Speaker
Description
It has been recently proposed that the production of negative ions with cesium sputter ion sources could be enhanced by laser-assisted resonant ion pair production [1]. We have tested this hypothesis by measuring the effect of pulsed diode lasers at various wavelengths on the O$^-$ beam current produced from Al$_2$O$_3$ cathode of a cesium sputter ion source [2]. Here we summarize the previously reported experimental results demonstrating enhancement of the extracted beam current owing to the laser exposure and amend them with data on photo-assisted Al$^-$ production. The experimental results provide evidence for the existence of a wavelength-dependent photo-assisted enhancement of negative ion currents but cast doubt on its alleged resonant nature as the effect is observed for both O$^-$ and Al$^-$ ions at laser energies above a certain threshold. 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. It is shown that the ions produced by the laser exposure originate from slightly different potential than the surface produced ions, which allows us to constrain the underlying physical mechanisms. It is concluded 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 cesium sputter ion source. We describe experiments designed to unambiguously confirm or dispute the relevance of the ion pair production for negative ion production. Finally, the possibility of ion pair production explaining the beneficial effect of xenon admixture on the negative ion yield of an RF-driven H$^-$ ion source [3] is discussed.
[1] J. S. Vogel, Nucl. Instrum. Methods Phys. Res. B 438, (2019), pp 89-95.
[2] O. Tarvainen et al., submitted to J. App. Phys. (2020); arXiv:2007.00521
[3] T. Kalvas, S.K. Hahto, J.H. Vainionpää, K.N. Leung, S.B. Wilde and P. Mandrillon, AIP Conf. Proc. 925, 136 (2007).
