Search

We report on experimental measurements of how an externally imposed magnetic field affects plasma heating by kJ-class, nanosecond laser pulses. The experiments reported here took place in gas cells analogous to magnetized liner inertial fusion targets. We observed significant changes in laser propagation and energy deposition scale lengths when a 12T external magnetic field was imposed in the gas cell. We find evidence that the axial magnetic field reduces radial electron thermal transport, narrows the width of the heated plasma, and increases the axial plasma length. Reduced thermal conductivity increases radial thermal gradients. This enhances radial hydrodynamic expansion and subsequent thermal self-focusing. Our experiments and supporting 3D simulations in helium demonstrate that magnetization leads to higher thermal gradients, higher peak temperatures, more rapid blast wave development, and beam focusing with an applied field of 12T.

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.

Streak Cameras are an essential diagnostic tool used in shock physics and high energy density physics experiments. Such experiments require well calibrated temporally resolved diagnostics for studying events that occur in the nanosecond to microsecond time scales. Although streak cameras are among the most common detectors used within the high energy density physics community, they require frequent calibration and typically lack reproducibility in the fine detail. A solid state device with similar temporal performance characteristics could provide several advantages to current streak camera systems by utilizing discrete spatial resolution set by the sensor diodes. National Security Technologies (NSTec) has built a multi-channel solid state streak camera (SSSC) prototype, in collaboration with Sandia National Laboratories, as part of an ongoing project to develop the technology to a level competitive with analog streak cameras. The device concept and results from electronic testing of our first prototypes will be discussed in this manuscript. These measurements will be used as a base for future SSSC development projects.

Streak Cameras are an essential diagnostic tool used in shock physics and high energy density physics experiments. Such experiments require well calibrated temporally resolving diagnostics for studying events that occur on the nanosecond to microsecond time scales. The Nevada National Security Site (NNSS) and Sandia National Laboratories (SNL) have built a 42-channel solid state streak camera (SSSC) prototype as a proof of concept for use as a streak camera replacement. This work is part of an ongoing project to develop the technology to a level competitive with analog streak cameras. The device concept, results from electronic and laser testing, along with recent improvements to increase the device's dynamic range will be discussed in this manuscript.

The Hippogriff camera developed at Sandia National Laboratories as part of the Ultra-Fast X-ray Imager (UXI) program is a high-speed, multi-frame, time-gated imager for use on a wide variety of High Energy Density (HED) physics experiments on both Sandia’s Z-Machine and the National Ignition Facility. The camera is a 1024 x 448 pixel array with 25 μm spatial resolution, containing 2 frames per pixel natively and has achieved 2 ns minimum integration time. It is sensitive to both optical photons as well as soft X-rays up to ~6 keV. The Hippogriff camera is the second generation UXI camera that contains circuitry to trade spatial resolution for additional frames of temporal coverage. The user can reduce the row-wise spatial resolution from the native 25 μm to increase the number of frames in a data set to 4 frames at 50 μm or 8 frames at 100 μm spatial resolution. This feature, along with both optical and X-ray sensitivity, facilitates additional experimental flexibility. Minimum signal is 1500 erms and full well is 1.5 million e-.

Gated x-ray images through the laser entrance hole (LEH) of a hohlraum can provide critical information for ICF experiments at the National Ignition Facility (NIF), such as the size of the LEH vs time, the growth of the gold bubble 1 , and the change in the brightness of inner beam spots due to time-varying cross beam energy transfer 2 . Incorporating a high-speed multi-frame CMOS x-ray imager developed by Sandia National Laboratories 3,4 into the existing Static X-ray Imager (SXI) diagnostic5 at NIF, the new Gated LEH Imager #1 (G-LEH-1) diagnostic is capable of capturing two to four LEH images per shot on its 1024x448 pixel photo detector array, with integration times as low as 2 ns per frame. The design of this diagnostic and its implementation on NIF will be presented.

The Ultra-Fast X-ray Imager (UXI) program is an ongoing effort at Sandia National Laboratories to create high speed, multi-frame, time gated Read Out Integrated Circuits (ROICs), and a corresponding suite of photodetectors to image a wide variety of High Energy Density (HED) physics experiments on both Sandia’s Z-Machine and the National Ignition Facility (NIF). The program is currently fielding a 1024 x 448 prototype camera with 25 μm pixel spatial resolution, 2 frames of in-pixel storage and the possibility of exchanging spatial resolution to achieve 4 or 8 frames of storage. The camera’s minimum integration time is 2 ns. Minimum signal target is 1500 e- rms and full well is 1.5 million e-. The design and initial characterization results will be presented as well as a description of future imagers.

A novel x-ray imager, which takes time-resolved gated images along a single line-of-sight, has been successfully implemented at the National Ignition Facility (NIF). This Gated Laser Entrance Hole diagnostic, G-LEH, incorporates a high-speed multi-frame CMOS x-ray imager developed by Sandia National Laboratories to upgrade the existing Static X-ray Imager diagnostic at NIF. The new diagnostic is capable of capturing two laser-entrance-hole images per shot on its 1024 × 448 pixels photo-detector array, with integration times as short as 1.6 ns per frame. Since its implementation on NIF, the G-LEH diagnostic has successfully acquired images from various experimental campaigns, providing critical new information for understanding the hohlraum performance in inertial confinement fusion (ICF) experiments, such as the size of the laser entrance hole vs. time, the growth of the laser-heated gold plasma bubble, the change in brightness of inner beam spots due to time-varying cross beam energy transfer, and plasma instability growth near the hohlraum wall.