Ion Acoustic Decay Instability Research Articles

Langmuir wave self-focusing versus decay instability

Electron trapping in a finite amplitude Langmuir wave (LW) leads to a frequency shift, \Delta\omega_{TP} < 0, and reduced Landau damping. These may lead to modulational instability. Its growth rate and damping threshold, due to escape of trapped electrons at rate \nu, are calculated for the first time in the short wavelength regime. If the background plasma is in thermal equilibrium, it is shown that this trapped particle modulational instability (TPMI) is not possible when k \lambda_D > 0.46, while for 0.33 < k \lambda_D < 0.46, TPMI requires that the fluctuation wavevector have a component perpendicular to k, the LW wavevector, with \lambda_D the electron Debye length. Its nonlinear evolution leads to self-focusing. Comparison is made with a re-evaluated LW ion acoustic decay instability (LDI): compared to classical estimates, the new LDI threshold is lowered by primary LW \Delta\omega_{TP} since frequency matching leads to wavenumber and hence damping reduction of the daughter LW. For parameters estimates relevant to a recent stimulated Raman scatter experiment (Kline et al., submitted to PRL), the LDI and TPMI thresholds cross in the range 0.28 < k \lambda_D < 0.34, consistent with the observed LDI regime change. However, if \nu exceeds a critical value, estimated to be order 1% of the electron plasma frequency, then TPMI is not possible at any wavenumber.

Saturation of radiation-induced parametric instabilities by excitation of Langmuir turbulence

Progress made in the last few years in the calculation of the saturation spectra of parametric instabilities which involve Langmuir daughter waves will be reviewed. These instabilities include the ion acoustic decay instability, the two plasmon decay instability (TPDI), and stimulated Raman scattering (SRS). In particular we will emphasize spectral signatures which can be directly compared with experiment. The calculations are based on reduced models of driven Langmuir turbulence. Thomson scattering from hf-induced Langmuir turbulence in the unpreconditioned ionosphere has resulted in detailed agreement between theory and experiment at early times. Strong turbulence signatures dominate in this regime where the weak turbulence approximation fails completely. Recent experimental studies of the TPDI have measured the Fourier spectra of Langmuir waves as well as the angular and frequency spectra of light emitted near 3/2 of the pump frequency again permitting some detailed comparisons with theory. Thomson scattering measurements of the Langmuir wave spectra from SRS are consistent with the saturation by secondary and tertiary decay of the primary SRS Langmuir waves. Scaling laws derived from a local model of SRS saturation are compared with full simulations and recent Nova experiments.

Measurements of the angular and temporal structure of second-harmonic emission from laser-produced plasmas

We have measured and analyzed the second harmonic emission, both in the plane of the laser electric field and perpendicular to it, at several angles near 135° from the laser wave vector. The experiments used from 1 to 80 J of 1.053 μm light to irradiate carbon–hydrogen (CH) targets with a 550 ps pulse. A random phase plate was used, producing characteristic intensities in the range of 1013–1014 W/cm2. This was sufficient to drive the Ion Acoustic Decay Instability, producing Stokes emission well-separated from the emission spike at the second harmonic of the laser frequency. The spectral structure of the Stokes emission was qualitatively similar for all intensities and angles of observation. The duration of the signals showed trends anticipated from linear theory. To explain the scaling of the signal strength and spectral width requires nonlinear theory.

Collisional regimes of radiation-driven Langmuir turbulence

Numerical solutions of the Zakharov equations for a plasma driven above the electron plasma frequency by a long-wavelength radiation pump can be applied to both ionospheric modification experiments and laboratory laser-plasma interactions. A key difference between these two environments is the much larger collisional damping of Langmuir waves near the critical density in laser plasmas. Zakharov equation simulations in one and two dimensions reveal a significant change in the character of the saturated turbulence state of the electromagnetic ion-acoustic decay instability for pump strengths near threshold as the collisional damping is increased to values appropriate to certain (low-intensity) laboratory laser-plasma experiments [Phys. Fluids B 3, 1983 (1991)]. The linear-instability is then characterized by the coupling of the up- and downshifted Langmuir decay modes. A new turbulence regime differing from existing models of both weak and strong turbulence is found, which is characterized by a sequence of narrow peaks in the Langmuir frequency spectrum and a nonlinearly broadened wave-vector spectrum centered near the linearly most unstable modes. Results in this regime may be relevant to second harmonic emission experiments [Phys. Fluids B 3, 1983 (1991)].

