Effects Of Isi On The Interaction Of 1.05 μm Light With High Z Plasmas

EFFECTS OF ISI ON THE INTERACTION OF 1.05 urn LIGHT WITH HIGH Z PLASMASD. R. Kania and P. Bell Lawrence Livermore National LaboratoryandS. P. Obenschain Naval Research LaboratoryHigh z materials can produce large amounts of x-rays in laser plasma interactions. The efficiency of this process depends on the average intensity and wavelength of the laser light.1 We have demonstrated that the detailed intensity distribution of the laser focal spot is also an important factor in determining the x-ray conversion efficiency. An increase in the low energy x- ray yield from gold targets was noted when induced spatial incoherence (ISI)2 was used to provide a smooth spatial laser intensity distribution when compared to a nominal high powered laser focal intensity distribution.The Pharos III laser at the Naval Research Laboratory was used to deliver a 2 ns pulse containing approximately 300 J of 1.05 urn laser light with a bandwidth of 0.15% to flat gold targets. Echelons were used in one arm of the laser to produce ISI and a smooth intensity distribution. The other arm of the laser did not have echelons and produced a non-uniform intensity distribution typical of high power lasers. Both arms operated from a common oscillator and were placed symmetrically with respect to the target normal. The focal diameter of both beams was adjust to yield near identical average intensities in an area containing 90% of the incident laser energy. Experiments were performed at an incident laser intensity of 10' 4W/cm2.Thermal (E 5 keV) x-ray emission was measured with photoconductive detectors (PCD). Experiments using the ISI and non-ISI arms of the laser were performed in sequence without any changes to the diagnostics. The data from the two experiments can be compared directly. Two detector channels were used to measure the soft x-rays. Channel one had a peak sensitivity at 450 eV and channel two peaked at 810 eV. Two channels are insufficient to determine the absolute conversion efficiency, but are sufficient to determine if any significant changes in the x-ray yield had occurred. The hard x-ray yield was monitored with a single filtered PCD.A systematic increase in the x-ray yield from the ISI interactions over the non-ISI interactions was observed. The signal amplitudes in channel one increased by 30 % and the signal levels in channel two increased by 10 % when normalized to the incident laser energy (see figures 1 and 2). The hard x-ray signals exhibited the opposite effect, the signal level decreased by 40 %. The scattered laser light signals also decreased with ISI, but the scattered laser light levels and hard x-ray yields were small enough to indicate that these changes were not the cause of the increase in x-ray yield.Our results are consistent with the measurements of scattered light and hard x-ray production of Obenschain et. al.3 with 0.53 urn light on plastic targets and are opposite their 1.05 urn results. This is a direct result of the increased collisionality of the gold targets for 1.05 urn light over the plastic targets resulting from the higher z of the plasma. As a result the laser light absorption (collisional absorption) profile for the 1.05 um interactions in gold more closely resemble the 0.53 um interactions in plastic targets with less laser light reaching the critical surface to excite instabilities.In conclusion, the intensity distribution of the laser light is important in determining the x-ray conversion efficiency in laser plasma interactions for 1.05 um interactions. This is not a result in the decrease in scattered light (increased absorption) or reduced hard x-ray production. Future experiments will address the nature of this result and examine the laser wavelength dependence.