G. D. Enright

Harmonic generation in CO2 laser target interaction

We report the observation of an extended series of integral harmonic lines in the spectrum of direct backscatter of 10.6‐μm radiation incident at intensities ≳1014 W/cm2 onto planar solid targets. We have observed and spectrally resolved up to the eleventh harmonic (0.95 μm) at intensities well above the plasma continuum background.

Harmonic generation in CO/sub 2/ laser target interaction

We report the observation of an extended series of integral harmonic lines in the spectrum of direct backscatter of 10.6‐μm radiation incident at intensities ≳1014 W/cm2 onto planar solid targets. We have observed and spectrally resolved up to the eleventh harmonic (0.95 μm) at intensities well above the plasma continuum background.

Stimulated Brillouin scattering in picosecond time scales: Experiments and modeling

This paper presents an experimental and theoretical study of stimulated Brillouin scattering (SBS) in laser produced plasma using a laser pump with a duration of 8–10 psec. The experiments were performed in a preformed plasma to minimize the flow velocity and have the same plasma conditions over a large range of laser intensities. The reflectivity was then compared to theoretical results over an intensity range of 1013–2×1015 W/cm2. A short pulse was used so that the SBS was in the temporally growing regime and saturation was not an issue.

Backscatter and beam refraction in underdense gas breakdown with nanosecond CO2 laser

A 50‐J 2‐ns FWHM CO2 laser pulse focused to intensities in excess of 1014 W/cm2 has been used to induce breakdown in underdense He gas. The laser radiation scattered from these plasmas has been temporally and spectrally analyzed. It is observed that ion wave scattering can result in reflectivities in excess of 50% of the incident intensity. At filling pressures above 30 Torr the incident laser radiation is explosively refracted around the focal region by plasma formed a few mm upstream from focus thus quenching the high‐intensity laser plasma interaction less than 1 ns subsequent to breakdown. A weak but well‐defined line at 7.06 μm (ω=3/2 ω0) was observed in the backscatter at filling pressures above 40 Torr.

Laser plasma sources for proximity printing or projection x‐ray lithography

In this article, we present a theoretical and experimental study of the x‐ray emission produced by laser plasma sources in different spectral ranges appropriate for x‐ray lithography either in proximity printing (XRL) or projection (XRPL) approaches. For XRPL application, experiments using 10 ns laser pulses show that the maximum conversion efficiency in the (80–250 eV) range is attained at I=1011 W cm−2 whereas for 25 ns pulses and I≤6×1011 W cm−2, it is still increasing with laser intensity. On the other hand, higher laser intensities are required to obtain a high conversion efficiency for XRL (0.9–1.4 keV). Efficient emission peaked at 1.1 keV can be achieved for I≥1013 W cm−2 with copper targets and pulse duration shorter than 5 ns. For iron line emission (peak at 0.9 keV), the laser intensity can be lower (I=5×1012 W cm−2) and the pulse duration longer (τp=10 ns). Finally, we discuss the different approaches which may lead to the appropriate laser design.

Novel laser line focus geometry applied to X-ray lasers

Abstract Producing a long narrow plasma column for X-ray laser studies requires focussing a laser beam into a line focus. Uniformity of intensity along the line is an important consideration. An array of wedges was used in conjunction with a cylindrical lens to provide averaging over many segments of the laser beam, resulting in a more uniform intensity distribution along the line focus. X-ray and infrared images of the resulting line plasmas are shown, to demonstrate the smoothing capability of the wedge array.

Quantitative measurements of fast ions from CO2 laser‐produced plasmas

The asymptotic behavior of the fast‐ion energy spectra has been studied by using a Thomson parabola ion spectrometer to analyze ionic species produced by a 10.6‐μm laser‐produced polyethylene plasma. The use of cellulose nitrate film detectors has yielded for the first time quantitative information about the charge state and energy distribution of these high‐energy ions as a function of laser energy and beam polarization.

X‐ray emission characteristics of plasmas created by a high‐intensity CO2 laser

The spectrum of the x‐ray continuum and line emission emanating from Al, Mg, and (CH2)n plasmas created with a nanosecond CO2 laser pulse has been investigated at irradiance levels up to 2×1014 W/cm2.

Superthermal x‐ray emission from CO2‐laser‐produced plasmas

The high‐energy continuum x‐ray emission from plasma created by intense (1014 W cm−2) nanosecond 10‐μm laser pulses has been characterized. The temperature of the superthermal electron component deduced from this emission was found to be strongly dependent on focus position, while displaying a weaker dependence on irradiation angle, beam polarization, and target composition. The variation of the hot‐electron temperature as a function of Iλ2 has been examined in detail in a range of Iλ2 from 1015 to 2×1016 W μm2 cm−2 for various target materials and has been found to be in qualitative agreement with the predictions of current theories of hot‐electron production based on resonance absorption.

X-ray laser gain measurements in a collisionally excited germanium plasma

We have used a novel beam-averaging technique that makes use of a segmented wedge array (SWA) to overlap several line foci and to produce a much more uniform linear plasma than has been obtained to date. With this focusing arrangement, laser gain was observed for the first time to our knowledge on a 2s2p63d–2s2p63p, J = 2–1 Ne-like transition at 198.99 A. The experiments were carried out on the LP2 Nd:glass laser system operating at 1.06 μm with incident energies of ~125 J/cm of line focus. We also observed gain on five 3p–3s transitions, using both the SWA and crossed cylindrical lenses to produce the line focus.