Observation of reflected electrons driven quasi- longitudinal (QL) whistlers in large laboratory plasma

Abstract

This paper reports experimental and theoretical investigations on plasma turbulence in the source plasma of a Large Volume Plasma Device. It is shown that a highly asymmetrical localized thin rectangular slab of strong plasma turbulence is excited by loss cone instability. The position of the slab coincides with the injection line of the primary ionizing energetic electrons. Outside the slab, in the core, the turbulence is weaker by a factor of 30. The plasma turbulence consists of oblique [θ=tan−1(k⊥/k||)≈87°] Quasi-Longitudinal (QL) electromagnetic whistlers in a broad band of 40kHz<f≤80 kHz with k⊥∼1.2 cm−1 and k||∼0.06cm−1. Experimental observations suggest that the primary agent for the turbulence is not driven by primary ionizing energetic electrons but by the loss cone feature in the velocity distribution of reflected energetic electrons. A magnetic mirror is formed in the Electron Energy Filter when it is energized. It is shown that it is this mirror which is responsible for both reflection of the energetic electrons and imposing loss cone feature on it. Theoretical framework is based upon Oblique whistler approximation by Sharma and Vlahos [Astrophys. J. 280, 405 (1984)] and Verkhoglyadova et al. [J. Geophys. Res. 115, A00F19 (2010)] and Quasi Longitudinal (QL) whistlers by Booker and Dyce [Radio Sci. J. Res 69D (1965)] for excitation of the plasma turbulence in the magnetosphere.

Investigations into growth of whistlers with energy of energetic electrons

“…Further justification for laboratory experiments on whistler destabilization lies in the difficulty of exact measurement of k ∥ and k ⊥ in the high-frequency domain in tokamaks, which limits the interpretation of the relevant data. In the present observations of LVPD being reported, whistlers are driven essentially by the energetic, reflected electrons from a loss cone or trapped particles in a partially magnetized plasma, ρ e ∼ 0.006 m and ρ i ∼ 0.45 m [17]. The whistler mode observed in LVPD has phase propagation highly oblique to the magnetic field (θ ∼ 85 • ) with k ∥ ≪ k ⊥ and the frequency domain where it resides has the ordering ω ci ≪ ω ∼ ω LH ≪ ω ce < ω pe .…”

“…The energetic electrons are confined along the magnetic field lines but, surprisingly, despite leakage of the magnetic field lines across the EEF, these energetic electrons remain confined within the EEF and are not observed in the target region (figure 2(b)). Subsequently, it has been observed and reported in Sanyasi et al [17], that these electrons are reflected from the magnetic mirror developed at the interface region of the EEF and source plasma at x = 0.65 m. The electron saturation current signals obtained for EEF ON and EEF OFF conditions are shown in figure 4(a). We observed a doubling of electron saturation current magnitude during the EEF ON condition in comparison to the EEF OFF condition.…”

“…The cross-correlation coefficient between the density (δne) and magnetic field (δBz) fluctuations obtained in the energetic belt region shows strong in-phase correlation ( [16], figure 9). Reprinted from [17], with the permission of AIP Publishing.…”

“…The ratio of density and temperature between the source and the target plasma are typically 8 and 2, respectively. Reprinted from [17], with the permission of AIP Publishing.…”

“…, where n r is the reflected particle density from a loss cone, B 0 is the background axial magnetic field, K is a constant and T e,h is the averaged energetic electron temperature, respectively. The γ i for the LVPD case is defined by equation ( 3) and γ d is defined as equation ( 6) [17,18]:…”