Tabletop Fast Neutron Sources Driven by High Repetition Rate Ultrashort Laser Systems

Abstract

The interaction of relativistic ultrashort laser pulses with matter can generate extreme acceleration gradients and localized plasma temperatures suitable for nuclear fusion. To date, experimental campaigns at various laboratories have successfully demonstrated sources of relativistic electrons and multi-MeV ions and secondary radiation in the form of intense ultrashort (fs-ps) pulses of X-rays, gamma rays, relativistic positrons, and neutrons from intense light-matter interaction. Historically, the chirped pulse amplification (CPA) lasers capable of generating Terawatt/Petawatt (high-field) peak power density necessary to drive such laser-plasma accelerators (LPA) have been limited to repetition rates of 1-10 Hz. Recently, solid-state diode-pumped optical parametric chirped-pulse amplifiers, Yb^3+ -doped thin-disk amplifiers, and Yb^3+ -doped large-core effectively single mode fiber optic amplifiers have demonstrated unprecedented scaling toward delivery of 0.1-1 J, <100 fs laser pulses at 1-10kHz. These advances in the field of high-power laser engineering have ushered in the era of third-generation femtosecond technology (3FST) with the stated objective of realizing terawatt-scale peak power at kilowatt-scale average power. Such “TW at a kW” photonic systems are ideal for driving high-brightness tabletop fast neutron sources.

This thesis work addresses the challenge of scaling relativistic laser targets to the kHz repetition regime with specific application to ultrashort laser-driven neutron sources (LDNS). First, the design, development, and testing of a novel pulsed electrohydrodynamic (EHD) microjet nozzle is presented. Proof-of-principle experiments carried out at 0.480 kHz on the University of Michigan relativistic lambda cubed laser resulted in 4x10^4 n/sec/sr isotropic yield from laser-driven D(d, n)He-3 fusion reactions in a 2.7 µm deuterated microjet. The fast neutron flux is confirmed via bubble dosimeters, solid-state Li6-doped microstructured semiconductor neutron detectors, He-3 proportional detectors and a neutron time-of-flight (nTOF) detection scheme using pulse shape discrimination enabled and time-gated EJ-309 liquid scintillation detectors. Building upon the suite of high repetition neutron diagnostics developed for the EHD targetry campaign, an experiment exploring neutron generation from relativistic optical vortex beams is presented. Irradiation of free-flowing D2O liquid microjets with orbital angular momentum (OAM) beams of topological charge l = 1 and l = 5 is shown to generate a record D-D neutron yield of over 10^6 n/sec/sr. To explain this marked source enhancement, a basic particle-in-cell (PIC) computational model is constructed to validate OAM beam self-focusing and subsequent filamentation due to plasma density inhomogeneity at the overdense target-vacuum boundary.

Lastly, the results from a collaborative experimental LDNS campaign utilizing a novel ultrashort fiber CPA laser based on coherent beam combination and coherent pulse stacking amplification are discussed. To the best of our knowledge, this experiment demonstrates the first high repetition (2-10 kHz) tabletop neutron source driven by an all-fiber femtosecond laser. As part of this effort, a synchronous off-color ultrashort laser backlight was developed as a high repetition optical diagnostic. This novel probe system is capable of arbitrary femtosecond-to-millisecond delay and is based on the application of gain-managed nonlinear amplification (GMNA) in Yb-doped fibers. Proof-of-principle operation of the backlight system is experimentally demonstrated through time-resolved imaging of a propagating laser-driven shock front with the frequency-doubled GMNA output beam.