We examine the effects of self-generated magnetic fields in a Kr gas pipe x-ray source platform. X-ray emission from Kr plasma is dependent on the plasma conditions, as the ionization state is largely a function of temperature. Magnetic fields are known to limit heat conduction, which increases temperature. We show that the emission in simulations of the gas pipe x-ray source is dependent on how self-generated magnetic fields are modeled. The inclusion of self-generated magnetic fields in simulations more accurately captures the emission of lower energy x-ray emission (L-shell), bringing results closer to experiments. The modeled x-ray emission and self-generated magnetic fields are shown to be particularly sensitive to the inclusion of the Nernst effect in simulations. Severely limiting the Nernst effect leads to a hotter Kr plasma, which can account for the discrepancy seen in earlier studies. By modifying the Nernst effect multiplier, we can achieve better experimental agreement in x-ray emission from gas pipes; the value of the multiplier that leads to the best agreement is dependent on the laser power of the drive. Currently, the suppression factor of the Nernst effect needed for high power drives (PL>200 TW) is more restrictive than what is currently put forward by non-local models.

We report on record brightness from Ag x-ray emission obtained using a novel laser-produced plasma source. The reported K-shell conversion efficiency of nearly 1% with a radiant energy of X 0 . 6 kJ/sr from Ag ions is the highest presently recorded and is about twofold greater than more conventional metal-lined cavity targets. He-like Ag ions are the dominant radiators at X 22 . 7 keV, which contrast the x-ray sources from other pulse power facilities that produce K-shell x-rays from Auger processes in near-neutral ions driven by the nonthermal hot electrons produced in the Z-pinch implosion. The reported x-ray emission was produced from a 4-mm-diameter, 4-mm-long underdense silver nanowire target at 10 mg/cm3. The National Ignition Facility laser beams deposited X 1000 kJ of 3 omega light into the target with a X 400 TW, 2.5 ns square pulse. Finally, we show via comparisons of data with radiation-hydrodynamics simulations that flux-limited classical thermal energy transport models are inadequate in correctly modeling the behavior of these non-local thermodynamic equilibrium plasmas.

X-ray diffraction is a powerful measurement technique for determining material properties, and it is now possible to perform these experiments at pressures exceeding 1 TPa [Rygg et al., Rev. Sci. Instrum. 91, 043902 (2020)] at the National Ignition Facility (NIF). The x-ray source for these experiments is the quasi-monochromatic Heα emission from metal foils heated to multi-keV temperatures. A critical aspect for understanding the sample's thermodynamic state is the amount of heating caused by absorption of the probe x-rays. In this work, we characterize the performance of the Ge foil x-ray source over more than 60 NIF x-ray diffraction experiments. We use this information to constrain the level of diffraction sample heating from the x-ray source and discuss the impact on the thermodynamic state.

Optical components for laser beams with high peak and averaged powers are being developed worldwide using stimulated plasma scattering that occurs when plasmas interact with intense, coherent light. After decades of pursuit of pulse compressors, mirrors, and other plasma based components that can be created by stimulated scattering from electron density perturbations forming on ultra-short time scales (e.g., via Stimulated Raman Scattering), more recent work has produced optical components on longer time scales allowing ion motion as well [via Stimulated Brillouin Scattering (SBS)]. In the most recent work, ion wave plasma optics have had success in producing pulses of focusable coherent light with high energy and fluence by operating on ns time scales and now promise to enable numerous applications. Experiments have further shown that in some parameter regimes, even simple plasma response models can describe the output of such optics with sufficient accuracy that they can be used as engineering tools to design plasma optics for future applications, as is already being done to control power deposition in fusion targets. In addition, the development of more sophisticated models promises to enable still higher performance from SBS driven plasma optical components under a wider range of conditions. The present status and most promising directions for future development of ion wave plasma optic techniques are discussed here.

Beam combination via an ion wave plasma optic is discussed, including measurement of the power transfer (pump depletion and seed amplification) for several seed pulse durations and total pump energies, with accompanying simulation studies.

