Prateek Varshney

Strong terahertz radiation generation by beating of two x-mode spatial triangular lasers in magnetized plasma

Resonant THz radiation generation is proposed by beating of two spatial-triangular laser pulses of different frequencies (ω1, ω2) and wave numbers ( k1, k2) in plasma having external static magnetic field. Laser pulses co-propagating perpendicular to a dc magnetic field exert a nonlinear ponderomotive force on plasma electrons, imparting them an oscillatory velocity with finite transverse and longitudinal components. Oscillatory plasma electrons couple with periodic density ripples n ′ = nq0e iqz to produce a nonlinear current, i.e., responsible for resonantly driving terahertz radiation at (ω = ω1 − ω2, k = k1 − k2 + q). Effects of THz wave frequency, laser beam width, density ripples, and applied magnetic field are studied for the efficient THz radiation generation. The frequency and amplitude of THz radiation were observed to be better tuned by varying dc magnetic field strength and parameters of density ripples (amplitude and periodicity). An efficiency about 0.02 is achieved for laser intensity of 2 × 10 W/cm in a plasma having density ripples about 30%, plasma frequency about 1 THz and magnetic field about 100 kG.

Tunable and efficient terahertz radiation generation by photomixing of two super Gaussian laser pulses in a corrugated magnetized plasma

A scheme of terahertz (THz) radiation generation is investigated by photo-mixing of two super Gaussian laser beams having different frequencies (ω1, ω2) and wave numbers (k→1, k→2) in a performed corrugated plasma embedded with transverse dc magnetic field. Lasers exert a nonlinear ponderomotive force, imparting an oscillatory velocity to plasma electrons that couples with the density corrugations ( n′=nα0eiαz) to generate a strong transient nonlinear current, that resonantly derives THz radiation of frequency ∼ ωh (upper hybrid frequency). The periodicity of density corrugations is suitably chosen to transfer maximum momentum from lasers to THz radiation at phase matching conditions ω=ω1−ω2 and k→=k→1−k→2+α→. The efficiency, power, beam quality, and tunability of the present scheme exhibit high dependency upon the applied transverse dc magnetic field along with q-indices and beam width parameters ( a0) of super Gaussian lasers. In the present scheme, efficiency ∼10−2 is achieved with the optimization of ...

Stimulated Raman scattering of beat wave of two counter-propagating X-mode lasers in a magnetized plasma

Effects of transverse static magnetic field on stimulated Raman scattering (SRS) of the beat wave excited by two counter-propagating lasers are studied. Two counter-propagating lasers with frequency difference, ω1∼ω2≥2ωp, drive a non resonant space charge beat mode at wave number k→0≈k→1+k→2 in a plasma, where k→1 and k→2 are wave vectors of lasers having frequencies ω1 and ω2, respectively. The driven beat wave acts as a pump for SRS and excites parametrically a pair of plasma wave (ω,k→) and side band electromagnetic wave (ω3,k→3) propagating in the sideward direction in such a way that momentum remains conserved. The growth rate of Raman process is maximum for side scattering at θs=π/2 for lower values of applied magnetic field (∼1 kG), which can be three fold by applying magnetic field ∼5.0 kG. Thus, optimum value of magnetic field can be utilized to achieve maximum electron acceleration in counter propagating geometry of beat wave acceleration by reducing the growth rate of Raman process.

Parametric excitation of fast upper hybrid waves by non-resonant beating of counter-propagating X-mode lasers in a magnetized plasma

Generation of fast and slow upper hybrid waves by two plasmon decay of non-resonant beating mode of two counter-propagating X-mode lasers is modelled in magnetized plasma. Two counter-propagating lasers having frequencies and wave-vectors (ω1,k1) and (ω2,k2), respectively, generate a non resonant beat wave at frequency difference ω0≈ω1∼ω2 and wave number k→0≈k→1+k→2 which parametrically excites a pair of copropagating fast and slow upper hybrid waves at ω0≈2ωh+(3k12vth2/ωh)  (1−ωh/ω1) where ωh and vth are the upper hybrid frequency and electron thermal speed, respectively. The fast upper hybrid wave can be utilized for electron acceleration because its phase velocity is close to c. The growth rate of decay process is Γ∼ωp/10 at scattering angle θs∼5π/6 and magnetic field ∼90  T, which is one order higher as compared to the growth rate of Raman process. The growth rate can be further enhanced (∼20%) by increasing the magnetic field ∼450 T.

Terahertz radiation generation by pulse slippage of Cosh-Gaussian lasers in a corrugated magnetized plasma

The combined effects of pulse slippage and the transverse magnetic field are studied on terahertz radiation excitation by nonlinear beating of two cosh-Gaussian (ChG) laser pulses propagating in a corrugated plasma. The beating lasers exert nonlinear ponderomotive force on plasma electrons. The oscillating electrons couple with corrugations present in the plasma and resonantly excite a nonlinear current (at different frequencies) which drive the terahertz wave at proper phase matching conditions. As the group velocity of THz radiation is higher than the group velocity of beating lasers, the THz pulse slips forward the pump lasers, and its saturation takes place. The effects of THz wave frequency, the decentred parameter (of beating lasers), the periodicity of the density structure, and the applied dc magnetic field are studied on THz emission. An efficiency of ∼10−4 is achieved for a laser intensity of ∼ 2 × 10 15 W/cm2 (at a laser wavelength of ∼10.6 μm for the CO2 laser).