Production and study of high-beta plasma confined by a superconducting dipole magnet

Abstract

The Levitated Dipole Experiment (LDX) [J. Kesner et al., in Fusion Energy 1998, 1165 (1999)] is a new research facility that is exploring the confinement and stability of plasma created within the dipole field produced by a strong superconducting magnet. Unlike other configurations in which stability depends on curvature and magnetic shear, magnetohydrodynamic stability of a dipole derives from plasma compressibility. Theoretically, the dipole magnetic geometry can stabilize a centrally peaked plasma pressure that exceeds the local magnetic pressure (β>1), and the absence of magnetic shear allows particle and energy confinement to decouple. In initial experiments, long-pulse, quasi-steady-state microwave discharges lasting more than 10s have been produced that are consistent with equilibria having peak beta values of 20%. Detailed measurements have been made of discharge evolution, plasma dynamics and instability, and the roles of gas fueling, microwave power deposition profiles, and plasma boundary shape. In these initial experiments, the high-field superconducting floating coil was supported by three thin supports. The plasma is created by multifrequency electron cyclotron resonance heating at 2.45 and 6.4GHz, and a population of energetic electrons, with mean energies above 50keV, dominates the plasma pressure. Creation of high-pressure, high-beta plasma is possible only when intense hot electron interchange instabilities are stabilized by sufficiently high background plasma density. A dramatic transition from a low-density, low-beta regime to a more quiescent, high-beta regime is observed when the plasma fueling rate and confinement time become sufficiently large.

Turbulent fluctuations during pellet injection into a dipole confined plasma torus

“…46 An accounting for particles in LDX has not been completed, but, based upon the relatively long period of outgassing observed after each discharge, we hypothesize that the cooled surface of the levitated cryostat acts as a temporary "getter pump" for the inward moving plasma.…”

Confinement improvement with magnetic levitation of a superconducting dipole

“…This multi-frequency ECRH creates a two component plasma containing a hot electron species. The plasma stored energy is dominated by a population of energetic electrons with Eeh > 50 keV and neh ~ 10 16 m -3 [11]. [12].…”

Observation of a new high-β and high-density state of a magnetospheric plasma in RT-1

“…It is characterized by high-β stability and excellent confinement properties [3]. Stable confinement of high-β plasmas has been demonstrated in dipole magnetic fields generated by levitated superconducting magnets in the Ring Trap 1 (RT-1) [4], the Mini Ring Trap (Mini-RT) [5], and the Levitated Dipole Experiment (LDX) [6,7]. In the first series of experiments in RT-1 [8][9][10][11], a plasma has been generated by using an electron cyclotron resonance heating (ECH) with 8.2 and 2.45 GHz microwaves [11,12].…”