X-ray Pinhole Camera Research Articles

Exploratory studies on a passively triggered vacuum spark

The results of an experimental investigation on a passively triggered vacuum spark device are presented. The diagnostics include the current, x-ray and optical emission measurements. The sharp dips in the current derivative signal indicate the occurrence of pinching at an early stage of the discharge (at current 5 kA). A well-confined plasma with a central hot region was recorded using a streak camera. The pinched plasma was observed to undergo kink-type oscillations with a time period of 10-15 ns. Repeated plasma fronts were seen to move from the anode to the cathode with an average velocity of 5 × 106 cm s-1. Soft x-ray emission having a radiation intensity of a few hundred mR per discharge was observed. The x-ray signals obtained using photodiodes showed multiple bursts. A soft x-ray pinhole camera recorded micro-pinches of 100 µm. The x-ray emitting regions were confined to the inter-electrode gap. The x-ray emission characteristics were influenced by the electrolytic resistance, which was connected across the spark gap to initiate discharge.

Potential diagnostics for the next-generation light sources

There is continued interest in developing a diffraction-limited soft or hard X-ray source. Candidate paths include the storage-ring-based free-electron laser (FEL) and the linac-based self-amplified spontaneous emission (SASE) FEL for the two regimes, respectively. As previously discussed, target beam parameters are σ x,y∼10 μm, σ x′,y′=1 μrad , and σ t ∼1 ps (0.1 ps for the linac case). We report the use of few- to sub-angstrom radiation emitted by a 7-GeV beam transitting a 198-period diagnostics undulator and a bending magnet to characterize the particle beam. A particle beam divergence as low as σ y′=3.3 μrad was measured using an X-ray monochromator. Additionally, a particle beam size of σ y<45 μm and a bunch length of σ t=28 ps with 4-ps resolution were measured using an X-ray pinhole camera with a unique synchroscan and dual-sweep X-ray streak camera as the detector. The adjustable pinhole aperture was varied by more than a factor of 100 to assess spatial resolution and dynamic range issues in the system. These diagnostics demonstrations should scale to next-generation applications.

Hohlraum symmetry measurements with surrogate solid targets (invited)

Time-resolved radiographic imaging of low density, solid spherical surrogate targets has been used to provide a time-dependent measurement of drive pressure symmetry in cylindrical hohlraums on both the Nova and Omega lasers. The experiments replace the usual capsule at the center of a gold hohlraum with a sphere of SiO2 foam (ρ=0.3 g/cm3). The laser generates an x-ray drive inside the hohlraum which does not produce perfectly symmetric drive pressure on a spherical target, giving rise to a distorted shock traveling radially inward. The rarefaction behind the shock generated in this sphere produces a rapid rise in x-ray transmission which is easily detectable experimentally by radiography. The position of this feature may be determined to within a few microns in our experimental setup using a gated x-ray pinhole camera. Time-dependent control of drive symmetry in a hohlraum requires the ability to adjust the laser power as a function of both time and position along the hohlraum axis. We have implemented this control on the Omega laser by delaying sets of beams (“beam staggering”) and on the Nova laser (“beam phasing”) by delivering two independent pulse shapes on inner and outer halves of the Nova beams. The improvement in P2 pressure symmetry with this control as measured with the foam ball technique was found to be as much as a factor of 3, compared with deliberately mistuned drives. On the National Ignition Facility (NIF), we expect to use this technique as part of the effort to tune drive symmetry to achieve higher gain. We present results from these experiments from both facilities and comparisons with LASNEX simulations. In addition, we discuss the application of the foam ball technique to NIF targets.

Investigation of XUV amplification with Ni-like xenon ions using laser-produced gas puff plasmas

This paper presents results of a study of a gas-puff plasma soft-X-ray amplifier using Ni-like xenon. The target was created by pulsed injection of xenon from a high-pressure solenoid valve through a nozzle in the form of a 2 cm long slit. The plasma was produced by irradiating the gas puff plume with six line-focused laser beams. The uniformity of the plasma column was monitored using a crossed-slit X-ray pinhole camera while the ionisation balance was assessed with the help of keV resonance spectra obtained by a crystal spectrometer. A modest amplification was seen at two J=0–1 lasing lines allowing however the first precise measurement of their wavelengths. Comparison of the results with a previously reported amplification in the gas-puff system is made, indicating issues essential to the performance of these systems.

