Generating optical supercontinuum and frequency comb in tenuous plasmas

There are several mechanisms by which the frequency spectrum of a laser broadens when it propagates at near-relativistic-intensity in tenuous plasma. Focusing on one dimensional effects, we identify two strong optical nonlinearities, namely, four-wave mixing (FWM) and forward Raman scattering (FRS), for creating octave-wide spectra. FWM dominates the interaction when the laser pulse is short and intense; Its combination with phase modulation produces a symmetrically broadened supercontinuum. FRS dominates when the laser pulse is long and relatively weak; It broadens the laser spectrum mainly towards lower frequencies and produces a frequency comb. The creation of the supercontinuum and frequency combs only frequency modulates, but does not compress, the laser pulse.

Plasma physics in strong-field regimes: theories and simulations

In strong electromagnetic fields, unique plasma phenomena and applications emerge, whose description requires recently developed theories and simulations [Y. Shi, Ph.D. thesis, Princeton University (2018)]. In the classical regime, to quantify effects of strong magnetic fields on three-wave interactions, a convenient formula is derived by solving the fluid model to the second order in general geometry. As an application, magnetic resonances are exploited to mediate laser pulse compression, using which higher intensity pulses can be produced in wider frequency ranges, as confirmed by particle-in-cell simulations. In even stronger fields, relativistic-quantum effects become important, and a plasma model based on scalar quantum electrodynamics (QED) is developed, which unveils observable corrections to Faraday rotation and cyclotron absorption in strongly magnetized plasmas. Beyond the perturbative regime, lattice QED is extended as a numerical tool for plasma physics, using which the transition from wakefield acceleration to electron-positron pair production is captured when laser intensity exceeds the Schwinger threshold.

Optical phase conjugation in backward Raman amplification

Compression of an intense laser pulse using backward Raman amplification (BRA) in plasma, followed by vacuum focusing to a small spot size, can produce unprecedented ultrarelativistic laser intensities. The plasma density inhomogeneity during BRA, however, causes laser phase and amplitude distortions, limiting the pulse focusability. To solve the issue of distortion, we investigate the use of optical phase conjugation as the seed pulse for BRA. We show that the phase conjugated laser pulses can retain focusability in the nonlinear pump-depletion regime of BRA, but not so easily in the linear amplification regime. This somewhat counter-intuitive result is because the nonlinear pump-depletion regime features a shorter amplification distance, and hence less phase distortion due to wave-wave interaction, than the linear amplification regime.

Pulsed RF Schemes for Tearing Mode Stabilization

The RF stabilization of tearing modes with current condensation has the potential to increase stabilization efficiency and loosen power localization requirements. Such benefits stem from the cooperative feedback between the RF deposition and resulting island temperature perturbation governed by diffusion. A self consistent treatment of the damping of an rf ray as it traverses the island shows that low damping scenarios can require unfavorably high powers to overcome initial power leakage and effectively capitalize on the nonlinear effect. In this work it is demonstrated that for such regimes,modulated stabilization schemes can achieve significant improvements in heating and current drive contributions to stabilization for the same average power as a continuous wave scheme. The impact of modulation frequency and duty cycle on the performance is explored, the results of which suggest modulation strategies in which the pulsing periods are kept on the order of a diffusive time.

On Seminal HEDP Research Opportunities Enabled by Colocating Multi-Petawatt Laser with High-Density Electron Beams

The scientific community is currently witnessing an expensive and worldwide race to achieve the highest possible light intensity. Within the next decade this effort is expected to reach nearly $10^{24},mathrm{W}/mathrm{cm^2}$ in the lab frame by focusing of 100 PW, near-infrared lasers. A major driving force behind this effort is the possibility to study strong-field vacuum breakdown and an accompanying electron-positron pair plasma via a quantum electrodynamic (QED) cascade [Edwin Cartlidge, The light fantastic, Science 359, 382 (2018)]. Whereas Europe is focusing on all-optical 10 PW-class laser facilities (e.g., Apollon and ELI), China is already planning on co-locating a 100 PW laser system with a 25 keV superconducting XFEL and thus implicitly also a high-quality electron beam [Station of Extreme Light (SEL) at the Shanghai Superintense-Ultrafast Lasers Facility (SULF)]. This white paper elucidates the seminal scientific opportunities facilitated by colliding dense, multi-GeV electron beams with multi-PW optical laser pulses. Such a multi-beam facility would enable the experimental exploration of extreme HEDP environments by generating electron-positron pair plasmas with unprecedented densities and temperatures, where the interplay between strong-field quantum and collective plasma effects becomes decisive.

