Manipulation of intense electromagnetic waves in plasma
Plasma is the ideal nonlinear medium inside which high-intensity laser pulses can be manipulated: compressed, chirped, frequency-shifted, and even brought to a complete halt. We are pursuing several such projects:
Microwave plasma interaction in the Undulator Induced Transparency (UIT) regime and its applications
The Undulator Induced Transparency (UIT) is a phenomenon originating from the coupling between the transverse and longitudinal electromagnetic waves (EM) in the magnetized plasma in the presence of the static helical magnetic undulator, which eliminates the absorption of an EM wave at the cyclotron frequency. A radical transfiguration of the plasma properties in the UIT regime results in slowing down the electromagnetic wave propagation waves and subsequent strong compression of the wave energy in the plasma. The UIT based pulse manipulation technique becomes very attractive due to the increasing demand for high power microwave sources, which requires the development of high power pulse compressors and power switching devices. Another signature of the UIT regime is that the phase velocity of the plasma wave is opposite to the phase and group velocity of the incident microwaves which opens up the possibility to design a plasma-based Backward Wave Oscillators (BWO). The direct application of the UIT to the electron and ion acceleration is feasible due to the fact that the polarization of the compressed waves is primarily longitudinal and their phase velocity is controllable by the undulator period.
Click on the icon below to download UIT simulation movie
(0.7MB, avi file)
Super-Radiant Amplification (SRA) of ultra-short pulses by a counter-propagating laser beam
Click on the icon below to download SRA process simulation movie
(0.1MB, avi file)
The tutorial lecture given by Dr. Shvets at the Lake Tahoe
School on Plasma Physics describes how laser pulses can be compressed in
plasma
Nonlinear Pulse Compression in Plasma: Beyond Chirped Pulse Amplification
Theory of generation of few-cycle electromagnetic pulses via electromagnetic cascading in plasmas
We propose an approach to compressing laser beams in plasmas via generation of coherent cascade of electromagnetic (EM) sidebands. The technique requires two co-propagating beams: a pump and a probe detuned by a near-resonant frequency \Omega<\omega_p.
Fig. 1. Principal scheme of the em cascade compressor using two plasma stages: one with the low GVD but high level of nonlinearity making for the periodic phase modulation and positive chirping of the laser field; and the second, high-density plasma with no resonant coupling between the sidebands where the compression occurs (non-resonant nonlinearities such as relativistic self-phase-modulation of the cascade in the dense Compressor plasma are admitted).
The ponderomotive force of laser beat wave drives a near-resonant electron density perturbation that modifies the refractive index of plasma so as to produce a periodic phase modulation of the em field with the beat period $\tau_b=2\pi/\Omega$. A train of chirped laser beatnotes (each of duration b) is thus created. For \Omega<\omega_p, the Stokes sidebands are advanced in time with respect to the anti-Stokes ones near the maximum of each beatnote.
The group velocity dispersion (GVD) of radiation in plasma can then compress each beatnote to a few-laser-cycle duration. As a result, a train of sharp electromagnetic spikes separated in time by $\tau_b$ is formed. Depending on the plasma and laser parameters, compression of the chirped beatnotes can be implemented either in a separate plasma of higher density, or in the same plasma concurrently with the phase modulation.
Nonlinearities of plasma other than ponderomotive excitation of the near-resonant plasma wave (nonlinear frequency shifts of the EM sidebands appearing due to the relativistic increase of electron mass and excitation of non-resonant harmonics of the electron density in the high-frequency electric field) do not disrupt the compression process even in the case when the laser intensity in the Compressor becomes relativistic (I~10^18 W/cm^2). Appropriate tuning the laser and plasma parameters can make manifestation of the forward Raman scattering tolerable in the Modulator plasma.
Fig. 2. The laser beat wave before (black) and after the Modulator (blue), and after the Compressor (red). Inlay shows one beatnote near the pulse center. Laser spectrum before (stairs) and after (bars) compresion.
References:
[1] S. Kalmykov and G. Shvets, "Application of detuned plasma beatwave for generation of few-cycle electromagnetic pulses", AIP Proc. 737, 552 (2004).
[2] S. Yu. Kalmykov and G. Shvets, "Laser pulse compression using electromagnetic cascading in plasma", submitted to Phys. Rev. Lett. (2004).
