J. Oh

Acceleration to high velocities and heating by impact using Nike KrF laser

The Nike krypton fluoride laser [S. P. Obenschain, S. E. Bodner, D. Colombant, et al., Phys. Plasmas 3, 2098 (1996)] is used to accelerate planar plastic foils to velocities that for the first time reach 1000 km/s. Collision of the highly accelerated deuterated polystyrene foil with a stationary target produces ∼Gbar shock pressures and results in heating of the foil to thermonuclear temperatures. The impact conditions are diagnosed using DD fusion neutron yield, with ∼106 neutrons produced during the collision. Time-of-flight neutron detectors are used to measure the ion temperature upon impact, which reaches 2–3 keV.

Observed transition from Richtmyer-Meshkov jet formation through feedout oscillations to Rayleigh-Taylor instability in a laser target

Experimental study of hydrodynamic perturbation evolution triggered by a laser-driven shock wave breakout at the free rippled rear surface of a plastic target is reported. At sub-megabar shock pressure, planar jets manifesting the development of the Richtmyer-Meshkov-type instability in a non-accelerated target are observed. As the shock pressure exceeds 1 Mbar, an oscillatory rippled expansion wave is observed, followed by the “feedout” of the rear-surface perturbations to the ablation front and the development of the Rayleigh-Taylor instability, which breaks up the accelerated target.

Laser plasma instability experiments with KrF lasers

Deleterious effects of laser-plasma instability (LPI) may limit the maximum laser irradiation that can be used for inertial confinement fusion. The short wavelength (248nm), 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 (150MW) with a rep-rate system that has a per pulse laser energy well below 1 MJ. Measurements of LPI using the Nike KrF laser are presented at and above intensities needed for the FTF (I∼2×1015W∕cm2). The results to date indicate that LPI is indeed suppressed. With overlapped beam intensity above the planar, single beam intensity threshold for the two-plasmon decay instability, no evidence of instab...

Absolute Hugoniot measurements for CH foams in the 2–9 Mbar range

Absolute Hugoniot measurements for empty plastic foams at ∼10% of solid polystyrene density and supporting rad-hydro simulation results are reported. Planar foam slabs, ∼400 μm thick and ∼500 μm wide, some of which were covered with a 10 μm solid plastic ablator, were directly driven by 4 ns long Nike krypton-fluoride 248 nm wavelength laser pulses that produced strong shock waves in the foam. The shock and mass velocities in our experiments were up to 104 km/s and 84 km/s, respectively, and the shock pressures up to ∼9 Mbar. The motion of the shock and ablation fronts was recorded using side-on monochromatic x-ray imaging radiography. The steadiness of the observed shock and ablation fronts within ∼1% has been verified. The Hugoniot data inferred from our velocity measurements agree with the predictions of the SESAME and CALEOS equation-of-state models near the highest pressure ∼9 Mbar and density compression ratio ∼5. In the lower pressure range 2–5 Mbar, a lower shock density compression is observed than that predicted by the models. Possible causes for this discrepancy are discussed.Absolute Hugoniot measurements for empty plastic foams at ∼10% of solid polystyrene density and supporting rad-hydro simulation results are reported. Planar foam slabs, ∼400 μm thick and ∼500 μm wide, some of which were covered with a 10 μm solid plastic ablator, were directly driven by 4 ns long Nike krypton-fluoride 248 nm wavelength laser pulses that produced strong shock waves in the foam. The shock and mass velocities in our experiments were up to 104 km/s and 84 km/s, respectively, and the shock pressures up to ∼9 Mbar. The motion of the shock and ablation fronts was recorded using side-on monochromatic x-ray imaging radiography. The steadiness of the observed shock and ablation fronts within ∼1% has been verified. The Hugoniot data inferred from our velocity measurements agree with the predictions of the SESAME and CALEOS equation-of-state models near the highest pressure ∼9 Mbar and density compression ratio ∼5. In the lower pressure range 2–5 Mbar, a lower shock density compression is observed th...

Observation of parametric instabilities in the quarter critical density region driven by the Nike KrF laser

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ=248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ=351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns≤τ≤1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for ...