Abstract
Stimulated Raman backscattering (SRS) has many unwanted effects in megajoule-scale inertially confined fusion (ICF) plasmas. Moreover, attempts to harness SRS to amplify short laser pulses through backward Raman amplification have achieved limited success. In high-temperature fusion plasmas, SRS usually occurs in a kinetic regime where the nonlinear response of the Langmuir wave to the laser drive and its host of complicating factors make it difficult to predict the degree of amplification that can be achieved under given experimental conditions. Here we present experimental evidence of reduced Landau damping with increasing Langmuir wave amplitude and determine its effects on Raman amplification. The threshold for trapping effects to influence the amplification is shown to be very low. Above threshold, the complex SRS dynamics results in increased amplification factors, which partly explains previous ICF experiments. These insights could aid the development of more efficient backward Raman amplification schemes in this regime.
Highlights
Stimulated Raman backscattering (SRS) has many unwanted effects in megajoule-scale inertially confined fusion (ICF) plasmas
The electromagnetic seed level increases as a function of time on a hydrodynamics timescale. This will be used to study the amplifier response for increasing initial electron plasma waves (EPWs) amplitude, as it follows the slow increase of the seed intensity
The additional contribution observed in the 6 mg cc À 1 foam in the period t 1⁄4 2.4 ns to t 1⁄4 2.8 ns in the shots with the compound target is interpreted as the reamplification of the foil signal as it propagates through the foam plasma, with a reamplification factor defined as (SRScompound À SRSfoil À SRSfoam)/SRSfoil
Summary
Stimulated Raman backscattering (SRS) has many unwanted effects in megajoule-scale inertially confined fusion (ICF) plasmas. As the EPW amplitude increases, trapping of the electrons in its potential well significantly modifies the electron distribution function in the neighbourhood of the EPW phase velocity, causing a reduction of the Landau damping rate compared with its linear value This reduction has been measured in lowtemperature plasmas (o10 eV)[14,15]. We demonstrate a sharp increase in the Raman amplification factor at scattered light wavelengths for which linear Landau damping was dominant This allows us to identify a laser and plasma parameter region for which kinetic nonlinearities in the Langmuir waves result in increased Raman amplification. The level of seed intensity that triggers nonlinearities in the plasma wave response corresponds to low levels of SRS reflectivities inside ICF targets and is close to the seed intensity available in BRA experiments
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