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.
A quasi-steady-state lower-hybrid current-drive operation is demonstrated in the Alcator C tokamak at densities up to n e -1 x 10 14 cm" 3 . The current-drive efficiency is measured experimentally over a wide range of densities and magnetic fields. The radial distribution of high-energy x rays indicates that the current-carrying electrons peak near the plasma axis. PACS numbers 52.40.Db, Generation of toroidal currents in tokamaks by injection of traveling lower-hybrid waves is currently of great interest in magnetically confined plasma-fusion research. 1 This interest arises from the possibility of steady-state tokamak operation which would make the tokamak concept a more attractive reactor candidate. 2 To present date, efficient "quasi-steady-state" operation has been demonstrated only at relatively low electron densities, namely at n e < 8x 10 12 cm" 3 in the PLT tokamak. 3 This mode of tokamak operation is achieved by terminating the inductive current sources (Ohmic power) as the rf power is injected. Eventually, a pure rf mode of operation appears feasible. 4The low-density limit in the PLT experiments is believed to be a consequence of the relatively low frequency (/ = 0.8 GHz) used. In other experiments current drive has been demonstrated by injecting rf power during full Ohmic heating (OH) operation. 5 " 8 In these experiments significant dc electric fields often exist during rf injection Avhich may modify the rf current drive figure of merit J/Pj. 1 In particular, the question of the influence of electron tails generated by the OH electric fields which existed before, or during rf injection, arises. Due to the relatively low frequencies (/=^1.3-GHz) even such "OH-assisted" experiments were carried out mostly at low densities (n e <2 xl0 13 cm-3 ).In this Letter we present experimental measurements of the efficiency of lower-hybrid current drive at reactor-relevant densities, namely at 10 13 <. /Hem" 3 ) < 10 14 . The experiments were carried out on the Alcator C tokamak (major radius R =64 cm, minor radius a = 16.5 cm) where the high rf frequency (/ = 4.6 GHz) and high rf powers (P < 1.1 MW) used allowed us to demonstrate rf current-drive operation at high densities. 9 The lower-hybrid waves were launched by two 4x4 wave-guide arrays located 180° relative to each other around the torus. For the current-drive experiments described in this paper the phases of adjacent waveguides in each row were set at 90°. The Brambilla rf power spectrum under such conditions is concentrated between N u = c/cn/co = 1 and 2.5 in the OH electron-drift direction, with approximately 30% of the power being in the opposite (negative) direction. 10 The negative portion of the spectrum is concentrated at relatively high values of N\\ (N n ~~ 6) and is expected to be absorbed by, and heat, the bulk plasma.In the present experiments the primary winding of the Ohmic heating transformer was opencircuited before application of the rf power. After the OH primary is opened the plasma current decays inductively with a typical time scale ...
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