H. Figueiredo

Turbulence and transport measurements with cold and emissive probes in ISTTOK

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.

Operation of the tokamak ISTOKK in a multicycle alternating flat-top plasma current regime

Operation of the tokamak ISTTOK in a multicycle alternating square wave plasma current regime is reported. Discharges with seven half-cycles without dwell times, over a total time span of about five times the maximum duration of a single DC discharge, were obtained by feeding the primary of the transformer with an electrolytic capacitor bank switched in polarity by a fast insulated gate bipolar transistor (IGBT) H bridge

Characterization of geodesic acoustic modes in the ISTTOK edge plasma

ISTTOK is equipped with two probe systems that allow the simultaneous measurement of the three-dimensional characteristics of the edge fluctuations with high temporal resolution. Electrostatic fluctuations consistent with the geodesic acoustic mode (GAM) are observed in the edge plasma. The radial, poloidal and toroidal structure of the fluctuations are investigated and good agreement with the GAM theoretical predictions is found. Furthermore, experimental evidence is presented suggesting that the GAM is modulating long-range correlations and the ambient turbulent fluctuations.

Edge plasma pressure measurements using a mechanical force sensor on the tokamak ISTTOK

In the present paper we report on a novel mechanical probe, which is able to measure the plasma pressure directly. The probe consists of two pendulums whose heads are exposed to the tokamak edge plasma, while the deflection is measured very sensitively outside the plasma by means of semi-conductor strain gauges. The plasma pressure was successfully measured in the ISTTOK edge plasma, its value being in good agreement with that derived from the electrical probe data (pp = 1–10 Pa). Furthermore, we discuss the possibility of determining the ion temperature Ti = pp/n − Te by combining the pressure measurement with those of n and Te from the electrical probes. Although the derived ion temperatures—besides that in the region close to the limiter—were reasonable, its uncertainty is still very large.

Control of the edge turbulent transport by emissive electrode biasing on the tokamak ISTTOK

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 E × B 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 to the low E × B shear induced by positive bias in the core periphery region associated with the appearance of large amplitude fluctuations. In addition, the effect of the electrode bias on the edge turbulent transport has been investigated identifying the changes induced in the fluctuations frequency spectrum and probability density function. 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.

Engineering aspects of the ISTTOK operation in a multicycle alternating flat-top plasma current regime

Abstract The main engineering aspects of the tokamak ISTTOK operation in a multicycle alternating flat-top plasma current regime are presented. AC discharges have been obtained feeding the ohmic and vertical magnetic field circuits with a specially designed alternating power supply, based on a single electrolytic capacitor bank and a fast insulated gate bipolar transistor (IGBT) H-bridge, feedback controlled by a discreet power DAC. The horizontal magnetic field has been created by an independent and pre-programmable DC power supply. The optimization of AC operation has also implied the installation of a new set of symmetric and more external windings for the vertical B-field and of an adequate gas puffing system. Discharges with seven half-cycles and flat-top plasma currents of about ±4 kA without dwell times were already achieved, in a total time span of 240 ms approximately equal to five times the maximum duration of a single DC discharge.

Comparison of fluctuations properties measured by Langmuir and ball-pen probes in the ISTTOK boundary plasma

In this work, the fluctuations properties measured by Langmuir and ball-pen probes are compared, aiming at investigating the influence of temperature fluctuations on Langmuir probe measurements, including the cross-field turbulent particle flux. With this aim, a 5-pin probe was designed to estimate the radial particle transport due to fluctuations simultaneously using Langmuir and ball-pen probes. A considerable difference is observed in the amplitude of the floating potential fluctuations measured by the two types of probes, but not in statistical properties such as skewness. The turbulent particle flux was found to be roughly four times larger when measured with Langmuir probes. However, quantities such as the phase velocity of the fluctuations or the poloidal correlation lengths are not significantly different, as floating potential fluctuations measured by both types of probes are highly correlated and roughly in phase. Finally, it is suggested that ball-pen and emissive probes may underestimate the amplitude of the plasma potential fluctuations and therefore probe measurements must be carefully validated.

Self-consistent analysis of the power-energy balance and determination of the runaway electron characteristics in the ISTTOK discharges

Low plasma density discharges at high longitudinal electric fields (E0≥1xa0Vxa0m-1) in the ISTTOK tokamak revealed characteristic features for the runaway (RA) generation process. The increase in soft x-ray emission was detected at the onset of the RA regime. The relaxations of the plasma parameters, a typical satellite effect of the instability driven by the RA electrons, were observed in experiments. In some discharges instability caused minor disruptions. Simultaneous determination of the RA electron characteristics and reconstruction of the electron temperature temporal evolution has been performed using self-consistent 0-D calculations of the plasma transport on the basis of the measured plasma parameters with the inclusion of the RA generation process. The kinetic energy as well as the confinement time of RA electrons was evaluated. The modelling procedure developed was applied in the investigation of a possible RA generation process at the thermal quench in large-scale tokamak disruption.

Characterization of the poloidal asymmetries in the ISTTOK edge plasma

Edge plasma in/out and up/down asymmetries have been directly investigated on ISTTOK with a probe system that simultaneously samples the plasma at four poloidal angles. An experimental investigation of the asymmetries is presented as a function of the direction of plasma current and toroidal magnetic field. The edge plasma parameters show significant in/out asymmetries in both equilibrium and fluctuating parameters. Asymmetries favoring the outboard side are observed in density, turbulent particle flux and diffusion coefficient independently of the toroidal magnetic field and plasma current directions, suggesting a ballooning-like transport. This interpretation is supported by the characteristics of the fluctuations as they are found to be more intermittent on the outboard side. In addition, smaller up/down asymmetries reversing with the reversal of the toroidal magnetic field direction and favoring the ion ∇B drift direction are found. Significant poloidal asymmetries were also measured in the parallel flow that can be attributed mainly to Pfirsch–Schluter flows.

Profile measurements of the electron temperature on the ASDEX Upgrade, COMPASS, and ISTTOK tokamak using Thomson scattering, triple, and ball-pen probes

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.