Design and performance studies of an MCP based pinhole camera for laser-produced plasma diagnostics

An X-ray pinhole camera system has been developed and tested to obtain spatially resolved time-integrated X-ray images of laser-produced plasmas (Nd: YAG laser of 75mJ energy in 35 picoseconds) using a single stage micro channel plate (MCP) detector coupled to a CCD camera-frame grabber system instead of X-ray film. Thick slab targets of aluminium, copper and gold were used to produce plasmas with different X-ray yields at laser intensities ranging from 8 × 1011 to 8 × 1012 W/cm2. MCP gain was increased with the help of biasing voltages to record low intensity X-ray images, which have been analysed using an image processing software “PROMISE” developed in-house. The experimental results are presented.

In situ calibration of microchannel-plate-based x-ray pinhole camera for observation of magnetically trapped plasma

The electronic amplification gain of a microchannel plate (MCP), as employed for detector and image amplifier of an x-ray pinhole camera, tends to decrease as the output current increases, posing problems both in quantitative analyses and in construction of a three-dimensional emissivity distribution. We report that the output-current dependent MCP gain is described in a simple empirical formula that is determined by an in situ calibration experiment using a steady-state low-temperature discharge plasma. We examine the validity of the formula affirmatively in correcting raw data of x-ray images of magnetically trapped hot plasma. It is also demonstrated that the correction leads to a three-dimensional distribution of soft x-ray emissivity in a quadrupole-mirror-trapped hot plasma that is consistent with other indirect measurements.

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.

Conceptual design for a time resolved x-ray imaging system useable in a strong hard x-ray background

A new type of time resolved soft x-ray pinhole camera has been conceptually designed for the measurement of the size of pinched electron beam of sub-mm size in the strong x-ray background. In the camera an x-ray image is converted to an optical image and transferred by an image guide of 1 m length to the place where hard x-ray background is weak. The optical image is intensified and gated by a multichannel plate image intensifier (MCP-II) and recorded on a film. In the test operation, an x-ray image of a flash x-ray source was successfully observed in an open shutter mode operation. The minimum exposure time of the MCP-II was estimated to be 6.5 ns from measurement with a short pulsed laser diode.

Los Alamos pinhole camera (LAPC): A new flexible x-ray pinhole camera

We have recently designed, built, and fielded a versatile, multichannel x-ray pinhole camera. The Los Alamos pinhole camera was designed to fit into any 6 in. manipulator (SIM) which is a standardized target chamber diagnostic tube. There are currently compatible SIMs available at the Trident, Omega, and NOVA laser systems. The camera uses nine pinholes in a 3×3 array to produce images at the film plane. The film housing is designed to hold multiple sheets of stacked x-ray film which also uses a dark slide to protect the film before exposure. Magnifications of 12, 8, 4, and 2X are selected by slip-on nose cones, which support pinholes, collimators, and blast shields. Individual channel filtering is provided by a 3×3 filter pack containing nine separate filter sub packs. Spatial resolution is limited by the pinhole diffraction limit and field of view is dependent on magnification and filter pack diameter.

Spatial resolution of gated x-ray pinhole cameras

We have conducted an investigation of the spatial resolution of a new gated x-ray pinhole camera (FXI). The spatial resolution, or its Fourier transform the modulation transfer function (MTF), is critical for quantitative interpretation of recent hydrodynamic instability data taken on the Nova laser. We have taken data corresponding to backlit straight edges, pinholes, and grids, both on the bench and in situ on Nova. For both the pinhole and edge data, the MTF at all wavelengths of interest can be deduced from a single image. Grids are of more limited usefulness, giving the value of the MTF only at the spatial period of the grid. These different techniques for characterizing the MTF of gated x-ray pinhole cameras will be discussed, with results specific to the FXI presented.

