Two-plasmon Decay Research Articles

Two-Plasmon Decay and Three-Halves Harmonic Generation in Filaments in a Laser-Produced Plasma

The two-plasmon decay of light propagating in filaments in a laser-produced plasma and the resulting $\frac{3{\ensuremath{\omega}}_{0}}{2}$ light emission is discussed with use of a simplified model. Experimental observations are consistent with the predictions of the model. Implications for use of the $\frac{3{\ensuremath{\omega}}_{0}}{2}$ doublet splitting as a temperature diagnostic are discussed.

Time-resolved32ω0spectrum from spherical laser-produced plasmas

The time-dependent spectrum of $\frac{3}{2}{\ensuremath{\omega}}_{0}$ harmonic emission from uniformly illuminated spherical layered targets has been measured. The features of the observed spectra are consistent with theoretical predictions of $\frac{3}{2}{\ensuremath{\omega}}_{0}$ emission from plasma waves generated by the two-plasmon decay instability in a hot, expanding plasma.

Production of hot electrons by two-plasmon decay instability in uv laser plasmas

Hot electron emission from a uv laser produced plasma with incident intensities 8×1013–2×1015 W/cm2 was measured by means of an electron spectrometer. Because of the small hot electron population, the self-consistent target potential due to escaped electrons may be considered for the first time in the data reduction. The total energy in hot electrons of temperature 50 keV is shown to be ≲10−3 of the incident laser energy. The target potential ≲80 kV. The hot electron temperature is essentially independent of intensity, the angular distribution of electron emission shows a weak polarization asymmetry, and the intensity threshold of hot electron production is 5×1013 W/cm2, all consistent with the two-plasmon decay instability being responsible for heating the electrons. The predicted spectrum of plasma waves is such that the accelerated electrons should be directed into the plasma almost parallel to the laser axis. Because only a small fraction of these electrons are reflected backwards to the detector, the x-ray yield is a factor of 20 higher than the escaped electrons would indicate.

Half- and three-halves harmonic measurements from laser-produced plasmas

Spectral measurements of the 3/2 harmonic emissions from 1.06 μm irradiated disk targets, and the 1/2 harmonic emissions from 0.53 μm irradiations, are presented. The splitting of the double-peaked spectra show a distinct target Z dependence, for constant laser irradiation parameters. Possible generation mechanisms in terms of the two-plasmon decay instability are discussed.

Kinetic theory of two-plasmon decay

The Vlasov–Maxwell theory of two-plasmon decay is developed for homogeneous and weakly inhomogeneous plasmas. Temporal growth rates in homogeneous plasmas and spatial amplification in weakly inhomogeneous plasmas are calculated and compared to fluid theory. For hot plasmas (Te≳10 keV) the kinetic analysis predicts lower growth rates and a reduced range of unstable wavenumbers compared to fluid theory.

Quenching of two-plasmon decay and stimulated-Raman-scattering instabilities by profile modification

The development of the electron density distribution in a CO2-laser irradiated underdense N2 gas jet is investigated interferometrically and is correlated to the observation of fast electrons generated by the two-plasmon decay and stimulated-Raman-scattering instabilities. During the 2 nsec laser pulse the quarter critical density scale length initially increases but soon ponderomotive forces steepen the profile resulting in a quenching of the fast electron emission.

Laser plasma coupling in long pulse, long scale length plasmas

Laser plasma coupling under near reactor-size target conditions is studied at Iλ2≂1014 W/cm2 μm2 with 1-mm size plasmas, 3-kJ, 3-ns irradiations on the 1.064-μm Shiva laser. Evidence for the occurrence of parametric processes, such as stimulated Brillouin sidescatter and the two-plasmon decay instability, is observed.

Growth and saturation of the two-plasmon decay instability

An experimental study of the two-plasmon decay instability is reported. An intense CO2 laser (10.6 μm) was used to generate the instability in a long scale length, preformed plasma. The principal experimental diagnostic measurement was time-resolved Thomson scattering, with resolution in either ω or k space. The results show that the instability evolves basically in agreement with linear theories, then saturates by the nonlinear process of mode coupling to short-wavelength ion fluctuations. At saturation, density fluctuations close to 20% are observed in both the electron and ion waves.

Stimulated Raman Scattering from uv-Laser-Produced Plasmas

Time-integrated, spectrally resolved measurements between 400 and 750 nm have been made of light backscattered from plasmas produced by 450-psec pulses from a 351-nm laser at ${10}^{13}$ to ${10}^{15}$ W/${\mathrm{cm}}^{2}$. Threshold and saturation behavior for the two-plasmon decay and the absolute and convective Raman instabilities have been observed. The scattered light spectra suggest the presence of a steepened density profile at the quarter critical density.

