The heating of polyethylene pellets with kilojoule, microsecond duration, CO₂laser pulses

Abstract

The energy absorption of 1-1.6 kJ gain-switched CO2 laser pulses focused by an approximately f/22 mirror to a spot of innermost diameter 300 mu m (FWHM) on 0.5 and 1 mm suspended cubes of polyethylene has been measured to be 75+or-13%. For plane targets the absorption was also encouragingly high, 71+or-10%. The relative insensitivity of refraction and other losses to pellet size (vis-a-vis previous short-pulse Nd laser experiments) are fully consistent with the ruby laser shadowgram and interferogram observations made towards the end of the 1.5 mu s CO2 pulse, which reveal dense cold material expanding almost randomly at velocities of approximately 0.5 mm mu s-1.

Scaling laws for plasma production and pellet acceleration by laser

“…This degree of conduction around the pellet is consistent with the holographic observation of Ref. [ 15]. The pellet velocities measured by Burgess [17] show an accurate 7/9 (or 0.78) power scaling with the incident laser power for energies ranging between 80 J and 1 kJ.…”

“…In the Appendix arguments are given to show that thermal conduction can support an ablation structure which extends to the side of the pellet away from the incident beam, provided that the beam size is comparable to the pellet size. Strong experimental confirmation of this has been obtained for 0.05-cm polythene cubes irradiated in the 0.07-cm diameter spot of a 10.6-//m laser beam [15]. In this holographic measurement of electron density, the ablation structure extends to an angle of ~ 140° from the incident laser axis.…”

“…We deduce from the holographic evidence in Ref. [15] and the discussion in the Appendix below that the ablation structure may extend outside the laser spot when the beam radius is less than the pellet radius. We conclude that in pellet acceleration schemes (such as that discussed by Felber [9]), this tendency of the ablation structure to surround the pellet due to plasma conductivity may lead to velocities substantially less than those which would be predicted on the basis of a plane-geometry model.…”

“…The laser was incident on a square face of the chip in a 750-/zm diameter spot. Detailed measurements of scattered and reflected light [15] showed that more than 70% of the incident light was absorbed. It was seen from shadowgraphs that the pellet did not break up or expand significantly during the 1 /JLS of irradiation.…”

“…It was mentioned in Section 2 above that the pellet radius could increase substantially over a period of ~ 1 /is, thereby affecting the ablation rate. The explosive expansion of initially solid pellet material following (or during) laser irradiation has been observed in deuterium [2,3], polythene [15,18] and polystyrene [18]. The very high expansion velocities (up to 3 X 10 6 cms" 1 for deuterium [2]) leave only a limited time for ionization by ablation before the pellet becomes underdense.…”