Laser plasma instability experiments with KrF lasers

Abstract

Deleterious effects of laser-plasma instability (LPI) may limit the maximum laser irradiation that can be used for inertial confinement fusion. The short wavelength (248 nm), large bandwidth, and very uniform illumination available with krypton-fluoride (KrF) lasers should increase the maximum usable intensity by suppressing LPI. The concomitant increase in ablation pressure would allow implosion of low aspect ratio pellets to ignition with substantial gain (>20) at much reduced laser energy. The proposed KrF laser based Fusion Test Facility (FTF) would exploit this strategy to achieve significant fusion power (150 MW) with a rep-rate system that has a per pulse laser energy well below 1 megajoule.

Shock ignition target design for inertial fusion energy

“…When the intensity increases past this limit, the instabilities show nonlinear behavior and are not well described by existing theory. Results from recent LPI experiments on the Nike KrF laser (0.248µm wavelength) are consistent with an intensity limit of about a factor of two higher 8,9 , and further experiments are planned. Whatever the limits, the compression intensity determines the pressure that drives the pellet, and ultimately the hydrodynamic instability growth and yield that can be achieved.…”

“…This limit will be determined by the plasma conditions (primarily, temperature and density scalelength) and laser wavelength. Although the exact limit is currently unknown, current measurements suggest that it is less than 10 15 W /cm 2 for 0.35 µm wavelength light [5][6][7] , and a factor of about two higher (1.5 − 1.7 × 10 15 W /cm 2 ) for 0.248 µm wavelength light 9 . The primary LPI concern in direct-drive is the twoplasmon decay instability (TPD), because the threshold is low and it can generate electrons hot enough to penetrate and preheat the fuel.…”

Collective stimulated Brillouin backscatter

“…If we additionally assume that T c ≫ L speckle /c then CBSBS threshold does not depend on T c . Such T c is accessible to KrF lasers [6], T c ≃ 0.7ps, but not for NIF glass lasers with beam smoothing up to 3 Å at 1ω, implying T c ≃ 4ps at 3ω. This is consistent with the numerical simulations which show that BSBS threshold in NIF emulation experiments is dominated by speckles [2,8].…”

Observation of Strong Oscillations of Areal Mass in an Unsupported Shock Wave

“…Detailed experimental study of the predicted strong oscillations has become possible recently, when the capability to produce strong short pulses (FWHM 350-500 ps, peak intensity up to 3.3×10 14 W/cm 2 in a Ø750 μm focal spot with a Ø400 μm flat top) was added to the main drive beams of the Nike laser [26]. The monochromatic x-ray imaging system on Nike is based on Bragg reflection from spherically curved crystals [8,10,16,27].…”