Development of the Talbot-Lau X-Ray Deflectometer (TXD) Diagnostic for OMEGA EP

July, 2021

The Johns Hopkins X-ray Imaging Group has developed an advanced refraction-based x-ray diagnostic for high-energy-density laboratory plasmas (HEDLP) under NNSA sponsorship. Talbot–Lau x-ray interferometry provides electron density diagnostics by measuring the refraction of a probing x-ray, produced by the changes in density in a probed plasma. In deflectometry mode,  phase-contrast, attenuation, and small-angle scattering can be inferred by Fourier analysis from a single moiré image. It can also provide elemental composition and detect the presence of microstructures.

Salaahuddin Karim, Sarah Fess, Tim Filkins, Pia Valdivia (Johns Hopkins U.), Michael Krieger, Mark Romanofsky, Chad Mileham, Christian Stoeckl (inset)

Talbot Lau X-Ray Deflectometer Team: Salaahuddin Karim; Sarah Fess; Tim Filkins; Pia Valdivia, Principal Investigator (Johns Hopkins University); Michael Krieger; Mark Romanofsky; Chad Mileham; and Christian Stoeckl (inset)

Pia Valdivia working on the Talbot–Lau X-ray Deflectometer

Principal Investigator, Pia Valdivia, (Johns Hopkins University)  working on the Talbot–Lau x-ray deflectometer

A Talbot–Lau x-ray deflectometer (TXD) capable of diagnosing electron density gradients relevant to inertial confinement fusion was implemented on OMEGA EP to characterize the evolution of an irradiated foil ablation front by mapping line-integrated electron density gradients. In the first experiment to benchmark the diagnostic platform, grating survival and moiré fringe formation with 60- to 80-µm spatial resolution was demonstrated. In preparation for a follow-up campaign, x-ray backlighter studies on the Multi-Terawatt (MTW) laser characterized source size and spectral quality of K-shell emission from irradiated copper foil, wire, and bookend targets to optimize photon flux and spatial resolution. A  second OMEGA EP campaign (sponsored by LaserNetUS) mapped the ablation front of a CH foil irradiated by a 150-J, 2-ns laser pulse  5 ns after laser incidence. The EP-TXD diagnostic recorded, for the first time, x-ray refraction angles <150 µrad with spatial resolutions of 10 to 30 µm.

A third campaign is currently underway to increase the moiré fringe contrast by developing monochromatic Talbot–Lau capabilities because the background signals observed in previous experiments significant decreases contrast and hampers the accuracy and sensitivity of phase-retrieval from moiré images. This monochromatic Talbot-Lau x-ray deflectometer (EP-M-TXD) includes a multilayer mirror that selects the K-alpha 8 keV emission from copper targets to further enhance the angular refraction sensitivity. A first prototype was designed, implemented, and tested on MTW and OMEGA EP.

A new National Laser Users Facility (NLUF) campaign will seek to optimize x-ray backlighting for EP-M-TXD diagnostics to characterize shell-release experiments as well as probing irradiated foils through TXD to help benchmark magnetohydrodynamical codes and simulations by mapping the evolution of the ablation front near critical density. The deflectometer was previously featured as a Quick Shot in March of 2019.

Monochromatic Talbot–Lau x-ray deflectometer in background with inset photo of Talbot–Lau x-ray deflectometer during shot 34890.

Four of the EP-M-TXD deflectometers are shown here. The inset image shows the EP-M-TXD in use during a target shot.