Quinn Marksteiner

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 demonstration of a compact stellarator magnetic trap using four circular coils

An experimental demonstration of a compact stellarator magnetic trap created from four circular coils is presented. The coil manufacturing and assembly tolerances were on the order of 0.5–1%, far less stringent than most other stellarators. The simplicity, loose mechanical tolerances, and low cost of the trap design makes it feasible for stellarators to be used for a variety of novel physics experiments, in addition to their present use for magnetic confinement fusion. The experiment, the Columbia Non-neutral Torus, has several other desirable features such as no significant internal island chains and the lowest aspect ratio, A⩽1.9, of any stellarator built to date.

CONSTRUCTION AND INITIAL OPERATION OF THE COLUMBIA NONNEUTRAL TORUS

Abstract We report on the results from initial testing and operation of the Columbia Nonneutral Torus, a new stellarator experiment constructed at Columbia University to study the confinement of nonneutral plasmas, electron-positron plasmas, and stellarator confinement in the presence of strong electrostatic fields. A new algorithm for automatic identification of good magnetic surfaces, island chains, and stochastic regions in Poincaré maps is also described. We present some of the details of the design of the interlocked in-vessel coils and the vacuum system and report on initial vacuum performance. Magnetic surface mapping and visualization results are also presented, confirming the existence of ultralow aspect ratio magnetic surfaces with excellent quality and good agreement with numerical calculations.

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.

Ion accumulation in an electron plasma confined on magnetic surfaces

Accumulation of ions can alter and may destabilize the equilibrium of an electron plasma confined on magnetic surfaces. An analysis of ion sources and ion content in 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 presented. In CNT ions are created preferentially at locations of high electron temperature, near the outer magnetic surfaces. A volumetric integral of neνiz gives an ion creation rate of 2.8×1011ions∕s. This rate of accumulation would cause neutralization of a plasma with 1011 electrons in about half a second. This is not observed experimentally, however, because currently in CNT ions are lost through recombination on insulated rods. From a steady-state balance between the calculated ion creation and loss rates, the equilibrium ion density in a 2×10−8Torr neutral pressure, 7.5×1011m−3 electron density plasma in CNT is calculated to be ni=6.2×109m−3,...

Confirmation of a large density variation along the magnetic axis of the Columbia Non-neutral Torus

Significant variations in the density and potential along the axis of a pure electron plasma in the Columbia Non-neutral Torus (CNT) stellarator have now been measured. Large variations along the magnetic field are predicted by three-dimensional equilibrium reconstructions of CNT plasmas and by simple electrostatic and geometric arguments [Lefrancois and Pedersen, Phys. Plasmas 13, 120702 (2006)]. The density variation, naxis,ϕ=0°∕naxis,ϕ=90°, is measured directly for several different plasma equilibria, and has a median value of 7.8, consistent with the predicted density variation of 4.4, because the error bars are large. The associated variation in potential predicted from the Boltzmann relation, eΔΦ∕Te=ln(4.4)=1.5, was also measured experimentally. The median measured, eΔΦ∕Te, was 1.1, which is of the predicted sign and in rough agreement with the measurements, but smaller than predicted. The difference is statistically significant, but might be related to the imperfect numerical modeling of the compli...

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 ...

First Studies of Pure Electron Plasmas in the Columbia Non‐neutral Torus

The first studies of pure electron plasmas confined on magnetic surfaces in the Columbia Non‐neutral Torus are overviewed. The electron plasma is created by a thermionic emitter filament and similar filaments mounted on ceramic rods are used as Langmuir and emissive probes. The equilibrium density, temperature and potential profiles are experimentally measured. Numerical calculations of the equilibrium agree well with measurements and also predict a toroidal density variation of a factor of four. The confinement time is found to decrease with increased neutral pressure and emitter bias voltage, and it is presently limited to 20 ms by the insulated emitter and probe rods. A retractable electron emitter and external diagnostics will be used to determine the confinement time in the absence of rods. Ion driven instabilities are observed at high neutral pressure and low magnetic field strength. Further research of these instabilities will be carried out.

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.