Experimental investigation of the interaction of 100 TW laser with solid targets (abstract)

We report on the first plasma experiments with the 100 TW laser at LLNL. We have experimentally investigated the interaction of 1 μm, 40 J, 400 fs, 5×1019 W/cm2 laser pulses with multilayer solid targets. Various diagnostics were used to characterize this interaction. An x-ray pinhole camera monitored laser spot size and plasma location on target. Part of the laser beam was split off to probe the density scale length of the preformed plasma. An optical system for Schlieren photography was designed to record the images of plasma with electron density gradients as low as 1021 cm−4 with characteristic scale length of 10 μm. The range of electron densities was 1018–2020 cm−3. CR-39 plastic track detectors were employed to measure the fast ion blowoff from the plasma. Using a foil-filter technique the proton energy spectrum up to 6 MeV was measured. It was found that the high energy tail usually had a narrow (FWHM ∼10°) cone of expansion. Energy conversion to the fast ions (above 0.5 MeV) was estimated to be ∼0.3%–1% of laser energy. In addition, charge-coupled devices were used for detecting characteristic K α emission caused by fast (&amp;gt;1 MeV) electrons. (See article by K. Wharton for further discussion of the fast electron measurements.) Estimated fast electron yield ∼3%.

XUV emission from an elongated plasma column produced using a high-power laser with a gas puff target

We report the first studies on the formation of an elongated plasma column suitable for X-ray laser experiments, using a gas puff target irradiated with a high-power laser. The gas puff targets, produced by pulsed injection of a small amount of gas from a high-pressure solenoid valve through a nozzle in the form of a slit, have been characterized by optical interferometry and X-ray backlighting. The formation of a hot plasma column up to 30-mm-long is demonstrated. The spatial uniformity of the column was monitored by means of an X-ray pinhole camera. XUV spectra measurements for SF6 gas puff targets show predominant 3–2 line of hydrogenic fluorine (λ = 8.1 nm), however, only linear increasing of its intensity with the target length was observed.

Neutron and x-ray emission studies in a low energy plasma focus

In a low energy Mather-type plasma focus energized by a single 32 μF capacitor, the x-ray and neutron emission is investigated using time-integrated and time-resolved detectors. The x-ray emission profile has a width (FWHM) of 40–50 ns. The neutron emission profile is broader compared to the x-ray emission profile and also delayed by 30–40 ns. To identify different regimes of x-ray emission, an x-ray pin-hole camera along with different absorption filters is employed. While the x-ray emission is high within a narrow pressure range of 2.0–2.5 mbar, the neutron emission is intense for a wider range of 1.0–4.5 mbar. The intense x-ray emission seems to originate from the axially moving shock wave. These results also indicate rather different production mechanisms for x-ray and neutron emission. Also on comparing the x-ray images with Al(2 μm), Al(5 μm), Al(9 μm) filters, we find that the bulk of x-rays from the focus filament have energies less than 2 keV.

System for Time-Resolved Observation of Impulsive Soft X-Ray Sources and Its Application to Plasma Focus Experiment

A soft X-ray pinhole camera system for time-resolved, one-dimensional (1D) measurement of an impulsive source whose energy is in the range from 1 keV to 20 keV is developed. The system is based on the filtered pinhole camera. The pinhole image is divided into six channels in the direction of the electrode axis through slits and bundled fiber optics. Each channel is detected with a scintillator-photomultiplier combination. The temporal resolution of this system is 5.2 ns. The absolute sensitivity of the channel is ∼4.2×10-20 A/[photons/(mrad2·s)]. This system was successfully used to observe the time evolution of the soft X-ray source generated with a plasma focus device with a gas-puffed target.

