Raman Amplification In Plasma Research Articles

Frequency Conversion of Lasers in a Dynamic Plasma Grating

When a dynamic medium in which the laser propagates changes its refractive index in time, the laser changes its frequency while keeping its wavevector unchanged to fullfil the dispersion relation. This is usually applied to upshift the laser frequency with ionizing plasma. We propose an alternative technique to modify light frequency. A transient plasma grating can be generated by two identical counterpropagating laser pulses via strongly coupled stimulated Brillouin scattering (SC SBS). The rapid evolution of the plasma grating affects the wave-dispersion relation and a band gap develops around the laser frequency, dependent on the grating amplitude. As a result, the lasers convert their frequency downward to the low edge of the band gap, while a free-traveling laser converts its frequency to both the upper and lower edge of the band gap. Depending on the considered setup, practical applications of this technique include either laser-frequency downshift or spectral splitting can be exploited. The former can be used for Raman amplification in plasma and the latter for dual-color x-ray generation by Thomson and/or Compton scattering.

Compression of laser pulses by near-forward Raman amplification in plasma

We propose a new scheme for compression of ultrashort laser pulses enabled by Near-Forward Raman Amplification (NFRA) in plasma. In contrast to traditional forward Raman amplification in plasma, the seed pulse in NFRA propagates faster than the pump through manipulation of the spatiotemporal properties of the pump beam. A “π-pulse” solution similar to that of stimulated Raman backscattering amplification is obtained, indicating that NFRA can be used for compression of ultrashort laser pulses. NFRA is numerically demonstrated with a 2D simulation using a front-tilt near-forward pump pulse.

The focusing effect in backward Raman amplification in plasma

In this paper, the focusing effect on backward Raman amplification in plasma is investigated. A fluid model, used to simulate the backward Raman amplification and including the relativistic, ponderomotive, and thermal self-focusing and the mutual-focusing effect simultaneously, is proposed and investigated. The focusing effect is shown to severely distort the profile of the seed when the seed intensity was as high as 1017 W/cm2. Reducing the plasma density can relax the focusing effect, but at the cost of decreasing the amplification efficiency. Changing the profile of the seed has a limited effect on mitigating the focusing effect. A Gaussian profile of the pump and a defocusing shape of the plasma density seem to be an effective way to mitigate the focusing effect without decreasing the amplification efficiency.

Beam cleaning of an incoherent laser via plasma Raman amplification

We show that backward Raman amplification in plasma can efficiently compress a temporally incoherent pump laser into an intense coherent amplified seed pulse, provided that the correlation time of the pump is longer than the inverse plasma frequency. An analytical theory for Raman amplification using pump beams with different correlation functions is developed and compared to numerical calculations and particle-in-cell simulations. Since incoherence on scales shorter than the instability growth time suppresses spontaneous noise amplification, we point out a broad regime where quasi-coherent sources may be used as efficient low-noise Raman amplification pumps. As the amplified seed is coherent, Raman amplification additionally provides a beam-cleaning mechanism for removing incoherence. At near-infrared wavelengths, finite coherence times as short as 50 fs allow amplification with only minor losses in efficiency.

An ultra-high gain and efficient amplifier based on Raman amplification in plasma

Raman amplification arising from the excitation of a density echelon in plasma could lead to amplifiers that significantly exceed current power limits of conventional laser media. Here we show that 1–100 J pump pulses can amplify picojoule seed pulses to nearly joule level. The extremely high gain also leads to significant amplification of backscattered radiation from “noise”, arising from stochastic plasma fluctuations that competes with externally injected seed pulses, which are amplified to similar levels at the highest pump energies. The pump energy is scattered into the seed at an oblique angle with 14 J sr−1, and net gains of more than eight orders of magnitude. The maximum gain coefficient, of 180 cm−1, exceeds high-power solid-state amplifying media by orders of magnitude. The observation of a minimum of 640 J sr−1 directly backscattered from noise, corresponding to ≈10% of the pump energy in the observation solid angle, implies potential overall efficiencies greater than 10%.

Backward Raman amplification in the long-wavelength infrared

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 μm with terawatt powers through backward Raman amplification. The effects of collisional damping, Landau damping, pump depletion, and wave breaking are examined, as well as the resulting design considerations for an LWIR Raman amplifier.

Amplification of ultra-short laser pulses via resonant backward Raman amplification in plasma

In this paper, we have examined the possibility of using resonant backward Raman amplification (BRA) as an efficient mechanism in amplifying the low intensity ultra-short (≤fs) pulses using plasma as intermediate amplifying medium; such pulses are anticipated to get produced in the form of the secondary sources at ALPS (Attosecond Light Pulse Source) center of ELI (Extreme Light Infrastructure). In preliminary assessment of the scheme, the analytical expressions for the pump/seed laser pulses and plasma characteristic features are obtained which concisely describe the parameter regime of resonant BRA applicability in achieving significant amplification. The consistency of the scheme in the context of ELI-ALPS sources has been validated through particle in cell (PIC) simulations. The peak intensity of the amplified seed pulse predicted via simulation results is found in reasonable agreement with the analytical estimates. Utilizing these analytical expressions as a basis in perspective of ELI-ALPS parameter access, a specific example displaying the key plasma and laser parameters for amplifying weak seed pulse has been configured; the limitations and conceivable remedies in resonant BRA implementation have also been highlighted.

