Shock wave transmission and propagation in solid media is studied using fiber optic pressure and velocity probes. Shock waves are generated in two experiments using a high power laser facility as well as conventional explosives. Shock wave properties including peak overpressure, mass velocity, shock duration, impulse, arrival time and shock velocity are characterized using fiber tip interferometric displacement sensors and Fabry-Perot pressure sensors. Measurements are conducted in polymethyl methacrylate and limestone. The probes recorded shock pressures up to 0.1 GPa (1 kbar). Measurements from the fiber optic sensors are shown to be in close agreement with measurements from an electrical sensor based on a Dremin loop.
We have measured the production of hν ⩾ 10 keV x rays from low-density, Ge-doped aerogel targets at the OMEGA laser (Laboratory
for Laser Energetics, University of Rochester). The targets were 1.2mm long by 1.5mm diameter beryllium cylinders filled with Ge-doped (20 atomic percent) SiO2 aerogel. The doped-aerogel density was 4.8 or 6.5 mg/cc. These targets are a major advance over previous doped aerogels: instead of suspending the dopant in the SiO2 matrix, the Ge atoms, with chemistry similar to Si, are incorporated directly in the matrix. Forty beams of the OMEGA laser (λ = 351 nm) illuminated the two cylindrical faces of the target with a total power of approximately 18 TW. The laser interaction strongly ionizes the target (ne/ncr⩽ 0.1-0.2), and allows the laser-bleaching wave to ionize supersonically the high-Z emitter ions in the sample. Ge K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and recorded with temporal resolution with a set of calibrated photoconductive devices (PCDs). The heating of the target was imaged with a gated (60 ps time resolution) x-ray framing camera, filtered to observe > 4 keV. 2-D radiative-hydrodynamic calculations predict rapid and uniform heating over the whole target volume with minimal energy losses into hydrodynamic motion. The calculations predict 150-200 J of x-ray energy output with hν ⩾ 10 keV.
Good agreement between measurements and the calculations is found.
Laser-based plasma spectroscopic techniques have been used with great success to determine the line shapes of atomic transitions in plasmas, study the population kinetics of atomic systems embedded in plasmas, and look at the redistribution of radiation. However, the possibilities for optical lasers end for plasmas with ne > 1022 cm-3 as light propagation is severely altered by the plasma. The construction of the Tesla Test Facility (TTF) at DESY (Deutsche Elektronen-Synchrotron), a short pulse tunable free electron laser in the vacuum-ultraviolet and soft X-ray regime (VUV FEL), based on the SASE (self amplified spontaneous emission) process, will provide a major advance in the capability for dense plasma-related research. This source will provide 1013 photons in a 200 fs duration pulse that is tunable from ~6 nm to 100 nm. Since an VUV FEL will not have the limitation associated with optical lasers the entire field of high density plasmas kinetics in laser produced plasma will then be available to study with the tunable source. Thus, one will be able to use this and other FEL x-ray sources to pump individual transitions creating enhanced population in the excited states that can be easily monitored. We show two case studies illuminating different aspects of plasma spectroscopy.
Starting from FCI2 simulations showing good multi-keV conversion efficiencies of a preformed plasm from thin foils heated by two laser pulses, experiments have been performed with titanium and copper on the Omega laser facility at University of Rochester. The advantages of using this method are efficiencies close to gas targets due to the under-dense plasma created by the pre-pulse and X-ray emissions available at high photon energies that cannot be reached with gas targets. Optimum parameters (laser intensities, delay between the two pulses and thickness of the foil) for titanium and copper foils were estimated from simulations. An increase in the multi-keV conversion efficiency (above 4 keV) by a factor of 2, compared to the case without pre-pulse, is clearly shown on titanium targets. X-ray emission was measured by different diagnostics in good agreement and close to simulations results.
We have measured the production of hν equal to or greater than 4.5 keV x-rays from low-density Ti-doped aerogel targets at the OMEGA laser facility (University of Rochester). The targets were 2.2 mm long by 2 mm diameter beryllium cylinders filled with Ti-doped (3 atomic percent) SiO2 foam. The doped-foam density was ≈ 3 mg/cc. Forty beams of the OMEGA laser (λ = 351 nm) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser interaction fully ionizes the target (formula available in paper), and allows the laser-bleaching wave to excite, supersonically, the high-Z emitter ions in the sample. The heating of the target was imaged with a gated (200 ps time resolution) x-ray framing camera filtered to observe > 4 keV. 2-D radiative-hydrodynamic calculations predict rapid and uniform heating over the whole target volume with minimal energy losses into hydrodynamic motion. An x-ray streak camera, also filtered to observe > 4 keV, was used to measure the rate of heat propagation in the target. Ti K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and also with a set of filtered aluminum x-ray diodes, both instruments provide absolute measurement of the multi-keV x-ray emission. Back-scattered laser energy is observed to be minimal. We find between 100 to 400 J of output with 4.67 equal to or less than hv equal to or less than 5.0 keV, predicted target performance is a factor of 2 - 3 too low in this range.
