D. L. Brower

The space potential in the tokamak text

A heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak TEXT [Nucl. Fusion Technol. 1, 479 (1981)]. The Ohmic discharges studied were perturbed by externally produced resonant magnetic fields (an ergodic magnetic limiter or EML). Without these perturbations the plasma central potential is generally consistent with the value calculated from radial ion momentum balance, using experimental values of density and ion temperature and assuming a neoclassical poloidal rotation velocity. Exceptions to the agreement are found when operating with reduced plasma parameters. Possible reasons for this discrepancy are explored, in particular, the effects of intrinsic magnetic field fluctuations, and modifications to the self‐consistent radial electric sheath. With the application of the EML fields the edge electric field and potential increase during periods of magnetic island overlap. A test particle calculation of electron transport shows increases in diffusivity also occur d...

Upgrade of far-infrared laser-based Faraday rotation measurement on MST

Recently, the far-infrared (FIR) laser (λ(0)=432u2002μm) Faraday rotation measurement system on MST has been upgraded. The dc flowing-gas discharge CO(2) pump laser is replaced by a rf-excited, sealed CO(2) laser at 9.27u2002μm (GEM select 100, Coherent Inc., Santa Clara, CA), which is subdivided equally into three parts to simultaneously pump three FIR cavities. The total infrared pump power is approximately 80 W on the 9R(20) line required to pump the formic acid molecule. Each FIR cavity produces ∼12u2002mW, sufficient for 11 simultaneous chord interferometry-polarimetry operations. Three key issues [(1) conservation of circularly polarized wave, (2) colinearity of two probe waves, and (3) stability of intermediate frequencies between lasers] affecting the Faraday rotation measurement have been resolved experimentally.

Biased limiter experiments on text

Experiments using an electrically biased limiter have been performed on the Texas Experimental Tokamak (TEXT). A small movable limiter is inserted past the main poloidal ring limiter (which is electrically connected to the vacuum vessel) and biased at VLim with respect to it. The floating potential, plasma potential and shear layer position can be controlled. With ¦VLim¦⩾ 50 V the plasma density increases. For VLim 0 the results obtained are inconclusive. Variation of VLim changes the electrostatic turbulence which may explain the observed total flux changes.

Digital complex demodulation applied to interferometry

The objective of this paper is to describe the principles of digital complex demodulation, and to summarize its advantages with respect to rapid time response and insensitivity to noise. These advantages are demonstrated by application to interferometry data collected on the Texas experimental tokamak (TEXT).

Electron density measurement by differential interferometry

A novel differential interferometer is being developed to measure the electron density gradient and its fluctuations. Two separate laser beams with slight spatial offset and frequency difference are coupled into a single mixer making a heterodyne measurement of the phase difference which is <1% of the total phase change experienced by each beam separately. This measure of the differential phase is made at multiple spatial points and can be inverted directly to provide the local density distribution.

Suppression of m=2 islands by electron cyclotron heating in the Texas Experimental Tokamak: Experiment and theory

Electron cyclotron heating (ECH) is used to suppress m=2 magnetohydrodynamic (MHD) oscillations in the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)]. The location of ECH power deposition is controlled by a movable antenna. The MHD activity is suppressed when the ECH beam is directed close to the q=2 surface. The experiment is simulated using a three‐dimensional resistive MHD code in cylindrical geometry. For fixed plasma current, the saturated m=2 island width is found to depend on the value of the safety factor at the magnetic axis (q0). The simulation suggests that the observed saturated m=2 island in the pre‐ECH plasma, which typically occupies 25% of the minor radius, corresponds to q0∼1.3. The suppression of the island in the presence of ECH is attributed to current profile modification. In some discharges, the m=2 activity does not resume even after the ECH pulse is turned off.

The effect of magnetic perturbations on edge transport in TEXT

The effect of magnetic islands and stochastic fields on thermal transport, particle transport, and the radial electric field in TEXT is discussed. Both intrinsic and externally produced magnetic islands are found to enhance particle transport. Large amplitude m = 2 islands (width/a less-than-or-equal-to 0.25) are produced by neon puffing. Smaller amplitude m congruent-to 6 islands (width/a less-than-or-equal-to 0.07) are produced with resonant external coils at low coil current. Higher currents produce stochastic magnetic fields for r/a greater-than-or-equal-to 0.85. Stochastic fields significantly alter the radial electric field and enhance thermal transport but not particle transport. Experimentally observed transport changes are compared with analytic theories and are in reasonable agreement.

Divertor interferometer diagnostic for ITER

In the harsh environment of the divertor region in ITER, plasmas spanning a huge density range from 1019to1022m−3 are anticipated making measurement of the electron density particularly challenging. For any reasonable wavelength choice, the total phase measured by a conventional two-color interferometer system is always ⪢2π and therefore subject to fringe counting errors. This problem can be remedied by adding a polarimeter capability whereby the Cotton-Mouton effect is measured or by employing differential interferometry. Using either approach, the total phase is always ⪡2π. The conceptual design of an interferometer system along with possible wavelength choices will be explored.

Vacuum compatible, variable cross‐section magnetic coil diagnostic used in digital feedback control of plasma position in TEXT‐Upgrade

A magnetic pickup coil diagnostic set is used to measure the position of the plasma column in the Texas Experimental Tokamak Upgrade (TEXT‐U) project. The output from this coil set is used in a digital feedback system to control the plasma position. To provide a fast time response for the feedback system, one complete coil set is located on the interior of the vacuum vessel. Another set with a slower time response is located on the outside of the vessel. To simplify and speed up data processing, the coils are constructed so that the X and Y coordinates of the plasma current centroid are each determined using the signals from only two separate coils. For each coordinate one coil is used to measure a tangential (relative to the coil surface) magnetic field component, while the second coil measures a normal field component. Due to physical constraints, the coils are not continuous around the vacuum vessel. The presence of gaps in the coils causes pickup of the external current flowing in the divertor coil wi...

Advanced Interferometry Techniques for Burning Plasmas

For future burning plasma experiments, all diagnostics must be re‐evaluated in terms of their measurement capabilities and robustness to the new environment. This is certainly true for interferometry measurements where conventional approaches may not be ideal and interpretation may require modification due to high plasma temperatures. Optimizing these systems to provide maximum information will be crucial to understanding burning plasma dynamics. This paper will explore a variety of phase measurement techniques for the main body and divertor regions that can be utilized on burning plasma experiments like ITER and beyond.