Louise Willingale

Direct laser acceleration (DLA) enables energy transfer from an ultra-high-intensity laser to plasma electrons and underpins many laser-driven particle and radiation-source concepts. A laser-driven azimuthal plasma magnetic field is a key player in this process: it confines energetic electrons, induces betatron oscillations, and makes possible a resonant interaction between the betatron motion and the laser field. While this betatron resonance can enhance electron energy gain, the gain itself generally drives frequency detuning and promotes largely reversible energy exchange that limits net acceleration. Here we show, using a test-electron model with prescribed fields, that a slow longitudinal increase of the quasi-static plasma magnetic field qualitatively changes DLA by introducing hysteresis in the ratio of the betatron frequency to the laser frequency experienced by the electron, so that this ratio depends on the prior evolution of the electron even at the same energy. This hysteresis enables phase control of the electron-laser energy exchange and suppresses the usual reversibility of DLA, allowing electrons to retain the acquired energy and sustain energy gain without intermittent losses.

The transfer of a high-intensity laser pulse energy to a high-energy electron beam via the direct laser acceleration mechanism is shown to be significantly enhanced through control of the plasma density gradient. Experiments performed using the OMEGA EP facility's high-intensity beams altered the plasma density and gradients by changing the Mach number and the angle of the gas-jet nozzle to the laser axis. When a long density gradient at the rear of the target is used, the total high-energy electron number measured was enhanced by 4.5 times compared to a shorter rear gradient. Complementary two-dimensional simulations, which follow the laser field evolution and the corresponding electron dynamics, strongly support the key trends observed in the experiment. The effect is twofold, the long density gradient provides the longest acceleration distance while it minimizes the formation of the sheath field as the electron beam exits into the vacuum. This study shows the importance of tailoring the plasma density.

The Gérard Mourou Center for Ultrafast Optical Science (CUOS) at the University of Michigan has constructed a new high-power laser user facility called ZEUS (the Zettawatt Equivalent Ultrashort pulse laser System), which is funded by the National Science Foundation (NSF). The ZEUS currently operates at a power exceeding 2 PW with a pulse duration of 25 fs. The facility has three shielded target areas mainly dedicated to external user experiments. The development of the laser facility, specifically designed to enable frontier plasma physics experiments, is described.

We report on a single-shot longitudinal phase-space reconstruction diagnostic for electron beams in a laser wakefield accelerator via the experimental observation of distinct periodic modulations in the angularly resolved spectra. Such modulated angular spectra arise as a result of the direct interaction between the ultrarelativistic electron beam and the laser driver in the presence of the wakefield. A constrained theoretical model for the coupled oscillator, assisted by a genetic algorithm, can recreate the experimental electron spectra and, thus, fully reconstructs the longitudinal phase-space distribution of the electron beam with a temporal resolution of approximately 1.3 fs. In particular, it reveals the slice energy spread of the electron beam, which is important to measure for applications such as x-ray free electron lasers. In our experiment, the root-mean-square energy spread retrieved is bounded at 9.9 MeV, corresponding to a 0.9%–3.0% relative spread, despite the overall GeV energy beam having approximately 100% relative energy spread.

The Zettawatt-Equivalent Ultrashort pulse laser System (ZEUS) at the University of Michigan is a user facility supported by the National Science Foundation. The name ZEUS refers to the collision of a petawatt (PW) laser pulse with a GeV energy electron beam, producing the equivalent of a “Zettawatt” power laser interaction (1021 W) in the rest frame of the electron beam. The facility aims to enable systematic exploration of strong-field quantum electrodynamics processes by producing synchronized 2.5 PW and 0.5 PW pulses. The ZEUS target area has been designed to enable two collision geometries: one in which the 2.5 PW pulse drives laser wakefield acceleration (LWFA) to generate high-energy electron beams that collide with the 0.5 PW pulse, and another in which the 0.5 PW pulse drives LWFA and electrons collide with the 2.5 PW pulse focused to ultrahigh intensity. This paper describes initial designs and preparations for zettawatt-equivalent experiments at ZEUS.