Generation of multi-hundred petawatt pulses by resonant Raman amplification in plasma

Backward Raman amplification (BRA) in plasma has been proposed to produce overcritical high-power laser pulses. In this paper, an application based on CPA and BRA is promoted to generate multi-hundred petawatt laser pulses. The compression of short-wavelength (around 351nm) and picosecond pulses has been proposed for high output intensity and short plasma length. This principle was employed in an application and a scheme is demonstrated using a full-kinetic particle-in-cell (PIC) code. The PIC code is also used to optimize key parameters in the resonant interaction. According to the simulated result using optimized parameters, the output seed fluence is amplified to 6.5kJ/cm2 and the full-width at half-maximum duration is compressed to 13fs, showing an energy transfer over 60%. Extending the result to the multidimensional case, a total energy of 3.9kJ and a laser power of 300PW are achievable, in a 0.6cm2 interaction spot. This result is helpful for the improvement of high-energy density physics.

Production of Single Pulse by Landau Damping for Backward Raman Amplification in Plasma

The effect of plasma wavebreaking has been proposed to obtain single-pulse output for backward Raman amplification in plasma. However, some experiments indicate that the scheme may not be effective. In this paper, we propose the effect of Landau damping for this purpose. According to the theoretical analysis, the single pulse is generated by setting a proper plasma temperature to fully Landau damp the secondary spikes. Although the main pulse will also be partly suppressed by the damping, the effective efficiency can be enhanced. Moreover, the scheme is numerically demonstrated feasible by using the parameters of Princeton experiment [1] , [2] . Additionally, the temperature to produce single-pulse output can keep below the critical temperature at various ratios of the plasma frequency to the pump pulse frequency.

Fast multidimensional model for the simulation of Raman amplification in plasma

We present Leap, a simulation model for Raman amplification in plasma, combining an envelope treatment of the laser fields with an electrostatic particle-in-cell solver. The code is fully two dimensional, with the model readily extendible to three dimensions, and includes dispersive and refractive effects. Simulations carried out for Raman amplification in a plasma channel show that guiding of both the pump and the probe contribute to the evolution of the probe, resulting in a shorter, more intense pulse.

Production of Picosecond, Kilojoule, and Petawatt Laser Pulses via Raman Amplification of Nanosecond Pulses

Raman amplification in plasma has been promoted as a means of compressing picosecond optical laser pulses to femtosecond duration to explore the intensity frontier. Here we show for the first time that it can be used, with equal success, to compress laser pulses from nanosecond to picosecond duration. Simulations show up to 60% energy transfer from pump pulse to probe pulse, implying that multikilojoule ultraviolet petawatt laser pulses can be produced using this scheme. This has important consequences for the demonstration of fast-ignition inertial confinement fusion.

Chirped pulse Raman amplification in plasma

Raman amplification in plasma has been proposed to be a promising method of amplifying short radiation pulses. Here, we investigate chirped pulse Raman amplification (CPRA) where the pump pulse is chirped and leads to spatiotemporal distributed gain, which exhibits superradiant scaling in the linear regime, usually associated with the nonlinear pump depletion and Compton amplification regimes. CPRA has the potential to serve as a high-efficiency high-fidelity amplifier/compressor stage.

Raman amplification in plasma: Wavebreaking and heating effects

A three-wave model has been developed to investigate the influence of wavebreaking and thermal effects on the Raman amplification in plasma. This has been benchmarked against a particle-in-cell code with positive results. A new regime, the “thermal chirp” regime, has been identified and illustrated. Here the shift in plasma resonance due to heating of the plasma by a monochromatic pump allows a probe pulse to be amplified and compressed without significant pump depletion. In regimes where damping dominates, it is found that inverse bremsstrahlung dominates at high densities, and improved growth rates may be achieved by preheating the plasma. At low densities or high pump intensities, wavebreaking acts to limit amplification. The inclusion of thermal effects can dramatically reduce the peak attainable intensity because of the reduced wavebreaking limit at finite temperatures.

Superradiant Linear Raman Amplification in Plasma Using a Chirped Pump Pulse

A theoretical and numerical investigation of small-signal Raman backscattering from a chirped pump pulse in plasma shows that an ultrashort probe pulse will grow superradiantly, i.e., with an amplitude that scales with the propagation length while contracting self-similarly. These features are commonly associated with the nonlinear stages of Raman amplification in the pump depletion and Compton regimes. We show that the superradiant scaling results in very broad-bandwidth amplification due to gain distributed in frequency as well as spatially. Since different frequencies excite the plasma at different positions, wave breaking is avoided, and prepulses and pedestals are substantially suppressed. Linear chirped pulse amplification in plasma could provide a very broad-bandwidth alternative to solid state laser amplifiers, potentially usable for optical pulses a few cycles in duration.