An intense short laser pulse or a millimetre wave propagating through a plasma channel may act as a wiggler for the generation of shorter wavelengths. When a relativistic electron beam is launched into the channel from the opposite direction, the laser radiation is Compton/Raman backscattered to produce coherent radiation at shorter wavelengths. The scheme, however, requires a superior beam quality with energy spread less than 1% in the Raman regime.

The self focusing of a quadruple Gaussian laser beam, comprising four coherent identical Gaussian beams with axes parallel to ẑ but shifted from z-axis by −x0x̂,x0x̂,−x0ŷ,x0ŷ in a collisionless plasma, is investigated in the paraxial ray approximation. The nonlinearity arises through the ponderomotive force led plasma density redistribution. As the beam propagates, it maintains the shape of its intensity profile with spot size r0 of each of the four distributions modified from r0 to r0f, x0, modified to x0f and the axial intensity enhanced by 1/f2, where f is the beam width parameter. In the regime of quadratic nonlinearity, the threshold beam power for self focusing, Pth, increases with x0 as the laser intensity gradient decreases and the ponderomotive force becomes weaker. At beam power greater than Pth, the beam focuses in an oscillatory manner, i.e., f varies periodically with z due to the saturating effect of nonlinearity. The locations of intensity maxima of the four beams also vary periodically with z. As x0 increases from 0 to 0.6r0, the transverse intensity gradient becomes weaker and the rate of self focusing, i.e., the rate of variation of f with z decreases.

A kinetic theory of nonlinear coupling of ion-Bernstein waves with the kinetic Alfvén wave in a plasma is developed using a guiding center formalism. The nonlinearity arises through the motion of ions. The coupling coefficient is simplified at the limit where the Alfvén wave’s wavelength is larger than an ion Larmor radius, whereas the ion-Bernstein waves may have arbitrary wavelengths. At power levels employed on large devices in the slow wave ion-cyclotron heating scheme, the excitation of magnetic fluctuations appears a genuine possibility.

A relativistic electron beam in the presence of a whistler wave excites a negative-energy beam space charge mode and a high frequency laser radiation (ω1≫ωc). The instability is explosive for a copropagating beam and the whistler pump, resulting in the excitation of an intense laser in a short pulse. The explosion time τ={(4/kv0)[(ω1−k1vb−ωc) γ0ω1/(ω1−k1vb)ω′pb]1/2}. For both the counterpropagating beam and the whistler wave, the instability is nonexplosive. The laser frequency in this case is somewhat larger, whereas the exponentiation time is comparable to the explosion time of the earlier case.

The process of second harmonic generation of an intense short pulse laser in a plasma is resonantly enhanced by the application of a magnetic wiggler. The wiggler of suitable wave number provides necessary momentum to second harmonic photons to make harmonic generation a resonant process. The laser imparts an oscillatory velocity to electrons and exerts a longitudinal ponderomotive force on them at , where ω1 and are the frequency and the wave number of the laser, respectively. As the electrons acquire oscillatory velocity at the second harmonic, the wiggler magnetic field beats with it to produce a transverse second harmonic current at , driving the second harmonic electromagnetic radiation. However, the group velocity of the second harmonic wave is greater than that of the fundamental wave, hence, the generated pulse slips out of the main laser pulse and its amplitude saturates.

Innate behaviours, though robust and hard wired, rely on modulation of neuronal circuits, for eliciting an appropriate response according to internal states and external cues. Drosophila flight is one such innate behaviour that is modulated by intracellular calcium release through inositol 1,4,5-trisphosphate receptors (IP3Rs). Cellular mechanism(s) by which IP3Rs modulate neuronal function for specific behaviours remain speculative, in vertebrates and invertebrates. To address this, we generated an inducible dominant negative form of the IP3R (IP3RDN). Flies with neuronal expression of IP3RDN exhibit flight deficits. Spatiotemporal expression of IP3RDN helped identify key flight-modulating dopaminergic neurons with axonal projections in the mushroom body. Attenuation of IP3R function in these presynaptic dopaminergic neurons resulted in flies with shortened flight bouts and a disinterest in seeking food, accompanied by reduced excitability and dopamine release upon cholinergic stimulation. Our findings suggest that the same neural circuit modulates the drive for food search and for undertaking longer flight bouts.

No abstract

Innate behaviours, although robust and hard wired, rely on modulation of neuronal circuits, for eliciting an appropriate response according to internal states and external cues. Drosophila flight is one such innate behaviour that is modulated by intracellular calcium release through inositol 1,4,5-trisphosphate receptors (IP3Rs). Cellular mechanism(s) by which IP3Rs modulate neuronal function for specific behaviours remain speculative, in vertebrates and invertebrates. To address this, we generated an inducible dominant negative form of the IP3R (IP3RDN). Flies with neuronal expression of IP3RDN exhibit flight deficits. Expression of IP3RDN helped identify key flight-modulating dopaminergic neurons with axonal projections in the mushroom body. Flies with attenuated IP3Rs in these presynaptic dopaminergic neurons exhibit shortened flight bouts and a disinterest in seeking food, accompanied by reduced excitability and dopamine release upon cholinergic stimulation. Our findings suggest that the same neural circuit modulates the drive for food search and for undertaking longer flight bouts.