Ultra-high intensity laser–plasma interactions can produce ultra-relativistic electrons via direct laser acceleration, assisted by quasi-static plasma magnetic and electric fields. These fields transversely confine electron motion and induce betatron oscillations. The net energy gain is strongly influenced by the interplay between two frequencies: the betatron frequency and the frequency of laser field oscillations experienced by the electron. Prior work has shown that energy gain is enabled by a resonance between the betatron oscillations and the oscillations of the laser field. In particular, higher-order resonances occur when the laser field completes multiple cycles during one betatron oscillation, allowing additional regimes of energy transfer beyond the fundamental (betatron) resonance. In this work, we demonstrate that such resonances become particularly effective when the laser's phase velocity is superluminal. Although the two frequencies generally evolve differently with increasing electron energy—leading to detuning—a superluminal phase velocity introduces a non-monotonic frequency ratio with a global minimum. This minimum allows sustained frequency matching over a broad energy range, thereby enabling enhanced energy gain. As the phase velocity increases, the betatron resonance becomes ineffective due to premature frequency detuning. At the same time, higher-order resonances become increasingly effective, emerging as the dominant mechanisms for enhanced energy gain in this regime of direct laser acceleration.

The OMEGA EP laser at the Laboratory for Laser Energetics was used to investigate high-power, relativistically intense laser interactions with near-critical density plasmas produced using low-density CH foam targets. A proton probe beam diagnostic, sensitive to the quasi-static electric and magnetic fields in the target, measured plasma-generated field structures along the foam interface with an aluminum target holder. Many discrete field structure sites were observed, suggesting filamentation of the electron beam during propagation through the plasma and magnetic field generation enhancement at plasma interfaces. Particle-in-cell simulations of the interaction were also performed, showing similar effects.

Magnetic field generation in ultraintense laser-solid interactions is studied over a range of laser intensities relevant to next-generation laser facilities ( a 0 = 50 – 500 ) using two-dimensional (2D) particle-in-cell simulations including strong-field quantum electrodynamic effects. It is found that fields O ( 0.1 ) MT (1 GG) may be generated by relativistic electrons traveling along the surface of the target. However, a significant fraction of the energy budget is converted to high-energy photons, approximately 38 % at a 0 = 500 , greatly reducing the available energy for field generation. A model for the evolution of the target-surface fields is created and the scaling of the field strength with a 0 is extracted from a set of 2D simulations. The simulated scaling allows for the estimation of field strengths and the model gives insight into the evolution of the fields on the next generation of laser facilities, a necessary component to the proposal of any future magnetized experiment.

Motivated by experiments employing picosecond-long, kilojoule laser pulses, we examined x-ray emission in a finite-length underdense plasma irradiated by such a pulse using two-dimensional particle-in-cell simulations. We found that, in addition to the expected forward emission, the plasma also efficiently emits in the backward direction. Our simulations reveal that the backward emission occurs when the laser exits the plasma. The longitudinal plasma electric field generated by the laser at the density down-ramp turns around some of the laser-accelerated electrons and re-accelerates them in the backward direction. As the electrons collide with the laser, they emit hard x rays. The energy conversion efficiency is comparable to that for the forward emission, but the effective source size is smaller. We show that the picosecond laser duration is required for achieving a spatial overlap between the laser and the backward energetic electrons. At peak laser intensity of 1.4×1020 W/cm2, backward-emitted photons (energies above 100 keV and 10° divergence angle) account for 2×10−5 of the incident laser energy. This conversion efficiency is three times higher than that for similarly selected forward-emitted photons. The source size of the backward photons (5 μm) is three times smaller than the source size of the forward photons.

Experiments using the OMEGA EP laser system were performed to study collisionless shock acceleration of ions driven by the interaction of a relativistically intense laser pulse with underdense plasma. The energy spectrum of accelerated ions in the direction transverse to laser propagation is measured to have several narrow-band peaks which are quasi-monoenergetic with a typical energy bandwidth of 3%. In deuterium plasmas, these ions generate a significant number of fast fusion neutrons. Particle-in-cell simulations confirm that these ions were accelerated by the interaction of transverse shocks and that the appearance of quasi-monoenergetic spectral features depends on the growth of an ion-electron two-stream instability during the interaction.