R. Costa

Gated x-ray detector for the National Ignition Facility

Two new gated x-ray imaging cameras have recently been designed, constructed, and delivered to the National Ignition Facility in Livermore, CA. These gated x-Ray detectors are each designed to fit within an aluminum airbox with a large capacity cooling plane and are fitted with an array of environmental housekeeping sensors. These instruments are significantly different from earlier generations of gated x-ray images due, in part, to an innovative impedance matching scheme, advanced phosphor screens, pulsed phosphor circuits, precision assembly fixturing, unique system monitoring, and complete remote computer control. Preliminary characterization has shown repeatable uniformity between imaging strips, improved spatial resolution, and no detectable impedance reflections.

Three-Dimensional Hydrodynamic Experiments on the National Ignition Facility

The production of supersonic jets of material via the interaction of a strong shock wave with a spatially localized density perturbation is a common feature of inertial confinement fusion and astrophysics. The behavior of two-dimensional (2D) supersonic jets has previously been investigated in detail [J. M. Foster et. al, Phys. Plasmas 9, 2251 (2002)]. In three-dimensions (3D), however, there are new aspects to the behavior of supersonic jets in compressible media. In this paper, the commissioning activities on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] to enable hydrodynamic experiments will be presented as well as the results from the first series of hydrodynamic experiments. In these experiments, two of the first four beams of NIF are used to drive a 40 Mbar shock wave into millimeter scale aluminum targets backed by 100 mg/cc carbon aerogel foam. The remaining beams are delayed in time and are used to provide a point-projection x-ray backlighter source for diagnosing the three-dimensional structure of the jet evolution resulting from a variety of 2D and 3D features. Comparisons between data and simulations using several codes will be presented.

Implementation of a near backscattering imaging system on the National Ignition Facility

A near backscattering imaging diagnostic system is being implemented on the first quad of beams on the National Ignition Facility. This diagnostic images diffusing scatter plates, placed around the final focus lenses on the National Ignition Facility target chamber, to quantitatively measure the fraction of light backscattered outside of the focusing cone angle of incident laser beam. A wide-angle imaging system relays an image of light scattered outside the lens onto a gated charge coupled device camera, providing 3 mm resolution over a 2 m field of view. To account for changes of the system throughput due to exposure to target debris the system will be routinely calibrated in situ at 532 and 355 nm using a dedicated pulsed laser source.

Angular sensitivity of gated microchannel plate framing cameras

Gated, microchannel-plate-based (MCP) framing cameras have been deployed worldwide for 0.2–9 keV x-ray imaging and spectroscopy of transient plasma phenomena. For a variety of spectroscopic and imaging applications, the angular sensitivity of MCPs must be known for correctly interpreting the data. We present systematic measurements of angular sensitivity at discrete relevant photon energies and arbitrary MCP gain. The results can been accurately predicted by using a simple two-dimensional approximation to the three-dimensional MCP geometry and by averaging over all possible photon ray paths.

Laser coupling to reduced-scale hohlraum targets at the Early Light Program of the National Ignition Facility

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization or...

ACHROMATICALLY FILTERED DIAMOND PHOTOCONDUCTIVE DETECTORS FOR HIGH POWER SOFT X-RAY FLUX MEASUREMENTS

A 1 mm square diamond photoconductive detector (PCD) has been installed on the LLNL Nova laser system, for use as a broad band soft x-ray power diagnostic. The PCD is installed behind an array of pinholes, which cast multiple, overlapping images of the source onto the diamond. This allows reduction of the x-ray intensity, to avoid saturation problems, while avoiding the spectral dependency of thin film filters. The diode current is read out on a 5 GHz bandwidth scope. The system is calibrated by comparison to an absolutely calibrated array of filtered vacuum x-ray photodiodes (XRD` s) (``dante``). The time response of the PCD and its bias electronics have been characterized using the 5th harmonic (210 nm) of a short pulse (< 1 ps) Ti:sapphire laser. The data show a fast rise, limited by the 5 GHz scope bandwidth, and a slower fall off, characterized by an RC time of order 200 ps.

Comparison of charge coupled device vs film readouts for gated micro-channel plate cameras

We have used a large format (4000×4000) high resolution (9 μm pixels) charge coupled device (CCD) to record images from the rear of a gated micro-channel plate (MCP) intensifier, and compared the results with conventional film recording. Measurements of linearity, dynamic range, dark noise, and distortion all show that the CCD is a superior replacement for film. Furthermore, its excellent registration allows for easy flat fielding, using data from a uniformly exposed MCP. As we increase the signal level to where the signal to noise is not dominated by photon counting statistics, we find that this flat fielding procedure produces a significant improvement in signal to noise. The small spatial scale of this noise has led to its identification as high spatial frequency variations in the MCP phosphor.

Characterization of x-ray streak cameras for use on Nova

There are many different types of measurements that require a continuous time history of x-ray emission that can be provided with an x-ray streak camera. In order to properly analyze the images that are recorded with the x-ray streak cameras operated on Nova, it is important to account for the streak characterization of each camera. We have performed a number of calibrations of the streak camera both on the bench as well as with Nova disk target shots where we use a time modulated laser intensity profile (self-beating of the laser) on the target to generate an x-ray comb. We have measured the streak camera sweep direction and spatial offset, curvature of the electron optics, sweep rate, and magnification and resolution of the electron optics.

Investigation of quantum efficiencies in multilayered photocathodes for microchannel plate applications

Microchannel plates (MCPs) are used in many diagnostic systems to study laser-plasma interactions. Typically the front surface of a MCP is coated with some photocathode (PC) material to convert x ray to electron. This is followed by electron multiplication along the microchannels. Materials such as CsI, Al, and Au have been identified as good PC materials, but the overall quantum efficiencies of these materials are low, so that electron multiplication along the channel has been the only path to signal amplification. This approach is known to have some problems (J. D. Wiedwald, University of California UCRL-JC-110906, August 1992). We investigated the effect of pairing transmission PC to standard MC’s and found that CsI and Au foils used in multilayered configuration, significantly enhance the overall quantum efficiency of the combination. The findings could lead to better MCP design and fabrication.

Time-resolved soft x-ray imaging diagnostic for use at the NIF and OMEGA lasers

The soft x-ray imager (SXRI) built for the first experiments at the National Ignition Facility (NIF) has four soft x-ray channels and one hard x-ray channel. The SXRI is a snout that mounts to a four strip gated imager. This produces four soft x-ray images per strip, which can be separated in time by ∼60ps. Each soft x-ray channel consists of a mirror plus a filter. The diagnostic was used to study x-ray burnthrough of hot Hohlraum targets at the NIF and OMEGA lasers. The SXRI snout design and issues involved in selecting the desired soft x-ray channels are discussed.