Thomson scattering measurements of ion-acoustic waves driven by ion-acoustic decay instabilities

A large angle, multichannel ultraviolet Thomson scattering diagnostic was developed to study ion-acoustic waves in laser-produced high-density plasma. The time evolution of the spectral density function of the ion-acoustic wave (IAW) was measured by the scattering system. When a weak IAW was excited, the measured spectrum had a well-defined narrow peak, which was consistent with plasma parameters predicted by computer simulations assuming an electron transport flux limit of 0.1. The spectrum of the IAW was quite different, and broad, when it was excited strongly.

Anomalous absorption due to ion-acoustic decay instability in a large-scale plasma relevant to laser fusion

The anomalous attenuation length of the laser light due to the ion-acoustic decay instability (IADI) can be much smaller than the instability width with a relatively weak laser intensity (Iλ2∼1014 W μm2/cm2) in a large-scale plasma. An instability width of only a few laser wavelengths is sufficient to absorb most of the laser energy. Two-dimensional electromagnetic computer simulation results and experimental results are consistent with strong absorption by the IADI.

Direct measurements of the ion acoustic decay instability in a laser-produced, large-scale, hot plasma.

The ion acoustic decay instability has been investigated using UV collective Thomson scattering in a large scale (\ensuremath{\sim}1 mm) and hot (\ensuremath{\sim}1 keV) plasma relevant to laser fusion. The instability was found to be easily excited. The spectrum of the electron plasma wave is consistent with simple theory. Collective Thomson scattering is shown to be a good tool for diagnosing the local electron temperature.

Linear ray-optics theory of the radiation-driven ion-acoustic decay instability in flowing, inhomogeneous plasmas

The linear Wentzel–Kramers–Brillouin (WKB) or ray-optics theory of the ion-acoustic decay instability is developed for flowing inhomogeneous, fluid plasmas of arbitrary ionic charge, as is needed in the planning and analysis of experiments. Equations are developed for the instability threshold in a homogeneous plasma and for the amplification in plasmas that may be inhomogeneous in more than one spatial direction and/or in time. These equations are formulated so that prior WKB theory of three-wave parametric instabilities may be directly applied to this problem. The results are compared with prior work on this instability, and are applied to several circumstances of practical interest. One finds that, for typical conditions, large amplification is possible over a wide range of angles. In addition, the flow properties of the plasma are found to be crucial in determining the angular distribution of the amplification produced by the instability. In particular, the magnitude of the plasma flow, when the flow is subsonic, directly controls the angle of maximum WKB amplification.

Reply to ‘‘Comments on ‘Ion acoustic parametric decay instability in laser-produced plasma with varying ionic charge’ [Phys. Fluids B 3, 1983 (1991)]’’

The authors of the comment have correctly identified an area in which our paper is neither sufficiently clear nor fully correct. Unfortunately, we do not agree with their equations or their physical arguments.

Comments on ‘‘Ion acoustic parametric decay instability in laser-produced plasma with varying ionic charge’’ [Phys. Fluids B 3, 1983 (1991)

an algebraic error is found in Equation (1) of the paper by Mizuno et al.1 Their measured threshold is analyzed following the corrected from of Equation (1). (AIP)

Ion acoustic parametric decay instability in laser-produced plasma with varying ionic charge

Studies of the ion acoustic decay instability (IADI) are reported in which the target material of the laser-produced plasma was varied. The IADI was monitored by observing the Stokes peaks of the second-harmonic spectrum. Its threshold was quite low (≊2×1013 W/cm2 ) even in high-Z plasma. The threshold increased only weakly with Z. On the other hand, the instability intensity decreased strongly with Z, which is attributed to the decrease of the growth rate. A simple theory explains these experimental results reasonably well.

Investigation of ion-acoustic-decay-instability thresholds in laser-plasma interactions.

