K. Piston

X-ray imaging in an environment with high-neutron background on National Ignition Facility

X-ray imaging instruments will operate in a harsh ionizing radiation background environment on implosion experiments at the National Ignition Facility. These backgrounds consist of mostly neutrons and gamma rays produced by inelastic scattering of neutrons. Imaging systems based on x-ray framing cameras with film and CCDs have been designed to operate in such harsh neutron-induced background environments. Some imaging components were placed inside a shielded enclosure that reduced exposures to neutrons and gamma rays. Modeling of the signal and noise of the x-ray imaging system is presented.

Investigation and suppression of artifacts in x-ray framing cameras due to advance radiation incident on microchannel plates

We present evidence of an artifact in gated x-ray framing cameras that can severely impact image quality. This artifact presents as a spatially-varying, high-intensity background and is correlated with experiments that produce a high flux of x-rays during the time before the framing camera is triggered. Dedicated experiments using a short pulse UV laser that arrives before, during, and after the triggering of the framing camera confirm that these artifacts can be produced by light that arrives in advance of the voltage pulse that triggers the camera. This is consistent with these artifacts being the result of photoelectrons produced uniformly at the active area of the camera by early incident light and then selectively trapped by the electromagnetic (EM) fields of the camera. Simulations confirm that the EM field above the active surface can act to confine electrons produced before the camera is triggered. We further present a method to suppress these artifacts by installing a conducting electrode in front of the active area of the framing camera. This device suppresses artifacts by attracting any electrons liberated by x-rays that arrive before the camera is triggered.

Radiation induced noise in x-ray imagers for high-yield inertial confinement fusion experiments

The large fluence of 14-MeV neutrons produced in high-yield inertial confinement fusion (ICF) experiments creates a variety of backgrounds in x-ray imagers viewing the implosion. Secondary charged particles produce background light by Cherenkov emission, phosphor screen excitation and possibly scintillation in the optical components of the imager. In addition, radiation induced optical absorption may lead to attenuation of the signal. Noise is also produced directly in the image recorder itself (CCD or film) via energy deposition by electrons and heavy charged particles such as protons and alphas. We will present results from CCD background measurements and compare them to Monte Carlo calculations. In addition we show measurements of luminescence and long-term darkening for some of the glasses employed in imagers.

Design and implementation of Dilation X-ray Imager for NIF "DIXI"

Gated X-Ray imagers have been used on many ICF experiments around the world for time resolved imaging of the target implosions. DIXI (Dilation X-ray Imager) is a new fixed base diagnostic that has been developed for use in the National Ignition Facility. The DIXI diagnostic utilizes pulse-dilation technology [1,2,3,4] and uses a high magnification pinhole imaging system to project images onto the instrument. DIXI is located outside the NIF target chamber approximately 6.5m from target chamber center (TCC). The pinholes are located 10cm from TCC and are aligned to the DIXI optical axis using a diagnostic instrument manipulator (DIM) on an adjacent port. By use of an extensive lead and poly shielded drawer enclosure DIXI is capable of collecting data at DT neutron yields up to Yn~ 1016 on CCD readout and up to Yn~ 1017 on film. Compared to existing pinhole x-ray framing cameras DIXI also provides a significant improvement in temporal resolution, <10ps, and the ability to capture a higher density of images due to the fact the pinhole array does not require collimators. The successful deployment of DIXI on the NIF required careful attention to the following subsystems, pinhole imaging, debris shielding, filtering and image plate (FIP), EMI protection, large format CsI photocathode design, detector head, detector head electronics, control electronics, CCD, film recording and neutron shielding. Here we discuss the initial design, improvements implemented after rigorous testing, infrastructure and commissioning of DIXI on the NIF.

