Spatially resolved measurements of two-dimensional turbulent structures in DIII-D plasmas

Abstract

Observation of modes consistent with the trapped-electron mode (TEM) has been made using the electron cyclotron emission imaging (ECEI) diagnostic on the DIII-D tokamak. The new measurements enable the extraction of spectral properties, including poloidal dispersion relations. The spatially correlated radial structure shows qualitative consistency with radially global linear gyrokinetic simulations, using the poloidal wavenumber selected in a narrow frequency band in the ECEI data. Simulations of trapped-electron modes (TEM) driven by the electron temperature gradient yield phase velocities in close agreement with measurements. As found previously in the outer core of DIII-D L-Mode plasmas, the electron temperature fluctuation levels from nonlinear gyrokinetic simulations fall below experiment.

Correlation electron cyclotron emission diagnostic and improved calculation of turbulent temperature fluctuation levels on ASDEX Upgrade

“…This section describes new analysis techniques used to calculate the fluctuation spectra and total fluctuation amplitudes from the CECE diagnostic and then compares these techniques to previous techniques [6][7][8][9][10][11][12][13][14][15][16][17] using synthetic and experimental data. The method pre-sented here does not require any sort of absolute or crosscalibration of radiometer channels, thus eliminating the possibility of calibration-induced error.…”

“…By correlating two closely spaced (radially) radiometer channels, however, one is able to eliminate uncorrelated thermal noise while retaining the turbulent fluctuation signal (assuming the structure is larger than the channel spacing). This technique has been utilized on a variety of machines worldwide [6][7][8][9][10][11][12][13][14][15][16][17].…”

Validation of nonlinear gyrokinetic transport models using turbulence measurements

“…Electron temperature fluctuations can currently only be accessed at long-wavelength or low k, with correlation electron cyclotron emission (CECE) diagnostics (Sattler & Hartfuss 1993Cima et al 1995;White et al 2008;Zemedkun et al 2015;Freethy et al 2016;Sung et al 2016;Fontana, Porte & Cabrera 2017) on tokamaks and stellarators; unfortunately this measurement is unavailable in spherical tokamaks, because the 2nd harmonic X-mode electron cyclotron emission is cutoff in these low toroidal field, high plasma beta plasmas. At low-k, it is also possible to measure the phase angle between electron temperature and density fluctuations, by coupling an ECE radiometer with a reflectometer (White et al 2010a).…”

“…The CECE measurement relies on correlating two such radiometer channels with the same turbulent signals, but with different thermal noise signals (Sattler & Hartfuss 1993;Cima et al 1995), and has been most widely deployed using a frequency-based decorrelation method (White et al 2008;Zemedkun et al 2015;Freethy et al 2016;Sung et al 2016). Correlation ECE radiometers have good spatial resolution, and can provide quantitative fluctuation levels in very straightforward manner, and are non-perturbative and passive measurements.…”

Role of the zonal flow in multi-scale multi-mode turbulence with small-scale shear flow in tokamak plasmas

“…In tokamak plasmas, another important short wave-length instability, the so-called collisionless trapped-electron mode (CTEM) commonly exists, which is driven by both electron temperature and/or density gradients. [23,24] The radial short wavelength spectrum of the zonal flow is mainly distributed in a wide range of q r ρ i ∼ [0.4, 1.5]. [25] It was analyzed theoretically which also exhibit a relatively short radial scale of the zonal flows with q r ρ θ i ∼ 1 (ρ θ i is the ion poloidal gyroradius) in CTEM turbulence.…”