D. K. Smith

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...

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.

Confinement of multiply charged ions in an electron cyclotron resonance heated mirror plasma

Multiply charged ions are studied in the Constance B quadrupole mirror experiment [Phys. Rev. Lett. 59, 1821 (1987)] in order to better understand the ion physics of electron cyclotron resonance (ECR) high charge‐state ion sources. By measuring the ion densities and end loss fluxes, the parallel confinement times for the first five charge states of oxygen plasmas are determined. The parallel ion confinement times increase with charge state and peak on axis, both indications of an ion‐confining potential dip created by the hot electrons. The radial profile of ion end loss is normally hollow, with the peak fluxes occurring at the edge of the ECR zone. An attempt is made to increase the end loss flux of a selected ion species by decreasing its parallel confinement time using minority ion cyclotron resonance heating (ICRH). In addition, an ion model is developed to predict the ion densities, end loss fluxes, and confinement times using the ion particle balance equations, the quasineutrality condition, and the...

Time‐of‐flight analyzer for ion end loss of a mirror plasma

A time‐of‐flight analyzer has successfully been used to measure the extracted charge‐state distribution and impurity level of an ECRH mirror plasma. End‐loss temperatures for each ion species have been determined from the rise time of the analyzer output pulses. Ions entering the line‐of‐sight device are first accelerated by a two‐aperture lens and are then focused with an einzel lens. Electrostatic deflection plates chop the ion beam into short pulses which travel down a 1.20‐m tube to an electron multiplier. Ion spectra are typically summed at a rate of 1 kHz using a hardware signal averager.

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.

TARA diagnostic set

The TARA Tandem Mirror Experiment has recently begun operation. The set of diagnostics available at this time is discussed. The following diagnostics are now in use: diamagnetic loops, a multichord microwave interferometer, Langmuir and emissive probes, pick‐up loops, and secondary‐emission detectors. End‐loss diagnostics include net current detector arrays, Faraday cup arrays, swept particle analyzer arrays, and calorimetry. Light‐emission measurements are made in the visible and VUV regions. A multichord fiber‐optic array for plasma position detection is also used. In addition, a three‐channel charge exchange analyzer, a hard x‐ray system, and fast pressure gauges are available.

Double parallel‐plate electrostatic analyzer for Tara end‐loss studies

An endloss spectrograph has been designed and is being fabricated for the Tara tandem mirror. This spectrograph is a double parallel‐plate electrostatic analyzer about 60 cm in length with a 30° entrance angle; the high‐voltage plate is a transparent wire cloth grid which allows higher energy particles to pass through for detection on a symmetric ground plane. The analyzer has three detection planes: first, the normal focal plane for low‐energy detection at high resolution (0.1–4 keV); second, the entrance grounded plate for mid energies (4–6 keV); and third, the symmetric ground plane for coarse resolution at higher energies (8–20 keV). The analyzer is defocusing on the symmetric plane. This spectrograph will be able to measure simultaneously the endloss potential cutoff and ions from the neutral beams; in conjunction with a loss cone neutral beam source the resolution is sufficient (60 eV) to measure the plasma potential.