High fluence K-shell and L-shell x-ray sources are desired for various high energy density physics experiments. One efficient method for creating such a source is the laser heating of materials that are underdense to laser light. Nano-wire foams are an ideal choice for an underdense material and have average densities of 6-15 mg/cm 3 . The manufacture of robust Cu, Ag and Au nano-wire foams into millimeter scale targets is possible through a technique of freeze casting an aqueous suspension of nano-wires. Cylindrical targets with sizes between 2 to 4 mm have been shot on both the NIF and the Omega laser facilities. For example, x-ray conversion efficiencies (XRCE) from silver nano-wire foams have been measured to be ~1.0% when heated with ~400 TW of 3w laser light in a 2.5 ns square pulse from the NIF laser system. The XRCE from foam targets have been found to be ~2 times that observed in metal lined cylindrical cavity targets and ~5 times that observed in prepulsed metal foils. Experimental results and comparisons with simulations will be presented.

We have developed the Sample Test Array and Recovery (STAR) platform for the National Ignition Facility (NIF) for studying the thermal and hydrodynamic responses of materials in extreme environments. The STAR platform expands the range of obtainable fluences and quadruples the rate that materials experiments can be conducted at the NIF. Example configurations are demonstrated for fluences spanning 0.56-34 J/cm2 with environmental isolation for post-shot material recovery and inspection and up to 1740 J/cm2 without isolation, with surface heating rates of up to 2 × 1014 K/s. An example experiment involving thermally driven shock and spallation of aluminum alloy 7075 is briefly discussed.

FY19 Annual Report 281 R. F. Heeter,1 F. Albert,1 S. J. Ali,1 L. R. Benedetti,1 N. Candeias Lemos,1 H. Chen,1 F. Coppari,1 A. Fernandez Pañella,1 D. E. Fratanduono,1 M. Ginnane,2 M. Gorman,1 M. Hohenberger,1 S. Jiang,1 G. E. Kemp,1 S. F. Khan,1 P. King,1 A. Krygier,1 A. E. Lazicki,1 T. Ma,1 M. J. MacDonald,1 D. Mariscal,1 E. V. Marley,1 M. C. Marshall,1 D. A. Martinez,1 M. A. Millot,1 C. A. J. Palmer,3 H. S. Park,1 J. Park,4 G. Perez Callejo,3 Y. Ping,1 B. B. Pollock,1 P. L. Poole,1 J. Ralph,1 A. M. Saunders,1 G. F. Swadling,1 R. Tommasini,1 K. Widmann,1 A. Zylstra,1 O. L. Landen,1 1 W. W. Hsing,1 and A. S. Wan1

Submitted for the DPP19 Meeting of The American Physical Society Ion Wave Plasma Optic Concepts for NIF and Other Lasers 1 ROBERT KIRKWOOD, P. L. POOLE, T. D. CHAPMAN, S. C. WILKS, P. MICHEL, L. DIVOL, Lawrence Livermore Natl Lab, N. J. FISCH, Princeton, P. NORREYS, Oxford, W. ROZMUS, University of Alberta, J. BUDE, B. E. BLUE, B. M. VAN WONTERGHEM, Lawrence Livermore Natl Lab — Recent demonstrations at NIF of plasma optics produced with ion waves driven by Cross Beam Energy Transfer (CBET) [1,2,3] have motivated work to develop concepts for similar optics to enhance laser performance at NIF and other laser facilities. The success of CBET models based on the linear response of ion waves in plasmas with minimal inverse Bremsstrahlung absorption [3,4], now motivates their use to design new plasma optics to produce beams with high performance in other respects, including: a beam combiner transferring energy to a beam with reduced focal spot size, a short pulse amplifier that transfers the power from many 1 ns beams to a single beam with <0.1 ns duration, and a pulse compressor that uses a plasma combined pump with >40 kJ in 1ns compressed to a duration of 10 to 100 ps in a second stage of interaction in a ̃15 cm plasma. The challenges associated with designing and fielding such optics at NIF and elsewhere, as well as the requirements plasma optics place on new or upgraded facilities will be discussed to identify the most promising concepts. [1] P. Poole in preparation [2] R. K. Kirkwood et al Nat. Phys. 14 , 80 (2018). [3] R. K. Kirkwood et al Phys. of Plas. 25 056701 (2018). [4] A Colaitis et al Physics of Plasmas 25, 033114 (2018) 1Work performed under the auspices of the U.S. DoE by Lawrence Livermore National Lab under Contract DE-AC52-07NA27344 Robert Kirkwood Lawrence Livermore Natl Lab Date submitted: 03 Jul 2019 Electronic form version 1.4

We describe a novel technology based on liquid crystal films for ultrathin targets and plasma mirrors for PW-class laser experiments, on-demand or rep-rated. We describe experiments on ion acceleration, relativistic transparency, and plasma mirror operation.

