A new scheme of the THz light source is proposed. The excitation of the Langmuir wave in a moderately relativistic electron beam via the two-stream instability and the subsequent interaction of the excited Langmuir wave with the visible light laser results in THz ligh via the Raman scattering. The cost of the current scheme could be cheaper than the conventional technologies by using the electron beam from cathodes and low-intensity infra-red laser.
A new type of the THz laser is proposed. A coherent tera-hertz light is emitted through the backward Raman scattering between a visible light laser and a relativistic electron beam. The threshold conditions for the laser intensity and the electron beam density are identified. The scheme may lead to one of the most intense tera-hertz coherent light sources.
A THz light amplifier and the corresponding methods are proposed, based on the Raman interaction between a THz light and visible light lasers. Two lasers, with their frequencies differing by that of a Langmuir wave, counter-propagate inside a background plasma, exciting a rather strong Langmuir wave. The THz light amplification occurs through the non-resonant Raman scattering between the THz light and one of the lasers in the presence of the mentioned Langmuir wave. It is normally the case that the non-resonant scattering does not exchange the energy between E \&M fields but the author demonst…
A scheme of THz radiation using two moderately intense lasers and a moderately relativistic electron beam is proposed. In the scheme, a laser encounters a co-propagating relativistic electron beam, and excites plasmons via the two-plasmon decay. The excited plasmons will emit the THz radiations, interacting with the second laser via the Raman scattering. Our estimation suggests that the mena-free-path of the pump laser to the THz radiation is much shorter than the Thomson scattering. The physical parametes for practical interests are presented.
The wealth of work in backward Raman amplification in plasma has focused on the extreme intensity limit, however backward Raman amplification may also provide an effective and practical mechanism for generating intense, broad bandwidth, long-wavelength infrared radiation (LWIR). An electromagnetic simulation coupled with a relativistic cold fluid plasma model is used to demonstrate the generation of picosecond pulses at a wavelength of 10 microns with terawatt powers through backward Raman amplification. The effects of collisional damping, Landau damping, pump depletion, and wave breaking are …
Experimentally observed decay rate of the long wavelength Langmuir wave in metals and dense plasmas is orders of magnitude larger than the prediction of the prevalent Landau damping theory. The discrepancy is explored, and the existence of a regime where the forward Raman scattering is stable and the backward Raman scattering is unstable is examined. The amplification of an x-ray pulse in this regime, via the backward Raman compression, is computationally demonstrated, and the optimal pulse duration and intensity is estimated.
The response of dense quantum plasmas in the backward Raman scattering is studied. The coefficients in the backward Raman scattering is found to be underestimated (overestimated) in the classical theory if the excited Langmuir wave has low-wave vector (high-wave vector). The second order quantum perturbation theory shows that the second harmonic of the pondermotive potential arises naturally even in a single particle motion contrary to the classical plasmas.
A scheme of soft x-ray lasers is proposed. The backward Raman scattering between an intense visible-light laser and a relativistic electron beam results in sofr x-ray via the Doppler shift. One of the most intense soft x-ray light sources is contemplated.
Compression of an intense laser pulse using backward Raman amplification (BRA) in plasma, followed by vacuum focusing to a small spot size, can produce unprecedented ultrarelativistic laser intensities. The plasma density inhomogeneity during BRA, however, causes laser phase and amplitude distortions, limiting the pulse focusability. To solve the issue of distortion, we investigate the use of optical phase conjugation as the seed pulse for BRA. We show that the phase conjugated laser pulses can retain focusability in the nonlinear pump-depletion regime of BRA, but not so easily in the linear a…
An intense terahertz (THz) radiation generated by the beating of two Langmuir waves, which are excited by the forward Raman scattering, is analyzed theoretically. The radiation energy per shot can be as high as 0.1 J, with the duration of 10 pico-second. Appropriate plasma density and the laser characteristics are examined.
