Michael S. Hahn

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.

Confinement of pure electron plasmas in the Columbia Non-neutral Torus

The Columbia Non-neutral Torus (CNT) [T. S. Pedersen, J. P. Kremer, R. G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgreen, and X. Sarasola, Fusion Sci. Technol. 50, 372 (2006)] is a stellarator used to study non-neutral plasmas confined on magnetic surfaces. A detailed experimental study of confinement of pure electron plasmas in CNT is described here. Electrons are introduced into the magnetic surfaces by placing a biased thermionic emitter on the magnetic axis. As reported previously, the insulated rods holding this and other emitter filaments contribute to the radial transport by charging up negatively and creating E×B convective transport cells. A model for the rod-driven transport is presented and compared to the measured transport rates under a number of different conditions, finding good agreement. Neutrals also drive transport, and by varying the neutral pressure in the experiment, the effects of rod-driven and neutral-driven transport are separated. The neutral-driven electron ...

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.

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.

Studies of a Parallel Force Balance Breaking Instability in a Stellarator

An instability has been observed in non‐neutral plasmas confined on magnetic surfaces in the presence of a finite ion fraction [Phys. Rev. Letters 100, 065002 (2008)]. The dependence of the frequency and amplitude of the instability on neutral pressure, magnetic field strength, and ion species show that the mode consists of interacting perturbations of ions and electrons. In the Columbia Non‐neutral Torus (CNT) the instability has a poloidal mode number of m = 1. This does not correspond to a rational surface, implying that the parallel force balance of the electron fluid is broken. Here the diagnostic methods used to study this instability are described in detail, and some key results are shown.

Pure Electron Plasmas Confined on Magnetic Surfaces

Summary form only given. Small Debye length, low temperature, pure electron plasmas have been successfully confined in the Columbia Non-neutral Torus (CNT). These plasmas are created by steady state emission of electrons from an electron emitter physically located on the axis of the magnetic surfaces. Equilibrium density, temperature, and potential profiles have been measured by emissive Langmuir probes and they agree well with numerical calculations. Toroidal density variations of a factor of four are also predicted numerically. Confinement time is measured by dividing the total number of electrons by the emission current, which is equal to the electron loss rate. Confinement is currently limited in CNT by the presence of insulating rods in the plasma. The rods charge up electrostatically and create an E x B convection of electrons out of the magnetic surfaces. Confinement has been studied as a function of magnetic field, emitter bias, and neutral pressure. Electron loss rate is found to be proportional to B-1 in the low neutral pressure regime, which is consistent with rod-driven transport being the dominant mechanism. At higher neutral pressures where the neutral-driven transport is dominant, the loss rate scales linearly with pn, as expected, and approximately as B-1.5. Ion driven instabilities are also observed at high neutral pressure. A retractable electron emitter has been installed in CNT that creates the plasma without having an insulating rod present in steadv state. Measurements of confinement time in this unperturbed plasma should help to clearly identify other transport mechanisms.

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.