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Notes: Experimental evidence for and against drift waves as the origin of the observed fluctuations and anomalous transport in the plasma interior is reviewed. Fluctuation spectra observed by far-infrared (FIR) scattering and a heavy ion beam probe (HIBP) are compared. The FIR system observes broad S(k,ω), which are spatially resolved at large k and readily identified with electron drift waves. At higher densities a clear ion feature, which may be associated with ion temperature gradient (ITG) modes, appears as well. A quasicoherent feature in the drift wave range of phase velocities is also found at the inner midplane. But interior HIBP measurements using a thallium beam have, by a two-point correlation method, measured wave numbers far too small (i.e., phase velocities far too high) to be those of drift waves of either variety. Some recent measurements with a cesium beam have produced phase velocities more closely in accord with drift waves at low frequencies. It is suggested that a new mode may be present, which is not drift wavelike. However, the phase velocity and coherence of the HIBP data exhibit qualitative features that suggest possible instrumental effects. Several of these are investigated, which individually do not appear to reconcile the data with a pure drift wave model. They are (1) sample volume size, (2) common mode effects such as cross-talk or path integrals of beam attenuation, and (3) two-stream instabilities. Reconciliation of these measurements is important to transport studies. That is, calculated particle and energy fluxes depend sensitively on the frequency and wave-number spectrum employed. The theoretical considerations about two-point correlations and possible instrumental effects are also relevant to other diagnostics using this technique.

Notes: The structure of the magnetic fluctuations in a tokamak has been determined from extensive measurements using a variety of probes outside the limiter in TEXT [Plasma Phys. Controlled Nucl. Fusion 27, 1335 (1985)]. The spectrum has been measured to 500 kHz, but little energy is present above 150 kHz. The spectrum ranges from low-frequency, low-m-number modes with high coherence to higher m values at higher frequencies, which have limited poloidal and temporal coherence but are specifically correlated with electrostatic fluctuations in the edge. Although these magnetic fluctuations are not directly significant for transport, they are a useful indication of edge turbulence. They are associated with turbulence only inside the limiter. The correlation length along field lines is long, and the phase variation of the correlated components suggests k(parallel)/k⊥ (approximately-equal-to) 0.005. These magnetic signals are consistent with a modest modulation of the plasma resistivity in the edge as in resistivity-gradient-driven modes, but the magnetic fluctuation signal is not proportional to the applied E(parallel). Their response to many other perturbations is reported.

Notes: Modifications to the spectrum and magnitude of turbulent density fluctuations in the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)], measured by far-infrared scattering, are related to changes in plasma profiles (e.g., density and potential) induced by naturally occurring magnetic islands (i.e., m=2) of varying amplitude. The statistical dispersion relation of the microturbulence that propagates in the electron diamagnetic drift direction is observed to be consistent with expectations for electron drift-wave turbulence when a spectral shift due to E×B plasma rotation is included. Observed spectral widths are several times larger than the linear mode frequency. The fluctuation amplitude is found to scale inversely with the electron density scale length.

Notes: Experimental results on the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion 1, 479 (1982)] and the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] indicate that electrostatic fluctuations can explain the edge transport of particles and perhaps energy. Certain mechanisms for drive (radiation and ionization, as well as density and temperature gradients) and stabilization (velocity shear) are suggested by these results. The experimental fluctuation levels and turbulence characteristics can be reproduced by considering the nonlinear evolution of fluid-type equations, incorporating thermal and ionization drives.

Notes: A heavy ion beam probe has been used to measure the plasma space potential profiles in the tokamak TEXT [Nucl. Fusion Technol. 1, 479 (1981)]. The Ohmic discharges studied were perturbed by externally produced resonant magnetic fields (an ergodic magnetic limiter or EML). Without these perturbations the plasma central potential is generally consistent with the value calculated from radial ion momentum balance, using experimental values of density and ion temperature and assuming a neoclassical poloidal rotation velocity. Exceptions to the agreement are found when operating with reduced plasma parameters. Possible reasons for this discrepancy are explored, in particular, the effects of intrinsic magnetic field fluctuations, and modifications to the self-consistent radial electric sheath. With the application of the EML fields the edge electric field and potential increase during periods of magnetic island overlap. A test particle calculation of electron transport shows increases in diffusivity also occur during periods of magnetic island overlap. These calculated changes in diffusivity are interpreted in terms of a stochastic layer width, which is itself used to predict a potential change for comparison with the experimental results.

