arXiv subjects

Y. C. Saxena

Publications and source records attributed to Y. C. Saxena.

5 recordsLinked to original sources

arXiv2026

Development of a thin-target hard X-ray bremsstrahlung detection system to study confined runaway electrons in Aditya-U Tokamak

A specially shielded CdTe detector based hard X-ray (HXR) monitoring system equipped with a lead collimator has been developed and installed on the Aditya-U tokamak to investigate the dynamics of fast electrons (~20-200 keV) generated during sawtooth activity. The pre-existing HXR monitor in Aditya-U is exposed to the entire HXR bremsstrahlung emission from the plasma volume, peripheral limiters, and other structural components, which limits its ability to separately study the dynamics of lost and confined runaway electrons (REs). In contrast, the newly developed diagnostic has successfully measured the chord-averaged thin-target HXR bremsstrahlung emission encompassing the core plasma region, particularly within and around the sawtooth inversion radius. The measured HXR spectra are validated through forward modelling code that incorporates plasma parameters, confined RE characteristics, and the geometric configuration of the diagnostic system. The results confirm the capability of the developed HXR monitor to probe the fast-electron dynamics during internal plasma instabilities.

physics.plasm-ph↗

arXiv2025

Stabilization of sawteeth instability by short gas pulse injection in ADITYA-U tokamak

Experiments on ADITYA-U tokamak show a marked enhancement in the sawtooth period by application of short gas puffs of fuel that cause a modification of the radial density profile. A consequent suppression of the trapped electron modes (TEMs) then leads to an increase in the core electron temperature. This slows down the heat propagation following a sawtooth crash, causing a delay in achieving the critical temperature gradient inside the q = 1 surface required for the next sawtooth crash to happen. The overall scenario has strong similarities with the behavior of sawtooth under electron cyclotron resonance heating (ECRH). Our findings suggest an alternate, simpler technique for sawtooth control that may be usefully employed in small/medium-sized tokamaks that do not have an ECRH or any other auxiliary heating facility.

physics.plasm-ph↗

arXiv2024

MHD activity induced coherent mode excitation in the edge plasma region of ADITYA-U Tokamak

In this paper, we report the excitation of coherent density and potential fluctuations induced by magnetohydrodynamic (MHD) activity in the edge plasma region of ADITYA-U Tokamak. When the amplitude of the MHD mode, mainly the m/n = 2/1, increases beyond a threshold value of 0.3-0.4 %, coherent oscillations in the density and potential fluctuations are observed having the same frequency as that of the MHD mode. The mode numbers of these MHD induced density and potential fluctuations are obtained by Langmuir probes placed at different radial, poloidal, and toroidal locations in the edge plasma region. Detailed analyses of these Langmuir probe measurements reveal that the coherent mode in edge potential fluctuation has a mode structure of m/n = 2/1 whereas the edge density fluctuation has an m/n = 1/1 structure. It is further observed that beyond the threshold, the coupled power fraction scales almost linearly with the magnitude of magnetic fluctuations. Furthermore, the rise rates of the coupled power fraction for coherent modes in density and potential fluctuations are also found to be dependent on the growth rate of magnetic fluctuations. The disparate mode structures of the excited modes in density and plasma potential fluctuations suggest that the underlying mechanism for their existence is most likely due to the excitation of the global high-frequency branch of zonal flows occurring through the coupling of even harmonics of potential to the odd harmonics of pressure due to 1/R dependence of the toroidal magnetic field.

physics.plasm-ph↗

arXiv2020

Characteristics of a Plasma Source with adjustable multi-pole line cusp geometry

Two magnetic configurations of Multi-cusp Plasma Device (MPD) have been explored to obtain high quiescence level, large uniform plasma region with nearly flat mean density and temperature profiles. In particular, properties of plasma in a six-pole six magnet (SPSM) and twelve pole six magnets (TPSM) cusp configurations are rigorously compared and reported here. It is found that more uniform plasma with nearly flat profiles is found in TPSM along with increased quiescence level. Findings are verified across various magnetic field strengths for both configurations.

physics.plasm-ph↗

arXiv2019

Observation of neutrals carrying ion-acoustic wave momentum in partially ionized plasma

An experimental study of Ion Acoustic (IA) wave propagation is performed to investigate the effect of neutral density for argon plasma in an unmagnetized linear plasma device. The neutral density is varied by changing the neutral pressure, which in turn allows the change in ion-neutral, and electron-neutral collision mean free path. The collisions of plasma species with neutrals are found to modify the IA wave characteristics such as the wave amplitude, velocity, and propagation length. Unlike the earlier reported work where neutrals tend to heavily damp IA wave in the frequency regime ω<ν_in (where ω is ion-acoustic mode frequency and ν_in is ion-neutral collision frequency), the experimental study of IA wave presented in this paper suggests that the collisions support the wave to propagate for longer distances as the neutral pressure increases. A simple analytical model is shown to qualitatively support the experimental findings.

physics.plasm-ph↗