Papers by Ronnie Shepherd
Abstract. We present progress in experiments for high efficiency Ne-like and Ni-like ion x-ray la... more Abstract. We present progress in experiments for high efficiency Ne-like and Ni-like ion x-ray lasers using the transient collisional excitation scheme. Experimental results have been obtained on the COMET 15 TW table-top laser system at the Lawrence Livermore National Laboratory (LLNL). The plasma formation, ionization and collisional excitation of the x-ray laser have been optimized using two sequential laser pulses of 600 ps and 1 ps duration with an optional pre-pulse. We have observed high gains up to 55 cm-' in Ne-like and Ni-like ion schemes for various atomic numbers. We report strong output for the 4d -4p line in lower Z Ni-like ion sequence for MO to Y, lasing from -190 A to 240 A, by pumping with less than 5 J energy on target.
Time-Resolved keV Emission Spectra from Hot, Dense Buried Layer K-Shell and L-Shell Targets
Springer Proceedings in Physics
ABSTRACT
Single-shot complete spatiotemporal measurement of terawatt laser pulses
Journal of Optics, 2021
We demonstrate, for the first time, a single-shot, complete spatiotemporal measurement of pulses ... more We demonstrate, for the first time, a single-shot, complete spatiotemporal measurement of pulses from a terawatt-scale, multi-stage-amplified, low repetition-rate laser source. The ultrashort pulse electric field, E(x,y,z,t), is spatiotemporally complex due to distortions that accrue from multiple chirped-pulse amplifiers, which requires a complete characterization. Meanwhile, the instability of the laser source introduces field profiles that vary significantly from pulse to pulse, which, together with the low repetition-rate (15 shots/hour), requires the use of a single-shot measurement technique. To accomplish the measurements, we used a wavelength-multiplexed, digital-holographic technique called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram, specially tailored to measure picosecond pulses at a wavelength of about 1 μm. Specifically, individual pulses from the compact multipulse terawatt laser were measured, with up to 0.3 J per shot of energy and ∼2 ps pulse durations, at 1052 nm. With these measurements, we characterized several major spatiotemporal distortions that affect the peak intensity at the laser focus, as well as the pulse-shape instability on a shot-to-shot basis. Our technique allows detailed diagnosis of laser pulses (especially high-order spatiotemporal distortions) and provides straightforward four-dimensional animations of pulse propagation to a focus.
Physical Review A, 2019
The shifts of the 1s3p 1 P 1 → 1s 2 1 S 0 He-β transition of Cl 15+ were measured in hot (≥ 600 e... more The shifts of the 1s3p 1 P 1 → 1s 2 1 S 0 He-β transition of Cl 15+ were measured in hot (≥ 600 eV), dense (1-2 g/cc) plasmas generated by a 66-151 J short-pulse laser beam at the Orion laser facility. The sub-picosecond laser beam irradiated optically thin, KCl microdot targets buried in layers of plastic. The measured red shifts ranged from 4.8 ± 1.1 eV for unshocked to 5.9 ± 1.2 eV for shocked targets. These values are significantly smaller than recent predictions from a self-consistent field ion-sphere model.
Light absorption and X-ray emission measurements from 100 femtosecond laser produced plasmas
AIP Conference Proceedings, 1994
Total reflected and scattered light measurements are used to define the net absorption for a 100 ... more Total reflected and scattered light measurements are used to define the net absorption for a 100 femtosecond, 400 nm laser pulse’s interaction with a wide range of solid density target materials. Absorption is determined over the intensity range of 1013 to 1018 W/cm2. This range is sufficiently broad to observe the transition in absorption from a low intensity region dominated
IEEE Conference Record - Abstracts. 2005 IEEE International Conference on Plasma Science, 2005
Laboratory experiments that employ Petawatt lasers are rapidly approaching parameter regimes once... more Laboratory experiments that employ Petawatt lasers are rapidly approaching parameter regimes once thought to be the exclusive domain of compact astrophysical objects. In * This work was performed under the auspice of the Department of Energy under Contract No. W-7405-Eng-48.
