L. P. Mix

Multiframe ion pinhole camera for intense ion beam transport and focusing experiments

We have developed a new ion pinhole camera to obtain energy density profiles of a focused multiterawatt ion beam as a function of ion energy. Beam ions that are elastically scattered from a thin gold foil are imaged through six baffled pinholes onto six separate areas of a sheet of CR‐39 nuclear track detector. Each of the areas is covered by an aluminum range filter and records an image with a different lower bound on the ion energy. Subtracting images with adjacent cutoff energies results in images with upper and lower bounds on the ion energy. Ion images are read from the CR‐39 with VERA, a fully automatic, image‐processor‐based nuclear track counting system. Images of the Particle Beam Fusion Accelerator‐II (PBFA‐II) proton beam have been obtained and show that the beam has been focused to achieve a horizontal FWHM of 5.7 mm and has delivered 9 kJ/cm2 to a target.

Intense ion beam diagnostics for particle beam fusion experiments on PBFA II (invited)

A review of the diagnostics used at Sandia National Laboratories to measure the parameters of intense proton and lithium beams generated on the PBFA‐II accelerator will be presented. These diagnostics consist of several types, namely: Kα x‐ray pinhole cameras, a multiframe dE/dx ion pinhole camera, a p‐i‐n array ion pinhole camera, Thomson parabola spectrographs, a Rutherford magnetic spectrograph, plasma visible spectroscopy, and several nuclear activation diagnostics. These components, when taken together, enable a rather thorough description of the 5‐MV, 10‐TW ion beams presently being produced. A unique feature of these diagnostics is that they are capable of operating in hard (several MeV) x‐ray bremsstrahlung backgrounds of some 109–1010 rad/s. Details of each diagnostic, its integration, data reduction procedures, and recent PBFA‐II data will be discussed.

Time and spatially resolved measurements of x-ray burnthrough and re-emission in Au and Au:Dy:Nd foils

In experiments at the Omega laser facility, x-ray framing and streak cameras were used to explore a technique for simultaneously measuring the relative x-ray burnthrough and re-emission properties of pure Au and high-Z mixture “cocktail” foils exposed to a Hohlraum radiation field. For the Au:Dy:Nd cocktail used in these preliminary experiments, the burnthrough measurements indicated a cocktail opacity ∼1.5 times that of pure Au. The x-ray re-emission fluxes from the cocktail and Au appeared to be equivalent. In the future, we propose to use this experimental arrangement to compare the relative x-ray burnthrough and re-emission properties of other potential wall materials proposed for use in National Ignition Facility Hohlraums.

Obtaining absolute spatial flux measurements with a time-resolved pinhole camera

A technique is described to determine the spatial x-ray flux emitted from a hohlraum wall and subsequently transmitted through a diagnostic hole. This technique uses x-ray diodes, bolometers, and a time-resolved pinhole camera to determine the spatial flux of x rays emitted through a hohlraum’s diagnostic hole. The primary motivation for this analysis was the relatively long duration, nearly 100 ns, of the x-ray drive present in z-pinch driven hohlraums. This radiation causes plasma to ablate from the hohlraum walls surrounding the diagnostic hole and results in a partial obscuration that reduces the effective area over which diagnostics view the radiation. The effective change in area leads to an underestimation of the wall temperature when nonimaging diagnostics such as x-ray diodes and bolometers are used to determine power and later to infer a wall temperature. An analysis similar to the one described below is then necessary to understand the radiation environment present in x-ray driven hohlraums whe...

Time‐resolved measurements of the focused ion beams on PBFA II

A time‐resolved camera has been developed to image the intense ion beam focus on PBFA II. Focused ions from a sector of the ion diode are Rutherford scattered from a thin gold foil on the diode axis and pinhole imaged onto an array of up to 49 PIN detectors to obtain the spatially and temporally resolved images. The signals from these detectors are combined to provide a movie of the beam focus with a time resolution of about 3 ns and a spatial resolution of 2 mm over a 12 mm field of view. Monte Carlo simulations of the camera response are used with the measured ion energy to account for the time‐of‐flight dispersion of the beam and to convert the recorded signals to an intensity. From measurements on an 81° sector of the diode, average intensities on a 6 mm sphere of about 5 TW/cm2 and energies approaching 80 kJ/cm2 are calculated for standard proton diodes. Corresponding numbers for a lithium diode are less than those measured with protons. The details of the analysis and image reconstruction will be pr...

