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

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 w(o) approximately 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 k(theta) < or = 1.8 cm(-1) and k(r) < or = 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 approximately 0.5% to approximately 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.

Energetic ion transport by microturbulence is insignificant in tokamaks

“…The Doppler backscattering system (DBS) 99 measures density fluctuations and the velocity of turbulent structures through an intermediate range of k h q s < 2. Electron temperature fluctuations are documented by the correlation electron cyclotron emission system (CECE) 100 with sensitivity of k h q s < 0:3. The CECE diagnostic is coupled with an X-mode reflectometer that measures density fluctuations within the same sampling volume.…”

A new fast two-color interferometer at Alcator C-Mod for turbulence measurements and comparison with phase contrast imaging

“…A standard Gaussian 24 is used to model the FTCI wavenumber response; similar Gaussian models have previously been applied to beam emission spectroscopy 2 and correlation electron cyclotron emission 26 fluctuation diagnostics. The model assumes that the response of a single detector element to line-integrated electron density fluctuations is a Gaussian with an e-folding length equal to one half of the effective detector width through the plasma,…”

The design of a correlation electron cyclotron emission system on J-TEXT

“…When it is close to the edge position, the level of temperature fluctuations is higher too. 3,4 In order to understand the physical mechanism of the anomalous transport, J-TEXT (27 cm minor radius and 105 cm major radius) also committed to the fluctuation characteristics of temperature measurement. In this paper, some basics of CECE measurement are presented in Sec.…”