Search

A double‐crystal x‐ray spectrometer (DCS) has been used to obtain high‐resolution x‐ray spectra of laser‐produced plasmas. The DCS has a high dispersion, and thus can obtain highly resolved spectra in circumstances where the resolution of a conventional single‐crystal Bragg spectrometer at comparable source‐to‐detector distances would be limited by the source size. In addition the DCS also yields unit‐magnification one‐dimensional spatial resolution in the direction perpendicular to the spectral direction. The instrument produces two spectra symmetrically disposed about a central wavelength, which makes it especially useful for measuring spectral line shifts. Analysis of the spectra from hydrogenic aluminium and helium‐like chlorine indicates a resolving power (λ/Δλ) greater than 2500 and a spatial resolution better than 30 μm.

Plasma electron temperatures from 3 eV to g1 keV have been measured or inferred in ${10}^{18}$ W ${\mathrm{cm}}^{\ensuremath{-}2}$ optical field ionization of helium and neon at electron densities ranging from ${10}^{17}$ to ${10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ by laser pulses of 350 fs duration and 248 nm wavelength. Increase of temperature with gas density is dominated at high densities by a combination of nonlinear inverse bremsstrahlung and stimulated Raman scattering heating which are identified as factors limiting progress to shorter extreme ultraviolet laser wavelength in recombination of optical field ionized plasmas.

We report on recent experiments utilizing 350–400 femtosecond pulses of KrF laser radiation to: i) generate high order odd‐integer harmonics in helium (37th, 67 A) and neon (35th, 71 A) and; ii) demonstrate nonlinear enhancement of xuv line emission in optically ionized nitrogen. The high harmonics are shown to arise from He+, Ne+ and Ne2+ ions which permit higher conversion efficiency in the xuv than is possible from neutrals. The observed density dependence of the xuv emission may imply a low electron temperature in the optically ionized, recombining plasma.

An injector-amplifier architecture for XUV lasers has been developed and demonstrated using the Ge XXIII collisional laser. Results are described for injection into single and double plasma amplifiers. Prismatic lens-like and higher order aberrations in the amplifier are considered. Limitations on ultimate brightness are discussed and also scaling to operation at shorter wavelengths. A preliminary study has been made of UV multiphoton ionization using 300 fs pulses at high intensity.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report results of Thomson scattering measurements of optically ionized gases produced using 12 psec pulses from a KrF-pumped Raman laser at intensities of 3 X 1017 W cm-2. Electron densities and temperatures are determined from the scattered Thomson spectra in both He and Ne (static and gas jet targets) at pressures of 10-3 - 1 bar. The central, low frequency region of the spectra from low pressure shots clearly shows an enhanced intensity above that given by the Salpeter approximation, and is indicative of a high electron-to-ion temperature ratios of at least 10 - 20. These observations are consistent with the long electron-ion collision time relative to the pulse length.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Current studies of optical ionization and heating of gases using KrF laser pulses are discussed. The Thomson scattered spectra reveal the evolution of the electron distribution. The stimulated Raman and Brillouin scattering instabilities are very useful for the study of the pressure dependent plasma heating, in particular the measurement of the electron temperature. (AIP)

Thomson scattering measurements have been used to determine the electron energy distribution in UV optically ionized He and Ne at pressures up to 1/2 bar irradiated at 3×10 17 W/cm 2 in 12 ps. The electrons evolve from a multiphoton ionization spectrum to a thermal distribution with increasing pressure. The low frequency part of the scattered spectrum clearly shows the «zero-damped» ion acoustic mode for high values of ZT e /T i . Experimental and modeling results indicate that nonlinear inverse bremsstrahlung heating and radial conduction cooling dominate the electron energy balance

Experiments are reported on optical ionization of gases produced by 12 ps pulses of focused KrF laser radiation at intensities of 3 X 1017 W/cm2. Simultaneous measurements of Thomson, stimulated Raman and stimulated Brillouin scattering have been used to determine the electron energy distribution and temperature in He and Ne (static and pulsed gas jet targets) at pressures of 10-3 - 1 bar. The electron velocity distribution evolves from a multiphoton ionization spectrum at low pressure to a thermal electron distribution at higher pressure. The ion determined portion of the Thomson scattered spectrum is highly non-thermal for pressures > a few mbar. At high pressure, significant Raman and Brillouin backscattering are observed. The results show that inverse bremsstrahlung absorption dominates heating for p > 10-2 bar except at the highest pressures where SRS may also be important.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Summary form only given. The authors report on studies of optical ionization of gases produced by 12-ps pulses of focused Raman-shifted KrF laser radiation (/spl gamma/=268 nm) at intensities of 3/spl times/10/sup 17/ W/cm/sup 2/. Measurements of Thomson, stimulated Raman, and stimulated Brillouin scattering have been used to determine the electron energy distribution and temperature in He and Ne (static and pulsed gas jet targets) at pressures of 10/sup -3/ to 1 bar and to investigate the role of parametric instabilities in the laser/plasma interaction. The electron velocity distribution evolves from a multiphoton (tunnel) ionization spectrum at low pressure to a thermal electron distribution at moderate pressure. The ion determined portion of the Thomson-scattered spectrum is highly nonthermal for pressures greater than a few mbar. At high pressure, significant Raman and Brillouin backscattering is observed. Inverse bremsstrahlung absorption is shown to dominate heating from p>10/sup -2/ bar except at the highest pressures where stimulated Raman scattering may also be important.

We report high harmonic generation from a 248.6-nm KrF laser giving harmonic orders up to the 37th (67 \AA{}) in a helium gas jet and the 35th (71 \AA{}) in neon, for laser intensities up to 4 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ W/${\mathrm{cm}}^{2}$ in 380-fs pulses. These observations are interpreted using theoretical modeling that identifies the ion species ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{Ne}}^{2+}$ as the sources of the highest harmonics.