Gated Microchannel Plate Detector Research Articles

Optimizing of Experimental Load of PFZ-200 Plasma Focus

This article presents a study of neutron emission on the PFZ-200 plasma focus at the Department of physics on FEE CTU in Prague, Czech Republic. In order to achieve the highest and most stable neutron yields, the deuterium working gas pressure and the anode shape were systematically varied. We observed the plasma time to the pinch and the discharge current by the Rogowski coil and neutron emission by the silver activation detector and scintillation time-of-flight detectors. The imploded plasma was visualized using a fast X-ray pinhole camera with a gated microchannel plate detector. The experiment presents the z-pinch discharges with the current maximum above 200 kA and the average neutron yields of ${3 \times 10^{8}}$ neutrons/shot. Measured pinch times were in the range from 1.65 to 1.85 $\mu \text{s}$ . The hollow round anode configuration performed the most stable neutron yields with a deviation under 20%.

First set of gated x-ray imaging diagnostics for the Laser Megajoule facility.

The Laser Megajoule (LMJ) facility located at CEA/CESTA started to operate in the early 2014 with two quadruplets (20 kJ at 351 nm) focused on target for the first experimental campaign. We present here the first set of gated x-ray imaging (GXI) diagnostics implemented on LMJ since mid-2014. This set consists of two imaging diagnostics with spatial, temporal, and broadband spectral resolution. These diagnostics will give basic measurements, during the entire life of the facility, such as position, structure, and balance of beams, but they will also be used to characterize gas filled target implosion symmetry and timing, to study x-ray radiography and hydrodynamic instabilities. The design requires a vulnerability approach, because components will operate in a harsh environment induced by neutron fluxes, gamma rays, debris, and shrapnel. Grazing incidence x-ray microscopes are fielded as far as possible away from the target to minimize potential damage and signal noise due to these sources. These imaging diagnostics incorporate microscopes with large source-to-optic distance and large size gated microchannel plate detectors. Microscopes include optics with grazing incidence mirrors, pinholes, and refractive lenses. Spatial, temporal, and spectral performances have been measured on x-ray tubes and UV lasers at CEA-DIF and at Physikalisch-Technische Bundesanstalt BESSY II synchrotron prior to be set on LMJ. GXI-1 and GXI-2 designs, metrology, and first experiments on LMJ are presented here.

OZSPEC-2: An improved broadband high-resolution elliptical crystal x-ray spectrometer for high-energy density physics experiments (invited)

A novel time, space, and energy-resolved x-ray spectrometer has been developed which produces, in a single snapshot, a broadband and relatively calibrated spectrum of the x-ray emission from a high-energy density laboratory plasma. The opacity zipper spectrometer (OZSPEC-1) records a nearly continuous spectrum for x-ray energies from 240 to 5800 eV in a single shot. The second-generation OZSPEC-2, detailed in this work, records fully continuous spectra on a single shot from any two of these three bands: 270-650, 660-1580, and 1960-4720 eV. These instruments thus record thermal and line radiation from a wide range of plasmas. These instruments' single-shot bandwidth is unmatched in a time-gated spectrometer; conversely, other broadband instruments are either time-integrated (using crystals or gratings), lack spectral resolution (diode arrays), or cover a lower energy band (gratings). The OZSPECs are based on the zipper detector, a large-format (100x35 mm) gated microchannel plate detector, with spectra dispersed along the 100 mm dimension. OZSPEC-1 and -2 both use elliptically bent crystals of OHM, RAP, and/or PET. Individual spectra are gated in 100 ps. OZSPEC-2 provides one-dimensional spatial imaging with 30-50 microm resolution over a 1500 microm field of view at the source. The elliptical crystal design yields broad spectral coverage with resolution E/DeltaE>500, strong rejection of hard x-ray backgrounds, and negligible source broadening for extended sources. Near-term applications include plasma opacity measurements, detailed spectra of inertial fusion Hohlraums, and laboratory astrophysics experiments.

Quantitative extraction of spectral line intensities and widths from x-ray spectra recorded with gated microchannel plate detectors

Plasma spectroscopy requires determination of spectral line intensities and widths. At Sandia National Laboratories Z facility we use elliptical crystal spectrometers equipped with gated microchannel plate detectors to record time and space resolved spectra. We collect a large volume of data typically consisting of five to six snapshots in time and five to ten spectral lines with 30 spatial elements per frame, totaling to more than 900 measurements per experiment. This large volume of data requires efficiency in processing. We have addressed this challenge by using a line fitting routine to automatically fit each spectrum using assumed line profiles and taking into account photoelectron statistics to efficiently extract line intensities and widths with uncertainties. We verified that the random data noise obeys Poisson statistics. Rescale factors for converting film exposure to effective counts required for understanding the photoelectron statistics are presented. An example of the application of these results to the analysis of spectra recorded in Z experiments is presented.

Multilayer mirror monochromatic self-emission x-ray imaging on the Z accelerator

A time-resolved, monochromatic soft x-ray diagnostic has been developed for self-emission imaging of imploding z pinches on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. Multiple pinhole images are reflected from a planar multilayer mirror with narrow photon energy bandwidth (<10eV) onto a 1ns gated microchannel plate detector. High-energy bremsstrahlung x rays are not reflected, providing improved signal to noise in comparison to a standard filtered pinhole camera included in the same beamline of the instrument. An example of data from the existing ∼277eV imaging system is presented, showing cooler imploding mass extending to larger radius than the on-axis K-shell emission of an aluminum wire array. Multilayer mirror and filter pair configurations ranging from 96to769eV are discussed.

