Confinement of pure-electron plasmas in a toroidal magnetic-surface configuration

A levitated magnetic dipole configuration as a compact charged particle trap

As a magnetic confinement configuration for electron-positron pair-plasmas, the APEX collaboration [T. S. Pedersen et al., New J. Phys. 14, 035010 (2012)] plans to construct a compact levitated dipole experiment with a high-temperature superconducting coil. In order to realize stable levitation of the dipole field coil, a simple feedback-controlled levitation system was constructed with conventional analog circuits. We report the properties of a prototype levitation system using a permanent magnet and compare its behavior to predictions from a stability analysis. We also present a practical review needed for the construction of a compact levitated dipole trap system based on the work of Morikawa et al. [Teion Kogaku, J. Cryo. Soc. Jpn. 39, 209 (2004)]. Numerical orbit analysis suggests improved confinement properties of charged particles in a dipole field trap by replacing the permanent magnet with a levitated superconducting coil magnet. Such a compact dipole field configuration is potentially applicable to the confinement of various charged particles including positrons and electrons.

A table top experiment to investigate production and properties of a plasma confined by a dipole magnet

We report a table top experiment to investigate production and properties of a plasma confined by a dipole magnet. A water cooled, strong, cylindrical permanent magnet (NdFeB) magnetized along the axial direction and having a surface magnetic field of ∼0.5 T is employed to create a dipole magnetic field. The plasma is created by electron cyclotron resonance heating. Visual observations of the plasma indicate that radiation belts appear due to trapped particles, similar to the earth's magnetosphere. The electron temperature lies in the range 2-13 eV and is hotter near the magnets and in a downstream region. It is found that the plasma (ion) density reaches a value close to 2 × 10¹¹ cm^-3 and peaks at a radial distance about 3 cm from the magnet. The plasma beta β (β = plasma pressure/magnetic pressure) increases radially outward, and the maximum β for the present experimental system is ∼2%. It is also found that the singly charged ions are dominant in the discharge.

Magnetospheric vortex formation: self-organized confinement of charged particles

A magnetospheric configuration gives rise to various peculiar plasma phenomena that pose conundrums to astrophysical studies; at the same time, innovative technologies may draw on the rich physics of magnetospheric plasmas. We have created a "laboratory magnetosphere" with a levitating superconducting ring magnet. Here we show that charged particles (electrons) self-organize a stable vortex, in which particles diffuse inward to steepen the density gradient. The rotating electron cloud is sustained for more than 300 s. Because of its simple geometry and self-organization, this system will have wide applications in confining single- and multispecies charged particles.

Diagnosing pure-electron plasmas with internal particle flux probes

Techniques for measuring local plasma potential, density, and temperature of pure-electron plasmas using emissive and Langmuir probes are described. The plasma potential is measured as the least negative potential at which a hot tungsten filament emits electrons. Temperature is measured, as is commonly done in quasineutral plasmas, through the interpretation of a Langmuir probe current-voltage characteristic. Due to the lack of ion-saturation current, the density must also be measured through the interpretation of this characteristic thereby greatly complicating the measurement. Measurements are further complicated by low densities, low cross field transport rates, and large flows typical of pure-electron plasmas. This article describes the use of these techniques on pure-electron plasmas in the Columbia Non-neutral Torus (CNT) stellarator. Measured values for present baseline experimental parameters in CNT are phi(p)=-200+/-2 V, T(e)=4+/-1 eV, and n(e) on the order of 10(12) m(-3) in the interior.

Experimental confirmation of stable, small-debye-length, pure-electron-plasma equilibria in a stellarator

The creation of the first small-Debye length, low temperature pure electron plasmas in a stellarator is reported. A confinement time of 20 ms has been measured. The long confinement time implies the existence of macroscopically stable equilibria and that the single particle orbits are well confined despite the lack of quasisymmetry in the device, the Columbia non-neutral torus. This confirms the beneficial confinement effects of strong electric fields and the resulting rapid E x B rotation of the electrons. The particle confinement time is presently limited by the presence of bulk insulating materials in the plasma, rather than any intrinsic plasma transport processes. A nearly flat temperature profile is seen in the inner part of the plasma.