Stephen John Moon

Picosecond resolution soft x-ray laser plasma interferometry

We describe a soft-x-ray laser interferometry technique that allows two-dimensional diagnosis of plasma electron density with picosecond time resolution. It consists of the combination of a robust high-throughput amplitude-division interferometer and a 14.7-nm transient-inversion soft-x-ray laser that produces approximately 5-ps pulses. Because of its picosecond resolution and short-wavelength scalability, this technique has the potential for extending the high inherent precision of soft-x-ray laser interferometry to the study of very dense plasmas of significant fundamental and practical interest, such as those investigated for inertial confinement fusion. Results of its use in the diagnostics of dense large-scale laser-created plasmas are presented.

High planarity x-ray drive for ultrafast shockless-compression experiments

A spatially planar (Δtime∕time∼0.2%) longitudinal stress drive extending over millimeter scale lengths is used to shocklessly compress an aluminum sample to a peak stress of 210GPa over nanosecond time scales. Direct laser irradiation onto the inner wall of an Au halfraum creates an x ray distribution with a near-uniform blackbody temperature of up to 137eV. The x rays ablate material from a low-Z foil in a region of planarity closely matched to the diameter of the halfraum. The resultant ablatively driven shock is converted into a ramp-stress-wave in a secondary aluminum target through unloading across an intermediate vacuum gap. Higher peak stresses and shorter associated risetimes result from increasing input laser energy. Ramp-compression experiments can provide single shot equation-of-state data close to the isentrope, information on the kinetics of phase transformations, and material strength at high pressures.

High resolution soft x-ray spectroscopy of low Z K-shell emission from laser-produced plasmas.

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.

Tabletop Transient Collisional Excitation X-Ray Lasers

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.

Picosecond 14.7 nm interferometry of high intensity laser-produced plasmas

We have developed a compact, 14.7 nm, sub-5 ps X-ray laser source at Lawrence Livermore National Laboratory ~LLNL! together with a Mach-Zehnder type diffraction grating interferometer built at Colorado State University for probing dense, high intensity laser-produced plasmas. The short wavelength and pulse length of the probe reduces refraction, absorption effects within the plasma and minimizes plasma motion blurring. This unique diagnostic capability gives precise two-dimensional ~2D! density profile snapshots and is generating new data for rapidly evolving laser-heated plasmas. A review of the results from dense, mm-scale line focus plasma experiments will be described with detailed comparisons to hydrodynamic simulations.

Taking Thin Diamonds to Their Limit: Coupling Static‐Compression and Laser‐Shock Techniques to Generate Dense Water

Laser‐driven Hugoniot experiments on precompressed samples access thermodynamic conditions unreachable by either static or single‐shock compression techniques alone. Recent experiments using Rutherford Appleton Laboratory’s Vulcan Laser achieved final pressures up to ∼200 GPa and temperatures up to 10,000 K in water samples precompressed to ∼1 GPa, thereby validating this new technique. Diamond anvils, used for sample precompression, must be thin in order to avoid rarefaction catch up; but thin diamonds fail under pressure. Anvils no more than 200 μm thick were encased in diamond cells modified to accommodate the laser‐beam geometry.

Traveling wave-pumping of ultra-short-pulse x-ray lasers

Pumping of proposed inner-shell photo-ionized (ISPI) x-ray lasers places stringent requirements on the optical pump source. We investigate these requirements for an example x-ray laser (XRL) in Carbon lasing on the 2p - 1s transition at 45 angstrom. Competing with this lasing transition is the very fast auger decay rate out of the upper lasing state, such that the x-ray laser would self-terminate on a femto-second time scale. XRL gain may be demonstrated if pump energy is delivered in a time short when compared to the auger rate. The fast self-termination also demands that we sequentially pump the length of the x-ray laser at the group velocity of the x- ray laser. This is the classical traveling wave requirement. It imposes a condition on the pumping source that the phase angle of the pump laser be precisely de-coupled from the pulse front angle. At high light intensities, this must be performed with a vacuum grating delay line. We also include a discussion of issues related to pump energy delivery, i.e. pulse-front curvature, temporal blurring and pulse fidelity. An all- reflective optical system with low aberration is investigated to see if it fulfills the requirements. It is expected that these designs together with new high energy (greater than 1 J) ultra-short pulse (less than 40 fs) pump lasers now under construction, may fulfill our pump energy conditions and produce a tabletop x-ray laser.

Soft X-Ray Laser Interferometry of a Dense Plasma Jet

Soft X-ray laser interferograms were acquired to map the evolution of a dense plasma jet created by the laser irradiation of a solid copper triangular target. The plasma is observed to rapidly expand along the symmetry plane of the target, forming a narrow plasma plume with measured electron densities of up to 1.2times1020 cm-3.

High energy density science with FELs: intense short pulse tunable x-ray sources

Short pulse (< 100 fs) tunable X-ray and VUV laser sources, based on the free electron laser (FEL) concept, will be a watershed for high energy density research in several areas. These new 4th generation light sources will have extremely high fields and short wavelength (~0.1 nm) with peak spectral brightness -photons/(s/mrad2/mm2/0.1% bandwidth- 1010 greater than 3rd generation light sources. We briefly discuss several applications: the creation of warm dense matter (WDM), probing of near solid density plasmas, and laser-plasma spectroscopy of ions in plasmas. The study of dense plasmas has been severely hampered by the fact that laser-based probes that can directly access the matter in this regime have been unavailable and these new 4th generation sources will remove these restrictions. Finally, we present the plans for a user-oriented set of facilities that will incorporate high-energy, intense short-pulse, and x-ray lasers at the first x-ray FEL, the LCLS to be opened at SLAC in 2009.

Improved Energy Coupling into the Gain Region of the Ni-like Pd Transient Collisional X-ray Laser

We present within this paper a series of experiments, which yield new observations to further our understanding of the transient collisional x-ray laser medium. We use the recently developed technique of picosecond x-ray laser interferometry to probe the plasma conditions in which the x-ray laser is generated and propagates. This yields two dimensional electron density maps of the plasma taken at different times relative to the peak of the 600ps plasma-forming beam. In another experimental campaign, the output of the x-ray laser plasma column is imaged with a spherical multilayer mirror onto a CCD camera to give a two-dimensional intensity map of the x-ray laser output. Near-field imaging gives insights into refraction, output intensity and spatial mode structure. Combining these images with the density maps gives an indication of the electron density at which the x-ray laser is being emitted at (yielding insights into the effect of density gradients on beam propagation). Experimental observations coupled with simulations predict that most effective coupling of laser pump energy occurs when the duration of the main heating pulse is comparable to the gain lifetime (~10ps for Ni-like schemes). This can increase the output intensity by more than an order of magnitude relative to the case were the same pumping energy is delivered within a shorter heating pulse duration (< 3ps). We have also conducted an experiment in which the output of the x-ray laser was imaged onto the entrance slit of a high temporal resolution streak camera. This effectively takes a one-dimensional slice of the x-ray laser spatial profile and sweeps it in time. Under some conditions we observe rapid movement of the x-ray laser (~ 3um/ps) towards the target surface.