P. S. Datte

Shock timing experiments on the National Ignition Facility: Initial results and comparison with simulation

Capsule implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium (DT) fuel to a sufficiently high areal density (ρR) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the DT fuel on a low adiabat. Initial experiments to measure the strength and relative timing of these shocks have been conducted on NIF in a specially designed surrogate target platform known as the keyhole target. This target geometry and the associated diagnostics are described in detail. The initial data are presented and compared with numerical simulations. As the primary goal of these experiments is to assess and minimize the adiabat in related DT implosions, a methodology is described for quantifying the adiabat from the shock velocity measurements. Results ...

Beam and target alignment at the National Ignition Facility using the Target Alignment Sensor (TAS)

The requirements for beam and target alignment for successful ignition experiments on the National Ignition Facility (NIF) are stringent: the average of beams to the target must be within 25 μm. Beam and target alignment are achieved with the Target Alignment Sensor (TAS). The TAS is a precision optical device that is inserted into target chamber center to facilitate both beam and target alignment. It incorporates two camera views (upper/lower and side) mounted on each of two stage assemblies (jaws) to view and align the target. It also incorporates a large mirror on each of the two assemblies to reflect the alignment beams onto the upper/lower cameras for beam alignment. The TAS is located in the chamber using reference features by viewing it with two external telescope views. The two jaws are adjusted in elevation to match the desired beam and target alignment locations. For some shot setups, a sequence of TAS positions is required to achieve the full setup and alignment. In this paper we describe the TAS, the characterization of the TAS coordinates for beam and target alignment, and summarize pointing shots that demonstrate the accuracy of beam-target alignment.

The preliminary design of the optical Thomson scattering diagnostic for the National Ignition Facility

The National Ignition Facility (NIF) is a 192 laser beam facility designed to support the Stockpile Stewardship, High Energy Density and Inertial Confinement Fusion programs. We report on the preliminary design of an Optical Thomson Scattering (OTS) diagnostic that has the potential to transform the communitys understanding of NIF hohlraum physics by providing first principle, local, time-resolved measurements of under-dense plasma conditions. The system design allows operation with different probe laser wavelengths by manual selection of the appropriate beamsplitter and gratings before the shot. A deep-UV probe beam (λ0 between 185-215 nm) will optimally collect Thomson scattered light from plasma densities of 5 x 1020 electrons/cm3 while a 3ω probe will optimally collect Thomson scattered light from plasma densities of 1 x 1019 electrons/cm3. We report the phase I design of a two phase design strategy. Phase I includes the OTS recording system to measure background levels at NIF and phase II will include the integration of a probe laser.

Simulated performance of the optical Thomson scattering diagnostic designed for the National Ignition Facility

An optical Thomson scattering diagnostic has been designed for the National Ignition Facility to characterize under-dense plasmas. We report on the design of the system and the expected performance for different target configurations. The diagnostic is designed to spatially and temporally resolve the Thomson scattered light from laser driven targets. The diagnostic will collect scattered light from a 50 × 50 × 200 μm volume. The optical design allows operation with different probe laser wavelengths. A deep-UV probe beam (λ0 = 210 nm) will be used to Thomson scatter from electron plasma densities of ∼5 × 1020 cm-3 while a 3ω probe will be used for plasma densities of ∼1 × 1019 cm-3. The diagnostic package contains two spectrometers: the first to resolve Thomson scattering from ion acoustic wave fluctuations and the second to resolve scattering from electron plasma wave fluctuations. Expected signal levels relative to background will be presented for typical target configurations (hohlraums and a planar foil).

Evaluating radiation induced noise effects on pixelated sensors for the National Ignition Facility

The National Ignition Facility (NIF) utilizes several different pixelated sensor technologies for various measurement systems that include alignment cameras, laser energy sensors, and high-speed framing cameras. These systems remain in the facility where they are exposed to 14MeV neutrons during a NIF shot. The image quality of the sensors degrades as a function of radiation-induced damage. This article reports on a figure-of-merit technique that aids in the tracking of the performance of pixelated sensors when exposed to neutron radiation from NIF. The sensor dark current growth can be displayed over time in a 2D visual representation for tracking radiation induced damage. Predictions of increased noise as a function of neutron fluence for future NIF shots allow simulation of reduced performance for each of the individual camera applications. This predicted longevity allows for proper management of the camera systems.

