A correlation electron cyclotron emission diagnostic and the importance of multifield fluctuation measurements for testing nonlinear gyrokinetic turbulence simulationsa

A. E. White, L. Schmitz, W. A. Peebles, T. A. Carter, T. L. Rhodes, E. J. Doyle, P. A. Gourdain, J. C. Hillesheim, G. Wang, C. Holland, G. R. Tynan, M. E. Austin, G. R. McKee, M. W. Shafer, K. H. Burrell, J. Candy, J. C. Deboo, R. Prater, G. M. Staebler, R. E. WaltzM. A. Makowski

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Abstract

A correlation electron cyclotron emission (CECE) diagnostic has been used to measure local, turbulent fluctuations of the electron temperature in the core of DIII-D plasmas. This paper describes the hardware and testing of the CECE diagnostic and highlights the importance of measurements of multifield fluctuation profiles for the testing and validation of nonlinear gyrokinetic codes. The process of testing and validating such codes is critical for extrapolation to next-step fusion devices. For the first time, the radial profiles of electron temperature and density fluctuations are compared to nonlinear gyrokinetic simulations. The CECE diagnostic at DIII-D uses correlation radiometry to measure the rms amplitude and spectrum of the electron temperature fluctuations. Gaussian optics are used to produce a poloidal spot size with wo ∼1.75 cm in the plasma. The intermediate frequency filters and the natural linewidth of the EC emission determine the radial resolution of the CECE diagnostic, which can be less than 1 cm. Wavenumbers resolved by the CECE diagnostic are kθ 1.8 cm-1 and kr 4 cm-1, relevant for studies of long-wavelength turbulence associated with the trapped electron mode and the ion temperature gradient mode. In neutral beam heated L -mode plasmas, core electron temperature fluctuations in the region 0.5<r/a<0.9, increase with radius from ∼0.5% to ∼2%, similar to density fluctuations that are measured simultaneously with beam emission spectroscopy. After incorporating "synthetic diagnostics" to effectively filter the code output, the simulations reproduce the characteristics of the turbulence and transport at one radial location r/a=0.5, but not at a second location, r/a=0.75. These results illustrate that measurements of the profiles of multiple fluctuating fields can provide a significant constraint on the turbulence models employed by the code.

Original languageEnglish
Article number103505
JournalReview of Scientific Instruments
Volume79
Issue number10
DOIs
StatePublished - 2008
Externally publishedYes

Funding

This research was supported by the U.S. Department of Energy under DE-FG03-01ER54615, JP333701, DE-FG02-89ER53296, DE-FG02-07ER54917, DE-FG03-97ER54415, DE-FG02-89ER53296, DE-FC02-04ER54698, and DE-AC52-07NA27344. AEW and JCH were supported by appointments to the Fusion Energy Sciences Fellowship Program administered by Oak Ridge Institute for Science and Education under a contract between the U.S. Department of Energy and the Oak Ridge Associated Universities. We thank the DIII-D team for their support of these experiments.

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