arXiv subjects
Publications and source records attributed to Nikhil Chakrabarti.
A stationary wave solution is obtained for the proton driven plasma wake field accelerator (PDPWFA). The wake field excitation by trains of equidistant proton microbunches produced due to self modulational instability has been discussed. Also, considering the necessity of the external magnetic field to control focusing of the beam, studies on the effect of magnetic field on the wake field structures have been done.
We report here an exact analytical travelling wave solution for nonlinear electron plasma wave excited by an intense ultra-relativistic proton beam. It brings out the underlying physics of longitudinal electric field characteristics of the excited wake wave formed behind the drive beam. The results are further supplemented by a fully relativistic particle in cell (PIC) code OSIRIS in 2D geometry. The plasma and beam parameters in the simulation are chosen in conformity with experimental work and it provides us the anticipated axial and transverse electric field profiles. The investigation is further extended by providing an analytical description of the wake wave excited by equi-spaced train of small proton bunches with the inclusion of the non-relativistic plasma ion dynamics. Our results show that the amplitude of the wake field does not grow indefinitely with the increase in the number of proton bunches. On the contrary, it saturates to a definitive limit.
Nonlinear stationary structures formed in a cold plasma with immobile ions in the presence of a relativistic electron current beam have been investigated analytically in the collisionless limit. These are cold plasma version of the relativistic Berstein-Greene-Kruskal (BGK) waves. The structure profile is governed by the ratio of maximum electrostatic field energy density to the relativistic kinetic energy density of the electron beam, {\it i.e.}, $κ_{R} = E_{m}/ (8 πn_{0} (γ_{0}-1)m_{0} c^{2})^{1/2}$, where $E_{m}$ is the maximum electric field associated with the nonlinear structure and $γ_{0}$ is the Lorentz factor associated with the beam velocity. It is found that, in the linear limit, {\it i.e.}, $κ_{R} \ll 1/\sqrt{γ_{0}}$, the fluid variables, {\it viz}, density, electric field, and velocity vary harmonically in space. In the range $0 < κ_{R} \leq 1/\sqrt{γ_{0}}$, the fluid variables exhibit an-harmonic behavior. For values of $κ_{R} > 1/\sqrt{γ_{0}}$, the electric field shows finite discontinuities at specific spatial locations indicating the formation of negatively charged planes at these locations.
Phase mixing of relativistic large amplitude nonlinear plasma wave in presence of a time independent space periodic ion density profile has been investigated. Inhomogeneous ion along with the relativistic variation of electron mass make the characteristic frequency of the wave to acquire a space dependency and thereby it breaks at arbitrarily small amplitude due to phase mixing. An approximate space time dependent solution is obtained in the weakly relativistic limit by Bogoliuboff and Kryloff method of averaging. We find that the change in the ion density perturbation and also the relativistic electron mass variation have significant effect in modifying the time at which phase mixing occurs.
In the field of fundamental plasma waves, direct observation of electron-acoustic wave (EAW) propagation in laboratory plasmas remains a challenging problem, mainly because of heavy damping. In the MaPLE device, the wave is observed and seen to propagate with phase velocity $\sim1.8$ times the electron thermal velocity. A small amount of cold, drifting electrons, with moderate bulk to cold temperature ratio ($\approx 2 - 3$), is present in the device. It plays a crucial role in reducing the damping. Our calculation reveals that the drift relaxes the stringent condition on the temperature ratio for wave destabilization. Growth rate becomes positive above a certain drift velocity even if the temperature ratio is moderate. The observed phase velocity agrees well with the theoretical estimate. Experimental realization of the mode may open up a new avenue in EAW research.
A generalized hydrodynamical model has been used to study low frequency modes in a strongly coupled, cold, magnetized dusty plasma. Such plasmas exhibit elastic properties due to strong correlations among dust particles and the tensile stresses imparted by the magnetic field. It has been shown that longitudinal compressional Alfven modes and elasticity modified transverse shear mode exist in such a medium. The features of these collective modes are established and discussed.