Physics of Plasmas, 2015
We propose a method for thermal conductivity measurements of high energy density matter based on ... more We propose a method for thermal conductivity measurements of high energy density matter based on differential heating. A temperature gradient is created either by surface heating of one material or at an interface between two materials by different energy deposition. The subsequent heat conduction across the temperature gradient is observed by various time-resolved probing techniques. Conceptual designs of such measurements using laser heating, proton heating, and x-ray heating are presented. The sensitivity of the measurements to thermal conductivity is confirmed by simulations.
Physics of Plasmas, 2015
The efficiency and uniformity of heating induced by hard x-ray free-electron laser pulse is inves... more The efficiency and uniformity of heating induced by hard x-ray free-electron laser pulse is investigated for 0.5 μm silver foils using the X-ray Pump Probe instrument at the Linac Coherent Light Source facility. Intense 8.9 keV x-ray pulses of 60 fs duration deposit energy predominantly via inner-shell ionization to create a non-equilibrium Ag solid density plasma. The x-ray pulses are focused to 14 × 17 μm2 by means of beryllium lenses and by varying the total beam energy, the energy deposition is varied over a range of irradiances from 4.4 to 6.5 × 1015 W/cm2. Two time-and-space resolved interferometers simultaneously probed the expansion of the front and rear sample surfaces and find evidence of a nearly symmetric expansion pointing to the uniformity of energy deposition over the full target thickness. The experimental results are compared with two different hydrodynamic simulations of the sample expansion. The agreement between experimental and theoretical results yields an esti...
X-Ray Laser Thomson Scattering at 21 nm of Laser-Heated High-Density Foil Plasmas
Springer Proceedings in Physics
Results of our preliminary studies for a demonstration of soft X-ray laser Thomson scattering in ... more Results of our preliminary studies for a demonstration of soft X-ray laser Thomson scattering in laser-produced dense plasma are presented. The investigated plasmas are produced by single-side heated foil targets using a 300-ps pulse of 438-nm wavelength at irradiances between 1013 and 1014 Wcm-2. The Nelike zinc X-ray laser, delivering ~ 1 mJ of focused energy at 21.2 nm, is
Simulations of K-shell Photoabsorption in Solid Density Plasmas
Only sparse experimental data currently exists for the equation of state of high-density material... more Only sparse experimental data currently exists for the equation of state of high-density materials heated to temperatures of 10's of eVs. In particular are questions of the ionization balance, line shapes, and spectral shifts of absorption edges and line positions. Using the 100-fs timescale of the laser heating pulse, ultrashort-pulse lasers heat material inside layered targets to 10's of eVs
EPJ Web of Conferences, 2013
We have performed an experiment aimed at measuring self-generated magnetic fields produced in sol... more We have performed an experiment aimed at measuring self-generated magnetic fields produced in solids by high electron currents following high-intensity and high contrast short-pulse laser irradiation. This was done using longitudinal high resolution proton deflectometry. The experiment was performed at the Titan-JLF laser facility with a high-power short-pulse beam (700 fs, ∼ 110 J) split into two beams irradiating two solid targets. One beam is used for the generation of protons and the other beam for the generation of the ultra-high currents of electrons and of the associated magnetic fields. This capability allows us to study the spatio-temporal evolution of the magnetic fields and its dependence on the laser intensity and target material.
Observation of Indirect, Time-Dependent Heating of Solid Targets from Ultraintense, Ultrashort Pulse Laser Light
IEEE Conference Record - Abstracts. 2005 IEEE International Conference on Plasma Science, 2005
Summary form only given. We have observed indirect, picosecond heating of solid targets from high... more Summary form only given. We have observed indirect, picosecond heating of solid targets from high intensity (>1017 W/cm 2), ultrashort pulse (500 fs) laser pulses. The study was performed on foil targets consisting of 1000 Aring Al and 12.5 mum Ti. The targets were illuminated from the aluminum side to eliminate direct laser heating of the Ti foil. Time-resolved measurements
Measurement of the relaxation time of hot electrons in laser-solid interaction at relativistic laser intensities
The authors have measured the relaxation time of hot electrons in short pulse laser-solid interac... more The authors have measured the relaxation time of hot electrons in short pulse laser-solid interactions using a picosecond time-resolved x-ray spectrometer and a time-integrated electron spectrometer. Employing laser intensities of 10¹, 10¹, and 10¹ W/cm², they find increased laser coupling to hot electrons as the laser intensity becomes relativistic and thermalization of hot electrons at timescales on the order of
Single-shot time resolved expansion and emission measurements of proton-heated warm dense matter
2007 Quantum Electronics and Laser Science Conference, 2007
The Sandia pulsed power machine Z produces both gammas and neutrons during Deuterium gas puff sho... more The Sandia pulsed power machine Z produces both gammas and neutrons during Deuterium gas puff shots. Currently, the time history of the gamma and neutron emission is measured during these experiments. Significant interest exists in imaging these neutrons. A prototype pinhole neutron imaging system has been fielded on Z with some success. Plans exist to field an imaging system on the upgraded ZR. Both the design and some results from Z will be presented. This work was performed by the University of California LLNL under the auspices of the DOE under contract W-7405-ENG-48.
