Laboratory study of the PDX/PLT laser blow-off trace element injectors

A laboratory study of the beams produced by laser blow-off trace element injectors used on the PDX and PLT tokamaks has been conducted. Eight target materials were studied, in the form of thin films on glass substrates: Al, Si, Sc, Cr, Fe, Ni, Cu, and Mo. Beam detectors included a quadrupole mass spectrometer, gridded retarding field analyzer, and beam collector plates. The collector plates were later analyzed by optical and electron microscopy and Auger spectroscopy. The results show that the injection produces the following species, in the usual order of decreasing velocity: free electrons, plasma, free ions, highly excited neutral atoms, unexcited neutral atoms, and metal clusters having diameters ranging up to several microns. The percentages of the various species, as well as their distributions of velocities, were found to be strong functions of the metal used, the film thickness, and the laser power density. Monoatomic species are generally contained within a half angle of approx.120, while clusters are within approx.50. Models of the production mechanism and implications for the use of the device as a tokamak diagnostic are discussed