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.

Results obtained on the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 13 1 ( 199 1) ] after installation of the design poloidal field winding are presented. Values of Be, = 2~, nd) T&/B s (a) -12% are achieved in low-current (I = 220 kA) operation; here, n, and T, are central electron density and temperature, and Be (a) is the poloidal magnetic field at the plasma edge. An observed decrease in pod) with increasing plasma current may be due to inadequate fueling, enhanced wall interaction, and the growth of a radial field error at the vertical cut in the shell at high current. Energy confinement time varies little with plasma current, lying in the range of OS-l.0 msec. Strong discrete dynamo activity is present, characterized by the coupling of m = 1, n = 5-7 modes leading to an m = 0, n = 0 crash (m and n are poloidal and toroidal mode numbers). The m = 0 crash generates toroidal flux and produces a small (2.5%) increase in plasma current.

Electrostatic fluctuations have been measured in a large reversed-field pinch, and are large ihe/n ~20%-40%, 7'^/r~10%-25%). Frequency and wave-number spectra are broad (AA2~70-150, Am-3-6), and differ from measured magnetic fluctuation spectra. The transport inferred from correlation measurements indicates that electrostatic fluctuations can account for significant particle losses, but contribute < 15% to energy loss.PACS numbers: 52.55.-s In toroidal magnetic confinement devices, cross-field transport exceeds diffusion predicted by collisional processes. Some theoretical models invoke electrostatic turbulence to explain this anomalous transport. Charge separation results in a fluctuating electric field, E = -VO; the ExB drift then drives transport. Electrostatic fluctuations may be responsible for particle transport in tokamak [1-4] and stellarator [5] edge plasmas, and perhaps also energy transport. Early measurements on ZETA [6] indicated that the role of electrostatic losses could be significant. More recent investigations have begun in several reversed-field-pinch (RFP) experiments [7][8][9]. The RFP and tokamak have similar edge plasma equilibrium density and temperature. However, the RFP contains greater magnetic shear, unfavorable magnetic curvature, and fewer magnetically trapped particles.In this Letter, we report measurements of edge electrostatic fluctuations (in density, potential, and electron temperature) in the MST RFP. We find that the amplitudes are large and the frequency and wave-number spectra are broad, similar to fluctuations in tokamaks. The deduced fluctuation-induced particle transport is comparable to the total particle losses. However, the fluctuation-induced energy transport is relatively small. MST [10,11] is a large RFP (^=0.52 m, 7^ = 1.5 m), with typical plasma parameters /^ < 600 kA, AZ^=(0.5-2.0) X 10'^ cm~^ r^o<500 eV, and pulse length < 80 msec. The present studies were conducted in low-current plasmas [/,, < 250 kA, /z,-=(0.6-0.8) x 10^^ cm~\ T^o < 180 eV], shown in Fig. 1. For these conditions, a single-turn loop voltage K/ = 15.5 V, pinch parameter O = Bp(a)/{Bt) = \.S5 (where (Bf) is the volume-averaged toroidal field), and reversal parameter F=Bt(a)/{Bt} = -0.15 were obtained.Probe measurements were made at r/a>:0.92 (where r/a = l at the wall), 40° above the outer midplane. Graphite toroidal rail limiters extend 1 cm from the wall at the inner and outer midplane. Triple probes were constructed using 0.5-mm-diam tungsten tips, spaced 1.6 mm apart. Two triple clusters, separated by 11.4 mm, were fixed on a single probe support. Measurements were made using a triple probe technique [12]. Ion saturation current Js was collected by a floating double probe biased to -300 V [> (5-10)A:rJ. The floating potential V/ was measured across a 100 kn impedance to the ground. The local plasma density rie, electron temperature Te, and (1) plasma potential Opi were then inferred bywhere ks is Boltzmann's constant, V^ is the potential of the positive-biased tip, and a, p, and f are consta...

Measurements of edge turbulence and the associated transport are ongoing in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 13 1 (199 1)] using magnetic and electrostatic probes. Magnetic fluctuations are dominated by m= 1 and n-R/a, tearing modes. Particle losses induced by magnetic field fluctuations have been found to be ambipolar ((4, g,.)/&=O). Electrostatic fluctuations are broadband and turbulent, with mode widths Am-3-7 and An-70-150. Particle, parallel current, and energy transport arising from coherent motion with the fluctuating ExB drift have been measured. Particle transport via this channel is comparable to the total particle loss from MST. Energy transport (from (FE+)/B,) due to electrostatic fluctuations is relatively small, and parallel current transport (from (J,, E,+)/B,) may be small as well.

Measurements of electron temperature using electron cyclotron emission (ECE) may be contaminated by density effects in plasmas with low optical depth. ECE intensity fluctuations in these plasmas will include a component dependent on density fluctuations as well. Correlation measurements, which will extract the temperature fluctuations from the dominant radiometer thermal noise, will include correlated density fluctuations in the result. In tokamak core plasmas, where normalized density fluctuations have been measured to be about 1%, this effect can be considerable (∼100%) at low levels of temperature fluctuations. However, at ‘‘relevant’’ levels, defined by the amount of anomalous heat transport, these effects are reduced, at least in TEXT-U. While exact interpretation of temperature fluctuation amplitudes is compromised, statements regarding the relevance of the inferred temperature fluctuation amplitudes can be made relatively unambiguously.