R. J. Dukart

ICF target diagnostics on PBFA II (invited)

Particle Beam Fusion Accelerator II is a light‐ion fusion accelerator that is presently capable of irradiating a 6‐mm‐diam sphere with ∼50 kJ of 5.5‐MeV protons in ∼15 ns. An array of particle and x‐ray diagnostics fielded on proton Inertial Confinement Fusion target experiments quantifies the incident particle beam and the subsequent target response. An overview of the ion and target diagnostic setup and capabilities will be given in the context of recent proton beam experiments aimed at studying soft x‐ray emission from foam‐filled targets and the hydrodynamic response of exploding‐pusher targets. Ion beam diagnostics indicate ∼100 kJ of proton beam energy incident within a 1.2‐cm radius of the center of the diode with an azimuthal uniformity which varied between 6% and 29%. Foam‐filled target temperatures of 35 eV and closure velocities of 4 cm/μs were measured.

Z‐pinch experiments on Saturn at 30 TW

We have recently completed the first gas‐puff Z‐pinch experiments on Saturn (32 TW, 1.4 MJ, 1.9 MV, 40‐ns FWHM, and 0.11 Ω). These experiments used the most powerful driver to date for fast Z‐pinch experiments. Saturn, a 36 module accelerator, uses a double post‐hole vacuum convolute to deliver the total machine current to the load. The 10‐nH Saturn Z‐pinch diode is capable of delivering a peak current of 10.5 MA. We diagnosed the current using segmented Rogowski coils at the insulator, resistive shunts in the vacuum transmission lines, and B‐dot loops and piezoelectric pressure gauges near the load. On most shots electrical losses in the vacuum convolute were minimal with nearly complete current delivery to the Z‐pinch load. We have conducted experiments with deuterium, neon, argon, krypton, and xenon gas puffs. A maximum total radiation yield of 505±25 kJ was obtained with xenon. The peak keV x‐ray yields were 100±5 kJ for neon L‐shell radiation, 30±4 kJ for krypton l‐shell radiation, and 39±4 kJ for ar...

Time-dependent, x-ray spectral unfolds and brightness temperatures for intense Li+ ion beam-driven hohlraums

X-ray-producing hohlraums are being studied as indirect drives for Inertial Confinement Fusion targets. In a 1994 target series on the PBFAII accelerator, cylindrical hohlraum targets were heated by an intense Li{sup +} ion beam and viewed by an array of 13 time-resolved, filtered x-ray detectors (XRDs). The UFO unfold code and its suite of auxiliary functions were used extensively in obtaining time- resolved x-ray spectra and radiation temperatures from this diagnostic. UFO was also used to obtain fitted response functions from calibration data, to simulate data from blackbody x-ray spectra of interest, to determine the suitability of various unfolding parameters (e.g., energy domain, energy partition, smoothing conditions, and basis functions), to interpolate the XRD signal traces, and to unfold experimental data. The simulation capabilities of the code were useful in understanding an anomalous feature in the unfolded spectra at low photon energies ({le} 100 eV). Uncertainties in the differential and energy-integrated unfolded spectra were estimated from uncertainties in the data. The time-history of the radiation temperature agreed well with independent calculations of the wall temperature in the hohlraum.

Plasma diagnostics using K α satellite emission spectroscopy in light ion beam fusion experiments

K α satellite spectroscopy can be a valuable technique for diagnosing conditions in high energy density plasmas. K α emission lines are produced in intense light ion beam plasma interaction experiments as 2p electrons fill partially open 1s shells created by the ion beam. In this paper, we present results from collisional-radiative equilibrium (CRE) calculations which show how K α emission spectroscopy can be used to determine target plasma conditions in intense lithium beam experiments on Particle Beam Fusion Accelerator-II (PBFA-II) at Sandia National Laboratories. In these experiments, 8-10 MeV lithium beams with intensities of 1-2 TW/cm 2 irradiate planar multilayer targets containing a thin Al tracer. K α emission spectra are measured using an X-ray crystal spectrometer with a resolution of λ/Δλ ≃ 1200. The spectra are analyzed using a CRE model in which multilevel (N L ∼ 10 3 ) statistical equilibrium equations are solved self-consistently with the radiation field and beam properties to determine atomic level populations. Atomic level-dependent fluorescence yields and ion-impact ionization cross sections are used in computing the emission spectra. We present results showing the sensitivity of the K α emission spectrum to temperature and density of the Al tracer. We also discuss the dependence of measured spectra on the X-ray crystal spectral resolution, and how additional diagnostic information could be obtained using multiple tracers of similar atomic number.

Progress In Pulsed Power Discharge Driven X-Ray Laser Research

In experiments performed during the past two years on Proto II (a 10-TW pulsed-power accelerator), we imploded annular plasmas onto thin-walled annular x-ray laser targets in order to create both a radiation pump source and an x-ray laser medium. For x-ray lasing the Z-pinch must be axially uniform, must efficiently produce the pump radiation, yet not destroy the laser medium on the cylindrical axis of symmetry until after the x-ray laser pulse. To characterize the pump source x-rays and lasant conditions, we regularly field a large number of x-ray diagnostics. In recent experiments, we produced over 15 kJ of >1-keV pump radiation with an imploding neon gas-puff load, recorded spectra from the pump source and lasant, and measured the axial pump source asymmetry. We are considering both recombination and resonance-pumped x-ray laser schemes.

Lithium beam‐driven target experiments at 1015 W/g on PBFA II at Sandia National Laboratories

A lithium beam is focused to an intensity 1–2 TW/cm2. The beam divergencies have been measured as low as 23 mrad. This lithium beam has the specific power deposition of ∼10 W/g, the beam‐driven target experiments have achieved radiation temperature of 58 eV.(AIP)

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 previou...

The Sandia X‐ray laser program

The Sandia X-ray Laser Program is based on the use of intense keV radiation produced by gas puff, Z-pinch implosions to photoionize Ne-like ions to F-like ions. A 3p-3s population inversion is generated via electron recombination processes. An annular stagnation shell is used to separate the imploding pump source from the lasant. We are also developing a converter technology for examining a Na-Ne line matching scheme. Experimental and computational results and comparisons will be presented.

Light ion beam ICF program overview

The light ion fusion program at Sandia National Laboratories is reviewed. The program is based on the indirect‐drive target concept and has the long‐term objective to obtain high yield in a microfusion facility. The near‐term technical effort will focus on increasing intensity and demonstrating 100 eV temperature in an ion hohlraum. Some concepts were tested experimentally confirming the accessibility of anticipated goals. (AIP)

Light ion hohlraum target experiments on PBFA II and Nova

The goal of the National Inertial Confinement Fusion (ICF) Program in the United States is a target yield in the range of 200 to 1000 MJ. To address this goal, the near‐term emphasis in the Light Ion Target Physics program is to design a credible high‐gain target driven by ion beams. Based on this target design, we have identified ion beam spatial parameters, ion beam energy and power deposition, the conversion of ion‐beam energy into soft x‐ray thermal radiation, the conversion of ion‐beam energy into hydrodynamic motion, radiation smoothing in low‐density foams, and internal pulse shaping as the critical physics issues. These issues are currently being addressed in both ion‐ and laser‐driven experiments.