Investigation of diocotron modes in toroidally trapped electron plasmas using non-destructive method

Abstract

Experiments with trapped electron plasmas in a SMall Aspect Ratio Toroidal device (SMARTEX-C) have demonstrated a flute-like mode represented by oscillations on capacitive (wall) probes. Although analogous to diocotron mode observed in linear electron traps, the mode evolution in toroids can have interesting consequences due to the presence of in-homogeneous magnetic field. In SMARTEX-C, the probe signals are observed to undergo transition from small, near-sinusoidal oscillations to large amplitude, non-linear “double-peaked” oscillations. To interpret the wall probe signal and bring forth the dynamics, an expression for the induced current on the probe for an oscillating charge is derived, utilizing Green's Reciprocation Theorem. Equilibrium position, poloidal velocity of the charge cloud, and charge content of the cloud, required to compute the induced current, are estimated from the experiments. Signal through capacitive probes is thereby computed numerically for possible charge cloud trajectories. In order to correlate with experiments, starting with an intuitive guess of the trajectory, the model is evolved and tweaked to arrive at a signal consistent with experimentally observed probe signals. A possible vortex like dynamics is predicted, hitherto unexplored in toroidal geometries, for a limited set of experimental observations from SMARTEX-C. Though heuristic, a useful interpretation of capacitive probe data in terms of charge cloud dynamics is obtained.

Ion-driven destabilization of a toroidal electron plasma -- A 3D3VPIC Simulation

“…Wall probes are located near the inner boundary wall, separated by θ = π/2 locations at L z = 0.16 m mid-plane are shown. The device size is identical to that of SMARTEX-C 39 .…”

“…In contrast to the above discussed devices, toroidal traps [34][35][36][37][38][39][40] of different aspect ratios (a/b, a is the major radius, b is the minor radius) apply an inhomogeneous external magnetic field with inherent curvature to confine an electron plasma. The electron plasma goes through rotational cross-field E × B drift where E field is the self electric field of the plasma and B is the external toroidal field.…”

“…The device was intended to utilize the potential well of the electron plasma to trap and accelerate heavy ions. Subsequent experiments were performed to confine electron plasma in axisymmetric toroidal geometry of different aspect ratios (a/b ∼ 6.2 36 and ∼ 1.5 37,38 ), followed by a number of experiments in partial toroidal geometry: small aspect ratio device SMARTEX-C (a/b ∼ 1.6) 39 , large aspect ratio device LNT II (a/b ∼ 13.7) 40 .…”

“…As indicated before, near-absolute equilibria of the electron plasma in PM traps with uniform magnetic fields allows various experiments to be performed in the device. Such near-absolute equilibrium in toroidal trap was recently addressed and a quiescent quasi-steady (QQS) 45,46 toroidal electron plasma has been found in a numerical experiment with similar parametric space of that of an existing small aspect ratio torus 39 . This finding also suggests the possibility of realizing such a quiescent pure ion plasma in toroidal equilibrium, which might provide excellent testbed/new directions for quantum information processing 8,28 in future.…”

Unprecedented confinement time of electron plasmas with a purely toroidal magnetic field in SMARTEX-C

“…Principal diagnostics of the trap are capacitive probes [31] which are essentially parts of wall insulated from the rest of it and are located at various toroidal and poloidal locations. These wall probes are utilized to monitor image currents that can be interpreted to obtain information about any electrostatic activity in the plasma [32]. Additionally, if the plasma is quiescent, these probes have been used to excite normal modes, namely the diocotron modes that are ubiquitously present in such single species plasma.…”