Simultaneous measurement of core electron temperature and density fluctuations during electron cyclotron heating on DIII-D

New measurements show that long-wavelength k s 0.5 electron temperature fluctuations can play an important role in determining electron thermal transport in low-confinement mode L-mode tokamak plasmas. In neutral beam-heated L-mode tokamak plasmas, electron thermal transport and the amplitude of long-wavelength electron temperature fluctuations both increase in cases where local electron cyclotron heating ECH is used to modify the plasma profiles. In contrast, the amplitude of simultaneously measured long-wavelength density fluctuations does not significantly increase. Linear stability analysis indicates that the ratio of the trapped electron mode TEM to ion temperature gradient ITG mode growth rates increases in the cases with ECH. The increased importance of the TEM drive relative to the ITG mode drive in the cases with ECH may be associated with the increases in electron thermal transport and electron temperature fluctuations. © 2010 American Institute of Physics. doi:10.1063/1.3318469 The importance of long-wavelength turbulent fluctuations in determining electron thermal transport in the core of tokamak plasmas is of considerable interest to the field of magnetically confined fusion. In the absence of magnetohydrodynamic MHD instabilities, small amplitude 1%, low frequency ci, where ci is the ion cyclotron frequency turbulent fluctuations associated with drift-wavetype instabilities are widely believed to drive radial transport. 1 The turbulence driven transport can lead to heat and particle losses that reduce the performance of magnetically confined fusion devices. 2 One expectation from driftwave theory that can be tested directly is that the ratio of electron temperature and density fluctuation amplitudes, T ˜ e /Te /n /n, should scale with the ratio of linear growth rates of the trapped electron mode TEM and the ion temperature gradient ITG mode, TEM/ITG. 3,4 To test this, multifield fluctuation measurements are needed. Electron temperature fluctuations and density fluctuations have previously been studied in tokamak experiments 5‐7 during electron cyclotron heating ECH, but the two fluctuating fields were not measured simultaneously. We present here the first simultaneous measurements of long-wavelength electron temperature fluctuations and density fluctuations in the core of an L-mode tokamak plasma heated with neutral beams and ECH. The experiment was performed on the DIII-D tokamak major radius R=1.67 m, minor radius a=0.61 m. 8 The discharges have magnetic field BT2.0 T, plasma current Ip=1 MA, with edge safety factor q955.2 in the time period of interest, and are inner wall limited. Afirst discharge is heated with 2.5 MW of neutral beam power injected in the direction of the plasma current beginning early in time t=300 ms. The L-mode plasma of interest 1500t 1800 ms is sawtooth-free and MHD-free. In a second