Continuing work in the design of shock ignition targets is described. Because of reduced implosion velocity requirements, low target adiabats, and efficient drive by short wavelength lasers, these targets produce high gain (> 100) at laser energies well below 1 megajoule. Effects of hydrodynamic instabilities like Rayleigh-Taylor or Richtmyer-Meshkov are greatly reduced in these low-aspect ratio targets. Of particular interest is the optimum ratio of ignitor to compression pulse energy. A simple pellet model and simulation-derived coupling coefficients are used to analyze optimal fuel assembly, and determine that shock ignition allows enough control to create theoretically optimum assemblies. The effects on target design due to constraints on the compression and ignitor pulse intensities are also considered and addressed. Significant sensitivity is observed from low-mode perturbations because of large convergence ratios, but a more powerful ignitor can mitigate this.

Articles you may be interested inExcitation temperature of a solution plasma during nanoparticle synthesis J. Appl. Phys. 116, 083301 (2014); 10.1063/1.4894156Fast electron propagation in Ti foils irradiated with sub-picosecond laser pulses at I λ 2 > 10 18 Wcm − 2 μ m 2High-resolution vacuum ultraviolet ͑VUV͒ spectra were recorded from plasmas generated by the NIKE KrF laser for the purpose of observing emission from the two-plasmon decay instability ͑TPDI͒ at 2 / 3 the NIKE wavelength ͑165 nm͒. The targets were irradiated by up to 43 overlapping beams with intensity up to Ϸ10 14 W/cm 2 and with beam smoothing by induced spatial incoherence ͑ISI͒. The targets consisted of planar foils of CH, BN, Al, Si, S, Ti, Pd, and Au. Titanium-doped silica aerogels in Pyrex cylinders were also irradiated. The spectra of the target elements were observed from charge states ranging from the neutral atoms to five times ionized. The spectrometer was absolutely calibrated using synchrotron radiation, and absolute VUV plasma emission intensities were determined. Emission from the TPDI at 165-nm wavelength was not observed from any of the irradiated targets. An upper bound on the possible TPDI emission was less than 4 ϫ 10 −8 the incident NIKE laser energy. The NIKE laser radiation backscattered from the silica aerogel targets at 248 nm was typically 6 ϫ 10 −6 the incident NIKE laser energy, and the spectral broadening corresponded to the 1-THz bandwidth of the ISI smoothing. The spectra from the moderately charged plasma ions ͑up to five times ionized͒, spectral linewidths, absolute continuum emission level, and slope of the continuum were consistent with plasma temperatures in the 100-300-eV range.

An imaging spectrometer was designed and fabricated for recording far ultraviolet spectra from laser-produced plasmas with wavelengths as short as 155 nm. The spectrometer implements a Cassegrain telescope and two gratings in a tandem Wadsworth optical configuration that provides diffraction limited resolution. Spectral images were recorded from plasmas produced by the irradiation of various target materials by intense KrF laser radiation with 248 nm wavelength. Two pairs of high-resolution gratings can be selected for the coverage of two wavebands, one grating pair with 1800 grooves/mm and covering approximately 155-175 nm and another grating pair with 1200 grooves/mm covering 230-260 nm. The latter waveband includes the 248 nm KrF laser wavelength, and the former waveband includes the wavelength of the two-plasmon decay instability at 23 the KrF laser wavelength (165 nm). The detection media consist of a complementary metal oxide semiconductor imager, photostimulable phosphor image plates, and a linear array of 1 mm(2) square silicon photodiodes with 0.4 ns rise time. The telescope mirrors, spectrometer gratings, and 1 mm(2) photodiode were calibrated using synchrotron radiation, and this enables the measurement of the absolute emission from the laser-produced plasmas with temporal, spatial, and spectral resolutions. The spectrometer is capable of measuring absolute spectral emissions at 165 nm wavelength as small as 5x10(-7) J/nm from a plasma source area of 0.37 mm(2) and with 0.4 ns time resolution.

A doppler velocimeter utilizing a spectrograph and a short-pulse laser probe is described which provides good spatial (20 μm) and subnanosecond temporal resolution. This system has been used to measure the velocity profiles of targets ablatively accelerated to very high velocities by a high-power laser beam. In comparison to systems employing interferometric techniques, this velocimeter significantly relaxes requirements on the target surface being examined and the time resolution needed to measure velocities of rapidly accelerating surfaces.