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X-ray lasers have been extensively studied around the world since the first laboratory demonstration on the Novette laser at LLNL in 1984. The characteristic properties of short wavelength, high monochromaticity, collimation and coherence make x-ray lasers useful for various applications. These include demonstrations of biological imaging within the water window, interferometry of laser plasmas and radiography of laser-heated surfaces. One of the critical issues has been the high power pump required to produce the inversion. The power scaling as a function of x-ray laser wavelength follows a {approx} {lambda}{sup -4} to {approx} {lambda}{sup -6} law. The shortest x-ray laser wavelength of {approx}35 {angstrom} demonstrated for Ni-like Au was at the limit of Nova laser capabilities. By requiring large, high power lasers such as Nova, the shot rate and total number of shots available have limited the rapid development of x-ray lasers and applications. In fact over the last fifteen years the main thrust has been to develop more efficient, higher repetition rate x-ray lasers that can be readily scaled to shorter wavelengths. The recent state of progress in the field can be found in references. The objective of the project was to develop a soft x-ray laser (XRL) pumped bymore » a short pulse laser of a few joules. In effect to demonstrate a robust, worlung tabletop x-ray laser at LLNL for the first time. The transient collisional scheme as proposed by Shlyaptsev et al. was the candidate x-ray laser for study. The successful endeavor of any scientific investigation is often based upon prudent early decisions and the choice of this scheme was both sound and fruitful. It had been demonstrated very recently for Ne-like Ti at 326 {angstrom} using a small tabletop laser but had not yet reached its full potential. We chose this scheme for several reasons: (a) it was a collisional-type x-ray laser which has been historically the most robust; (b) it had the promise of high efficiency and low energy threshold for lasing; (c) the principal architect of this laser (Dr. Shlyaptsev) was part of the team; (d) the laser driver requirements matched closely to the existing Physics facilities. There were additional important reasons for study. The higher repetition rate and therefore total number of shots available in a smaller facility would allow a more complete characterization of the x-ray laser properties. Optimized plasma irradiation conditions should then lead to the extraction of the maximum x-ray laser energy at the shortest possible wavelength. Also, tabletop XRLs are a relatively new phenomenon and so new transient {approx}1 ps timescale atomic kinetic physics could be studied. In the remainder of this report we give a brief summary of the DRD activities and give concluding remarks with future directions. More details of the work can be found in the publications, proceedings and documents listed in the Appendix at the end of this report.« less

Recent experiments at the Lawrence Livermore National Laboratory using the 10 TW Novette laser demonstrated increased absorption and conversion to X-rays and decreased production of suprathermal electrons with short-wavelength (

Experiments at LLNL using the 10 TW Novette laser have led to significantly increased understanding of laser/plasma coupling. Tests using 1.06 ..mu..m, 0.53 ..mu..m and 0.26 ..mu..m light have shown increased light absorption, increased efficiency of conversion to x-rays, and decreased production of suprathermal electrons as the wavelength of the incident light decreases. The data indicate that stimulated Raman scattering is the source of the excessive hot electrons and that the effect can be controlled by the proper selection of laser frequency and target material. The control of these effects has led to achievement of higher inertial fusion target compressions and to production of the first laboratory x-ray laser.

A laser triggered photoconductive switch based microchannel‐plate (MCP) gating system has been developed. The gating pulse provided to the MCP has a fall time on the order of 200 ps. Use of this system has resulted in increased signal‐to‐noise ratio on three MCP intensified x‐ray spectrometers during recent x‐ray experiments at the Novette laser facility. The x‐ray signals of interest have a duration of approximately 300 ps while background in the same spectral region continues for several nanoseconds. As a result, signal‐to‐noise ratio is maximized by turning the MCP off just after the signals of interest are recorded. The MCP’s are turned on by charging to approximately 1 kV with a slow rise‐time (500 ns) pulse and rapidly gated off by a photoconductive switch connected to the MCP through transmission lines. Staggering the turnoff times of three microstrip lines across the MCP by 250 ps provides some time resolution. Details of the system, including pulse charging system, trigger beam delivery optics, and system diagnostics will be discussed.

This paper describes the computer hardware and software used for acquisition and processing of data from experiments at the NOVETTE laser fusion facility. Nearly two hundred sensors are used to measure the performance of millimeter extent targets irradiated by multi-kilojoule laser pulses. Sensor output is recorded on CAMAC based digitzers, CCD arrays, and film. CAMAC instrument outputs are acquired and collected by a network of LSI-ll microprocessors centrally controlled by a VAX 11/ 780. The user controls the system through menus presented on color video displays equipped with touch panels. The control VAX collects data from all microprocessors and CCD arrays and stores them in a file for transport to a second VAX 11/780 which is used for processing and final analysis. Transfer is done through a high speed fiber-optic link. Relational data bases are used extensively in the processing and archiving of data.