XRD array, grazing incidence spectrometer, and x‐ray imaging camera on PBFA II

Target diagnostics on PBFA II will include a time‐resolved x‐ray diode array, an energy‐resolved, time‐integrated x‐ray camera, and a grazing incidence spectrometer. A clean vacuum, independent of that on PBFA II, will be supplied to the detectors to maintain more reproducible calibrations. We also describe the diagnostics and outline the calculations which determined the shielding required. Based on the calculations the background due to bremsstrahlung in the least sensitive diagnostic is 12 eV/pixel. The appropriate shielding is being built to field these diagnostics.

Ion movie camera for particle‐beam‐fusion experiments

A camera with a 3 ns time resolution and a continuous (≳100 ns) record length has been developed to image a 1012–1013 W/cm2 ion beam for inertial‐confinement‐fusion experiments. A thin gold Rutherford‐scattering foil placed in the path of the beam scatters ions into the camera. The foil is in a near‐optimized scattering geometry and reduces the beam intensity∼seven orders of magnitude. The scattered ions are pinhole imaged onto a 2D array of 39 p‐i‐n diode detectors; outputs are recorded on LeCroy 6880 transient‐waveform digitizers. The waveforms are analyzed and combined to produce a 39‐pixel movie which can be displayed on an image processor to provide time‐resolved horizontal‐ and vertical‐focusing information.

Data analysis for the Rutherford magnetic spectrograph on PBFA‐II

A magnetic spectrograph employing a gold Rutherford scattering foil has been constructed and fielded on Sandia National Laboratories’ PBFA‐II accelerator to diagnose intense ion beams. The instrument simultaneously records time‐integrated data on nuclear track recording film, CR‐39, and time‐resolved data on a set of p‐i‐n diodes. The details of the analysis of these data to provide time‐resolved power density, ion current density, ion energy spectra (including mean ion energy), and ion species identification will be presented.

Optimized Rutherford‐scattering geometries for intense ion‐beam diagnostics

Rutherford scattering is used routinely for diagnostic purposes to reduce the intensity of ion beams in inertial‐confinement‐fusion experiments. A limitation of the scattering technique is the strong dependence of the Rutherford cross section on the scattering angle [σ∝1/sin4(θs/2)] because the angular distribution of the beam ions is not precisely known and is difficult to measure. It is shown that this limitation can be overcome with the use of a thin scattering foil, and an analytic expression is derived that defines geometries for which the scattering probability is, to first order, independent of θs.

An integrated diagnostic package for intense proton and lithium-ion beam measurements on the sandia national laboratories' PBFA-II accelerator☆

Abstract A review of the diagnostic packages used at Sandia National Laboratories to measure the parameters of intense proton and lithium beams generated on the PBFA-II accelerator will be presented. These diagnostics consist of several types, namely: K α X-ray pinhole cameras, a multiframe dE/dx ion pinhole camera, a p-i-n diode array ion pinhole camera, Thomson parabola spectrographs, a Rutherford magnetic spectrograph, plasma visible spectroscopy and several nuclear activation diagnostics. These components, when taken together, provide a rather thorough description of the 5 MV, 10 TW ion beams presently being produced. Specifically, the beam parameters measured by the diagnostic array include spatial profile (off and on axis), absolute number, species, voltage, current density and focal power density. A unique feature of these diagnostics is that they are capable of operating in hard (several MeV) X-ray bremsstrahlung backgrounds of some 109–1011rad/s. The operating principles of each diagnostic will be summarized in the paper, along with a discussion of how the diagnostics are integrated together to form a complete system. The paper will close with a discussion of a new nuclear track counting system that has been developed for automatic counting of solid-state nuclear track detectors.