Spatially dependent photodetachment measurements in a low-pressure high-power modulated volume ion source

Spatially resolved photodetachment measurements have been carried out in a pulsed low-pressure multicusp discharge. Previous H- extraction measurements and photodetachment measurements of the H" density have shown targe increases in the negative ion density in the post discharge of a pulsed plasma. However these increases are shown here to be strongly dependent upon the spatial position of the measurements. The post discharge increases in the negative ion density are explained by the rapid reduction of the fast electron density and the cooling of the electron temperature after switch off. The spatial dependence of the H- density is attributed to the variation of plasma species and temperatures throughout the source due to the magnetic fields of the confining magnets. It is also shown that the rise time of negative ions in the post discharge is strongly dependent upon position from the filaments. In the region close to the filaments the negative ion rise time in the post discharge is 10 to 20 /us but away from this region the rise time reaches values of over 50 μs, which indicates that transport properties of the negative ions play an important role in the temporal behaviour at positions away from the central region where the filaments are located.