The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (ΦBPP) and the floating potential (Vfl) of the Langmuir probe (LP), which is compared with the Thomson scattering diagnostic on ASDEX Upgrade and COMPASS. Excellent agreement between the two diagnostics is obtained for circular and diverted plasmas and different heating mechanisms (Ohmic, NBI, ECRH) in deuterium discharges with the same formula Te = (ΦBPP - Vfl)/2.2. The comparative measurements of the electron temperature using BPP/LP and triple probe (TP) techniques on the ISTTOK tokamak show good agreement of averaged values only inside the separatrix. It was also found that the TP provides the electron temperature with significantly higher standard deviation than BPP/LP. However, the resulting values of both techniques are well in the phase with the maximum of cross-correlation function being 0.8.
Edge polarization experiments were carried out on ISTTOK using an emissive electrode. The dependence of the bias current on the radial electric field, measured by a radial array of Langmuir probes, is in qualitative agreement with the expression derived by Stringer for the transitional region between plateau and collisional transport regimes (Stringer T.E. 1993 Nucl. Fusion 33 1249). Improved particle confinement is clearly observed for negative bias associated with a large radial electric field. In some cases, a double-peaked profile of the radial electric field is observed just after biasing, evolving afterwards to a single-peaked profile as confinement improves. These types of profiles may be related to the multiple solutions of the non-linear equation for the radial electric field, recently put forward by Kasuya et al (2003 Nucl. Fusion 43 244). Above a certain threshold of the bias current stronger improved confinement events are observed, during short periods, for both bias polarities.
A probe array consisting of three emissive probes and one cold cylindrical probe was developed for edge plasma measurements in ISTTOK. Emissive probes are particularly suitable for turbulence studies as they are able to deliver a more accurate measure of the plasma potential by reducing the effect of temperature fluctuations. The probe array has the advantage of recording the density, the electric field and their fluctuations simultaneously. Radial plasma profiles were recorded with and without negative edge biasing by an emissive electrode. The statistical properties of the poloidal electric field and of the turbulent particle flux, measured with cold and emissive probes, were compared. Both the root mean square of the poloidal electric field and the fluctuation-induced particle flux were found to be significantly larger when measured with the emissive probes, indicating that temperature fluctuations are important for the measurement of the particle flux. The probability distribution of the particle flux was also found to be more peaked and asymmetric when measured with the emissive probes.
The interplay between large scale fluctuations and the local turbulent transport has been investigated in the tokamak ISTTOK [C. A. F. Varandas et al., Fusion Technol. 29, 105 (1996)] plasma boundary region. It has been found that the floating potential fluctuations, dominated by low frequency oscillations, exhibit a significant toroidal correlation at large distances that can be attributed to the geodesic acoustic mode. The level of long-distance correlations is strongly bursty, showing a significant degree of coupling with the local electrostatic turbulent transport. Experimental findings show the key role of multiscale physics in the regulation of transport in the edge region of fusion plasmas.
In this paper results are presented on the changes induced by emissive electrode biasing in the ISTTOK edge transport. The boundary plasma is characterized with focus on the relation between ExB sheared flows and particle transport. We suggest that the distinct behaviour of the particle confinement for positive and negative bias observed in ISTTOK is related with the low ExB shear induced by positive bias in the core periphery region associated with the appearance of large amplitude fluctuations. In addition, the effect of electrode bias on the edge turbulent transport has been investigated identifying the changes induced on the fluctuations frequency spectrum and PDF. We have shown that negative electrode bias reduces the propagation of large-scale events, making the fluctuations distribution more Gaussian and resulting in low amplitude fluctuations across most of the edge plasma region. For positive bias, large amplitude, broad spectrum fluctuations appear in the core periphery, which increase the cross-field diffusion and contribute to the observed asymmetry in particle transport with the bias polarity.
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