Efficient multi-keV X-ray sources from Ti-doped aerogel targets

Soft x-ray power diagnostics for fusion experiments at NIF, Omega, and Z facilities

In this Review Article, we discuss a range of soft x-ray power diagnostics at inertial confinement fusion (ICF) and pulsed-power fusion facilities. This Review Article describes current hardware and analysis approaches and covers the following methods: x-ray diode arrays, bolometers, transmission grating spectrometers, and associated crystal spectrometers. These systems are fundamental for the diagnosis of ICF experiments, providing a wide range of critical parameters for the evaluation of fusion performance.

Conversion efficiency of multi-keV L-shell-band X-ray emission

This study explored the influence of foil thickness, laser pulse width, and laser intensity to optimize the multi-keV X-ray conversion efficiency of a sandwiched (CH/Sn/CH) planar target under laser irradiation at the Shenguang II laser facility. The X-ray photon field values were measured using a set of elliptically bent crystal spectrometers and the conversion efficiencies (ξ_x) of photon energies were in the range of 3.7-4.3 keV. The experimental results indicate that the X-ray yields of 3.7 to 4.3 keV radiation strongly depend on the laser pulse width, target thickness, and laser intensity. The results also demonstrate that three-layer thin foils can provide an efficient multi-keV X-ray source because they can change the distribution of emitted multi-keV X-rays and target dynamics versus nanosecond laser pulses to produce large, hot, and underdense plasma. However, the underdense plasma produced as a rarefaction wave causes the overdense plasma generated by the laser pulse to expand. Therefore, the laser parameters and foil thickness must be carefully optimized to produce an efficient 3.7 to 4.3 keV X-ray source. Otherwise, the rarefaction waves from both sides of the thin foil may suppress multi-keV X-ray emission. This study represents an important advancement in the development of an efficient multi-keV L-shell-band X-ray source.

A compact filtered x-ray diode array spectrometer for the National Ignition Facility: SENTINEL

Sentinel is a 16-channel, filtered x-ray diode array spectrometer that has been developed to measure ∼1 keV-20 keV x-ray emission generated by the National Ignition Facility (NIF) laser. Unlike the large, fixed-port versions of this diagnostic that currently exist on the NIF (known as Dante), Sentinel is a Diagnostic Instrument Manipulator compatible such that it can be fielded along the polar or equatorial lines-of-sight-an essential new capability for characterizing the often anisotropic x-ray emission from laser-driven sources. We present the diagnostic design along with preliminary diode calibrations and performance results. The novel, small-form-factor x-ray diode design allows for ≳5×-25× increased channel areal density over that of Dante, simultaneously enabling improved diagnostic robustness and fidelity of spectral reconstructions. While the Sentinel diagnostic is anticipated to improve line-of-sight spectral characterization of x-ray sources for a wide variety of programs on the NIF, the compact and portable design is also attractive to small- and mid-scale facilities with limited diagnostic real estate.

Atomic layer deposition-derived ultra-low-density composite bulk materials with deterministic density and composition

A universal approach for on-demand development of monolithic metal oxide composite bulk materials with air-like densities (<5 mg/cm(3)) is reported. The materials are fabricated by atomic layer deposition of titania (TiO2) or zinc oxide (ZnO) using the nanoscale architecture of 1 mg/cm(3) SiO2 aerogels formed by self-organization as a blueprint. This approach provides deterministic control over density and composition without affecting the nanoscale architecture of the composite material that is otherwise very difficult to achieve. We found that these materials provide laser-to-X-ray conversion efficiencies of up to 5.3%, which is the highest conversion efficiency yet obtained from any foam-based target, thus opening the door to a new generation of highly efficient laser-induced nanosecond scale multi-keV X-ray sources.

Demonstration of a 13-keV Kr K-shell x-ray source at the National Ignition Facility

We report 3% conversion efficiency of laser energy into Kr K-shell (≈13 keV) radiation, consistent with theoretical predictions. This is ≈10× greater than previous work. The emission was produced from a 4.1-mm-diameter, 4-mm-tall gas pipe target filled with 1.2 or 1.5 atm of Kr gas. 160 of the National Ignition Facility laser beams deposited ≈700 kJ of 3ω light into the target in an ≈140 TW, 5.0-ns-duration square pulse. The Dante diagnostics measured ≈5 TW into 4π solid angle of ≥12 keV x rays for ≈4 ns, which includes both continuum emission and flux in the Kr He_{α} line at 13 keV.

