Observations of an Ion-Driven Instability in Non-Neutral Plasmas Confined on Magnetic Surfaces

Abstract

The first detailed experimental study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma confined on magnetic surfaces is presented. The instability has a poloidal mode number m=1, implying that the parallel force balance of the electron fluid is broken and that the instability involves rotation of the entire plasma, equivalent to ion-resonant instabilities in Penning traps and toroidal field traps. The mode appears when the ion density exceeds approximately 10% of the electron density. The measured frequency decreases with increasing magnetic field strength, and increases with increasing radial electric field, showing that the instability is linked to the E x B flow of the electron plasma. The frequency does not, however, scale exactly with E/B, and it depends on the ion species that is introduced, implying that the instability consists of interacting perturbations of ions and electrons.

First experimental studies of the physics of plasmas of arbitrary degree of neutrality

“…In addition to the already known poloidal mode number m = 1 of CNT's single mode iondriven oscillations [21], the toroidal mode number is now known: n = 0. The m = 1, n = 0 structure of this mode is identical to that of the diocotron mode seen in pure toroidal traps [30].…”

“…When more magnetized ions are present in the plasma (either by introducing lighter ion species (He + ) or increasing the B field), the plasma reverts to single mode behaviour in the range 0.03 < η < 0.8. This single mode was previously discovered and studied in CNT's standard configuration (64 • tilt angle) and has been identified as an ion-driven instability [21]. In the standard configuration, the rotational transform of the magnetic surfaces varied from ι = 0.12 on the magnetic axis to 0.23 at the plasma edge [12].…”

“…The mode travels in the E × B direction and the poloidal mode number is m = 1 (as was the case in the 64 • configuration [21]). The ion-related mode does not align itself to the field even with this low shear rotational profile, instead again chooses a m = 1 poloidal mode number.…”

“…Fluctuations in the plasma potential ( φ) are detected by measuring the oscillating current in an internal floating emissive (hot) filament [21]. Perturbations in the plasma were also measured with an ion probe, a set of capacitive probes, and a high-speed camera.…”

Pure electron plasmas confined for 90 ms in a stellarator without electron sources or internal objects

“…In steady state plasmas, an ion density on the order of 10% of the electron density leads to instabilities and decreased confinement in CNT [21]. We therefore believe that the ion contamination causes an instability that destroys confinement rapidly, perhaps by first increasing the electron temperature, or generating a hot tail in the electron distribution function, either of which would then increases the ionization rate [15].…”