A new way of the THz light generation is considered. A tera-hertz light is emitted from the interaction between a high frequency microwave and a relativistic electron beam, via the backward Raman scattering. The up-shifting of the frequency occurs through the relativistic Doppler's effect. This scheme may lead to a cheap and compact tera-hertz light sources.
This paper describe a numerical simulation method for the interaction between laser pulses and low density plasmas based on hydrodynamic approximation. We investigate Backward Raman Amplifying (BRA) experiments and their variants. The numerical results are in good agreement with experiments.
A scheme of soft-xray radiation is proposed. In the scheme, a moderately intense laser excites plasmons via the two-plasmon decay in a relativistic electron beam. As the second laser encounters the electron beam in the opposite direction, it emits soft x-rays via the backward Raman scattering. Our analysis suggests that the effective cross-section of this scattering is higher than the Thomson scattering and that the conversion efficiency from the pump laser to the soft x-ray could be very high in the optimal scenario. USing the plasmon pump with duration of 1-100 pico-second and the electron b…
Laser beams carrying orbital angular momentum (OAM) provide an additional degree of freedom and have found wide applications ranging from optical communications and optical manipulation to quantum information. The efficient generation and operation of ultra-intense OAM beams is a big challenge that has to be met, currently setting a limit to the potential applications of ultra-intense OAM beams in high-energy-density physics studies. Here, we theoretically and numerically demonstrate for the first time that a pump beam with a new OAM state is generated by coupling of the seed pulse with OAM La…
Backward Raman amplification of laser pulses in plasmas can produce nearly relativistic unfocused output intensities and multi-exawatt powers in compact devices. The largest achievable intensity depends on which of major competitive processes sets this limit. It is shown here that the relativistic electron nonlinearity can cause saturation of the leading amplified spike intensity before filamentation instabilities develop. A simple analytical model for the saturation, which supports numerical simulations, is suggested. The upper limit for the leading output spike unfocused intensity is calcula…
In plasma-based backward Raman amplifiers, the output pulse intensity increases with the input pump pulse intensity, as long as the Langmuir wave mediating energy transfer from the pump to the seed pulse remains intact. However, at high pump intensity, the Langmuir wave breaks, at which point the amplification efficiency may no longer increase with the pump intensity. Numerical simulations presented here, employing a 1D Vlasov-Maxwell code, show that, although the amplification efficiency remains high when the pump only mildly exceeds the wavebreaking threshold, the efficiency drops precipitou…
The non-linear convection for the THz generation from two-colored lasers are analyzed in the context of the forward Raman scattering. The energy transfer from the lasers to the THz light can be efficient. The possible peak intensity of the generated THz light is estimated and the optimal duration time is estimated.
Backward Raman amplification is limited by relativistic nonlinear dephasing resulting in saturation of the leading spike of the amplified pulse. Pump detuning is employed to mitigate the relativistic phase mismatch and to overcome the associated saturation. The amplified pulse can then be reshaped into a mono-spike pulse with little precursory power ahead of it, with the maximum intensity increasing by a factor of two. This detuning can be employed advantageously both in regimes where the group velocity dispersion is unimportant and where the dispersion is important but small.
Through resonant backward Raman scattering, the plasma wave mediates the energy transfer between long pump and short seed laser pulses. These mediation can result in pulse compression at extraordinarily high powers. However, both the overall efficiency of the energy transfer and the duration of the amplified pulse depend upon the persistence of the plasma wave excitation. At least with respect to the recent state-of-the-art experiments, it is possible to deduce that at present the experimentally realized efficiency of the amplifier is likely constrained mainly by two effects, namely the pump c…
The leading amplified spike in backward Raman amplifiers can reach nearly relativistic intensities before the saturation by the relativistic electron nonlinearity, which sets an upper limit to the largest achievable leading spike intensity. It is shown here that this limit can be substantially exceeded by the initially sub-dominant spikes, which surprisingly outgrow the leading spike after its nonlinear saturation. Furthermore, an initially negligible group velocity of the amplified pulse in strongly under-critical plasma appears to be capable of delaying the filamentation instability in the n…
A new scheme of hard x-ray or gamma ray light is considered. The excitation of the Langmuir wave in an ultra dense electron beamm via the two-stream instabilities and the interaction of the excited Langmuir wave with the visible light-laser results in hard x-ray or gamma ray via three-wave interaction. The analysis suggests that the hard x-ray with the wave-lenght as small as 0.03 nm can be achieved. The plausible parameter for the practical use is proposed and the comparison with the conventional methods are provided.
