D. DePonte | Searcharxiv

arXiv2020

Design and Characteristics of a Population Inversion X-ray Laser Oscillator

Oscillators are at the heart of optical lasers, providing stable transform limited pulses. In contrast, X-ray free electron lasers use self-amplified spontaneous emission (SASE), resulting in large stochastic intensity and spectral fluctuations. Amplified spontaneous emission (ASE) of the $Kα_1$ line has been recently observed for Ne gas, Cu compounds and Mn solutions at the LCLS and SACLA X-ray free electron lasers (XFELs), using an X-ray SASE pulse as a pump to create population inversion. Here we describe the physics and realization of an X-ray laser oscillator (XLO) based on periodically pumping a Cu compound gain medium in a tunable Bragg cavity with a SASE pulse train, generating intense ($\sim$ 5 x 10$^{10}$ ph/pulse), fully coherent, transform limited 8 keV pulses with 48 meV spectral resolution. We also discuss extending these results to other elements to operate XLO from about 5 to 12 keV, improving X-ray-based research beyond current capabilities.

physics.acc-ph↗

arXiv2008

X-Ray Powder Diffraction from Sub-Micron Crystals of Photosystem-1 Membrane Protein

We demonstrate that powder diffraction data can be collected from sub-micron crystals of a mbrane protein with nearly two orders of magnitude more atoms than the molecules commonly used for powder diffraction. The crystals of photosystem-1 protein were size-selected using a 500 nm pore- size filter and delivered to a soft x-ray beam with a photon energy of 1.5 keV using a dynamically focused micro-jet developed for the serial crystallography experiment at beamline 9.0.1. The 10-micron jet places many such randomly oriented crystals in the x-ray beam simultaneously resulting in a powder diffraction pattern which extends to 28 angstrom resolution with just 200 seconds of x-ray exposure. The use of the jet for particle delivery allows for a thin sample, appropriate for the soft x-rays used, and continuously refreshes the crystals so that radiation damage is not possible. The small size of the crystals requires the use of lower energy photons for increased scattering strength and increased spacing between powder rings. The powder patterns obtained in this way, from abundant nano-crystals, could be used to provide low resolution molecular envelopes if phased using techniques such as compressive sensing which do not require atomic resolution data. The results also serve to test our aerojet injector system, with future application to femtosecond diffraction in Free Electron X-ray Laser schemes, and for Serial Crystallography using a single-file beam of aligned hydrated molecules.

physics.bio-ph↗