Source geometric considerations for OMEGA Dante measurements

The Dante is a 15 channel filtered diode array which is installed on the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The system yields the spectrally and temporally resolved radiation flux from 50 eV to 10 keV from various targets (i.e., Hohlraum, gas pipes, etc.). The absolute flux is determined from the radiometric calibration of the x-ray diodes, filters, and mirrors and an unfold algorithm applied to the recorded voltages from each channel. The unfold algorithm assumes an emitting source that is spatially uniform and has a constant area as a function of photon energy. The emitting x-ray source is usually considered to be the laser entrance hole (LEH) of a given diameter for Hohlraum type targets or the effective wall area of high conversion efficiency K-shell type targets. This assumption can be problematic for several reasons. High intensity regions or "hot spots" in the x-ray are observed where the drive laser beams strike the target. The "hot spots" create non-uniform emission seen by the Dante. Additionally, thinned walled (50 μm) low-Z targets (C(22)H(10)N(2)O(5)) have an energy dependent source size since the target's walls will be fully opaque for low energies (E < 2-3 keV) yet fully transmissive at higher energies. Determining accurate yields can be challenging for these types of targets. Discussion and some analysis will be presented.

The x-ray source application test cassette for radiation exposures at the OMEGA laser

We have designed a sample cassette that can be used to position up to six samples in the OMEGA laser chamber. The cassette accommodates round samples up to 38.1 mm (1.5(")) in diameter and square samples up to 27 mm on a side, any of which can be up to 12.7 mm thick. Smaller specimens are centered with spacers. The test cassette allows each sample to have a unique filter scheme, with multiple filter regions in front of each sample. This paper will present mechanical design considerations and operational aspects of the x-ray source application cassette.

Enhanced inner-shell x-ray emission by femtosecond-laser irradiation of solid cone targets

The possibility of enhancing inner-shell x-ray emission, especially Kα emission, by femtosecond-laser irradiation of solid cones instead of foils was investigated theoretically. In a model for hot electron (HE) transport and Kα x-ray generation, Kα emission from laser-irradiated solid cones and foils is investigated. As a complementarity to the model, the contributions from electric and magnetic fields generated by the HE current in solid cones and foils are discussed. The results indicate that the efficiency of HE energy conversion to Kα photons is improved and the optimum HE temperature is increased.

Note: Application of laser produced plasma K alpha x-ray probe in radiation biology

A dedicated radiation biology x-ray generation and exposure system has been developed. 8.0 keV in energy x-ray pulses generated with a femtosecond-laser pulse was used to irradiate sample cells through a custom-made culture dish with a silicon nitride membrane. The x-ray irradiation resulted in DNA double-strand breaks in the nucleus of a culture cell that were similar to those obtained with a conventional x-ray source, thus demonstrating the feasibility of radiobiological studies utilizing a single burst of x-rays focused on single cell specimens.

Conversion efficiency of high-Z backlighter materials

High-Z backlighter materials are commonly used as x-ray sources for diagnosing laser-driven experiments. In order to properly plan for experiments and analyze the data, it is important to understand both the number and distribution of photons emitted by the x-ray source when it is irradiated by a laser. The conversion efficiency of L-shell and M-shell emitters is not as well understood as K-shell emitters. The conversion efficiency of the former is typically presented in terms of the entire L- or M-shell spectral region. However, for some applications, one may only want to use a subset of this spectral region. Laser conversion efficiency for L-shell and M-shell emitters suitable for high-energy (>3 keV) absorption spectroscopy is presented at multiple laser intensities. The measured conversion efficiency of the materials ranges from 0.2% to 0.6%.

Measurement of 2l-nl' x-ray transitions from approximately 1 microm Kr clusters irradiated by high-intensity femtosecond laser pulses

X-ray line emission from 2l-nl' transitions in Ne-like Kr and nearby ions has been observed from approximately 1 microm Kr clusters irradiated by fs-scale laser pulses at the JAERI facility in Kyoto, Japan. The incident laser intensity reached 10(19) W/cm2, with pulse energies from 50 to 300 mJ and pulse durations from 30 to 500 fs. The dependence of the x-ray spectral features and intensity on the incident laser intensity is rather weak, indicating that the 1-2 ps cluster lifetimes limit the number of ions beyond Ne-like Kr that can be produced by collisional ionization. Lines from F- to Al-like Kr emitted from the cluster plasmas have been identified using data from the relativistic multiconfiguration flexible atomic code. A collisional-radiative model based on these data has been constructed and used to determine that the cluster plasma has electron densities near 10(22) cm(-3), temperatures of a few hundred eV, and hot electron fractions of a few percent.