Dynamics of flows, fluctuations, and global instability under electrode biasing in a linear plasma device

Abstract

Grid biasing is utilized in a large-scale helicon plasma to modify an existing instability. It is shown both experimentally and with a linear stability analysis to be a hybrid drift-Kelvin–Helmholtz mode. At low magnetic field strengths, coherent fluctuations are present, while at high magnetic field strengths, the plasma is broad-band turbulent. Grid biasing is used to drive the once-coherent fluctuations to a broad-band turbulent state, as well as to suppress them. There is a corresponding change in the flow shear. When a high positive bias (10Te) is applied to the grid electrode, a large-scale (ñ/n≈50%) is excited. This mode has been identified as the potential relaxation instability.

Self-Organization, Structures, and Anomalous Transport in Turbulent Partially Magnetized Plasmas with Crossed Electric and Magnetic Fields

“…In this Letter, we investigated the nonlinear gradient-drift and lower-hybrid instabilities in partially magnetized plasmas with crossed electric and magnetic fields. These modes are expected to play a central role in formation of long wavelength structures and transport in various devices employing E × B configurations for electric propulsion [1,3,28], material processing [2,6,29], and cylindrical Penning type devices [4,30,31]. The large scale structures (shear zonal flows and vortices) are produced via the inverse cascade of the energy flow from short wavelength modes.…”

On the nature of blob propagation and generation in the large plasma device: Global GRILLIX studies

“…There are three relevant plasma instabilities in LAPD that may be present simultaneously [46,47,48], the resistive driftwave instability (DW), the rotational interchange instability (RII) and the Kelvin-Helmholtz instability (KHI).…”

“…17drive the KHI due to a radially sheared flow. The KHI is discussed for linear machines in detail in [35,45,50] and exhibits a phase difference of δ nφ ≈ π/2 to π [48,51]. The first term on the right hand side of Eq.…”

“…It represents a centrifugal force and is destabilising for a radially decreasing background potential d(log φ 0 )/dr < 0. The RII exhibits a phase difference between δ nφ ≈ π/4 to π/2 [48,47] and typically mode number of m = 1 or m = 2 [51]. Snapshots from a full 3D turbulence simulation are shown in Fig.…”

Experimental investigation of coaxial-gun-formed plasmas injected into a background transverse magnetic field or plasma

“…Experiments have also been carried out in which coaxialgun-formed plasmas are injected into a background plasma (temperature T e ∼ 1 − 5eV , density n e ∼ 10 17 m −3 ) that has been characterized elsewhere. 40,41 The background plasma pressure is typically 10−100 times less than the background magnetic pressure (i.e., low β ∼ 0.01 − 0.1). Therefore, the background magnetic field dominates the interaction, and the plasma itself has a relatively small effect on the observations reported in this paper.…”