Observation of Electromagnetic Fluctuation Induced Particle Transport in ETG Dominated Large Laboratory Plasma

TL;DR

This study demonstrates electromagnetic fluctuation-induced particle transport in a laboratory plasma dominated by Electron Temperature Gradient (ETG) turbulence, with experimental results aligning with theoretical models.

Contribution

First experimental observation of electromagnetic particle flux in high-beta ETG turbulence in a large laboratory plasma device, supported by theoretical modeling.

Findings

Electromagnetic flux is much smaller than electrostatic flux, about 10^-5 times.

Electromagnetic flux is non-ambipolar, contrary to slab ETG models.

Theoretical estimates match experimental measurements.

Abstract

The Large Volume Plasma device(LVPD), a cylindrically shaped, linear plasma device of dimension( $\phi =2m $, $ L =3m $) have successfully demonstrated the excitation of Electron Temperature Gradient(ETG) turbulence[ Mattoo et al.,[1]]. The observed ETG turbulence shows significant power between $\omega \sim (25-90 krad/s )> \Omega_{ci}(= 8 krad/s )$, corresponding to the wavenumber, $ k_y \rho_e ~ (0.1- 0.2)$ where, $ \Omega_{ci}$ and $\rho_e(=5 mm ) $, ion cyclotron frequency and electron Larmor radius, respectively. The observed frequency and wave number matches well with theoretical estimates corresponding to Whistler-ETG mode. We investigated electromagnetic (EM) fluctuations induced plasma transport in high beta ( $ \beta \sim 0.01 - 0.4 $ ) ETG mode suitable plasma in LVPD. The radial electromagnetic (EM) electron (ion) flux ( $ \Gamma^{e,i}_{em} $) results primarily from the correlation between fluctuations of parallel electron current ($\delta J_{\parallel ,e,i}$ ) and radial magnetic field ($ \delta B_r $ ). The electromagnetic particle flux is observed to be much smaller than the electrostatic particle flux,$ \Gamma_{em}\approx 10^{-5} \Gamma_{es} $ .The EM flux is small but finite contrary to the conventional slab ETG model. The electromagnetic flux is non-ambipolar. A theoretical model is obtained for the EM particle flux in straight homogeneous magnetic field geometry. The theoretical estimates are seen to compare well with the experimental observations. Sluggish parallel ion response is identified as the key mechanism for generation of small but finite electromagnetic flux.