Broadband high-resolution elliptical crystal x-ray spectrometer for high energy density physics experiments

The spectroscopic investigation of high temperature laser produced plasmas in general, and x-ray opacity experiments in particular, often requires instruments with both a broad coverage of x-ray energies and high spectral, spatial, and temporal resolution. We analyze the design, model the response, and report the commissioning of a spectrometer using elliptical crystals in conjunction with a large format, gated microchannel plate detector. Measurements taken with this instrument at the Janus laser facilities demonstrate the designed spectral range of 0.24–5.8keV and spectral resolution E∕ΔE>400, resulting in two to three times more spectral data than achieved by previous spectrometer designs. The observed 100ps temporal resolution and 35μm spatial resolution are consistent with the requirements of high energy density opacity experiments.

Time- and space-resolved elliptical crystal spectrometers for high energy density physics research

X-ray spectrometers used in high energy density plasma experiments must provide high time, space, and spectral resolution while overcoming the difficulties imposed by x-ray background, debris, and mechanical shocks. At the Z facility these problems are addressed using a suite of elliptical crystal spectrometers. The elliptical geometry isolates the detector from the line of sight with a slit placed at the elliptical focus, while the sensitivity enables locating the crystal 2–4 m from the plasma source. Space and time resolution are obtained by using an array of slits to project one dimensional plasma images onto the crystal and recording the spectrally dispersed images with a gated microchannel plate detector.

Rayleigh–Taylor instability evolution in ablatively driven cylindrical implosions

The Rayleigh–Taylor instability is an important limitation in inertial confinement fusion capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects of material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes (e.g., Bell–Plesset growth). Amplitude thresholds for the nonlinear regime are reduced, since the wavelength λ of a mode m decreases with convergence λ∼R/m, where R is the radius. Convergent effects have been investigated using an imploding cylinder driven by x-ray ablation on the NOVA laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)]. By doping sections of the cylinder with opaque materials, in conjunction with x-ray backlighting, the growth and feedthrough of the perturbations from the ablation front to the inner surface of the cylinder for various initial modes and amplitudes from early time through stagnation was measured. Mode coupling of illumination asymmetries with material perturbations is observed, as well as phase reversal of the perturbations from near the ablation front to the inner surface of the cylinder. Perturbation growth is observed due to convergence and compressibility alone, without the effects of acceleration, and scales as ∼1/ρR, where ρ is the mass density. Imaging is performed with an x-ray pinhole camera coupled to a gated microchannel plate detector.

Ion probe for tightly bunched charged particle beams

A nonperturbing probe that utilizes a focused ion beam has been developed to measure the position and profile of a tightly bunched charged particle beam (CPB). The ions are injected across the path of the CPB and are deflected by the electromagnetic field of the CPB. Deflected ions are detected by a gated microchannel plate detector and video camera. From the magnitude of the deflections, one can determine the position, radial profile, and in some cases, the axial length of the CPB. Ion parameters can be adjusted to tailor the probe to a wide range of CPB parameters, from submicron, ps bunches to millimeter-sized, quasicontinuous beams. We present the theory of operation of the probe, provide a number of numerical examples to illustrate its capabilities, and describe a proof-of-principle version of the probe that we built and successfully tested at SLAC. The experimental results obtained with this prototype version have shown the ion probe diagnostic to be feasible, reliable, and well understood by the theory and models.

X-ray radiographic imaging of hydrodynamic phenomena in radiation-driven materials—Shock propagation, material compression, and shear flow*

One- and two-dimensional, time-resolved x-ray radiographic imaging at high photon energy (5–7 keV) is used to study shock propagation, material motion and compression, and the effects of shear flow in solid density samples which are driven by x-ray ablation with the Nova laser. By backlighting the samples with x rays and observing the increase in sample areal density due to shock compression, the trajectories of strong shocks (∼40 Mbars) in flight are directly measured in solid density plastic samples. Doping a section of the samples with high-Z material (Br) provides radiographic contrast, allowing a measurement of the shock-induced particle motion. Instability growth due to shear flow at an interface is investigated by imbedding a metal wire in a cylindrical plastic sample and launching a shock in the axial direction. Time-resolved radiographic measurements are made with either a slit-imager coupled to an x-ray streak camera or a pinhole camera coupled to a gated microchannel plate detector, providing ∼10 μm spatial and ∼100 ps temporal resolution.

Electrical characteristics of short pulse gated microchannel plate detectors

Gating of microchannel plates (MCPs) can produce shutter times of less than 40 ps. This is accomplished with high-fidelity electronic circuitry and short pulse (sub 80 ps) high-amplitude electronic drivers. Measurements have shown that the fidelity of the electrical circuitry is one of the limiting factors in producing shorter x-ray gating.

Four-frame gated Wolter x-ray microscope

A four-frame gated x-ray microscope has been developed for the study of inertial confinement fusion. Four x-ray images were obtained by introducing flat, grazing incidence mirrors into the converging annular ray bundle of a previously developed Wolter x-ray microscope system. Gated microchannel plate detectors and associated drive electronics were developed which can record a sequence of gated images of less than 100 ps duration, with an interframe timing accuracy of ±50 ps. The resulting diagnostic can record one time-resolved image and four gated images simultaneously. The system is described, and images of an imploding capsule are presented which demonstrate the unique capabilities of the instrument.

Large-area 200-ps gated microchannel plate detector

Results are presented with a 15-mm-wide gated microchannel plate UV and x-ray detector. The active area is part of a 6-Ω transmission line driven by an electronically generated gate pulse. The microchannel plate is coated with CsI allowing tests with a frequency-quadrupoled, high-repetition-rate 1.05-μm laser. Results showing optical gate widths as short as 100 ps are presented.