E. Sevillano

Plasma production and heating in a tandem mirror central cell by radio‐frequency waves in the ion cyclotron frequency range

Plasma production and heating in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto, Japan (IAEA, Vienna, 1987), Vol. 2, p. 251] have been studied. Using radio‐frequency excitation by a slot antenna in the ion cyclotron frequency range (ICRF), plasmas with a peak β⊥ of 3%, density of 4×1012 cm−3, ion temperature of 800 eV, and electron temperature of 75–100 eV were routinely produced. The plasma radius decreased with increasing ICRF power, causing reduced ICRF coupling and saturation of the plasma beta. About 70% of the applied ICRF power can be accounted for in direct heating of both ions and electrons. Wave field measurements have identified the applied ICRF to be the slow, ion cyclotron wave. In operation without end plugging, the plasma parameters were limited by poor axial confinement and the requirements for maintenance of magnetohydrodynamic stability and micros...

Two‐color interferometer system for Alcator C‐MOD

High densities with sharp, fast rising profiles are expected during pellet injection in Alcator C‐MOD. Consideration of interferometer systems other than far infrared will be needed to avoid refractive effects and make profile measurements during pellet injection possible. CO2 wavelengths are proving to be the most interesting from the standpoint of density measurements, with the necessary vibration subtraction provided by a coaxial He:Ne system. We will describe the two‐color interferometer as it is presently designed and present results from simulations made of the system to determine noise levels, minimum measurable density, and optimal number and location of chords. Ray tracing results both with and without pellet injection will be compared. Possible fringe counting schemes will also be discussed.

Experimental studies of divertor stabilization in an axisymmetric tandem mirror

A divertor coil set has been installed on the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1984 (IAEA, Vienna, 1985), Vol. 2, p. 285] for stabilization of m=1 flutelike modes. The effectiveness of divertor stabilization is discussed in experiments where m=1 modes are driven to instability by plug electron cyclotron heating (ECH) in an ion cyclotron heated (ICH) plasma. The instability onset is characterized by thresholds in ECH power, fueling rate, ICH power, and mapping radius of the divertor null. In general, the stability is enhanced by mapping the null radially inwards into the plasma. The interdependence of these parameters and their effect on equilibrium profiles and stability boundaries are discussed.

Plasma potential enhancement by rf heating near the ion‐cyclotron frequency

The observation of enhanced plasma potentials, i.e., potentials greater than the Boltzmann values, in a mirror device is reported. The potential structure is driven by strong radio frequency heating near the ion‐cyclotron resonance and near the local electron bounce frequency. The potentials and their effect on losses from the central cell of a tandem mirror are discussed.

Experimental study of nonlinear M = 1 modes in the Tara tandem mirror

The nature of a rigid, flutelike M=1 instability as seen in the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, 1985), Vol. 2, p. 285] is discussed. Radial density and light emission profiles obtained by inverting chord measurements are compared to end loss radial profiles during the evolution of the mode to its nonlinear saturated state. This final state is characterized by a coherent, flutelike motion of the plasma as a whole about the machine axis.

Stability of plasmas sustained by ion cyclotron wave excitation in the central cell of the Tara tandem mirror

The stability of plasmas produced by radio‐frequency heating in the ion cyclotron frequency range (ICRF) has been studied in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1986, Proceedings of the 11th International Conference, Kyoto (IAEA, Vienna, 1987), Vol. II, p. 251]. Ion cyclotron wave excitation by a slot antenna provided stability against macroscopic plasma motions in an axisymmetric configuration. The maintenance of macroscopic stability depended on the ICRF power, gas fueling rate, ion cyclotron resonance location, and ω/ωci at the antenna location. The ICRF ponderomotive force model is consistent with many of the observed stability features and predicts that the E+ component of the ion cyclotron wave was responsible for the stabilization. The Alfven ion cyclotron microinstability was observed when the plasma β⊥ and anisotropy were sufficiently high. Magnetic probe measurements of the unstable mode identified it as a...

Gas pressure measurements and control in the Tara Tandem Mirror experiment

The Tara Tandem Mirror has a 10 m long, 22 cm diameter central cell plasma heated by fundamental ion cyclotron heating. Typical central cell parameters in unplugged operation are n = 3 × 1012/cm3, Ti⊥ = 300 eV, Ti∥ ≃ 75 eV. The axisymmetric plug cell incorporates sloshing ions and ECH to generate axial confining potentials. The axisymmetric central cell and plug comprise a max B mirror which is observed to operate in both flute stable and unstable regimes. The flute instability is m = 1 and can be stabilized by an outboard anchor. The anchor plasma is formed by electron and ion cyclotron heating. Satisfactory operation of a tandem mirror requires extensive control of neutral gas from neutral beam (NB) sources [1] and startup. Tara makes extensive use of Ti gettering in the beamlines, beam dumps and plasma surfaces for both hydrogen pumping and reflux control. A description of this technology along with its impact on plasma performance is discussed.

Stabilization of the Tara tandem mirror plasma by MHD anchors

The effectiveness of a warm ion and hot electron population in the Tara outboard minimum-B anchors in stabilizing MHD flute-like modes in the central cell and axicells is assessed. With a combination of ECH and ICRF heating in the anchors, βhe > 15% and βi ~ 0.5% have been obtained. The ICRH component has a generally stabilizing effect on global MHD activity, but the stabilization is not linear in ion beta. Pinhole camera pictures indicate that the hot electron density profile is radially peaked. The resulting creation of a deeper magnetic well for the warm ions was expected to enhance the MHD stabilizing properties of the anchor. However, the addition of hot electron beta to an ICRF heated anchor plasma was observed to have no beneficial effect on MHD stability.

Observation of trapped particle modes in a tandem mirror

An instability with azimuthal mode number m≥3 localized to an axisymmetric end cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982)] has been observed, most prominently during strong ion cyclotron resonance heating in the end cell. The instability, which causes enhanced radial losses, becomes either more stable or flutelike when the connection (passing fraction) between the central cell and end cell is increased, depending on whether sufficient stabilization is provided in the central cell. The beta is sufficiently low to rule out the possibility of magnetohydrodynamic ballooning modes. Based on the plasma parameters, the instability appears to be a collisionless curvature‐driven trapped particle mode that has been predicted to be unstable in linked minimum‐ and maximum‐B mirror devices.

Ion radial transport in the Tara central cell

The radial transport rate of bulk ions in the Tara central cell, in a globally axisymmetric configuration, is determined by comparing the ionization source profile deduced from axial two-dimensional measurements of H? emission with the axial electron end-loss profile. The radial transport of an impurity ion is also calculated, from the departure of the radial profile of O V from coronal equilibrium. The two techniques yield comparable transport coefficients: D ~ 3 ? 104cm2 ? s?1 and v ~ ? 1000r cm ? s?1, near the plasma core. A separate technique is used to deduce a lower bound for the edge bulk ion radial transport rate of 3 ? 105 cm2 ? s?1. Azimuthal electric fields are proposed as the likely source of particle transport.