Paul W. Brenner

Magnetic Surface Visualizations in the Columbia Non-Neutral Torus

Visualizations of magnetic surfaces are a valuable diagnostic in the Columbia Non-neutral Torus (CNT). The CNT is a compact stellarator, which is currently being used to study non-neutral plasmas confined on magnetic surfaces. The full 3-D shapes of magnetic surfaces created by CNTs simple four circular coil geometry are readily visualized by using an electron beam and neutral gas. These visualizations are useful for probe alignment and the confirmation of the magnetic surface topology, and they were necessary for the recent installation of a conducting boundary conforming to the last closed magnetic surface.

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

We report on the creation and up to 90 ms sustainment of pure electron plasmas confined in a stellarator without internal objects. Injection of positrons into such plasmas is expected to lead to the creation of the first electron-positron plasma experiments. These newly created plasmas will also allow a study of pure electron plasmas without the perturbing presence of internal objects. The plasmas were created by thermionic emission of electrons from a heated, biased filament that was retracted in 20 ms. The confinement of these transient plasmas is different from that of steady state plasmas with internal objects and emissive filaments, and is generally shorter, limited by ion buildup. The decay time is increased by lowering the neutral pressure, lowering the electron plasma temperature, or operating with neutrals with high ionization energies (helium). These findings are all consistent with ion accumulation being the cause for the shorter than expected confinement times. The magnetic field strength also...

Confinement jumps in a non-neutral plasma

Measurements of confinement jumps in pure electron plasmas confined on magnetic surfaces are presented and discussed. The experiments were performed in the Columbia non-neutral torus stellarator [T. S. Pedersen, J. P. Kremer, R. G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgren, and X. Sarasola, Fusion Sci. Technol. 50, 372 (2006)]. The jumps exhibit hysteresis and are associated with a negative differential resistance. The jumps occur at particular emission currents of the biased emissive filaments that create and sustain the electron plasmas independent of the methods used to affect the emission current. This observation, as well as other experimental evidence, supports that the jumps are caused by a cathode instability. The jumps can also be triggered by the application of a bias potential on a nearby mesh. In most circumstances, the jumps occur between two stable but measurably different equilibrium states. These different equilibrium states have substantially different confinement ...

Confinement of pure electron plasmas in the CNT stellarator

The Columbia Non‐neutral Torus is a stellarator devoted to non‐neutral and electron‐positron plasma research. Confinement and transport processes have been studied in some detail now, and an understanding of these processes has emerged. Transport is driven in two ways: The presence of internal rods, and the presence of neutrals. Both transport processes are clearly distinguished experimentally, and a model of the rod driven transport has been developed, yielding very good agreement with experimental data. The neutral driven transport is faster than originally expected and indicates the presence of unconfined orbits in CNT. Numerical modeling of the electron orbits in CNT confirms the existence of loss orbits and shows that a flux surface conforming electrostatic boundary will greatly improve confinement. Such a boundary has now been installed in CNT, with initial results showing an order of magnitude improvement in confinement.

Studies Of Enhanced Confinement In The Columbia Non‐Neutral Torus

Recently the measured confinement time in the Columbia Non‐neutral Torus (CNT) has been increased by nearly an order of magnitude to 190 ms. Previously, enhanced transport caused in part by the mismatch of constant potential and magnetic surfaces limited confinement times to 20 ms. A conducting boundary conforming to the last closed magnetic flux surface has been installed to minimize potential variation along magnetic surfaces, provide new methods to influence the plasma, and act as an external diagnostic. A summary of new results with the conducting boundary installed will be presented, including discussion of how confinement is influenced by neutral pressure, magnetic field strength, and the effect of biasing individual sectors of the mesh.

Equilibrium, stability, and transport of electron plasmas in the Columbia Non-neutral Torus

The equilibrium, stability and transport properties of electron plasmas confined on magnetic surfaces in the Columbia Non-neutral Torus are discussed. The equilibrium is characterized by a Poisson-Boltzmann equation. Measured potential and temperature profiles are presented. These plasmas are generally stable but can be destabilized by an ion driven instability that involves the interaction of the ion and electron fluids and has a poloidal mode number of m = 1. The transport of electrons driven by collisions with neutrals is much greater than the neoclassical prediction. A code has been written to follow single particle motion to determine why. Finally, sudden jumps between different equilibria with different transport levels are being investigated.