arXiv subjects

Roopendra Singh Rajawat

Publications and source records attributed to Roopendra Singh Rajawat.

9 recordsLinked to original sources

arXiv2026

Monoenergetic acceleration of charge-neutralized ion bunches to GeV-scale energies by the combination of a high-current electron beam and an ionization front

Compact heavy ion accelerators have numerous applications, ranging from heavy ion fusion to carbon ion radiotherapy, and testing radiation-hardened electronics. The demand could be met by developing high-gradient traveling wave plasma accelerators of high-charge ($\simμ\mathrm{C}$) relativistic ion beams. We will discuss a novel ion acceleration regime -- Counter-propagating ionization Front Acceleration (CFA) -- utilizing counter-propagating Ionization Front (IF) and high-current Relativistic Electron Beam (REB). Theoretical modeling and 3D PIC simulations demonstrate the possibility of using typical REBs produced by induction voltage adders propagating through a gas-filled tube undergoing laser ionization to achieve acceleration gradients in excess of $\sim 250 {\rm MeV/m}$ while accelerating micro-Coulombs of ions over meters distance. A unique energy conversion mechanism -- from the REB to electromagnetic fields to the ions is discussed, as well as the limits on the accelerated ions charge and the degree of its neutralization, acceleration gradient, and ion energy spread.

physics.plasm-ph↗

arXiv2025

Propagation and collisionless damping of topologically-protected surface plasma waves in non-uniformly magnetized plasma columns

Recent theoretical studies revealed the existence of topologically-protected surface plasma waves (TSPWs) in cold magnetized plasmas assumed uniform along the direction of a uniaxial magnetic field. Reflections-free propagation of the TSPWs along arbitrarily-shaped plasma boundaries oriented perpendicularly to the magnetic field was shown to be preserved even when their collisionless damping by localized upper-hybrid resonances was accounted for. Here we extend this theory to the realistic case of three-dimensional magnetic field produced by finite-sized magnetic coils. We demonstrate that when TSPWs are launched in the direction of the decreasing magnetic field, they are collisionlessly absorbed within a highly localized (evanescent) region as they propagate away from the magnetic coil. We show that the resulting wave reflection can be negligible -- in clear contrast with conventional wave reflection from the corresponding evanescence regions.

physics.plasm-ph↗

arXiv2025

Nontrivial low-frequency topological waves at the boundary of a magnetized plasma

The topological properties of a magnetized cold gaseous plasma have recently been explored and the existence of topologically protected edge states has been established. These studies are limited to a magnetized plasma, where ions are infinitely massive and provide a neutralizing background. When ion motion is included, a new class of low-frequency unidirectional topological waves (TSPWs) emerges in the dispersion relation. The group velocity of these waves is in the opposite direction of high-frequency topological electron waves for a given magnetic field direction. The Berry curvature and Chern numbers are calculated to establish nontrivial topological phase. Additionally, we demonstrate a unique characteristic of ion dominated TSPW propagating above the ion cyclotron frequency: their collisionless damping via coupling to the continuum of lower-hybrid resonant modes localized inside a smooth plasma-vacuum interface. These finding broadens the possible applications and observations of these exotic excitations in space and laboratory plasmas.

physics.plasm-ph↗

arXiv2024

Effects of Laser Polarization on Target Focusing and Acceleration in a Laser-Ion Lens and Accelerator

We present the process of ion acceleration using ultra-thin foils irradiated by elliptically polarized, high-intensity laser pulses. Recently, efficient generation of monoenergetic ion beams was introduced using the concept of laser-ion lensing and acceleration (LILA). LILA is an innovative technique where the target's radially varying thickness enables simultaneous acceleration and focusing of a proton beam. In this work, we extend the LILA framework to incorporate elliptically polarized (EP) laser pulses. While it's commonly assumed that EP lasers are unsuitable for radiation pressure acceleration (RPA) due to excessive electron heating that compromises ion acceleration, our multidimensional particle-in-cell simulations challenge this notion. We show that, with proper optimization of the target's average thickness, EP laser pulses can successfully drive the LILA mechanism. We also demonstrate that with a non-uniform thickness target, even linearly polarized laser pulses can efficiently generate low-emittance focused ion beams, with the overall laser-to-ions energy conversion comparable to those predicted for circularly polarized laser pulses.

physics.plasm-ph↗

arXiv2024

Continuum damping of topologically-protected edge modes at the boundary of a magnetized plasma