Charge and mass resolved time of flight observations of 140 fs laser produced ions
Review of Scientific Instruments, 1995
Ions from this plasma are self-extracting with energies of 0.1 to 100 keV, the extraction potenti... more Ions from this plasma are self-extracting with energies of 0.1 to 100 keV, the extraction potential being one of the desired observables. The charge/mass (z/μ, μ is mass in AMU) separation is provided by a static magnetic field (B⋅L=1280 G cm, FWHM=14 mm) located 5 cm from the linear (1−d) detector. Displacement along the detector axis is thus proportional to z/μ(1/v). The detector is a gold cathode MCP with a fast (sub-ns) phosphor. The phosphor output is coupled into a streak camera (typical sweep 8.5 or 24 μs total) through a coherent fiber bundle. Streak images are grabbed with a 14-bit CCD. The signature of any specific ion is a straight line of slope proportional to z/μ. Since there are usually more than one charge state of a given ion, integer multiple slopes appear. Thus z and m can be found. Absolute calibration is taken from the slope of the proton streak, which is always present with our plasmas. While providing the same information as a Thomson parabola, the straight lin...
Physical Review Letters, 2006
Energy relaxation of the hot electron population generated by relativistic laser pulses in overde... more Energy relaxation of the hot electron population generated by relativistic laser pulses in overdense plasma is analyzed for densities ranging from below to 1000 times solid density. It is predicted that longitudinal beam-plasma instabilities, which dominate energy transfer between hot electrons and plasma at lower densities, are suppressed by collisions beyond solid density. The respective roles of collisional energy transfer modes, i.e., direct collisions, diffusion, and resistive return current heating, are identified with respect to plasma density. The transition between the kinetic and the collisional regimes and scalings of collisional process are demonstrated by a fully integrated one-dimensional collisional particle simulation.
Enhanced hot-electron localization and heating in high-contrast ultraintense laser irradiation of microcone targets
Physical Review E, 2009
High resolution soft x-ray spectroscopy of low Z K-shell emission from laser-produced plasmas
The Review of scientific instruments, 2008
A large radius, R=44.3 m, high resolution grating spectrometer (HRGS) with 2400 lines/mm variable... more A large radius, R=44.3 m, high resolution grating spectrometer (HRGS) with 2400 lines/mm variable line spacing has been designed for laser-produced plasma experiments conducted at the Lawrence Livermore National Laboratory Jupiter Laser Facility. The instrument has been run with a low-noise, charge-coupled device detector to record high signal-to-noise spectra in the 10-50 A wavelength range. The instrument can be run with a 10-20 microm wide slit to achieve the best spectral resolving power, approaching 1000 and similar to crystal spectrometers at 12-20 A, or in slitless operation with a small symmetrical emission source. We describe preliminary spectra emitted from various H-like and He-like low Z ion plasmas heated by 100-500 ps (full width at half maximum), 527 nm wavelength laser pulses. This instrument can be developed as a useful spectroscopy platform relevant to laboratory-based astrophysics as well as high energy density plasma studies.
Journal de Physique IV (Proceedings), 2006
Deposition of maximum laser energy into a small, high-Z enclosure in a short laser pulse creates ... more Deposition of maximum laser energy into a small, high-Z enclosure in a short laser pulse creates a hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technology 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, the Raman backscatter spectrum contains features consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Physical Review Letters 89, 015003 (2002)]. Also, NIF Early Light diagnostics indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization orthogonal to that of the incident light.
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Papers by Ronnie Shepherd