A new mechanism for the THz light amplification is identified ina non-resonant Raman scattering between the THz light and a visible light lasers. The non-resonant scattering normally does not exchange the energy between E \&M fields, but the presence of the plasma density gradientcreates an condition in which a visible light laser could transfer its energy into a tera-hertz (THz) light via the laser-plasma interaction. The gain per length could reach 100 (1000) per centimeter for the THz light (far infra-red light) amplification.
In a laser-irradiated plasma, the Langdon effect makes the electron energy distribution function (EEDF) tend to a super-Gaussian distribution, which has important influences on laser plasma instabilities. In this work, the influences of a super-Gaussian EEDF on the convective stimulated backward Raman scattering (SRS) and stimulated backward Brillouin scattering (SBS) are studied systematically for a wide range of typical plasma parameters in the inertial confinement fusion (ICF). Distinct behaviors are found for SRS and SBS in the variation trend of the peak spatial growth rate and the corres…
It is proposed to replace the traditional counterpropagating laser seed in backward Raman amplifiers with a plasma wave seed. In the linear regime, namely, for a constant pump amplitude, a plasma wave seed may be found by construction that strictly produces the same output pulse as does a counterpropagating laser seed. In the nonlinear regime, or pump-depletion regime, the plasma-wave-initiated output pulse can be shown numerically to approach the same self-similar attractor solution for the corresponding laser seed. In addition, chirping the plasma wave wavelength can produce the same benefic…
The decay of the Langmuir waves in dense plasmas is not accurately predicted by the prevalent Landau damping theory. A dielectric function theory is introduced, predicting much higher damping than the Landau damping theory. This strong damping is in better agreement with the experimentally observed data in metals. It is shown that the strong plasmon decay leads to the existence of a parameter regime where the backward Raman scattering is unstable while the forward Raman scattering is stable. This regime may be used to create intense x-ray pulses, by means of the the backward Raman compression.…
We show via particle-in-cell simulations that small normalized magnetic fields ($ω_c/ω_p \ll 1$) applied perpendicularly to a light wave can significantly modify the evolution of backward stimulated Raman scattering (SRS) in the kinetic regime. The presence of the magnetic field increases the threshold for kinetic inflation and decreases the amount of reflectivity when SRS is driven significantly above threshold. Analysis indicates this arises because trapped electrons are accelerated as they surf across the wave, leading to the continual dissipation of the electron plasma waves over a wider r…
Backward stimulated Raman scattering (BSRS) with Langmuir decay instability (LDI) and Langmuir collapse has been researched by Vlasov simulation for the first time. The decay productions of LDI cascade and their evolution with time is clearly demonstrated, which occurs simultaneously with Langmuir collapse. The BSRS reflectivity will be decreased largely through LDI cascade and Langmuir collapse. In CH plasmas, when $T_i/T_e=1/3$, the Landau damping of the slow ion-acoustic wave (IAW) is lower than that in H plasmas. Therefore, the BSRS can be further suppressed through LDI cascade by the way …
Raman and Brillouin amplification of laser pulses in plasma have been shown to produce picosecond pulses of petawatt power. In previous studies, filamentation of the probe pulse has been identified as the biggest threat to the amplification process, especially for Brillouin amplification, which employs the highest plasma densities. Therefore it has been proposed to perform Brillouin scattering at densities below $n_{cr}/4$ to reduce the influence of filamentation. However, parastic Raman scattering can become a problem at such densities, contrary to densities above $n_{cr}/4$, where it is supp…
