Dustin H. Froula

The flying focus features an intensity peak that propagates at a focal velocity that is decoupled from the group velocity of the medium. Control over the velocity of the focal point is predicted to revolutionize secondary source experiments by allowing velocity matching of physical processes at the core of nonlinear optics and high-field physics. Despite this promise, experimental demonstrations have lagged pioneering theoretical work due to the difficulty of creating an ultrashort flying focus in the laboratory. In this article, the spatiotemporal shaping of a terahertz pulse generated via gas-phase photoionization by a two-color, ultrashort flying focus is described. The combination of radial group delay provided by reflective, radially-stepped echelons and spherical aberration produced by an axiparabola generates a flying focus with sub- and superluminal focal point velocities. The flying focus efficiently generates terahertz radiation that exhibits fundamentally different spatiotemporal properties based on its velocity. We present detailed measurements of the spatial and temporal properties of the terahertz radiation and measure its focal velocity. This demonstration shows the utility of terahertz generation with a flying focus and enables future experiments in nonlinear optics with spatiotemporal control.

We formulate a theory of Thomson scattering (TS) from inhomogeneous plasmas using a ray-tracing approach. The scattered radiation is described within the eikonal approximation, including convective gain arising from scattering instabilities driven by the TS probe laser. In the limit of a weak probe and vanishing gain, the theory recovers the standard TS cross section and scattered power due to volumetric particle noise. The ray-tracing formalism is implemented in a TS fitting framework, enabling accurate analysis of inhomogeneous plasmas whose background profiles can be provided by large-scale hydrodynamic simulations or experimental diagnostics. Several examples of synthetic TS spectra are presented, and further extensions of the theory are discussed.

The flying focus features an intensity peak that propagates at a focal velocity that is decoupled from the group velocity of the medium. Control over the velocity of the focal point is predicted to revolutionize secondary source experiments by allowing velocity matching of physical processes at the core of nonlinear optics and high-field physics. Despite this promise, experimental demonstrations have lagged pioneering theoretical work due to the difficulty of creating an ultrashort flying focus in the laboratory. In this article, spatiotemporal shaping of a terahertz pulse generated via gas-phase photoionization by a two-color, ultrashort flying focus is described. The combination of radial group delay provided by reflective, radially-stepped echelons and spherical aberration produced by an axiparabola generate a flying focus with sub- and superluminal focal point velocities. The flying focus efficiently generates terahertz radiation that exhibits fundamentally different spatiotemporal properties based on its velocity. We present detailed measurements of the spatial and temporal properties of the terahertz radiation and measure its focal velocity. This demonstration shows the utility of terahertz generation with a flying focus and enables future experiments in nonlinear optics with spatiotemporal control.

Optical techniques for spatiotemporal control can produce laser pulses with custom amplitude, phase, or polarization structure. In nonlinear optics and plasma physics, the use of structured pulses typically follows a forward design approach, in which the efficacy of a known structure is analyzed for a particular application. Inverse approaches, in contrast, enable the discovery of new structures with the potential for superior performance. Here, an implementation of the unidirectional pulse propagation equation that supports automatic differentiation is combined with gradient-based optimization to design structured pulses with features that are advantageous for a range of nonlinear optical and plasma-based applications: (1) a longitudinally uniform intensity over an extended region, (2) a superluminal intensity peak that travels many Rayleigh ranges with constant duration, spot size, and amplitude, and (3) a laser pulse that ionizes a gas to form a uniform column of plasma. In the final case, optimizing the full spatiotemporal structure improves the performance by a factor of 15 compared to optimizing only spatial or only temporal structure, highlighting the advantage of spatiotemporal control.

The generation of dense electron–positron pair beams in the laboratory can enable direct tests of theoretical models of γ -ray bursts and active galactic nuclei. We have successfully achieved this using ultrarelativistic protons accelerated by the Super Proton Synchrotron at CERN. In the first application of this experimental platform, the stability of the pair beam is studied as it propagates through a meter-length plasma, analogous to TeV γ -ray-induced pair cascades in the intergalactic medium. It has been argued that pair beam instabilities disrupt the cascade, thus accounting for the observed lack of reprocessed GeV emission from TeV blazars. If true, this would remove the need for a moderate strength intergalactic magnetic field to explain the observations. We find that the pair beam instability is suppressed if the beam is not perfectly collimated or monochromatic, hence the lower limit to the intergalactic magnetic field inferred from γ -ray observations of blazars is robust.

Optical techniques for spatiotemporal control can produce laser pulses with custom amplitude, phase, or polarization structure. In nonlinear optics and plasma physics, the use of structured pulses typically follows a forward design approach, in which the efficacy of a known structure is analyzed for a particular application. Inverse approaches, in contrast, enable the discovery of new structures with the potential for superior performance. Here, an implementation of the unidirectional pulse propagation equation that supports automatic differentiation is combined with gradient-based optimization to design structured pulses with features that are advantageous for a range of nonlinear optical and plasma-based applications: (1) a longitudinally uniform intensity over an extended region, (2) a superluminal intensity peak that travels many Rayleigh ranges with constant duration, spot size, and amplitude, and (3) a laser pulse that ionizes a gas to form a uniform column of plasma. In the final case, optimizing the full spatiotemporal structure improves the performance by a factor of 15 compared to optimizing only spatial or only temporal structure, highlighting the advantage of spatiotemporal control.

We present results from an experiment carried out at the OMEGA-EP laser facility that investigated the effect of a perpendicular magnetic field on stimulated Raman scattering (SRS) and report the first direct measurement of magnetic mitigation of SRS. A 13-T magnetic field generated by pulsed-power coils was imposed on a gas jet plasma, and a novel three-picket interaction beam was used to explore SRS reflectivity for several plasma conditions within single shots. The time-resolved backscattered light shows that SRS was mitigated by the external 13-T magnetic field in the kinetic regime (kλD∼0.3, where k is the electron plasma wave's wavenumber and λD is electron Debye length) and at a density of ne/ncr∼0.10. On the other hand, we also measured an enhancement of SRS reflectivity at lower density (ne/ncr<0.08). We discuss experimental results in the context of magnetohydrodynamic and particle-in-cell simulations that scan SRS dynamics for a variety of plasma conditions. While the experimental evidence of SRS mitigation validates prior work on the kinetic simulation of SRS in an external magnetic field, we also find that other mechanisms such as SRS rescatter can lead to an enhancement of measured SRS reflectivity in simulations of parameters relevant to our experiment.

Dephasingless laser wakefield acceleration (DLWFA), a novel laser wakefield acceleration concept based on the recently demonstrated “flying focus” technology, offers a new paradigm in laser-plasma acceleration that could advance the progress toward a TeV linear accelerator using a single-stage system without guiding structures. The recently proposed NSF OPAL laser facility could be the transformative technology that enables this grand challenge in laser-plasma acceleration. We review the viable parameter space for DLWFA based on the scaling of its performance with laser and plasma parameters, and we compare that performance to traditional laser wakefield acceleration. These scalings indicate the necessity for ultrashort, high-energy laser architectures such as NSF OPAL to achieve groundbreaking electron energies using DLWFA. Initial results from MTW-OPAL, the platform for the 6-J DLWFA demonstration experiment, show a tight, round focal spot over a distance of 3.7 mm. New particle-in-cell simulations of that platform indicate that using hydrogen for DLWFA reduces the amount of laser light that is distorted due to refraction at ionization fronts. An experimental path, and the computational and technical design work along that path, from the current status of the field to a single-stage, 100-GeV electron beam via DLWFA on NSF OPAL is outlined. Progress along that path is presented.