T. L. Wang

Development of a nanosecond-laser-pumped Raman amplifier for short laser pulses in plasma

Progress on developing a plasma amplifier/compressor based on stimulated Raman scattering of nanosecond laser pulses is reported. Generation of a millijoule seed pulse at a wavelength that is redshifted relative to the pump beam has been achieved using an external Raman gas cell. By interacting the shifted picosecond seed pulse and the nanosecond pump pulse in a gas jet plasma at a density of ∼1019 cm−3, the upper limit of the pump intensity to avoid angular spray of the amplified seed has been determined. The Raman amplification has been studied as a function of the pump and seed intensities. Although the heating of plasma by the nanosecond pump pulse results in strong Landau damping of the plasma wave, an amplified pulse with an energy of up to 14 mJ has been demonstrated, which is, to the best of our knowledge, the highest output energy so far by Raman amplification in a plasma. One-dimensional particle-in-cell simulations indicate that the saturation of amplification is consistent with onset of partic...

Multi-beam effects on backscatter and its saturation in experiments with conditions relevant to ignitiona)

To optimize the coupling to indirect drive targets in the National Ignition Campaign (NIC) at the National Ignition Facility [E. Moses et al., Phys. Plasmas 16, 041006 (2009)], a model of stimulated scattering produced by multiple laser beams is used. The model has shown that scatter of the 351 nm beams can be significantly enhanced over single beam predictions in ignition relevant targets by the interaction of the multiple crossing beams with a millimeter scale length, 2.5 keV, 0.02−0.05 × critical density, plasma. The model uses a suite of simulation capabilities and its key aspects are benchmarked with experiments at smaller laser facilities. The model has also influenced the design of the initial targets used for NIC by showing that both the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) can be reduced by the reduction of the plasma density in the beam intersection volume that is caused by an increase in the diameter of the laser entrance hole (LEH). In this model, a linea...

Particle-in-cell simulations of kinetic effects in plasma-based backward Raman amplification in underdense plasmas

A one dimensional particle-in-cell study of the kinetic effects involved in plasma-based backward Raman amplification is presented for nonrelativistic laser pulses interacting in underdense thermal plasmas. Simulations are performed to study how effects such as particle heating and trapping, frequency modulations, and wave breaking of the plasma wave can change with different plasma conditions. The result of this parametric scan of plasma density and temperature is the identification of optimal plasma conditions for amplification of an ultrashort seed laser pulse by a pump laser of nonrelativistic intensity. The relevance of this study to possible experimental scenarios is discussed.

Feasibility study for using an extended three-wave model to simulate plasma-based backward Raman amplification in one spatial dimension

Results are reported of a one-dimensional simulation study comparing the modeling capability of a recently formulated extended three-wave model [R. R. Lindberg, A. E. Charman, and J. S. Wurtele, Phys. Plasmas 14, 122103 (2007); Phys. Plasmas 15, 055911 (2008)] to that of a particle-in-cell (PIC) code, as well as to a more conventional three-wave model, in the context of the plasma-based backward Raman amplification (PBRA) [G. Shvets, N. J. Fisch, A. Pukhov et al., Phys. Rev. Lett. 81, 4879 (1998); V. M. Malkin, G. Shvets, and N. J. Fisch, Phys. Rev. Lett. 82, 4448 (1999); Phys. Rev. Lett. 84, 1208 (2000)]. The extended three-wave model performs essentially as well as or better than a conventional three-wave description in all temperature regimes tested, and significantly better at the higher temperatures studied, while the computational savings afforded by the extended three-wave model make it a potentially attractive tool that can be used prior to or in conjunction with PIC simulations to model the kinet...

Study of x‐ray radiation from a laser wakefield accelerator

A Laser Wakefield Accelerator (LWFA) is under development at Lawrence Livermore National Laboratory (LLNL) to produce electron bunches with GeV class energy and energy spreads of a few‐percent. The interaction of a high power (200 TW), short pulse (50 fs) laser with neutral He gas can generate quasi‐monoenergetic electron beams at energies up to 1 GeV [1]. The laser pulse can be self‐guided over 1 cm overcoming the limitation of vacuum diffraction. X‐ray betatron radiation is emitted while the accelerated electrons undergo oscillations in the wakefield electrostatic field. Here we present electron spectra measurements with a two screen spectrometer allowing to measure both the electron energy and the transverse deflection at the plasma exit. We have measured monoenergetic electron beams above 300 MeV. Furthermore a forward directed x‐ray beam is observed. Preliminary measurements of the spectrum are in reasonable agreement with the calculated betatron spectrum in the synchrotron asymptotic limit using the...