Short-Pulse Underdense Laser–Plasma Interactions

The Plasma and Ultrafast Physics Group studies short-pulse (<10-ps), high-power (>100-TW) laser beam propagation 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 Amplifier 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.

Ultrafast Thomson Scattering for Underdense Plasma Dynamics

The rapid evolution of electron density and temperature in a laser-produced plasma were measured using collective Thomson scattering (see Figure). Unprecedented picosecond time resolution, enabled by a pulse-front-tilt compensated spectrometer, revealed a transition in the plasma-wave dynamics from an initially cold, collisional state to a quasi-stationary, collisionless state. The hydrogen gas jet was ionized at an intensity near 10¹⁴ W/cm², where the initial electron plasma temperature and density were measured to be 3 eV and 8.4 × 10¹⁸ cm^–3, respectively. Over the first 18 ps, the plasma temperature increased modestly (16 eV) as the plasma density became fully ionized (1.1 × 10¹⁹ cm^–3) and then rapidly increased to a statured level of 93 eV over the next 20 ps. During this evolution the plasma transitioned from a nonideal to an ideal plasma. These picosecond electron temperature and density measurements can be applied to laser-plasma devices that require knowledge of the rapidly evolving plasma conditions. Laser-plasma Raman amplifiers require frequency matching between an electromagnetic beat wave and the plasma frequency for efficient energy transfer from a pump laser to the seed, but if the plasma frequency is rapidly evolving, as these experiments show, the amplifier will be detuned and the efficiency will be poor. With measurements of the plasma evolution, the system could be properly tuned to recover efficient energy transfer.

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 instabilities (e.g., stimulated Raman scattering, filamentation…), and (3) provides control over the plasma conditions in the amplifier.

Wakefield Acceleration/Betatron X-Rays

In laser wakefield acceleration [6] (LWFA), 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, high-energy-density science, and astrophysics research.

Relevant Publications

[1] "Flying Focus: Spatiotemporal Control of the Laser Focus," LLE Review Quarterly Report 151, 115, Laboratory for Laser Energetics, University of Rochester, Rochester, NY, LLE Document No. DOE/NA/1944-1347 (2017); "Spatiotemporal Control of Laser Intensity," D. H. Froula, D. Turnbull, A. S. Davies, T. J. Kessler, D. Haberberger, J. P. Palastro, S.-W. Bahk, I. A. Begishev, R. Boni, S. Bucht, J. Katz, and J. L. Shaw, Nat. Photonics 12, 262 (2018).
[2] "Raman Amplification with a Flying Focus," LLE Review Quarterly Report 151, 122, Laboratory for Laser Energetics, University of Rochester, Rochester, NY, LLE Document No. DOE/NA/1944-1347 (2017).
[3] Plasma Scattering of Electromagnetic Radiation: Theory and Measurement Techniques, D. H. Froula, S. H. Glenzer, N. C. Luhmann, Jr., and J. Sheffield, 2nd ed. (Academic, Amsterdam, 2011).
[4] "Superradiant Amplification of an Ultrashort Laser Pulse in a Plasma by a Counterpropagating Pump," G. Shvets, N. J. Fisch, A. Pukhov, and J. Meyer-ter-Vehn, Phys. Rev. Lett. 81, 4879 (1998).
[5] "Fast Compression of Laser Beams to Highly Overcritical Powers," V. M. Malkin, G. Shvets, and N. J. Fisch, Phys. Rev. Lett. 82, 4448 (1999).
[6] "Laser Electron Accelerator," T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979).
[7] "Femtosecond X Rays From Laser-Plasma Accelerators," S. Corde, K. Ta Phuoc, G. Lambert, R. Fitour, V. Malka, A. Rousse, A. Beck, and E. Lefebvre, Rev. Mod. Phys. 85, 1 (2013).
[8] "Angular Dependence of Betatron X-Ray Spectra from a Laser-Wakefield Accelerator," F. Albert, B. B. Pollock, J. L. Shaw, K. A. Marsh, J. E. Ralph, Y. H. Chen, D. Alessi, A. Pak, C. E. Clayton, S. H. Glenzer, and C. Joshi, Phys. Rev. Lett. 111, 235004 (2013).
[9] "Observation of Betatron X-Ray Radiation in a Self-Modulated Laser Wakefield Accelerator Driven with Picosecond Laser Pulses," F. Albert, N. Lemos, J. L. Shaw, B. B. Pollock, C. Goyon, W. Schumaker, A. M. Saunders, K. A. Marsh, A. Pak, J. E. Ralph, J. L. Martins, L. D. Amorim, R. W. Falcone, S. H. Glenzer, J. D. Moody, and C. Joshi, Phys. Rev. Lett. 118, 134801 (2017).