A necessary condition for perpendicular electric field control in magnetized plasmas

The electrostatic model proposed by Poulos [Phys. Plasmas (2019), $mathbf{26}$, 022104] to describe the electric potential distribution across and along a magnetized plasma column is used to shed light onto the ability to control perpendicular electric fields. The effective electrical connection between facing end-electrodes is shown to be conditioned upon the smallness of a dimensionless parameter $tau$ function of the plasma column aspect ratio and the square root of the conductivity ratio $sigma_perp/sigma_{parallel}$. The analysis of a selected set of past end-electrodes biasing experiments confirms that this parameter is small in experiments that have successfully demonstrated perpendicular electric field tailoring. On the other hand, this parameter is $mathcal{O}(1)$ in experiments that failed to demonstrate control, pointing to an excessively large ion-neutral collision frequency. A better understanding of the various contributions to $sigma_perp$ is needed to gain further insights into end-biasing experimental results.

Laser frequency upconversion in plasmas with finite ionization rates

Laser frequency can be upconverted in a plasma undergoing ionization. For finite ionization rates, the laser pulse energy is partitioned into a pair of counter-propagating waves and static transverse currents. The wave amplitudes are determined by the ionization rates and the input pulse duration. The strongest output waves can be obtained when the plasma is fully ionized in a time that is shorter than the pulse duration. The static transverse current can induce a static magnetic field with instant ionization, but it dissipates as heat if the ionization time is longer than a few laser periods. This picture comports with experimental data, providing a description of both laser frequency upconverters as well as other laser-plasma interaction with evolving plasma densities.

Creating localized plasma wave by ionization of doped semiconductors

Localized plasma waves can be generated by suddenly ionizing extrinsic semiconductors with spatially periodic dopant densities. The built-in electrostatic potentials at the metallurgical junctions, combined with electron density ripples, offer the exact initial condition for exciting long-lasting plasma waves upon ionization. This method can create plasma waves with a frequency between a few terahertz to sub-petahertz without substantial damping. The lingering plasma waves can seed backward Raman amplification in a wide range of resonance frequencies up to the extreme ultraviolet regime. Chirped wavevectors and curved wavefronts allow focusing the amplified beam in both longitudinal and transverse dimensions. The main limitation to this method appears to be obtaining sufficiently low plasma density from solid-state materials to avoid collisional damping.

Radiation in equilibrium with plasma and plasma effects on cosmic microwave background

The spectrum of the radiation of a body in equilibrium is given by Plancks law. In plasma, however, waves below the plasma frequency cannot propagate; consequently, the equilibrium radiation inside plasma is necessarily different from the Planck spectrum. We derive, using three different approaches, the spectrum for the equilibrium radiation inside plasma. We show that, while plasma effects cannot be realistically detected with technology available in the near future, there are a number of quantifiable ways in which plasma affects cosmic microwave background (CMB) radiation.

Favorable Collisional Demixing of Ash and Fuel in Magnetized Inertial Fusion

Magnetized inertial fusion experiments are approaching regimes where the radial transport is dominated by collisions between magnetized ions, providing an opportunity to exploit effects usually associated with steady-state magnetic fusion. In particular, the low-density hotspot characteristic of magnetized liner inertial fusion results in diamagnetic and thermal frictions which can demix thermalized ash from fuel, accelerating the fusion reaction. For reactor regimes in which there is substantial burnup of the fuel, increases in the fusion energy yield on the order of 5% are possible.