During recent months we have continued investigations of many different aspects of x-ray lasers to characterize and improve the source and applications. This work has included temporal characterization of existing laser-heated x-ray lasers under a wide range of pumping conditions. We have also looked into more details at different applications of x-ray lasers among which was the interferometry of laser-produced and capillary discharge plasmas in several irradiation conditions for different target Z materials. The reduction of pump energy remains the most important for the generation of new compact x-ray lasers. Numerical studies show that there are some ways to improve several of the key parameters of x-ray lasers specifically repetition rates and efficiency.
In this work we report our numerical modeling results of laser-generated transient inversion and capillary discharge X-ray lasers. In the search for more efficient X-ray lasers we look closely at other approaches in conjunction with experiments at LLNL. In the search for improved X-ray lasers we perform modeling and experimental investigations of low density targets including gas puff targets. We have found the importance of plasma kinetics in transient X-ray lasers by expanding the physical model beyond hydrodynamics approach with Particle In Cell (PIC) and Fokker-Planck codes. The evidence of the Langdon effect was inferred from the recent experimental data obtained with the Ni-like Pd X-ray laser. We continue modeling different kinds of capillary discharge plasma configurations directed toward shorter wavelength X-ray lasers, plasma diagnostics and other applications.
We present experimental results of a high efficiency Ne-like Fe transient collisional excitation x-ray laser using the COMET 15 TW table-top laser system at LLNL. The plasma formation, ionization and collisional excitation of the x- ray laser have been optimized using two sequential laser pulses: a plasma formation beam with 5 J energy of 600 ps duration and a pump beam with 5 J energy of 1.2 ps duration. Since the observation of strong lasing on the 255 angstroms 3p - 3s J equals 0 - 1 transition of Ne-like Fe, we have achieved high gains of 35 cm-1 and saturation of the x-ray laser. A five-stage traveling wave excitation enhances the strongest Fe 3p - 3s 255 angstroms lasing line as well as additional x-ray lines. A careful characterization of the plasma column conditions using L-shell spectroscopy indicates the degree of ionization along the line focus.
Recent transient collisional excitation x-ray laser experiments are reported using the COMET tabletop laser driver at the Lawrence Livermore National Laboratory. Ne- like and Ni-like ion x-ray laser schemes have been investigated with a combination of long 600 ps and short approximately 1 ps high power laser pulses with 5 - 10 J total energy. We show small signal gain saturation for x-ray lasers when a reflection echelon traveling wave geometry is utilized. A gain length product of 18 has been achieved for the Ni-like Pd 4dyields4p J equals 0 - 1 line at 147 angstroms, with an estimated output of approximately 10 (mu) J. Strong lasing on the 119 angstroms Ni-like Sn line has also been observed. To our knowledge this is the first time gain saturation has been achieved on a tabletop laser driven scheme and is the shortest wavelength table-top x-ray laser demonstrated to date. In addition, we present preliminary results of the characterization of the line focus uniformity for a Ne-like ion scheme using L-shell spectroscopy.
Kr-like ions are good candidates for FUV lasing since they can be produced in plasmas quite easily. We present results from a spectroscopic investigation of Y IV emission from a high current density, cold cathode reflex discharge. The Y II to Y V emission is recorded in the 200 - 3000 angstroms range using photometrically calibrated spectrometers, while the emission of trace aluminum ions serves for plasma diagnostics. The intensities of the Y IV 4d - 5p and 5s - 5p transitions strongly increase relative to lines from Y II and Y III with increasing plasma current. The spectra studied here are obtained at a current density of 1.75 A/cm2. Experimental Y IV intensity ratios spanning several excited configurations are compared with collisional radiative predictions of the HULLAC atomic physics package. Good agreement is found for the measured and predicted ratios of 4p55p to 4p55s level populations per statistical weight. Finally, the response of the Kr-like system to a fast, transient excitation pulse is examined using the RADEX code. Large transient gains are predicted for several 5s - 5p transitions in Y IV, Zr V, Nb VI and Mo VII.