Study of early laser-induced plasma dynamics: Transient electron density gradients via Thomson scattering and Stark Broadening, and the implications on laser-induced breakdown spectroscopy measurements

In this paper a study of stimulated Raman scattering (SRS) from relatively planar plasmas irradiated with short‐wavelength (0.53 μm) laser light is reported. The Novette Laser Facility [Laser Part. Beams 3, 173 (1985)] produced several kilojoules of light in 1 nsec, which allowed it to irradiate a large spot with enough intensity to produce significant Raman scattering. These experiments measured the fluence, angular distribution, spectrum, and timing of the Raman light, as a function of the average laser intensity. Reductions in the Raman fluence at low laser intensity are attributed to collisional damping. The measured SRS fluence was larger than that predicted from convective amplification of bremsstrahlung noise, as calculated using the average properties of the laser beam and the plasma. Possible contributions to the observed scattering from enhanced noise, Raman scattering within filaments, and the absolute Raman instability at density extrema are discussed.

Studies of Raman scattering from overdense targets irradiated by several kilojoules of 0.53 μm laser light

A modified version of the Plasma Beat-Wave Accelerator scheme is introduced and analyzed, which is based on autoresonant phase-locking of the nonlinear Langmuir wave to the slowly chirped beat frequency of the driving lasers via adiabatic passage through resonance. This new scheme is designed to overcome some of the well-known limitations of previous approaches, namely relativistic detuning and nonlinear modulation or other non-uniformity or non-stationarity in the driven Langmuir wave amplitude, and sensitivity to frequency mismatch due to measurement uncertainties and density fluctuations and inhomogeneities. As in previous schemes, modulational instabilities of the ionic background ultimately limit the useful interaction time, but nevertheless peak electric fields at or approaching the wave-breaking limit seem readily attainable. Compared to traditional approaches, the autoresonant scheme achieves larger accelerating electric fields for given laser intensity, or comparable fields for less laser power; the plasma wave excitation is much more robust to variations or uncertainties in plasma density; it is largely insensitive to the precise choice of chirp rate, provided only that chirping is sufficiently slow; and the quality and uniformity of the resulting plasma wave and its suitability for accelerator applications may be superior. In underdense plasmas, the total frequency shift required is only of the order of a few percent of the laser carrier frequency, and for possible experimental proofs-of-principle, the scheme might be implemented with relatively little additional modification to existing systems based on either solid-state amplifiers and Chirped Pulse Amplification techniques, or, with somewhat greater technological effort, using a CO{sub 2} or other gas laser system.

/pdf/robust-autoresonant-excitation-in-the-plasma-beat-wave-24eiz25136.pdf

Robust Autoresonant Excitation in the Plasma Beat-Wave Accelerator

Stimulated Raman scattering of light waves by an underdense plasma is affected by the presence of a density ripple caused by a simultaneously occurring stimulated Brillouin instability. The problem is treated kinetically for the particularly interesting case where the ripple has nearly the same wavelength as the plasma wave. The ripple is found to reduce the growth rate of the usual Raman instability but allows other decay modes to occur. Numerical results for the frequencies, growth rates, and k spectra of these modes are obtained. A physical explanation is given for a baffling result of the calculation. The physical picture is also of interest to particle acceleration by plasma waves.

Raman scattering in a nearly resonant density ripple

Previous absolute instability results for significant growth from a parametric instability involving oppositely directed modes coupled by a uniform pump with phase inflection coupling (Δk=k‘x2/2) are extended to higher modes and to thresholds. The results are applied to stimulated Raman scattering experiments done with exploding foils.

Phase‐inflection parametric instability behavior near threshold with application to laser–plasma stimulated Raman scattering (SRS) instabilities in exploding foils

Electron plasma waves excited by optical mixing of two antiparallel CO/sub 2/ laser beams have been detected by ruby-laser Thomson scattering. The density resonance is confirmed by two separate methods.

Thomson-scattering detection of plasma waves excited by two laser beams

The threshold intensity for excitation of the stimulated Raman backscatter instability has been measured at 9.58 μm in a well‐diagnosed plasma free of Brillouin scattering. The initial level of density fluctuations was independently measured by ruby laser scattering. The observed threshold of 1×1011 W/cm2 is somewhat below the calculated ‘‘absolute’’ threshold for underdense plasmas, and the apparent noise level is higher than that measured directly.

The stimulated Raman threshold. I. Experiment

It is shown here that the Rayleigh–Taylor instability in a device like a theta pinch has two different stages. The first stage is during the implosion phase of the theta pinch, and the second stage initiates after the formation of the plasma column. Termination of the first stage and evolution of the second stage from growth to a highly nonlinear stage and eventual wave breaking is presented by holographic interferometry. Though the instability in the first stage has been investigated before, here by employing microscopic theory it is shown that the magnetic field gradient is the driving mechanism of the instability in the latter stage.

Evolution of the Rayleigh–Taylor instability driven by a magnetic field gradient

The time‐dependent Stark effect in a hydrogenlike atom is used to detect a low‐frequency (close to the ion plasma frequency) instability in a theta‐pinch plasma. Detection required the observation of a sequence of harmonics emitted by the 4686 A line of Heii. The instability is believed to be a two‐stream instability between the ions and electrons which develops during the imploding stage of the theta pinch.

Detection of low‐frequency instability through the satellites of a hydrogenlike atom

Etude de l'excitation d'ondes de plasma electroniques dans un plasma chaud et entierement ionise par 2 faisceaux laser. Le taux de croissance et l'amplitude absolue des ondes excitees, mesurees par diffusion de la lumiere, indiquent un tres fort couplage de la lumiere laser au plasma

Behrouz Amini

Papers

Thomson-scattering detection of plasma waves excited by two laser beams

The stimulated Raman threshold. I. Experiment

Evolution of the Rayleigh–Taylor instability driven by a magnetic field gradient

Detection of low‐frequency instability through the satellites of a hydrogenlike atom

Absolute amplitude and growth time of electron plasma waves excited by two laser beams.