Short-Pulse Underdense Laser–Plasma Interactions

The Plasma and Ultrafast Physics Group studies short-pulse (<10-ps), high-power (>100-TW) laser beam propgation through underdense plasmas. This team works on laser systems around the county including the Jupiter Laser Facility at Lawrence Livermore National Laboratory, the Multi-Terawatt (MTW) Laser and the Optical Parametric Amplfier Line (OPAL) lasers at the Laboratory for Laser Energetics at the University of Rochester. These experiments are intimate hands-on experiences that involve developing advanced diagnostics and several-weeks-long campaigns over multiple years.

Flying Focus: Spatiotemporal Control of Laser Intensity

An advanced focusing scheme called a "flying focus" has been demonstrated in which a diffractive lens combined with a chirped laser pulse enables a small-diameter laser focus to propagate nearly 100× its Rayleigh length [1]. Furthermore, the speed at which the focus—and therefore the peak intensity—moves is decoupled from the group velocity of the laser; it was demonstrated to co- or counter-propagate along the laser axis at any velocity. In addition to plasma amplifiers [2], the spatiotemporal control of laser intensity achieved by the flying focus has the potential to change the way other plasma devices are optimized, including photon accelerators, laser wakefield accelerators, high-harmonics generation, and THz generation.

Nonlinear Electron Plasma Waves

The dynamics of strongly driven electron plasma waves (EPW's) is a rich area of plasma physics that involves many complex phenomena that are difficult to predict in simulations or diagnose experimentally. As an electron plasma wave is driven to high amplitude, a multitude of effects can occur such as nonlinear frequency shifts, wave breaking, acceleration of high-energy electrons, and cascading to shorter-wavelength plasma waves.

Experiments are carried out that focus on probing nonlinear EPW dynamics in the underdense plasma (UDP) target chamber of the MTW laser. To control the amplitude of the EPW's, two counter-propagating laser pulses (pump and seed), whose frequency difference equals the plasma frequency, are being developed. The pump pulse will be provided by the current 1053-nm, 25-ps, 75-J MTW laser and the seed pulse will be provided by the ultrashort OPAL, which will deliver 50 mJ in 50 fs with a central wavelength tunable from 1100 nm to 1300 nm. To measure the plasma conditions and probe the amplitude and frequency of the driven electron plasma wave, optical Thomson scattering is used [3].

One prominent application, Raman amplification [4,5], is the use of large amplitude EPW's to transfer a large amount of energy from the pump to the seed, thereby amplifying the seed to powers in excess of the damage thresholds that currently limit optical parametric chirped-pulse–amplification (OPCPA) laser systems today. The PUPG has proposed a novel concept to use a flying focus for laser-plasma based amplifiers [2]. This concept (1) enables constant longitudinal intensities over distances that are 100s of times the Rayleigh length of the system, (2) eliminates deleterious pump beam intstabilities (e.g., stimulated Raman scattering, filamenation...), and (3) provides control over the plasma conditions in the amplifer.

Wakefield Acceleration/Betatron X-Rays

In laser wakefield acceleration [6] (LWFA) (figure on right), a high-intensity, short-pulse laser is propagated through an underdense plasma where it drives a relativistic plasma wave (a wake), that has accelerating gradients 1000 × higher than those of traditional radiofrequency particle accelerators. Electrons can become trapped in this plasma wake and accelerated to relativistic energies. PUPG is pursuing methods to utilize LWFA's driven with both OMEGA EP and MTW OPAL to develop x-ray sources [7–9] toward the eventual realization of an x-ray source for inertial confinement fusion, hiegh-energy-density science, and astrophysics research.

Relevant Publications