The ion acoustic decay instability has been studied by monitoring the Stokes sideband of the second-harmonic emission from a laser-produced plasma. The intensity threshold decreased as the laser spot size increased. It reached homogeneous-plasma collisional values in planar plasmas produced using large laser spot sizes ({ital D}{approx gt}600 {mu}m), for laser wavelengths of either 1.06 or 0.53 {mu}m. These observations show that the ion acoustic decay instability may well play a more important role in laser fusion that had been generally believed.

The effect of the ion acoustic decay instability on microwave‐plasma interactions

Laboratory measurements are presented that show the ion acoustic decay instability strongly effects p‐polarized microwave‐plasma interactions. It is shown that the ion acoustic decay instability is only weakly excited if the unperturbed density profile is relatively steep. Only suprathermal (hot) electrons are heated due to resonance absorption. For a relatively shallow density profile, the ion acoustic decay instability is strongly excited in the underdense region near the critical surface. In this case, suprathermal electrons traveling both up and down the density profile are heated due to the electron waves driven by the ion acoustic decay instability. This additional heating results in a higher return current of thermal electrons. This current of thermal electrons must flow through the turbulent region so the thermal electrons are strongly heated due to anomalous dc resistivity. In addition, a strong dc electric field is driven in the turbulent region to maintain the required return current. These measurements can guide our understanding of the effect of strongly driven ion acoustic decay instability on microwave‐plasma interactions in the ionosphere.

Hot-electron production due to the ion acoustic decay instability in a long underdense plasma.

Laboratory and particle simulation experiments are used to show that moderate-power microwaves (laser light) (Ilambda/sup 2//T/sub e/(W/cm/sup 2/)(..mu..m/sup 2//keV)approx. <5 x 10/sup 14/ for ZT/sub e//T/sub

Electron heating caused by the ion-acoustic decay instability in a finite-length system

The ion-acoustic decay instability is investigated for a finite-length plasma with density somewhat below the cutoff density of the electromagnetic driver (n∼0.7nc). For this regime, the heating in a very long system can overpopulate the electron tail and cause linear saturation of the low phase velocity electron plasma waves. For a short system, the instability is nonlinearly saturated at larger amplitude by ion trapping. Absorption can be significantly increased by the large-amplitude ion waves. These results compare favorably with microwave experiments.

Ion acoustic parametric decay instability in a plasma with β > 1

It is found that in a plasma with β > 1 a finite amplitude ion acoustic wave is unstable and decays into two shear Alfvén waves propagating in opposite directions. The threshold and growth rate of the instability are determined.

Control of density correlations in laser-heated plasma

The excitation of plasma density perturbations of selected wavenumber spectra by two oppositely directed short-wavelength laser beams and the simultaneous interaction with these perturbations of a long-wavelength laser beam whose frequency is comparable to the electron Langmuir frequency is analyzed. Perturbations of the plasma density having a spatial period equal to one-half the wavelength of the short-wavelength beams are produced at zero frequency, and at the ion-acoustic frequency. The third beam excites the oscillating two-stream instability and the ion-acoustic decay instability, so that these initial perturbations will grow rapidly and dominate the density fluctuation spectrum. The present scheme may be useful in avoiding superthermal electrons that are typically produced when radiation from a single beam is absorbed and long-wavelength density fluctuations become significant.

Anomalous High-Frequency Resistivity of a Plasma

In one- and two-dimensional computer simulations an anomalous high-frequency resistivity in a plasma driven by a large electric field oscillating near the electron plasma frequency is investigated. The large field excites the oscillating two-stream and the ion-acoustic decay instabilities in agreement with the linear theory. When the ion and electron fluctuations saturate, a strong anomalous heating of the plasma sets in. This strong heating is due to an efficient coupling of the externally imposed large electric field to the plasma by ion fluctuations. The anomalous collision frequency and the saturation fluctuation amplitudes are determined as a function of the external field amplitude and frequency, and the electron-ion mass ratio. A simple nonlinear theory gives results in reasonable agreement with simulations.

Saturation of the Decay Instability for Comparable Electronic and Ion Temperatures

We present a model for the nonlinear evolution of the electron-plasma ion-acoustic decay instability when the ratio of electron and ion temperatures is of order 1. Ion nonlinear Landau damping is found to be the dominant saturation mechanism. We calculate the saturated spectral intensity and pump absorption coefficient. An estimate is given of the energy that the enhanced fluctuations absorb from an electromagnetic wave incident on an inhomogeneous plasma.