We present an experimental demonstration of the efficient acceleration of electrons beyond 60 MeV using micro-channel plasma targets. We employed a high-contrast, 2.5 J, 32 fs short pulse laser interacting with a 5 \mu m inner diameter, 300 \mu m long micro-channel plasma target. The micro-channel was aligned to be collinear with the incident laser pulse, confining the majority of the laser energy within the channel. The measured electron spectrum showed a large increase of the cut-off energy and slope temperature when compared to that from a 2 \mu m flat Copper target, with the cutoff energy enhanced by over 2.6 times and the total energy in electrons >5 MeV enhanced by over 10 times. Three-dimensional particle-in-cell simulations confirm efficient direct laser acceleration enabled by the novel structure as the dominant acceleration mechanism for the high energy electrons. The simulations further reveal the guiding effect of the channel that successfully explains preferential acceleration on the laser/channel axis observed in experiments. Finally, systematic simulations provide scalings for the energy and charge of the electron pulses. Our results show that the micro-channel plasma target is a promising electron source for applications such as ion acceleration, Bremsstrahlung X-ray radiation, and THZ generation.

This paper discusses experimental techniques and considerations associated with the transition to high repetition-rate experiments in High Energy Density Physics (HEDP). We particularly highlight approaches to experimentation that become practical only at a threshold of repetition rate. We focus on the transition from operation at several-shots-per-day towards operation in the range of 1/min. to 1 Hz.

We report on the performance of high x-ray fluence Kr K-shell sources that are being developed for high energy density experiments. These targets are 4.1 mm in diameter 4.4 mm tall hollow epoxy tubes having a 40 μm thick wall holding 1.5 atm of Kr gas. For these shots, the National Ignition Facility laser delivered a nominally constant total energy of ≈750 kJ of 351 nm (3ω) light at the three power levels [e.g., ≈120 (low), ≈145 (medium), and ≈210 TW (high)]. The Kr K-shell (Ephoton = 8–20 keV) x-ray radiant intensity and radiant energy (kJ/sr) of these sources were found to increase as a function of laser power but began to plateau at the highest laser power. The Kr K-shell radiant energy increased from ≈1 kJ/sr at ≈120 TW to ≈2 kJ/sr at ≈210 TW. Radiation hydrodynamics simulations predict radiant energies to be always higher than these measurements. The increase in K-shell emission is attributed to its strong dependence on the electron temperature. Electron temperature distributions were inferred from measured Heα and Lyα line emission through the use of a genetic algorithm and Scram modeling. The inferred temperatures from the experiment are 20% to 30% higher than those predicted from modeling.

Pulse-cleaning plasma mirrors are widely employed to improve ultraintense laser contrast, but much of the literature concerning their effect on the reflected pulse is empirical. A simulation study of pulse-cleaning plasma mirrors using the particle-in-cell code large scale plasma is presented. The importance of capturing initial ionization from neutral atoms, collisional effects, and simulation dimensionality is considered. Excellent agreement with experimental data is obtained when a multiphoton ionization model is employed. Furthermore, a series of 2D simulations is shown to accurately replicate both the reflected light intensity and mode obtained from full 3D simulations at significantly reduced computation cost.

We report on the setup and commissioning of a compact recollimating single plasma mirror (PM) for temporal contrast enhancement at the Draco 150 TW laser during laser-proton acceleration experiments. The temporal contrast with and without PM is characterized single-shot by means of self-referenced spectral interferometry with extended time excursion at unprecedented dynamic and temporal range. This allows for the first single-shot measurement of the PM trigger point, which is interesting for the quantitative investigation of the complex pre-plasma formation process at the surface of the target used for proton acceleration. As a demonstration of high contrast laser plasma interaction we present proton acceleration results with ultra-thin liquid crystal targets of ∼ 1 μm down to 10 nm thickness. Focus scans of different target thicknesses show that highest proton energies are reached for the thinnest targets at best focus. This indicates that the contrast enhancement is effective such that the acceleration process is not limited by target pre-expansion induced by laser light preceding the main laser pulse.