Measuring Beam Sizes and Ultra-Small Electron Emittances Using an X-ray Pinhole Camera

A very simple pinhole camera set-up has been built to diagnose the electron beam emittance of the ESRF. The pinhole is placed in the air next to an Al window. An image is obtained with a CCD camera imaging a fluorescent screen. The emittance is deduced from the size of the image. The relationship between the measured beam size and the electron beam emittance depends upon the lattice functions alpha, beta and eta, the screen resolution, pinhole size and photon beam divergence. The set-up is capable of measuring emittances as low as 5 pm rad and is presently routinely used as both an electron beam imaging device and an emittance diagnostic.

Investigation on the pseudospark electron beam and its application for the generation of soft X-rays

An investigation on the pseudospark electron beam is presented in this paper. Time-integrated and time-resolved pictures of the X-rays generated by the electron beam are taken with a special X-ray pinhole camera. They show the spatial and temporal development of the pseudospark electron beam. Time-integrated pictures indicate spatial instabilities of the electron beam caused by a bent or broadened electron beam due to space charge effects. The results of the time-resolved measurements show X-ray emission only during two short periods at the very beginning of the discharge before the beam current reaches its maximum. Only during these periods does the electron beam contain high-energy electrons. In between these two periods the energy of the beam electrons decreases because of a space-charge-limited beam transport. >

X-ray diagnostics of dense plasmas produced by high-energy heavy ions

Plasmas with solid-state density at temperatures of a few eV will be produced in the near future at GSI by high-energy heavy ions. To diagnose these plasmas, novel methods–based on hard X-ray diagnostics–have to be applied. An investigation of the hydrodynamic expansion of the target requires a space- and time-resolved measurement of the plasma density. The determination of the target density independently of its temperature is obtained by a special X-ray pinhole camera enabling a selective registration of the target K-shell emission in combination with space- and time-resolved beam intensity measurements.

Characterization of a high-speed x-ray imager (abstract)a)

We are building a new time-resolving x-ray pinhole camera to image z pinches at the Saturn facility. The instrument is designed to have a spatial resolution of 64 μm for 2 keV photons, and a temporal resolution of 100 ps. The microchannel-plate-intensified detector will have nine, 20-mm-wide gold striplines. Each stripline will have an impedance of 5.55 Ω. We will present measurements of the temporal resolution of the detector, and describe new high-voltage gating circuitry designed to drive the low-impedance striplines.

X-ray sensitive charge-coupled device instrumentation for short and ultrashort pulse laser-produced plasma experiments

We describe the use of x-ray sensitive charge-coupled device (CCD) detectors for measuring the 1–15 Å emission from high temperature plasmas produced at the L-Division JANUS and USP (pulse length∼100 fs) laser facilities. Instruments based upon commercially available video format cameras have been designed to replace x-ray film, as the two-dimensional recording medium, for imaging and x-ray spectroscopic applications. Used in conjunction with a high sensitivity x-ray pinhole camera, magnification 5–20, the CCD detectors can give spatial resolution approaching 5 μm and energy band information by the use of edge filters. A compact wavelength dispersive spectrometer equipped with the CCD detector and a flat Bragg crystal analyzer can record survey spectra.

PBFA II lithium beam characterization from inner-shell x-ray images

The Particle Beam Fusion Accelerator (PBFA II) is now driving targets with ICF-relevant lithium ion beams. During the most recent lithium beam target series, time-integrated x-ray pinhole cameras viewed the ion-induced inner-shell x-ray fluorescence from the central gold cone target and a titanium-coated strip. Ion-beam profiles at a nominal 10-mm radius and fixed azimuthal direction were obtained from images of the Ti Kα fluorescence of a Ti-coated Al diagnostic wire. The gold cone gave us beam profiles at a nominal 3-mm radius and at all azimuthal angles from the Au Lα fluorescence. From these profiles, we obtained the ion-beam vertical focus position, the full width at half maximum, and the degree of azimuthal uniformity for the lithium target shots. For these initial results, beam steering problems were evident. Azimuthal uniformity was measured from the ion-beam footprint on the outer Au case (predominantly Au Lα) of the hohlraum target and was found to be in the same range (up to 30%) as for previous proton beam target series.