Recent extension of the topological ideas to continuous systems with broken time-reversal symmetry, such as magnetized plasmas, provides new insights into the nature of scattering-free topologically-protected surface plasma waves (TSPWs). We demonstrate a unique characteristic of TSPWs propagating above the electron cyclotron frequency: their collisionless damping via coupling to the continuum of resonant modes localized inside a smooth plasma-vacuum interface. Damped TSPWs retain their unidirectional nature and robustness against backscattering. When sheared magnetic field creates a boundary between damped and undamped TSPWs, the two refract into each other without reflections

physics.plasm-ph↗

arXiv2021

Fermi acceleration in relativistic collisionless plasma shocks correlates with anisotropic energy gains

Collisionless shocks generated by two colliding relativistic electron-positron plasma shells are studied using particle-in-cell (PIC) simulations. Shocks are mediated by the Weibel instability (WI), and the kinetic energy of the fastest accelerated particles is found to be anisotropically modified by WI-induced electric fields. Specifically, we show that all particles interacting with the shock bifurcate into two groups based on their final relativistic Lorentz factor $γ$: slow ($γ< γ_{bf}$) and fast ($γ> γ_{bf}$), where $γ_{bf}$ is the bifurcation Lorentz factor that was found to be approximately twice the initial (upstream) Lorentz factor $γ_0$. We have found that the energies of the slow particles are equally affected by the longitudinal and transverse components of the shock electric field, whereas the fast particles are primarily accelerated by the transverse field component.

physics.plasm-ph↗

arXiv2019

Stationary Bernstein-Greene-Kruskal structures in a current carrying relativistic cold plasma

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.

physics.plasm-ph↗

arXiv2017

Particle-in-cell simulation of Buneman instability beyond quasilinear saturation

Spatio-temporal evolution of Buneman instability has been followed numerically till its quasilinear quenching and beyond, using an in-house developed electrostatic 1D particle-in-cell simulation code. For different initial drift velocities $k_{L}v_{0}/ω_{pe} \approx 0.1 \, - \, 1$ and for a wide range of electron to ion mass ratios (m/M), growth rate obtained from simulation agrees well with the numerical solution of the fourth order dispersion relation. Quasi-linear saturation of Buneman instability occurs when ratio of electrostatic field energy density ($\sum\limits_{k} |E_{k}|^{2}/8π$) to initial electron drift kinetic energy density ($W_{0} = \frac{1}{2} n_{0}m v^{2}_{0}$) reaches up to a constant value, which as predicted by Hirose [Plasma Physics 20, 481(1978)], is independent of initial electron drift velocity but depends on electron to ion mass ratio m/M as $\sum\limits_{k} |E_{k}|^{2}/16πW_{0} \approx (m/M)^{1/3}$. This result stands verified in our simulations. Growth of the instability beyond the first saturation (quasilinear saturation ) till its final saturation [Ishihara et. al., PRL 44, 1404(1980)] follows an algebraic scaling with time. In contrast to the quasilinear saturation, the ratio of final saturated electrostatic field energy density to initial kinetic energy density, is relatively independent of electron to ion mass ratio and is found to depend only on the initial drift velocity. Beyond the final saturation, electron phase space holes coupled to large amplitude ion solitary waves, a state known as coupled hole-soliton , are seen in our simulations. The propagation characteristics ( amplitude - speed relation ) of these coherent modes is found to be consistent with the theory of Saeki et. al. [PRL 80, 1224(1998)].

physics.plasm-ph↗

arXiv2016

One dimensional PIC simulation of relativistic Buneman instability

Spatio-temporal evolution of the relativistic Buneman instability has been investigated in one dimension using an in-house developed particle-in-cell simulation code. Starting from the excitation of the instability, its evolution has been followed numerically till its quenching and beyond. As compared to the well understood non-relativistic case, it is found that the maximum growth rate ($γ_{max}$) reduces due to relativistic effects and varies with $γ_{e0}$ and m/M as $γ_{max} \sim \frac{\sqrt{3}}{2\sqrt{γ_{e0}}}\biglb(\frac{m}{2M}\bigrb)^{1/3}$, where $γ_{e0}$ is Lorentz factor associated with the initial electron drift velocity ($v_{0}$) and (m/M) is the electron to ion mass ratio. Further it is observed that in contrast to the non-relativistic results[Hirose,Plasma Phys. 20, 481(1978)] at the saturation point, ratio of electrostatic field energy density ($\sum\limits_{k} |E_{k}|^{2}/8π$) to initial drift kinetic energy density ($W_{0}$) scales with $γ_{e0}$ as $\sim 1/γ^{2}_{e0}$. These simulation results are found to be in good agreement with that derived using fluid theory.

physics.plasm-ph↗