A characterization technique for nanosecond gated CMOS x-ray cameras

We present a characterization technique for nanosecond gated CMOS cameras designed and built by Sandia National Laboratory under their Ultra-Fast X-ray Imager program. The cameras have been used to record images during HED physics experiments at Sandia’s Z Facility and at LLNL’s National Ignition Facility. The behavior of the camera’s fast shutters was not expected to be ideal since they propagate over a large pixel array of 25 mm x 12 mm, which could result in shutter timing skew, variations in the FWHM, and variations in the shutter’s peak response. Consequently, a detailed characterization of the camera at the pixel level was critical for interpreting the images. Assuming the pixel’s photo-response was linear, the shutter profiles for each pixel were simplified to a pair of sigmoid functions using standard non-linear fitting methods to make the subsequent analysis less computationally intensive. A pixel-level characterization of a ”Furi” camera showed frame-to-frame gain variations that could be normalized with a gain mask and significant timing skew at the sensor’s center column that could not be corrected. The shutter profiles for Furi were then convolved with data generated from computational models to forward fit images collected with the camera.

Single Line of Sight CMOS radiation tolerant camera system design overview

This paper covers the preliminary design of a radiation tolerant nanosecond-gated multi-frame CMOS camera system for use in the NIF. Electrical component performance data from 14 MeV neutron and cobalt 60 radiation testing will be discussed. The recent development of nanosecond-gated multi-frame hybrid-CMOS (hCMOS) focal plane arrays by the Ultrafast X-ray Imaging (UXI) group at Sandia National Lab has generated a need for custom camera electronics to operate in the pulsed radiation environment of the NIF target chamber. Design requirements and performance data for the prototype camera system will be discussed. The design and testing approach for the radiation tolerant camera system will be covered along with the evaluation of commercial off the shelf (COTS) electronic component such as FPGAs, voltage regulators, ADCs, DACs, optical transceivers, and other electronic components. Performance changes from radiation exposure on select components will be discussed. Integration considerations for x-ray imaging diagnostics on the NIF will also be covered.

Managing NIF safety equipment in a high neutron and gamma radiation environment.

AbstractThe National Ignition Facility (NIF) is a 192 laser beam facility that supports the Inertial Confinement Fusion program. During the ignition experimental campaign, the NIF is expected to perform shots with varying fusion yield producing 14 MeV neutrons up to 20 MJ or 7.1 × 1018 neutrons per shot and a maximum annual yield of 1,200 MJ. Several infrastructure support systems will be exposed to varying high yield shots over the facility’s 30-y life span. In response to this potential exposure, analysis and testing of several facility safety systems have been conducted. A detailed MCNP (Monte Carlo N-Particle Transport Code) model has been developed for the NIF facility, and it includes most of the major structures inside the Target Bay. The model has been used in the simulation of expected neutron and gamma fluences throughout the Target Bay. Radiation susceptible components were identified and tested to fluences greater than 1013 (n cm−2) for 14 MeV neutrons and &ggr;-ray equivalent. The testing includes component irradiation using a 60Co gamma source and accelerator-based irradiation using 4- and 14- MeV neutron sources. The subsystem implementation in the facility is based on the fluence estimates after shielding and survivability guidelines derived from the dose maps and component tests results. This paper reports on the evaluation and implementation of mitigations for several infrastructure safety support systems, including video, oxygen monitoring, pressure monitors, water sensing systems, and access control interfaces found at the NIF.

Impulse responses of visible phototubes used in National Ignition Facility neutron time of flight diagnostics

Neutron-induced visible scintillation in neutron time of flight (NToF) diagnostics at the National Ignition Facility (NIF) is measured with 40 mm single stage micro-channel plate photomultipliers and a 40 mm vacuum photodiode, outside the neutron line of sight. In NIF experiments with 14 MeV neutron yields above Y > 10 × 1015 these tubes are configured to deliver of order 1 nC of charge in the nominally 5 ns NToF into a 50 Ω load. We have examined a number of 40 mm tubes manufactured by Photek Ltd. of St. Leonards on Sea, UK, to determine possible changes in the instrument impulse response as a function of signal charge delivered in 1 ns. Precision NToF measurements at approximately 20 m require that we characterize changes in the impulse response moments to <40 ps for the first central moment and ∼2% rms for the square root of the second central moment with ∼500 ps value. Detailed results are presented for three different diode configurations.

Gated photocathode design for the P510 electron tube used in the National Ignition Facility (NIF) optical streak cameras

The optical streak cameras currently used at the National Ignition Facility (NIF) implement the P510 electron tube from Photonis1. The existing high voltage electronics provide DC bias voltages to the cathode, slot, and focusing electrodes. The sweep deflection plates are driven by a ramp voltage. This configuration has been very successful for the majority of measurements required at NIF. New experiments require that the photocathode be gated or blanked to reduce the effects of undesirable scattered light competing with low light level experimental data. The required ~2500V gate voltage is applied between the photocathode and the slot electrode in response to an external trigger to allow the electrons to flow. Otherwise the slot electrode is held approximately 100 Volts more negative than the potential of the photocathode, preventing electron flow. This article reviews the implementation and performance of the gating circuit that applies an electronic gate to the photocathode with a nominal 50ns rise and fall time, and a pulse width between 50ns and 2000ns.