Notes: This is a review of what is known about fluctuations and anomalous transport processes in tokamaks. It mostly considers experimental results obtained after, and not included in, the reviews of Liewer [Nucl. Fusion 25, 543 (1985)], Robinson [in Turbulence and Anomalous Transport in Magnetized Plasmas (Ecole Polytechnique, Palaiseau, France, 1986), p. 21], and Surko [in Turbulence and Anomalous Transport in Magnetized Plasmas (Ecole Polytechnique, Palaiseau, France, 1986), p. 93]. Therefore much of the pioneering work in the field is not covered. Emphasis is placed on results where comparisons between fluctuations and transport properties have been attempted, particularly from the tokamak TEXT [Nucl. Technol./Fusion 1, 479 (1981)]. A brief comparison of experimentally measured total fluxes with the predictions of neoclassical theory demonstrates that transport is often anomalous; fluctuations are thought to be the cause.The measurements necessary to determine any such fluctuation-driven fluxes are described. The diagnostics used to measure these quantities, together with some of the statistical techniques employed to analyze the data, are outlined. In the plasma edge detailed measurements of the quantities required to directly determine the fluctuation-driven fluxes are available. The total and fluctuation-driven fluxes are compared: the result emphasizes the importance of edge turbulence. No model adequately describes all the measured properties. In the confinement region experimental observations are presently restricted to measurements of density and potential fluctuations and their correlations. Various distinct turbulence features that have been observed are described, and their characteristics compared with the predictions of various models. Correlations observed between these fluctuations and plasma transport properties are summarized. A separate section on magnetic fluctuations shows there is very little information available inside the plasma, generally prohibiting quantified comparisons between fluctuation levels and transport. Both coherent and turbulent magnetic fluctuations are addressed, and the differences between low and high plasma pressure (low and high beta) are noted. The contributions of alternate confinement devices, such as stellarators and reversed field pinches, to understanding tokamak anomalous transport are discussed. Finally, future directions are proposed.

Notes: The nonlinear coupling coefficient and the energy transfer associated with three-wave interactions are computed from measured data of the turbulent edge plasma of the Texas Experimental Tokamak(TEXT) [Nucl. Technol. Fusion 1, 479 (1981)]. The results show the presence of three-wave interactions. The interactions cause energy to cascade away from the dominant waves of the spectrum primarily toward lower, but also toward higher frequencies. The results are obtained with a new digital spectral analysis technique based on the estimation of higher-order cumulants. The method is discussed and tested on a simulation experiment. The same technique is useful for neutral fluids as well.

Notes: A compact of self-contained portable system has been designed to diagnose the edge plasma of devices with a wide range of sizes and configurations. The system is capable of measuring mean and fluctuation quantities of density, temperature, potential from a standardized Langmuir probe array using a fast reciprocating probe drive, but can also be used for other fluctuation sources, e.g., magnetic probes. The signal processing software allows an immediate analysis of the turbulent characteristics. The results are stored in a database. The portable device is available for collaborative measurements with the goal of allowing cross-device comparison of edge fluctuation characteristics and transport.

Notes: We have combined several techniques in a new and unique way that allows us to measure edge profiles (ne, Te, and φp), their gradients, fluctuations (ñe and φ˜f), and fluctuation-induced heat and particle transport on a single discharge of the TEXT tokamak. To do this we utilize a fast reciprocating probe drive, a four pin Langmuir probe array, and suitable data analysis techniques. With this method, many of the quantities of interest to turbulence studies can be obtained on one discharge. Scaling studies of fluctuations versus free-energy drives (gradients) are easily carried out and data from such a scan are presented. This method also allows a study of changes to the profiles, fluctuations, and transport caused by perturbations to the plasma (ECH, pellets, etc.) during a single discharge.

Notes: Measurements of electrostatic fluctuations on the edge of the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] are used to study the role of the edge turbulence in the particle transport in this current-free magnetic configuration. Spatial profiles of the plasma electron density ne, temperature Te, and fluctuations in density (ñe) and in the plasma floating potential (φ˜f ) are measured at the edge in electron cyclotron heated plasmas using a Langmuir probe array. At the last closed flux surface (LCFS), r/a≈1, Te≈20–40 eV, and ne≈1012 cm−3 for a line-averaged electron density n¯e=(3–6)×1012 cm−3. The relative fluctuation levels decrease as the probe is moved into the core plasma. For Te(approximately-greater-than)20 eV, ñe/ne≈5%, and eφ˜f /Te≈2ñe/ne at r/a=0.95. The measured fluctuation spectra are broadband (40–300 kHz) with k¯ρs≈(0.05–0.1), where k¯ is the average wave number of the fluctuations and ρs is the ion Larmor radius at the sound speed. Near the LCFS, the density fluctuations can be approximated by ñe/ne ≈0.4/k¯Ln, where Ln is the gradient scale length of ne. The propagation direction of the fluctuations reverses to the electron diamagnetic direction at r/a〈1. The phase velocity of the fluctuations and the electron drift velocity are comparable. The particle flux estimated from the fluctuations is consistent with fluxes obtained from the particle balance using the Hα spectroscopic measurements. Many of the features seen in the ATF edge fluctuations resemble those of Ohmically heated plasmas in the Texas Experimental Tokamak (TEXT) [Phys. Rev. Lett. 62, 1844 (1989)].