Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

A multi-frame shadowgraphy diagnostic has been developed and applied to laser preheat experiments relevant to the Magnetized Liner Inertial Fusion (MagLIF) concept. The diagnostic views the plasma created by laser preheat in MagLIF-relevant gas cells immediately after the laser deposits energy as well as the resulting blast wave evolution later in time. The expansion of the blast wave is modeled with 1D radiation-hydrodynamic simulations that relate the boundary of the blast wave at a given time to the energy deposited into the fuel. This technique is applied to four different preheat protocols that have been used in integrated MagLIF experiments to infer the amount of energy deposited by the laser into the fuel. The results of the integrated MagLIF experiments are compared with those of two-dimensional LASNEX simulations. The best performing shots returned neutron yields ∼40-55% of the simulated predictions for three different preheat protocols.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Here we present details of the design, simulation, and performance of a 100-GW linear transformer driver (LTD) cavity at Sandia National Laboratories. The cavity consists of 20 “bricks.” Each brick is comprised of two 80 nF, 100 kV capacitors connected electrically in series with a custom, 200 kV, three-electrode, field-distortion gas switch. The brick capacitors are bipolar charged to ±100 kV for a total switch voltage of 200 kV. Typical brick circuit parameters are 40 nF capacitance (two 80 nF capacitors in series) and 160 nH inductance. The switch electrodes are fabricated from a WCu alloy and are operated with breathable air. Over the course of 6,556 shots the cavity generated a peak electrical current and power of 1.03 MA (±1.8%) and 106 GW (±3.1%). Experimental results are consistent (to within uncertainties) with circuit simulations for normal operation, and expected failure modes including prefire and late-fire events. New features of this development that are reported here in detail include: (1) 100 ns, 1 MA, 100-GW output from a 2.2 m diameter LTD into a 0.1 Ω load, (2) high-impedance solid charging resistors that are optimized for this application, and (3) evaluation of maintenance-free trigger circuits using capacitive coupling and inductive isolation.

Abstract not provided.

A series of Magnetized Liner Inertial Fusion (MagLIF) experiments have been conducted in order to investigate the mix introduced from various target surfaces during the laser preheat stage. The material mixing was measured spectroscopically for a variety of preheat protocols by employing mid-atomic number surface coatings applied to different regions of the MagLIF target. The data show that the material from the top cushion region of the target can be mixed into the fuel during preheat. For some preheat protocols, our experiments show that the laser-entrance-hole (LEH) foil used to contain the fuel can be transported into the fuel a significant fraction of the stagnation length and degrade the target performance. Preheat protocols using pulse shapes of a few-ns duration result in the observable LEH foil mix both with and without phase-plate beam smoothing. In order to reduce this material mixing, a new capability was developed to allow for a low energy (∼20 J) laser pre-pulse to be delivered early in time (-20 ns) before the main laser pulse (∼1.5 kJ). In experiments, this preheat protocol showed no indications of the LEH foil mix. The experimental results are broadly in agreement with pre-shot two-dimensional HYDRA simulations that helped motivate the development of the early pre-pulse capability.

Abstract not provided.

The Magnetized Liner Inertial Fusion concept (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] is being studied on the Z facility at Sandia National Laboratories. Neutron yields greater than 1012 have been achieved with a drive current in the range of 17-18 MA and pure deuterium fuel [Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)]. We show that 2D simulated yields are about twice the best yields obtained on Z and that a likely cause of this difference is the mix of material into the fuel. Mitigation strategies are presented. Previous numerical studies indicate that much larger yields (10-1000 MJ) should be possible with pulsed power machines producing larger drive currents (45-60 MA) than can be produced by the Z machine [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. To test the accuracy of these 2D simulations, we present modifications to MagLIF experiments using the existing Z facility, for which 2D simulations predict a 100-fold enhancement of MagLIF fusion yields and considerable increases in burn temperatures. Experimental verification of these predictions would increase the credibility of predictions at higher drive currents.

Abstract not provided.

X-ray diffraction measurements to characterize phase transitions of dynamically compressed high-Z matter at Mbar pressures require both sufficient photon energy and fluence to create data with high fidelity in a single shot. Large-scale laser systems can be used to generate x-ray sources above 10 keV utilizing line radiation of mid-Z elements. However, the laser-to-x-ray energy conversion efficiency at these energies is low, and thermal x-rays or hot electrons result in unwanted background. We employ polycapillary x-ray lenses in powder x-ray diffraction measurements using solid target x-ray emission from either the Z-Beamlet long-pulse or the Z-Petawatt (ZPW) short-pulse laser systems at Sandia National Laboratories. Polycapillary lenses allow for a 100-fold fluence increase compared to a conventional pinhole aperture while simultaneously reducing the background significantly. This enables diffraction measurements up to 16 keV at the few-photon signal level as well as diffraction experiments with ZPW at full intensity.

Amplification of the transverse scattered component of stimulated Brillouin scattering (SBS) can contribute to optical damage in the large aperture optics of multi-kJ lasers. Because increased laser bandwidth from optical phase modulation (PM) can suppress SBS, high energy laser amplifiers are injected with PM light. Phase modulation distributes the single-frequency spectrum of a master oscillator laser among individual PM sidebands, so a sufficiently high modulation index β can maintain the fluence for all spectral components below the SBS threshold. To avoid injection of single frequency light in the event of a PM failure, a high-speed PM failsafe system (PMFS) must be employed. Because PM is easily converted to AM, essentially all PM failsafes detect AM, with the one described here employing a novel configuration where optical heterodyne detection converts PM to AM, followed by passive AM power detection. Although the PMFS is currently configured for continuous monitoring, it can also detect PM for pulse durations ≥2 ns and could be modified to accommodate shorter pulses. This PMFS was deployed on the Z-Beamlet Laser (ZBL) at Sandia National Laboratories, as required by an energy upgrade to support programs at Sandia's Z Facility such as magnetized liner inertial fusion. Depending on the origin of a PM failure, the PMFS responds in as little as 7 ns. In the event of an instantaneous failure during initiation of a laser shot, this response time translates to a 30-50 ns margin of safety by blocking a pulse from leaving ZBL's regenerative amplifier, which prevents injection of single frequency light into the main amplification chain. The performance of the PMFS, without the need for operator interaction, conforms to the principles of engineered safety.

Abstract not provided.