Fusion Research Center Contributions:

  1. Electron Cyclotron Emission (ECE) as a Density Fluctuation Diagnostic (Poster 7.5MB - Paper 400kB ) - A.G. Lynn, P.E. Phillips, FRC, UT, A. Hubbard, PSFC, MIT.
    An ECE diagnostic measures the electron cyclotron emission from an optically thick plasma that is limited to the blackbody radiation limit of the plasma at the cyclotron resonate location. This provides an electron temperature profile for plasmas with spatial varying magnetic fields, such as a Tokamak. In Alcator C-Mod, a high resolution heterodyne ECE observes the plasma at an 18° angle with respects to the mid-plane. The system was designed with a 1MHz bandwidth and a poloidal resolution of <1cm. The 32 channels, with 7mm separation, provide a temperature profile (2nd harmonic ECE, 270-304GHz) over the low field side plasma radius at 5.4T.
    During discharges with internal transport barriers (ITB), fluctuations were observed in the ECE signal in the high confinement regime of the plasma. Depending on conditions, these would observe either broadband fluctuations at ~100kHz with a width of 100kHz or an ~80kHz mode. These fluctuations would only be observed during the ITBs with the strong density peaking. Changes in the temperature profile indicated that refractive effects were important. Normal incidence ECE systems would observer refractive effects only for densities within 5-10% of the cut-off frequency. Small changes in the density profile have been observed in the ECE signal for densities greater than 30% of the cut-off density and at levels of ∆n/n<1%. Broadband fluctuations have also been observed in the plasma interior following high-density pellet injections.
    These fluctuations have been associated with density fluctuations. TORAY-GA[1], a ray tracing code, has been used to investigate the sensitive of the ECE system to density perturbations. The localization and sensitivity of these measurements and its suitability as a density fluctuation diagnostic will be discussed in this paper.
    This work was supported by US. Dept of Energy Grant     DE-FG03-96ER54373
    [1] D.B. Batchelor, R.C. Goldfinger, and H. Weitzner. IEEE Transactions on Plasma Science, PS-8(2):78, 1980.
  2. Interpretation of Neutral Beam Emission Spectras the Beam-Component Density Distribution (Poster (600kB) - Paper(600kB)) - William L. Rowan, R. V. Bravenec, M. B. Sampsell, Fusion Research Center, The University of Texas at Austin, Austin, TX 78712, R. S. Granetz, Plasma Science and Fusion Center, MIT, Cambridge, MA 02139.
    Diagnostic neutral beams (DNB) are used on tokamaks and stellarators for measurement of ion temperature, plasma current density, and other critical quantities. It is important tooptimize the density in the energy components of the DNB for the diagnostic application. Measured component densities are also useful for interpretation of beam diagnostics andfor design of new diagnostics. The spectrally resolved photon emission from the interaction of the neutral beam with a background neutral gas is commonly used for thismeasurement. The emission cross sections available for interpretation of the measurement are reviewed here. Simulations employ these cross sections with other datato infer the properties of the plasma sources from which the beams are extracted. The simulations are also reviewed. Empirical examples are drawn from beam emissionspectra measured at Alcator C-Mod.
    Work supported by USDOE Grant DE-FG03-96ER54373 and by USDOE Coop.Agree. No. DE-FC02-99-ER54512
  3. Neutral Beam Diagnostics for the HT-7 Tokamak (Poster(1.8MB) - Paper(400kB)) - L.Q. Hu1, William L. Rowan2, B.N. Wan1, C.D. Hu1, H. He2, B.H. Liu1, K. Gentle2, and HT-7 DNB Team1
    The major research areas for the HT-7 tokamak are advanced steady-state operation, high-performance plasma discharges, and plasma fueling studies. A diagnostic neutral beam (DNB) will be added to the diagnostic set so that charge-exchange recombination spectroscopy (CXRS) can be used to acquire ion temperature and rotation and so that beam-emission spectroscopy can be used to measure density fluctuations. In normal operation, the DNB can produce 5.5 A of extracted beam current in hydrogen at an energy of 50 keV with a pulse length of 0.1 s. The full-, half-, and third-energy current fractions are 0.34, 0.30, and 0.36. D and He beams can also be produced, and the beam can be modulated at 500 Hz. The CXRS system will have 15 spatial observation chords and employ a spectrometer with 0.02-nm resolution which can acquire spectra at the rate of 100 Hz. In this paper, the detailed technical capabilities and physical implementation of the DNB and CXRS systems are presented.
    1 Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China
    2 Fusion Research Centre, The University of Texas at Austin, Austin, Texas 78712, USA
  4. Synthetic Turbulence Diagnostics for Nonlinear Gyrokinetic Simulations (pdf 650KB) - R. V. Bravenec, Fusion Research Center, The University of Texas at Austin, W. M. Nevins Lawrence Livermore National Laboratory, D. R. Ernst Plasma Science and Fusion Center, MIT, J. Candy, General Atomics.
    Gyrokinetic turbulence codes are able to calculate the nonlinearly saturated fluctuations, due to microinstabilities, of plasma parameters in plasma confinement configurations such as the tokamak. These results, however, must be benchmarked against experimental data. This is challenging because of limitations in the diagnostics ( e.g., finite spatial resolution) and the fact that the diagnostics typically do not measure basic plasma parameters. For example, beam-emission spectroscopy measures Balmer-alpha emission from a neutral beam, not electron density per se. In this work, we present a system based on the GKV1 suite of IDL™ routines in which user-supplied "filters" are applied to the code outputs to simulate individual diagnostics. Digital time-series analysis may then be performed on the processed outputs. The results, such as rms fluctuation level, mean wave numbers, correlation lengths and times, etc., can then be compared directly to the experimental analysis.
  5. Improved spatial resolution for the DIII-D ECE radiometer (Poster 3.5MB - Paper 1MB) - M.E. Austin,University of Texas, R.F. Ellis, University of Maryland
    A scheme is presented for improving the spatial resolution of the ECE heterodyne radiometer diagnostic on the DIII-D tokamak. This is primarily accomplished with an optimized optics system replacing the current long horn antenna. A Gaussian beam system has been designed employing a scalar horn and ellipsoidal mirror that achieves a 3 dB spot size of less than 3 cm at the DIII-D magnetic axis and 4 cm at the edge providing a factor of 4 improvement.  It is shown that further improved localization can be realized by reducing the intermediate frequency bandwidth of the radiometer filters from 1 GHz to 500 MHz and that for many conditions of interest the relativistic broadening width will remain less than the instrumental width.