X-ray gain experiments at λ ~ 200A in laser-induced exploding selenium foil targets were successfully demonstrated at LLNL’s Novette laser facility in 1984. Recent experiments at KMS were designed to examine parameter variations of target thickness, laser duration and intensity, and one-sided vs two-sided irradiation, as a means of improving the performance of the gain experiments. Preliminary experiments were also performed on double-foil targets in attempts to prolong the duration and spatial extent of the x-ray lasing conditions in the space between the two foils. Four-frame holographic interferometry was used in determining the time dependence of the evolving density profiles obtained by Abel inversion of the interferometric fringe field and x-ray crystal spectroscopy was applied to monitor the x-ray emission.

Novanet is a control system oriented fiber optic local area network that was designed to meet the unique and often conflicting requirements of the Nova laser control system which will begin operation in 1984. The computers and data acquisition devices that form the distributed control system for a large laser fusion research facility need reliable, high speed communications. Both control/status messages and experimental data must be handled. A subset of NOVANET is currently operating on the two beam Novette laser system.

Data are presented from three different x-ray laser inversion schemes tested on the Novette laser facility at LLNL: collisional excitation of neon-like selenium (λ = 206, 209, 220A); resonant photoexcitation of hydrogen-like fluorine (λ = 81A); and recombination of hydrogen-like magnesium (λ = 130A). The data illustrate the measurement capabilities of a new, time resolved imaging spectrometer (TGSS-EM), and provide insight to the dynamics and emission characteristicts of a new class of laser produced plasmas.

The design and performance of the Novette laser system, which is a two-beam, two-wavelength (1.05 ..mu.. and 0.53 ..mu..) target irradiation facility using phosphate glass laser chains, are discussed with information on the glass properties, controlling factors in the design selection, and projected performance with varying operating conditions. (LCL)

Recent experiments conducted with the multi-terrawatt Novette laser have demonstrated substantial amplification of soft x rays in laser-produced plasmas. Gains at wavelengths of 206 and 209 A in neonlike selenium and at 153 A in neonlike yttrium have been observed.

Three high-temperature, high-density experments were conducted recently with the 10-terawatt, short-wavelength Novette laser system at the Lawrence Livermore National Laboratory. The experiments demonstrated successful solutions to problems that arose during previous laser-plasma interaction experiments with long-wavelength (greater than 1 micrometer) lasers: (i) large-scale plasmas, with dimensions approaching those needed for high-gain inertial fusion targets, were produced in which potentially deleterious laser-plasma instabilities were collisionally damped; (ii) deuterium-tritium fuel was imploded to a density of 20 grams per cubic centimeter and a pressure of 10(10) atmospheres under the improved laser conditions, and compression conditions (preheating and pressure) were consistent with code calculations that predict efficient (high-gain) burn of a large thermonuclear fuel mass when driven with a large, short-wavelength laser; and (iii) soft x-rays were amplified by a factor of 700 by stimulated emission at 206 and 209 angstroms (62 electron volts) from selenium ions in a laser-generated plasma. These small, short-pulse x-ray sources are 10(10) to 10(11) times brighter than the most powerful x-ray generators and synchrotron sources available today. The plasma conditions for these experiments were made possible by advances in Nd:glass laser technology, in techniques to generate efficiently its short-wavelength harmonics at 0.53, 0.35, and 0.26 micrometers, and in diagnostic and computational modeling.

Imaging high-energy x-ray emission from laser fusion targets is important because such images provide views deep inside the target and reveal the features of transport of hot/superhot electrons. These electrons can cause target preheat and are detrimental to efficient target implosion. We report here successful imaging of 20-, 45-, 70-, and 100keV x-ray emissions from the fusion targets at our Novette laser facility.

We present the design and construction of a sensitive pinhole camera for imaging suprathermal x rays. Our device is a pinhole camera consisting of four filtered pinholes and microchannel‐plate electron multiplier for x‐ray detection and signal amplification. We report successful imaging of 20‐, 45‐, 70‐, and 100‐keV x‐ray emissions from the fusion targets at our Novette laser facility. Such imaging reveals features of the transport of hot electrons and provides views deep inside the target.

In this paper a study of stimulated Raman scattering (SRS) from relatively planar plasmas irradiated with short‐wavelength (0.53 μm) laser light is reported. The Novette Laser Facility [Laser Part. Beams 3, 173 (1985)] produced several kilojoules of light in 1 nsec, which allowed it to irradiate a large spot with enough intensity to produce significant Raman scattering. These experiments measured the fluence, angular distribution, spectrum, and timing of the Raman light, as a function of the average laser intensity. Reductions in the Raman fluence at low laser intensity are attributed to collisional damping. The measured SRS fluence was larger than that predicted from convective amplification of bremsstrahlung noise, as calculated using the average properties of the laser beam and the plasma. Possible contributions to the observed scattering from enhanced noise, Raman scattering within filaments, and the absolute Raman instability at density extrema are discussed.