Core temperature fluctuations and related heat transport in the Texas Experimental Tokamak-Upgrade

Abstract

The mechanism(s) responsible for anomalous heat transport in the tokamak plasma core has remained elusive to experimental verification. In this paper the hypothesis that high-frequency electrostatic turbulence can account for the measured electron heat transport in Ohmically heated tokamak discharges of the Texas Experimental Tokamak-Upgrade (TEXT-U) [Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier, Amsterdam, 1989), Vol. 1, p. 342] is tested. To accomplish this, core temperature fluctuations have been determined from the measured correlation between two electron cyclotron radiation signals detected by a multichannel high-frequency-resolution heterodyne radiometer. It is found that long wavelength modes (poloidal wave number ≲1 cm−1) are present, with an electron temperature fluctuation amplitude comparable to the density fluctuation amplitude. However, these modes cannot account for observed transport. An extrapolation of the observed turbulent temperature spectrum to the shorter wavelengths measured by far infrared scattering shows that shorter-wavelength electrostatic turbulence could still be responsible for transport.

Drift waves and transport

“…The long-wavelength ␦T e /T e fluctuation amplitudes, which are free of this problem, are reported to be comparable to the amplitude of the density fluctuations. First, Cima et al (1995) conclude that the directly measured long-wavelength component of the turbulent flux q e ϭ 3…”

Recent Advancements in Microwave Imaging Plasma Diagnostics

“…In addition, the relatively poor spatial resolution of conventional ECE has limited its sensitive wavenumber spectral range due to sample volume effects 17,18,27 . These problems were solved when the high spatial resolution ECE imaging diagnostic was developed and implemented on TEXT-U, which measured the poloidal wavenumbers, statistical dispersion relations, and poloidal correlation lengths of electron temperature fluctuations in the plasma core for the first time 23,28 .…”

“…Despite the above, the spectra as well as the root-mean-square (rms) values of turbulent T e fluctuations can be measured by applying intensity interferometric techniques to the ECE signals 6,[17][18][19][20][21][22][23][24][25][26] . The key is to obtain two signals that have uncorrelated radiation noise, so that the (4) Here, c 1 and c 2 are constant sensitivity coefficients of the detection system channels.…”

Correlation ECE diagnostic in Alcator C-Mod

“…The CECE diagnostic in Alcator C-Mod measures long wavelength (k θ ρ s < 0.3) T e fluctuations. The spectral decorrelation method [3] is used in the CECE diagnostic to measure the small amplitude(≤ 1%) turbulent T e fluctuations. Figure 1 shows a diagram a e-mail: sck83@mit.edu Figure 1.…”

“…If we have two channels with uncorrelated thermal noise and common T e fluctuations, the fluctuations can be resolved by correlating these two channels. More details about the principles of CECE can be found in [2][3][4].…”