An Oscillator Configuration for Full Realization of Hard X-ray Free Electron Laser

At a Glance

Metadata	Details
Publication Date	2016-01-01
Journal	OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information)
Authors	K. -J. Kim, Tomasz Kołodziej, Ryan Lindberg, Deming Shu, Yuri Shvyd’ko
Citations	2
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond X-ray Optics for Hard XFELO

This document analyzes the requirements for diamond X-ray optics as detailed in the research paper “AN OSCILLATOR CONFIGURATION FOR FULL REALIZATION OF HARD X-RAY FREE ELECTRON LASER.” It connects these stringent specifications directly to the advanced material science capabilities offered by 6CCVD.

Executive Summary

Application Validation: The research confirms the feasibility of a Hard X-ray Free Electron Laser Oscillator (XFELO) based on the LCLS-II 4 GeV linac, utilizing diamond crystals as essential Bragg mirrors.
Material Selection: Single Crystal Diamond (SCD) was chosen as the optimal material due to its superior optical, thermal, and mechanical properties, which are critical for high-power X-ray applications.
Performance Achieved: Experimental results demonstrated near-100% Bragg reflectivity (> 99%) for 13.9 keV and 23.7 keV X-rays using high-quality, defect-free SCD.
High Power Resilience: Diamond samples (100 µm thick) successfully withstood extreme X-ray power densities (1-10 kW/mm²) comparable to XFELO operating conditions, confirming material survivability.
Thermal Requirement: Successful XFELO operation necessitates cryogenic cooling (T ≤ 100K) to leverage diamond’s unmatched thermal diffusivity and near-zero thermal expansion coefficient, ensuring lattice homogeneity.
Stability Challenge: The system requires extremely high angular stability (< 10 nrad rms) and positional stability (3 µm rms), demanding ultra-precise, high-purity SCD optics and advanced mounting techniques.

Technical Specifications

The following hard data points summarize the critical operational parameters and material requirements for the LCLS-II based XFELO.

Parameter	Value	Unit	Context
Electron Energy	4	GeV	LCLS-II Linac Source
Photon Energy (@ 5^th harm.)	14.4	keV	XFELO Output
Spectral Bandwidth (FWHM)	5	meV	Required Output Coherence
Peak Power Density on Diamond	1 - 10	kW/mm²	XFELO Cavity Exposure
Required Operating Temperature	T ≤ 100	K	Thermal Management Requirement
Required Angular Stability	< 10	nrad (rms)	Crystal/Mirror Stabilization Goal
Required Positional Stability	3	µm (rms)	Crystal/Mirror Stabilization Goal
Tested Bragg Reflectivity	> 99	%	Near normal incidence (13.9/23.7 keV)
Tested Diamond Thickness	100	µm	Resilience Study Sample
Norm. Emittance	0.3	µm	Electron Beam Quality

Key Methodologies

The experimental validation focused heavily on material characterization and resilience testing under simulated XFELO conditions.

Material Selection and Growth: High-quality, defect-free Single Crystal Diamond (SCD) was sourced from multiple manufacturers, confirming that modern MPCVD techniques can produce material suitable for XFELO optics.
Reflectivity Measurement: X-ray optics experiments were conducted using 13.9 keV and 23.7 keV photons to establish that the predicted Bragg reflectivity of diamond is feasible, achieving greater than 99% at near normal incidence.
Thermal Management Modeling: Simulations determined that the radiation heat load requires the diamond crystal to be cryogenically cooled (T ≤ 100K). This ensures sufficient time for heat conduction and maintains crystal temperature homogeneity before the arrival of the subsequent pulse.
High-Power Resilience Testing: A 100 µm thick diamond sample was irradiated at the APS 35-ID beamline using 8 keV X-rays focused to an 8 kW/mm² power density, simulating XFELO cavity conditions.
Damage and Spectral Analysis: The irradiated spots were analyzed using rocking curve FWHM maps and high-resolution topography (10^-8 accuracy). This confirmed no discernible change in spectral response after up to 4 hours of exposure, demonstrating material resilience.
Stability Demonstration: A pilot experiment using a six-crystal X-ray monochromator achieved an angular stability of 13 nrad (rms), demonstrating progress toward the stringent < 10 nrad stability requirement needed for the XFELO cavity.

6CCVD Solutions & Capabilities

The realization of the Hard XFELO relies entirely on the availability of ultra-high-purity, precisely manufactured SCD optics. 6CCVD is uniquely positioned to supply the materials and customization required to meet or exceed the specifications outlined in this research.

Applicable Materials

To replicate or extend this research, the following 6CCVD material is required:

Optical Grade Single Crystal Diamond (SCD): Required for the Bragg mirrors. Our MPCVD growth process yields high-purity, low-defect SCD necessary to achieve near-100% reflectivity and maintain stability under high thermal load at cryogenic temperatures.

Customization Potential

The paper highlights the need for specific thicknesses (e.g., 100 µm) and extremely high surface quality. 6CCVD offers the following capabilities essential for XFELO development:

Requirement from Paper	6CCVD Capability	Specification
Precision Thickness Control	Custom SCD Thickness	0.1 µm - 500 µm (Routinely supplied at 100 µm ± 5 µm tolerance)
Ultra-Smooth Surface Finish	Advanced Polishing	Ra < 1 nm (SCD)
Large Area Optics	Custom Dimensions	Plates/wafers up to 125 mm (PCD) or large SCD plates
Robust Mounting & Stability	Custom Metalization Services	In-house deposition of Au, Pt, Pd, Ti, W, Cu for low-stress bonding and thermal contact at T ≤ 100K.
Global Logistics	Shipping	Global shipping (DDU default, DDP available)

Engineering Support

6CCVD’s in-house PhD team specializes in the thermal, mechanical, and optical properties of diamond for extreme environments. We can assist researchers with:

Material Selection: Optimizing SCD grade and orientation for specific X-ray energies (e.g., 14.4 keV) and Bragg angles.
Thermal Management: Providing consultation on material integration and thermal contact design to ensure the crystal maintains homogeneity at cryogenic temperatures (T ≤ 100K).
Surface Preparation: Ensuring the required Ra < 1 nm surface finish is achieved to minimize scattering and support the stringent < 10 nrad angular stability requirements for X-ray optical components.

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

View Original Abstract

An X-ray free electron laser can be built in an oscillator (XFELO) configuration by employing an X-ray cavity with Bragg mirrors such as diamond*. An XFELO at the 5th harmonic frequency may be implemented at the LCLS II using its 4 GeV superconducting linac. The XFELO will provide stable, coherent, high-spectral-purity hard x-rays. In addition, portions of its output may be enhanced by the LCLS amplifier for stable pulses of ultrashort duration determined by the electron bunch length. Much progress has been made recently on the feasibility of an XFELO: Analytical and numerical methods have been developed to compute the performance of a harmonic XFELO. The energy spread requirement over a sufficient length of the bunch can be met by temporal shaping of the photo-cathode drive laser**. Experiments at the APS have shown that Be-compound refractive lenses are suitable for a low-loss focusing and that the synthetic diamond crystals can withstand the intense x-ray exposure, in accord with estimates based on molecular dynamics considerations***. A strain-free mounting of thin diamond crystal (< 100 microns) can be realized by shaping a thick diamond into a blind alley****.

Tech Support

Original Source

DOI: https://doi.org/10.18429/jacow-ipac2016-mopow039