Aspects of the Physics of Laser–Plasma Interactions

A variety of topics in laser-plasma interaction physics is reviewed. The trend from exploding-pusher experiments to ablative compression studies already evident at the time of the Nagoya Conference in 1980 has gathered momentum. The particular emphasis placed at present on the scaling of interaction phenomena with wavelength, in particular stimulated Raman scattering and two-plasmon decay, filamentation and jet formation together with aspects of absorption and transport phenomena is essential for a proper understanding of laser–plasma interactions.

Experimental Observations of Nonlinear Saturation of the Two-Plasmon Decay Instability

Direct measurements by time-resolved Thomson scattering of the saturation of the two-plasmon decay instability driven by a C${\mathrm{O}}_{2}$ laser in an underdense plasma are presented. A time-correlated growth in ion fluctuations is also seen, enabling the saturation mechanism to be identified with the ion motions driven by the ponderomotive force associated with the large-amplitude electrostatic waves.

Ion fluctuation effects on the two-plasmon decay and stimulated Raman scattering

Secondary Langmuir-wave decay into another Langmuir wave and an ion-acoustic wave is shown to provide a very efficient saturation mechanism for two-plasmon decay and stimulated Raman scattering. Near the quarter critical density, where two-plasmon decay dominates Raman scattering, large ion fluctuations are expected at the saturated stage of the two-plasmon instability.

Self-Focusing of 10.6-μm Radiation in an Underdense Plasma

Self-focusing filaments have been observed in a long-scale-length plasma irradiated with a 10.6-\ensuremath{\mu}m beam containing hot spots. Filaments were observed by use of Thomson scattering from electron plasma waves produced by two-plasmon decay in the walls of the filament, and Fresnel reflection of a visible probe beam from the density discontinuity at the filament boundary.

Hot Electron Generation by the Two-Plasmon Decay Instability in the Laser-Plasma Interaction at 10.6 μm

Hot electron generation in a long-density-scale-length (\ensuremath{\sim}300 \ensuremath{\mu}m) underdense plasma has been studied. For a plasma of maximum electron density of $\frac{1}{4}{n}_{c}$ the hot electron emission is localized in the plane of polarization of the incident C${\mathrm{O}}_{2}$ laser and peaked about 45\ifmmode^\circ\else\textdegree\fi{} with respect to the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ vector of the laser beam, in both forward and backward directions. These observations suggest that the hot electrons are generated by the two-plasmon decay instability at quarter-critical density.

Nonlinear Saturation and Recurrence of the Two-Plasmon Decay Instability

Two-dimensional computer simulations show that short-wavelength ion fluctuations and density-profile steepening are dominant factors in the saturation of the $2{\ensuremath{\omega}}_{\mathrm{pe}}$ instability. The long-term evolution of this instability is described and the implications of this complex saturation process for absorption and harmonic emission are discussed.

Two-Plasmon Decay and Profile Modification Produced by 10.6-μm Radiation at Quarter-Critical Density

Two-plasmon decay of 10.6-\ensuremath{\mu}m radiation has been studied in a long density scale length (\ensuremath{\sim}300 \ensuremath{\mu}m) plasma using Thomson scattering and interferometry. Enhanced levels of both electron and ion plasma waves have been observed near $\frac{1}{4}{n}_{c}$ which are quenched during the C${\mathrm{O}}_{2}$-laser pulse. Profile modification localized at $\ensuremath{\sim}\frac{1}{4}{n}_{c}$ has also been observed.

Two-plasmon decay in a CO2-laser-plasma interaction experiment

Two-plasmon decay has been observed in a C${\mathrm{O}}_{2}$-laser-gas-target-plasma interaction experiment. The spectrum of $\frac{3}{2}$-harmonic radiation has been measured and the threshold intensity for decay determined. An experimental value of \ensuremath{\sim} 2 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ W/${\mathrm{cm}}^{2}$ is in reasonable agreement with theoretical predictions for an inhomogeneous plasma.

Effect of self-focusing on parametric back-scattering and absorptive instabilities in a plasma

This paper presents an investigation of the effect of self-focusing on the absolute growth rates of parametric back-scattering (e.g. stimulated Raman and Brillouin scatterings) and absorptive instabilities (e.g. two-plasmon decay and ion–plasmon decay) in a homogeneous and collisionless plasma. Gaussian intensity distribution of the pump laser beam causes the redistribution of carriers in the plane transverse to the beam propagation on account of the ponderomotive force, and leads the beam to propagate in an oscillatory wave guide mode. The modifications in plasma frequency and intensity of the beam cause the growth rates of the above parametric instabilities to show an oscillatory behaviour with the distance of propagation.

Soliton Formation and Saturation of Decay Instability of an Electromagnetic Wave into Two Plasma Waves

The soliton formation with its detuning of the resonance is shown to be an important saturation mechanism for the two-plasmon decay instability at the quarter critical density in laser-irradiated plasmas.

Raman scattering and two-plasmon instability in laser-created plasmas

Couplings in which a plasma wave can be generated through either one of two mechanisms are investigated. It is shown that the Raman up-conversion associated with the two-plasmon decay might account for the observation of the 3 ω 0/2 harmonic.