Gallium Nitride (GaN) is a wide-bandgap semiconductor having excellent radiation properties. GaN crystal is ionic-covalent with significant iconicity resulting in stronger molecular bond strength, which in in turn leads to excellent radiation hardness. Further, GaN has ultrafast carrier relaxation time. GaN transistors are promising for high-frequency applications due to their large bandgap (3.9eV) and higher breakdown field (<5MV/cm). These exceptional characteristics make GaN suitable to operate in high radiation flux environment such as fusion plasma facilities, for ultrafast detection. The expected detector temporal response is faster than 0.01-1 ns.
We have been systematically testing neutron radiation effects in GaN devices and materials at Los Alamos Neutron Science Center (LANSCE) with ever increased neutron fluence levels, and at National Ignition Facility (NIF) high foot, high yield shots. In 2013 LANSCE run cycle, we tested GaN UV LED devices at 3.1E11 neutrons/cm^2. In 2015-2016 LANSCE run cycles, we have been operating three neutron beam lines with fluence level 1.2E11, 1.5E13, and 1E15 neutrons/cm^2. The irradiated samples include GaN UV LEDs, GaN HEMTs, and GaN substrates. In the experiments up to 2015 run cycle, we have characterized electrical and optical performances of GaN device before and after neutron irradiation, including the device IV curve measurements monitored at over the three months neutron irradiation time, and device IV curve measurements before and after NIF high yield shot irradiation. We observed no substantial degradation. These experiments firmly established GaN devices as the radiation hard platform of the next generation fusion plasma diagnostic instruments.
This paper will review the specifications, test and experiment performance features of Bechtel Nevada's Phase 2 X-ray Streak Camera (P2XSC). The P2XSC was developed to meet stringent inertial confinement fusion (ICF) and high energy density (HED) science requirements for experiments at Omega laser facility at Laboratory for Laser Energetics (LLE), and National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). The paper reports recent progress in developing a large format, high dynamic range, and high-reliability Xray streak camera at Bechtel Nevada. We have designed, built, and tested an advanced X-ray camera. Bechtel Nevada's P2XSC substantially outperforms first generation streak cameras developed over a decade ago. Recent laboratory tests of P2XSC show that the channel dynamic range reaches 6000, the resolution reaches 50 micrometers at the photocathode (6~7 pixels at the image plane) at deep ultraviolet (UV) input wavelength, and 35 micrometers (4~5 pixels) at X-ray wavelength. The image resolution varies less than 30% across the photocathode. However, the 50 mm photocathode has a usable length of approximately 34 mm due to charge coupled device (CCD) camera limitations. The total number of resolution elements is approximately 900 in both spatial and temporal directions. The P2XSC is integrated into a compact airbox enclosure compatible with the ten-inch manipulator (TIM) specifications at LLE, Omega. The system is remotely controllable. The P2XSC system has been operated in the airbox for several thousands of shots for tests at Bechtel Nevada calibration facilities in Livermore and at the LLNL Janus laser facility. High-resolution data will be shown.
This paper discusses the use of a reference streak camera (RSC) to diagnose laser performance and guide modifications to remove high frequency noise from Bechtel Nevada’s long-pulse laser’s output. The upgraded laser used now exhibits less than 0.1% high frequency noise in cumulative spectra, exceeding NIF calibration specifications.
ICF experiments require full characterization of streak cameras over a wide range of sweep speeds (10 ns to 480 ns). This paradigm of metrology poses stringent spectral requirements on the laser source for streak camera calibration. Recently, Bechtel Nevada worked with a laser vendor to develop a high performance, multi-wavelength Nd:YAG laser to meet NIF calibration requirements. For a typical NIF streak camera with a 4096x4096 pixel CCD, flat field calibration at 30 ns requires a smooth laser spectrum over 33 MHz to 68 GHz. Streak cameras are the appropriate instrumentation for measuring laser amplitude noise at very high frequencies since the upper end spectral content is beyond the frequency response of typical optoelectronic detectors for a single shot pulse.
The SC was used to measure a similar laser at its second harmonic wavelength (532 nm) establishing baseline spectra for testing signal analysis algorithms. The RSC was then used to measure the custom calibration laser. In both spatial-temporal measurements and cumulative spectra, 6~8 GHz oscillations were identified. The oscillation was diagnosed as inter-surface reflections between amplifiers. In addition, RSC spectral data changes were found due to temperature instabilities in the seeding laser. Upgrades were made on the findings and high frequency noises were removed from the laser output.
A streak camera is a recording instrument in which spatial image is swept in time, creating a spatial-temporal image on a charge-coupled device (CCD). Traditional analysis for captured image data has been using uniform grid as sampling points, in which a block of CCD pixel readouts are summed to give one reading. Equivalently simple area moving averages are applied concurrently while sampling, and high frequency content is reduced. To solve this problem, we use peak-value sampling procedure, based on the view from photoelectron statistics. After background correction, maximum values in spatial dimensions are selected to obtain time series data. A DSP filter is then applied and optimized for this time series. A Welch algorithm fast Fourier transform is applied to obtain power spectra. Segmented cumulative spectra is then calculated for global statistics and related to time domain fluctuations. Self similarity at different sweeping time-scales is used to recognize CCD pattern noise. Sinusoidal pattern noise is automatically corrected by peak-value sampling. Computational results show that time-frequency analysis using peak-value sampling algorithm and similar variants is far more effective in discovering high frequency oscillatory noise than traditional uniform binned sampling. We have applied this algorithm to analyze data produced by a 4096x4096 CCD streak camera illuminated with a macro pulse laser.
High frequency oscillations in 6~10 GHz region were found in laser spectra. Spatial-temporal oscillations of this range are difficult to diagnose with conventional optoelectronic detectors on a per-shot basis. This work has led to improvement of laser design.
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