowing photoelectrons to overcome the surface potential barrier and become bound to the affinity level of the ejected atom, thus supplementing the described electron capture by tunneling. To our knowledge the possible role of photon absorption in the electron capture process has not hitherto been studied whereas Blahins et al have recently used a laser to study photodetachment with a cesium sputter source 17 .…”