A multi-species powder dropper for magnetic fusion applications.

We present a device for controlled injection of a variety of materials in powder form. The system implements four independent feeder units, arranged to share a single vertical drop tube. Each unit consists of a 80 ml reservoir, coupled to a horizontal linear trough, where a layer of powder is advanced by piezo-electric agitation at a speed proportional to the applied voltage, until it falls into a drop tube.

New steady-state quiescent high-confinement plasma in an experimental advanced superconducting tokamak.

A critical challenge facing the basic long-pulse high-confinement operation scenario (H mode) for ITER is to control a magnetohydrodynamic (MHD) instability, known as the edge localized mode (ELM), which leads to cyclical high peak heat and particle fluxes at the plasma facing components. A breakthrough is made in the Experimental Advanced Superconducting Tokamak in achieving a new steady-state H mode without the presence of ELMs for a duration exceeding hundreds of energy confinement times, by using a novel technique of continuous real-time injection of a lithium (Li) aerosol into the edge plasma. The steady-state ELM-free H mode is accompanied by a strong edge coherent MHD mode (ECM) at a frequency of 35-40 kHz with a poloidal wavelength of 10.

Methods and preliminary measurement results of liquid Li wettability.

A test of lithium wettability was performed in high vacuum (< 3 × 10(-4) Pa). High magnification images of Li droplets on stainless steel substrates were produced and processed using the MATLAB(®) program to obtain clear image edge points. In contrast to the more standard "θ/2" or polynomial fitting methods, ellipse fitting of the complete Li droplet shape resulted in reliable contact angle measurements over a wide range of contact angles.

Continuous improvement of H-mode discharge performance with progressively increasing lithium coatings in the National Spherical Torus Experiment.

Lithium wall coatings have been shown to reduce recycling, improve energy confinement, and suppress edge localized modes in the National Spherical Torus Experiment. Here, we show that these effects depend continuously on the amount of predischarge lithium evaporation. We observed a nearly monotonic reduction in recycling, decrease in electron transport, and modification of the edge profiles and stability with increasing lithium.

Experimental setup for tungsten transport studies at the NSTX tokamak.

Tungsten particles have been introduced into the National Spherical Torus Experiment (NSTX) in Princeton with the purpose to investigate the effects of tungsten injection on subsequent plasma discharges. An experimental setup for the study of tungsten particle transport is described where the particles are introduced into the tokamak using a modified particle dropper, otherwise used for lithium-powder injection. An initial test employing a grazing-incidence extreme ultraviolet spectrometer demonstrates that the tungsten-transport setup could serve to infer particle transport from the edge to the hot central plasmas of NSTX.

Preliminary results from the Alcator C-Mod polarimeter.

A poloidally viewing far infrared polarimeter diagnostic is being developed for the Alcator C-Mod tokamak, and will be used to determine the q-profile and to study density and magnetic field fluctuations. A three-chord version of what will eventually be up to a ten-chord system has been designed and fabricated and will be installed on C-Mod before the end of the current run period. Bench tests of a single chord mock-up of this system show acceptable noise levels for the planned measurements.

Edge-localized-mode suppression through density-profile modification with lithium-wall coatings in the National Spherical Torus Experiment.

Reduction or elimination of edge localized modes (ELMs) while maintaining high confinement is essential for future fusion devices, e.g., the ITER.

Stark-tuned, far-infrared laser for high-frequency plasma diagnostics.

A Stark-tuned optically pumped far-infrared CH(3)OH laser operating at 119 microm has been built. The laser is designed to operate at high power while exhibiting a well-separated Stark doublet. At a pump power of 65 W and an electric field of 1 kV/cm the laser has delivered over 100 mW cw while exhibiting a frequency splitting of 34 MHz.

Multichannel far-infrared laser interferometer for electron density measurements on the tokamak fusion test reactor.

A ten-channel far-infrared laser interferometer which is routinely used to measure the spatial and temporal behavior of the electron density profile on the tokamak fusion test reactor is described, and representative results are presented. This system has been designed for remote operation in the very hostile environment of a fusion reactor. The possible expansion of the system to include polarimetric measurements is briefly outlined.

Theory of the rooftop resonator: resonant frequencies and eigenpolarizations.

The eigenpolarizations and resonant frequencies are derived for a circular dielectric waveguide resonator (EH11 mode) terminated by perfectly reflecting metallic rooftop mirrors. It is demonstrated that, in the far-IR region, circularly polarized helical standing waves result from an arbitrary angular misalignment of the rooftop creases. Further, the right and left circularly polarized modes have different resonant frequencies.

Effects of passive isolation on several optically pumped far-infrared laser lines.

The effects of a recently introduced passive-isolation scheme for optically pumped far-infrared lasers have been investigated on several well-known far-infrared laser lines. In addition to increasing the laser output stability, the isolation scheme alters the pump-to-far-infrared power-conversion efficiency and the polarization state of the far-infrared output.