Flux and estimated spectra from a low-intensity laser-driven X-ray source

At a Glance

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

Technical Documentation & Analysis: Laser-Driven X-ray Source Characterization

Reference Paper: Mix LT, Maslow JA, Jaworski MA, Coleman JE (2024) Flux and estimated spectra from a low-intensity laser-driven X-ray source. Laser and Particle Beams 42, e1, 1-9.

Executive Summary

This research validates the use of modest-intensity laser systems (4 x 10¹³ W/cm²) to generate nanosecond bursts of soft X-rays (1-5 keV), a critical capability for detector calibration and high-energy-density (HED) physics diagnostics.

Diagnostic Validation: High-speed, time-resolved measurements were successfully achieved using Diamond Radiation Detectors (DRDs), confirming their essential role in ultra-fast X-ray diagnostics (response time ~200 ps).
Optimal Source Material: Copper (Cu) foils (5 µm thick) were identified as the optimal target material, yielding the highest X-ray flux with peak emission centered around 2 keV.
Hydrodynamic Insights: The ratio of forward/backward X-ray emission was used to calculate material ablation rates (up to 1.78 µm/ns for Cu), providing key data for modeling foil disassembly dynamics.
Material Requirement: The DRDs utilized in this study require high-purity, high-quality Single Crystal Diamond (SCD) to ensure rapid carrier collection and minimal signal noise.
6CCVD Relevance: 6CCVD is the expert supplier of the custom SCD material, precision dimensions, and metalization required to replicate and advance these high-performance DRDs for next-generation HED experiments.
Future Scaling: The paper suggests future intensity scaling (up to 3.6 x 10¹⁴ W/cm²), necessitating robust, high-damage-threshold SCD detectors, a core offering of 6CCVD.

Technical Specifications

The following table summarizes the key experimental parameters and performance metrics extracted from the research paper.

Parameter	Value	Unit	Context
Laser Intensity (on target)	4 x 10¹³	W/cm²	Modest intensity regime
Laser Pulse Energy	5	J	Nd:YAG, frequency doubled
Laser Pulse Duration (FWHM)	8	ns	Time resolution for X-ray generation
X-ray Detector Type	Diamond Radiation Detectors (DRDs)	N/A	Single Crystal Diamond (SCD)
DRD Response Time	~200	ps	Required for time-resolved measurements
DRD Dimensions	3 x 1 x 1	mm	Custom size requirement
X-ray Sensitivity Range	< 6	keV	Primary sensitivity range of DRDs
Optimal Target Material	Copper (Cu)	N/A	Produced highest X-ray yield
Optimal Cu Foil Thickness	5	µm	Thickness used for highest yield
Peak X-ray Emission (Cu)	~2	keV	Estimated spectral peak
Maximum Cu Ablation Rate	1.78	µm/ns	Instantaneous rate (10 µm foil)
Target Chamber Vacuum	10^-7	Torr	Required experimental environment
Future Intensity Goal	3.6 x 10¹⁴	W/cm²	Goal for increased X-ray flux (9x increase)

Key Methodologies

The experiment focused on characterizing the X-ray emission from laser-ablated metal foils using high-speed diamond detectors and filtered imaging.

Laser Setup: A flashlamp-pumped Nd:YAG laser, frequency doubled to 532 nm, delivered a 5 J, 8 ns pulse.
Targeting: The beam was focused via a 150 mm lens to a 1/e² diameter of ~40 µm, achieving 4 x 10¹³ W/cm² intensity on thin metal foils (Al, Ti, Fe, Cu, Sn, SS-304) mounted in a 10^-7 Torr vacuum chamber.
Time-Resolved Detection: Single Crystal Diamond Radiation Detectors (DRDs) (3 x 1 x 1 mm) were mounted in two arrays:
- Backward Direction: Facing the front surface of the target (10 cm distance).
- Forward Direction: Behind the target, measuring X-rays transmitted through the thinning foil (18 cm distance).
Signal Acquisition: DRDs were biased at -150 V, and the induced current was recorded, leveraging the detector’s ~200 ps response time.
Spectral Estimation: A Sophia-XO CCD camera, filtered by eight different metal foils, was used to estimate the X-ray spectrum via an expectation-maximization algorithm.
Ablation Rate Calculation: The ratio of forward (I_For) to backward (I_Back) signals was used in the transmission equation (I_For / I_Back = e^{-µ/ρ * ρz(t)}) to calculate the time-dependent foil thickness z(t) and ablation rate (dz/dt).

6CCVD Solutions & Capabilities

This research highlights the critical need for high-performance SCD diamond materials in advanced X-ray diagnostics. 6CCVD is uniquely positioned to supply the custom materials and fabrication services required to replicate and scale the DRDs used in this study.

Applicable Materials

To achieve the required ~200 ps response time and radiation hardness for HED physics applications, the following 6CCVD material is explicitly recommended:

Optical Grade Single Crystal Diamond (SCD): Required for high-speed photoconductive detectors (DRDs). Our SCD material ensures high purity, low defect density, and excellent carrier mobility, which are essential for minimizing signal rise time and maximizing sensitivity to soft X-rays (< 6 keV).

Customization Potential

The success of the DRDs relies on precise dimensions and robust electrical contacts. 6CCVD offers full customization to meet or exceed the specifications used in this research:

Research Requirement	6CCVD Capability	Specification Match/Exceedance
Detector Dimensions	Custom Laser Cutting & Fabrication	We provide SCD plates in custom sizes (e.g., 3 mm x 1 mm x 1 mm) and wafers up to 125 mm (PCD). SCD thickness available from 0.1 µm up to 500 µm.
Electrical Contacts	In-House Metalization Services	We offer custom deposition of Au, Pt, Ti, W, Pd, and Cu contacts, ensuring stable, low-resistance ohmic contacts necessary for the -150 V bias and high-fidelity signal extraction.
Surface Finish	Precision Polishing	SCD surfaces can be polished to an industry-leading roughness of Ra < 1 nm, critical for minimizing surface leakage current and maximizing detector uniformity.
Substrate Robustness	Thick Substrates for HED	For future high-intensity experiments (up to 3.6 x 10¹⁴ W/cm²), we supply robust SCD substrates up to 10 mm thick, offering superior thermal management and radiation damage resistance compared to standard materials.

Engineering Support

The successful implementation of diamond detectors in complex laser-plasma experiments requires specialized material knowledge.

Material Selection for HED: 6CCVD’s in-house PhD team can assist with material selection for similar Laser-Driven X-ray Source Characterization and High-Energy-Density Physics projects, ensuring the optimal balance of purity, thickness, and metalization scheme for specific fluence and time-resolution requirements.
Boron Doped Diamond (BDD) Extension: While this paper focused on intrinsic SCD, 6CCVD also offers Boron-Doped Diamond (BDD) for applications requiring stable, conductive electrodes or specialized electrochemical sensing in related plasma environments.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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 × 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-μm-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 ∼1.5 μm/ns for the materials tested implying thermal temperature around 0.6 eV. The X-ray spectra from Cu targets peaks at ∼2 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.

Tech Support

Original Source

DOI: https://doi.org/10.1017/lpb.2024.1