arXiv subjects

A. K. Ram

Publications and source records attributed to A. K. Ram.

6 recordsLinked to original sources

arXiv2026

Parametric Decay of Broadband Lower Hybrid Waves

We derive a nonlinear dispersion relation for the parametric decay of an arbitrary lower hybrid pump spectrum, generalizing the standard three-wave treatment to the broadband case. Finite pump bandwidth is found to reorganize the decay spectrum, collapsing the nonresonant continuum predicted by monochromatic theory into narrow bands localized at ion cyclotron harmonics. This reorganization is distinct from the gradual suppression usually associated with finite pump coherence: it occurs at modest bandwidth, while growth rates remain large. Accounting for finite bandwidth restores agreement between theoretical growth rate spectra and sideband measurements from lower hybrid current drive experiments on Alcator C-Mod. More broadly, these results suggest that pump coherence can qualitatively determine the accessible decay channels in magnetized plasmas with kinetic resonances.

physics.plasm-ph↗

arXiv2022

Two Dimensional Electromagnetic Scattering form Dielectric Objects using Qubit Lattice Algorithm

A qubit lattice algorithm (QLA) is developed for Maxwell equations in a two-dimensional Cartesian geometry. In particular, the initial value problem of electromagnetic pulse scattering off a localized 2D dielectric object is considered. A matrix formulation of the Maxwell equations, using the Riemann-Silberstein-Weber vectors, forms a basis for the QLA and a possible unitary representation. The electromagnetic fields are discretized using a 16-qubit representation at each grid point. The discretized QLA equations reproduce Maxwell equations to second order in an appropriate expansion parameter $ε$. The properties of scattered waves depend on the scale length of the transition layer separating the vacuum from the core of the dielectric. The time evolution of the fields gives interesting physical insight into scattering when propagating fields are excited within the dielectric medium. Furthermore, simulations show that the QLA recovers Maxwell equations even when $ε\sim 1$.

physics.optics↗

arXiv2022

The effect of the width of the incident pulse to the dielectric transition layer in the scattering of an electromagnetic pulse -- a quantum lattice algorithm simulation

The effect of the thickness of the dielectric boundary layer that connects a material of refractive index $n_1$ to another of index $n_2$ is considered for the propagation of an electromagnetic pulse. For very thin boundary layer the scattering properties of the pulse mimics that found from the Fresnel jump conditions for a plane wave - except that the transmission to incident amplitudes are augmented by a factor of $\sqrt{n_2/n_1}$. As the boundary layer becomes thicker one finds deviations away from the Fresnel conditions and eventually one approaches WKB propagation. However there is found a small but unusual dip in part of the transmitted pulse that persists in time. The quantum lattice algorithm (QLA) used recovers the Maxwell equations to second order in a small parameter -- but QLA still recovers Maxwell equations when this parameter is unity. The expansion parameter is the speed of the pulse in medium $n_1$.

physics.plasm-ph↗

arXiv2018

Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

In spherical tokamaks, the electron plasma frequency is greater than the electron cyclotron frequency. Electromagnetic waves in the electron cyclotron range of frequencies are unsuitable for directly heating such plasmas due to their reduced accessibility. However, mode-conversion of the extraordinary wave to the electron Bernstein wave (X-B mode-conversion) at the upper hybrid resonance makes it possible to efficiently couple externally-launched electromagnetic wave energy into an overdense plasma core. Traditional mode-conversion models describe an X-mode wave propagating in a potential containing two cutoffs that bracket a single wave resonance. Often, however, the mode-conversion region is in the edge, where turbulent fluctuations and blobs can generate abrupt cutoffs and scattering of the incident X-mode wave. We present a new framework for studying the X-B mode-conversion which makes the inclusion of these fluctuations analytically tractable. In the new approach, the high-field cutoff is modelled as an infinite barrier, which manifests as a boundary condition applied to a wave equation involving only one cutoff adjacent to the resonance on the low-field side. The new model reproduces the main features of the previous approach, yet is more suitable for analyzing experimental observations and extrapolating to higher dimensions. We then develop an analytical estimate for the effect of small-amplitude, quasi-monochromatic density fluctuations on the X-B mode-conversion efficiency using perturbation theory. We find that Bragg backscattering of the launched X-mode wave reduces the mode-conversion efficiency significantly when the fluctuation wavenumber is resonant with the wavenumber of the incident X-mode wave. These analytical results are corroborated by numerically integrating the mode-conversion equations.

physics.plasm-ph↗

arXiv2018

Scattering of radio frequency waves by cylindrical blobs in the plasma edge in tokamaks

Radio frequency waves are routinely used in tokamak fusion plasmas for plasma heating, current control, and as well as in diagnostics. These waves are excited by antenna structures placed near the tokamak's wall and they have to propagate through a turbulent layer known as scrape-off layer, before reaching the core plasma (which is their target). This layer exhibits coherent density fluctuations in the form of blobs and filaments. The scattering processes of RF plane waves by single blob is studied, with the assumption that the blob has cylindrical shape and infinite length. The axis of the blob is not necessarily aligned with the externally applied magnetic field in the case of plane waves. The investigation concerns the case of Electron Cyclotron (EC) waves ($f_0$=170 $GHz$) for ITER-like and Medium Size Tokamak applications (such as TCV, ASDEX-U, DIII-D, etc) as well as the case of Low Hybrid (LH) waves ($f_0$=4.6 $GHz$) for ALCATOR C-MOD Tokamak device. The study covers for a variety of density contrasts between the blobs and the ambient plasma and a wide range of blob radii.

physics.plasm-ph↗

arXiv2003

Coherent Acceleration of Magnetized Ions by Electrostatic Waves with Arbitrary Wavenumbers

This paper studies the coherent acceleration of ions interacting with two electrostatic waves in a uniform magnetic field B0. This is a generalization of an earlier analysis of waves propagating perpendicularly to B0 to include the effect of wavenumbers along B0. The Lie transformation technique is used to develop a perturbation theory describing the ion motion, and results are compared with numerical solutions of the complete equations of motion. Coherent energization occurs when the Doppler-shifted wave frequencies differ by nearly an integer multiple of the ion cyclotron frequency. When the difference in the parallel wavenumbers of the two waves is increased the coherent energization of ions is limited to a small part of the phase space. The energization of ions and its dependence on wave parameters is discussed.

physics.plasm-ph↗