2D magnetic field warp reversal in images taken with DIXI (dilation x-ray imager)

DIXI utilizes pulse-dilation technology to achieve x-ray imaging with temporal gate times below 10 ps. The longitudinal magnetic eld used to guide the electrons during the dilation process results in a warped image, similar to an optical distortion from a lens. Since the front end, where x-rays are converted into electrons at the beginning of the magnetic eld, determines the temporal resolution these distortions in uence the temporal width of the images at the back end, where it is captured. Here we discuss the measurements and methods used to reverse the magnetic warp e ect in the DIXI data. The x-ray measurements were conducted using the COMET laser facility at the Lawrence Livermore National Laboratory.

Optimizing the input and output transmission lines that gate the microchannel plate in a high speed framing camera

We present new designs for the launch and receiver boards used in a high speed x-ray framing camera at the National Ignition Facility. The new launch board uses a Klopfenstein taper to match the 50 ohm input impedance to the ~10 ohm microchannel plate. The new receiver board incorporates design changes resulting in an output monitor pulse shape that more accurately represents the pulse shape at the input and across the microchannel plate; this is valuable for assessing and monitoring the electrical performance of the assembled framing camera head. The launch and receiver boards maximize power coupling to the microchannel plate, minimize cross talk between channels, and minimize reflections. We discuss some of the design tradeoffs we explored, and present modeling results and measured performance. We also present our methods for dealing with the non-ideal behavior of coupling capacitors and terminating resistors. We compare the performance of these new designs to that of some earlier designs.

Demonstration of enhanced DQE with a dual MCP configuration

X-ray framing cameras based on proximity-focused micro-channel plates (MCP) have been playing an important role as diagnostics of inertial confinement fusion experiments [1]. Most of the current x-ray framing cameras consist of a single MCP, a phosphor, and a recording device (e.g. CCD or photographic films). This configuration is successful for imaging x-rays with energies below 20 keV, but detective quantum efficiency (DQE) above 20 keV is severely reduced due to the large gain differential between the top and the bottom of the plate for these volumetrically absorbed photons [2]. Recently developed diagnostic techniques at LLNL require recording backlit images of extremely dense imploded plasmas using hard x-rays, and demand the detector to be sensitive to photons with energies higher than 40 keV [3]. To increase the sensitivity in the high-energy region, we propose to use a combination of two MCPs. The first MCP is operated in low gain and works as a thick photocathode, and the second MCP works as a high gain electron multiplier [4,5]. We assembled a proof-of-principle test module by using this dual MCP configuration and demonstrated 4.5% DQE at 60 keV x-rays.

M-ARIANE (Mirror-assisted Active Readout In A Neutron Environment): an x-ray imaging system for implosion experiments on the National Ignition Facility at ignition neutron yields

X-ray imaging diagnostics instruments will operate in a harsh ionizing radiation background environment during ignition experiments at the National Ignition Facility (NIF). This background consists of mostly neutrons and gamma rays produced by inelastic scattering of neutrons. An imaging system, M-ARIANE (Mirror-assisted Active Readout In A Neutron Environment), based on an x-ray framing camera with film, has been designed to operate in such a harsh neutron-induced background environment. Multilayer x-ray mirrors and a shielding enclosure are the key components of this imaging system which is designed to operate at ignition neutron yields of ~1e18 on NIF. Modeling of the neutronand gamma-induced backgrounds along with the signal and noise of the x-ray imaging system is presented that display the effectiveness of this design.

Efficiency and decay time measurement of phosphors for x-ray framing cameras usable in harsh radiation background

Phosphors are key components of x-ray framing cameras. On implosion experiments at the National Ignition Facility, the x-ray framing cameras must operate in a harsh neutron induced ionizing radiation. One promising approach of neutron induced background reduction is separation of the neutron background with using difference of x-ray and neutron time-of- flight. To complete x-ray imaging before arrival of the neutron induced radiation to the detector, it is crucial to find a phosphor which has high efficiency and fast decay time. We tested various phosphor materials to optimize design of framing cameras for implosion experiments.