CUOS Seminar: Momentum and absorption, Phase transitions, and Transparency

Nick Ernst

Experimental application of orbital angular momentum (OAM) for enhanced laser absorption and neutron generation

Laser driven neutron sources boast several desirable characteristics including synchronized ultrashort pulse duration (sub-ns), small source size (sub-mm), and intrinsic proliferation safety – but have struggled to provide widespread application for the likes of neutron imaging and prompt gamma ray activation given their low-average flux when compared to conventional devices. Here we take a significant step in bridging this gap and present the first demonstration of enhanced absorption by optical vortex beams in free-flowing D₂O streams to generate unprecedented neutron yields of 1.45 × 10⁶ n/s/sr on the high-repetition, λ³ Laser at the University of Michigan (16 mJ, 480 Hz, 67 fs). We show taking advantage of lasers carrying orbital angular momentum (OAM) with topological charge l = 1 & l = 5 impinging at grazing incidence on a free-flowing, ∼20 μm heavy-water jet provides marked enhancement in laser absorption and resulting fast neutron yields. The increased heating of the plasma by OAM beam is observed despite a decrease in intensity by two orders of magnitude relative to similar arrangements and relaxed target conditions. Tabletop neutron flux of this level is comparable to that characteristic of commercial generators and we anticipate repetition rate scaling (1-10 kHz) of this interaction could easily generate ∼10⁸ n/s and be well suited for a variety of commercial, industrial, scientific, and national security implications.

André Antoine

Characterization of Non-Thermal Phase Transitions in MgO and NaCl with Two-color X-ray Pulses

With x-ray Free Electron Lasers (FEL), intense x-ray FEL pulses interact with samples, which can alter their electronic and atomic structures. Previous experiments on x-ray FEL-matter interactions have mostly focused on semiconductors like diamond and silicon. However, little is known about x-ray-induced bond breaking in multi-element solids or ionic solids. Recent calculations have predicted a crystalline-to-disordered phase transition in sodium chloride (NaCl) upon high-intensity x-ray interaction. Using FEL x-ray pump and x-ray probe pulses, we can induce non-thermal phase transitions and detect new material phases. We have investigated the time-dependent intensity of diffraction peaks in NaCl and magnesium oxide (MgO). Our experimental observations are compared with density functional theory and particle-in-cell simulations. Our findings reveal the ultrafast responses of these materials, analyzed through the variations in diffraction peak intensities.

Brendan Stassel

Experimental measurements of relativistically induced transparency at BELLA iP2

Petawatt laser systems provide researchers access to the relativistic transparency regime for solid density targets. Ongoing work in particle acceleration and laser energy absorption shows that the relativistically induced transparency regime is important to the future of laser-plasma physics. Analytical theory provides an approximation for the onset-time to relativistic transparency in thin foils, but there is a lack of experimental data that supports the model. At the BELLA Center’s iP2 beamline, we developed a suite of optical diagnostics to characterize the transmitted and reflected light from a high intensity (>10²¹ W/cm²) laser plasma interaction on silicon nitride membranes and near-critical density foams. A frequency resolved optical gating (FROG) device measured the temporal and spectral profiles of the reflected light, while the transmitted light was measured by a GRENOUILLE device and spectrometer. Additional diagnostics were used to verify the high intensity laser pulse interacted with an overdense plasma. We present preliminary experimental data showing the temporal and spectral changes to the laser pulse as it relativistically interacts with the various targets, and how it compares to the most recent theoretical predictions.

Pizza will be served!