Flux and estimated spectra from a low-intensity laser-driven X-ray source
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-01-01 |
| Journal | Laser and Particle Beams |
| Authors | L. Tyler Mix, James A. Maslow, Michael Jaworski, J. E. Coleman |
| Institutions | Los Alamos National Laboratory |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the development and characterization of a benchtop soft X-ray source driven by a moderate-intensity nanosecond laser pulse incident on thin metal foils.
- Optimal Source Material: 5 ”m thick Copper (Cu) foils yielded the highest soft X-ray flux compared to Al, Ti, Fe, SS-304, and Sn targets at 4 x 1013 W/cm2 intensity.
- Spectral Characteristics: The estimated Cu X-ray spectrum peaks sharply near 2 keV, dominated by L-shell emission, with negligible flux observed above 4 keV.
- Hydrodynamic Dynamics: Time-resolved measurements using Diamond Radiation Detectors (DRDs) revealed a strong dependence of X-ray transport on foil opacity and hydrodynamic disassembly time.
- Ablation Rate Quantification: The average material ablation rate was measured to be approximately 1.64 ”m/ns for Cu, suggesting a thermal temperature for the sublimating material of about 0.6 eV (6900 K).
- Modeling Validation: One-dimensional radiation-magneto-hydrodynamic simulations (HELIOS-CR) provided qualitative agreement with experimental observations regarding X-ray timing and flux dependence on foil thickness.
- Core Value: The system provides reliable, nanosecond bursts of soft X-rays, making it an excellent source for detector calibration and specialized imaging.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Laser System Type | Flashlamp-pumped Nd:YAG | N/A | Frequency doubled |
| Laser Wavelength | 532 | nm | |
| Laser Energy (Max) | ~5 | J | Per pulse |
| Pulse Width (FWHM) | 8 | ns | |
| Laser Intensity (On Target) | 4 x 1013 | W/cm2 | Modest intensity regime |
| Focal Spot Diameter (1/e2) | ~40 | ”m | |
| Optimal Target Material | Cu | N/A | Highest X-ray yield |
| Optimal Cu Foil Thickness | 5 | ”m | |
| Measured Max Cu Ablation Rate | 1.78 | ”m/ns | Instantaneous rate for 10 ”m Cu |
| Average Cu Ablation Rate | 1.64 ± 0.07 | ”m/ns | Average across tested intensities |
| Estimated Thermal Temperature | ~0.6 | eV | Corresponds to 6900 K |
| Peak X-ray Emission (Cu) | 1.7 to 1.9 | keV | Estimated spectral peak |
| X-ray Emission Cutoff | >4 | keV | Negligible emission observed |
| DRD Instrument Response Time | ~200 | ps | Diamond Radiation Detectors |
| Sophia Camera QE | >60 | % | For X-rays between 0.5 and 5.0 keV |
| Simulated Max Plasma Temp | ~800 | eV | HELIOS-CR simulation (ablated material) |
Key Methodologies
Section titled âKey MethodologiesâThe experimental setup utilized a single-beam, moderate-intensity laser incident on thin metal foils, coupled with time-resolved and filtered imaging diagnostics.
- Laser Irradiation: A 5 J, 8 ns, 532 nm Nd:YAG pulse was focused to a 40 ”m spot, achieving 4 x 1013 W/cm2 intensity on various metal foils (Al, Ti, Fe, Cu, Sn, SS-304) mounted in a 10-7 Torr vacuum chamber.
- Time-Resolved Flux Measurement: Diamond Radiation Detectors (DRDs) were used to measure the induced current from X-rays. Detectors were placed in both the âforward directionâ (X-rays transiting the foil) and the âbackward directionâ (X-rays emitted from the laser incidence surface).
- Ablation Rate Calculation: The time-dependent ratio of the forward signal (IFor) to the backward signal (IBack) was used in conjunction with the transmission equation (IFor/IBack = e-”/Ï * z(t)) to calculate the material thickness z(t) over time, yielding the ablation rate (dz/dt).
- Coarse X-ray Spectroscopy: X-ray images were captured using a Sophia-XO CCD camera viewing the source through an array of eight different metal filters (Al, Ti, Fe, Cu).
- Spectral Estimation: An Expectation-Maximization (EM) algorithm, commonly used in medical computed tomography, was applied to the filtered images to iteratively estimate the X-ray spectrum, ensuring the calculated signal matched the measured signal within 10% error.
- Hydrodynamic Simulation: One-dimensional radiation-magneto-hydrodynamic simulations were performed using the HELIOS-CR software, coupled with SPECT3D, to model the laser-foil interaction, plasma heating (up to ~800 eV), and resulting X-ray emission spectra.
Commercial Applications
Section titled âCommercial ApplicationsâThis low-intensity, laser-driven soft X-ray source is valuable for applications requiring high-flux, nanosecond X-ray bursts in the 1-5 keV range.
- X-ray Detector Calibration: Used for calibrating sensitive X-ray detectors (like DRDs and CCD cameras) and spectrometers prior to installation in large-scale high-energy-density physics facilities (e.g., DARHT, NIF).
- Biological and Medical Imaging: The high yield in the 1-5 keV range is ideal for soft X-ray biological imaging, offering high contrast for organic materials.
- Plasma Diagnostic Development: Serves as a compact, benchtop source for fundamental studies of laser-plasma interactions and the development of new X-ray diagnostics.
- Materials Science Research: Used to probe material response and hydrodynamic disassembly processes under moderate laser irradiation conditions.
- Specialized Lithography: Potential application in X-ray lithography requiring moderate energy and short pulse widths for high efficiency.
View Original Abstract
Abstract Laser-driven X-rays as probes for high-energy-density physics spans an extremely large parameter space with laser intensities varying by 8 orders of magnitude. We have built and characterized a soft X-ray source driven by a modest intensity laser of 4 Ă 10 13 W/cm 2 . Emitted X-rays were measured by diamond radiation detectors and a filtered soft X-ray camera. A material-dependence study on Al, Ti, stainless steel alloy 304, Fe, Cu and Sn targets indicated 5-ÎŒm-thick Cu foils produced the highest X-ray yield. X-ray emission in the laser direction and emission in the reverse direction depend strongly on the foil material and the thickness due to the opacity and hydrodynamic disassembly time. The time-varying X-ray signals are used to measure the material thinning rate and is found to be âŒ1.5 ÎŒm/ns for the materials tested implying thermal temperature around 0.6 eV. The X-ray spectra from Cu targets peaks at âŒ2 keV with no emission >4 keV and was estimated using images with eight different foil filters. One-dimensional hydrodynamic and spectral calculations using HELIOS-CR provide qualitative agreement with experimental results. Modest intensity lasers can be an excellent source for nanosecond bursts of soft X-rays.