Energy efficiency of laser produced C- and T-ion sources

Abstract

The conversion efficiency of laser energy into kinetic ion energy in a laser-produced plasma has been investigated for two quite different targets: graphite and tantalum. The laser energy (intensity) varied from several mJ to 200 ml (I O 9 to 7 x 10" W cm-') which is appropriate to many applications of a lwr produced ion source. The conversion efficiency as a function of the laser energy was directly determined by differential measurements of the charge. kinetic energy and angular emission distribution of the plasma ions in absolute units. Whilst for the Ta target a nearly constant efficiency of about 30% was observed, the graphite result shows an unexpectedly strong enhancement of the transfer efficiency of up to 80% in the laser intensity range around 1.5 x loio Wm-'. It is assumed that the results are related to the difference in the surface roughness of the targets.

Angular emission distributions of neutrals and ions in laser ablated particle beams

“…In these models, the particle cloud is always treated as a neutral gas. Therefore, coulomb interactions or the coupling of the electrons to the laser field are not considered, although the ions essentially contribute to the total energy in the plasma cloud (Rupp and Rohr 1991).…”

The mass dependence of the jet formation in laser-produced particle beams

“…A major argument for taking into account explicitly the role of the ion component in the energy balance during the expansion-at least for plasmas produced by laser intensities above about 10 9 W cm −2 -arises from the fact that in this case the ions always carry the majority of the total energy. Their energy can amount to up to 90% of the total (Rupp and Rohr 1991) even if their relative number is only of the order of 10% and the degree of ionization is as small as about 1% (Gorbonov andKonov 1989, Mann andRohr 1992). Furthermore, one should be aware that ratios of ionic to neutral particles are measured with particle detectors placed at relatively large distances from the target compared with the focal size (typically 0.1-1 m) and that they by no means represent the distributions in the dense focal interaction range.…”

Study of angular dependences of ion component parameters in CO2-laser-produced plasma

“…The total ion current was measured at 308 cm distance from the target using tripleelectrode grid-less collector, where two electrodes were used for ion beam extraction. The extraction voltage on collector U ext = − (12)(13)(14)(15) kV provided the saturation of the measured current signal. The data of measurements on the ion collector and ion analyser were registered using digital oscilloscope LeCroy 9450 and then post-processed by original software.…”

“…Careful absolute measurements of the charge, energy and angular emission distribution of the plasma particles (neutral and ion component) as a function of the laser energy were done in works [15][16][17][18] with C, Al, Ti, Ni, Mo and Ta targets for Nd laser pulse duration τ = 5, 14 and 20 ns and laser intensity of P = 4 × 10 9 -7 × 10 10 W cm −2 . It was shown that characteristic ion expansion angle decreases with increase of atomic mass of the target element and charge state of ions.…”