J. H. Irby

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...

2D full-wave simulation of ordinary mode reflectometry

A 2D full-wave simulation of ordinary mode propagation has been developed in an effort to model effects seen in reflectometry experiments but not properly explained by 1D analysis. The geometric fall-off of the fields, together with the effects of both refraction and diffraction, considerably modify the results obtained. The now commonly seen experimental observations of large amplitude and phase variations of the echo signal and occasional ramping of the phase can be explained by these 2D effects in the presence of fluctuations.

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.

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.

CO2 laser polarimeter for measurement of plasma current profile in Alcator C‐Mod

A multichannel infrared polarimeter system for measurement of the plasma current profile in Alcator C‐Mod has been designed, constructed, and tested. The system utilizes a cw CO2 laser at a wavelength of 10.6 μm. An electro‐optic polarization‐modulation technique has been used to achieve the high sensitivity required for the measurement. The recent results of the measurements as well as the feasibility of its application on international thermonuclear experimental reactor are presented.

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.

Simulation of experimental data from the proposed C‐Mod reflectometer

A six‐channel, narrowband reflectometer is expected to go into operation on Alcator C‐Mod during its first year of operation. Several programs have been developed to model the effects of both refraction and diffraction on the received signal. Results indicate that careful design and alignment will be necessary to avoid large variations in signal phase and amplitude caused by small variations in plasma position. Simulations of the effects of coherent fluctuations and driftwave turbulence on the echo indicate that both can in some cases degrade the localization of the reflection layer.