C.A.F. Varandas

Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results

Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques. In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly. In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor. In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence. On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field Er and confinement, limiter biasing acting mainly on the SOL. In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through time-resolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

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.

A fully computerized and distributed VME system for control and data acquisition on the tokamak ISTTOK

Abstract The ISTTOK operation and research programme require a very reliable control and data acquisition system due to the pulsed nature of the tokamak discharges. This system (SCAD) was designed in a distributed, modular, multivendor, integrated and transparent approach, taking advantage of recent improvements in networking, front-end processing and database management. This paper reports some of the most relevant aspects involved in the design of the SCAD current configuration. The computer system is based on personal computers (PCs) and Motorola 68 000 family microprocessors, linked by an ethernet local area network. Control is made by a vacuum controller unit as well as by VME digital I/O, timing and digital-to-analog converter modules. Data acquisition is provided by VME instrumentation as well as by PC based modules and by a digitizing oscilloscope. All VME modules were developed on site. The ISTTOK operation is controlled by a supervisory program. Data from the engineering and physics diagnostics is stored in a central database.

ATCA/xTCA-based hardware for control and data acquisition on Nuclear Fusion fast control plant systems

In contemporary control and data acquisition systems for Nuclear Fusion devices, the galloping need for high channel density and real-time multi-input-multi-output (MIMO) support gave rise to a new generation of hardware architecture based on the Advanced Telecommunications Computing Architecture (ATCA) specification. In addition, ATCA successfully delivered solutions in other sensitive issues such as form-factor component area, power dissipation and redundancy, complying with the high complexity and security required for such systems. Experience has showed, however, that due to its aforementioned complexity, such hardware devices can yield to a lengthy development. Furthermore, the ATCA specification is, as yet, somewhat undefined for instrumentation applications, more so within the specificities of Plasma Physics applied devices. The entitled “xTCA” specification is currently being developed for those purposes. Based on the ATCA itself, it will define new functionalities that standardize and facilitate hardware development for device operation in a Fusion control plant environment - most notably, dedicated timing and input-output (IO) port assignment on the Rear Transition Module (RTM). The prototype hereby presented is an xTCA Peripheral Component Interface (PCIe) switch Advanced Mezzanine Card (AMC) carrier blade. The device serves as a hub, as to control and handle I/O data from its parent nodes existing within the same xTCA shelf through its proprietary fabric channels in dual-star topology. Parent node blades, under development, are equally linked through xTCAs agnostic fabric in full-mesh topology, as to attain system MIMO functionality from all I/O endpoints. The switch blade carries up to four AMC modules, adding up to modularity and versatility. This allows for a much more independent and speedier hardware development, as dedicated AMC modules, such as data processing and storage devices, can be simultaneously projected. Commercial off-the-shelf (COTS) AMC products are readily available and may also be immediately integrated in the system.

A high performance real-time plasma control and event detection DSP based VME system

This paper describes the digital signal processors module of a high performance system, specially designed for real-time plasma control and event detection on the next generation fusion experiments with long duration discharges. The system is composed of a commercial CPU board and several on-site developed intelligent modules inserted in the same VME crate.

Digital control system for the TCV tokamak

A new digital feedback control system has been developed, integrated and used successfully to control the TCV plasma. The system is designed to be modular, distributed, and easily expandable, accommodating hundreds of diagnostic inputs and actuator outputs. It offers the possibility to design advanced control algorithms using more information on the plasma state, as well as the ability to control all TCV actuators, including PF coils, gas valves, the gyrotron powers and launcher angles of the ECRH/ECCD system, as well as diagnostic triggering signals. The system consists of multiple PC nodes connected to a sharing memory network. The control algorithms are programmed as block diagrams in Simulink. Using Embedded Coder, the C code is generated automatically from the Simulink model, then compiled into a Linux shared library (“.so” file) and copied to target nodes. When the TCV discharge is progressing, an application on each node is executed and dynamically loads the shared library at runtime. During the real time process, to dedicate the whole CPU performance for the algorithms, all interrupts to the CPU on each node are suspended. Since its inception, the new digital control system has enabled a multitude of plasma control applications, ranging from basic experiments of coil current and density control to advanced experiments of MHD and plasma profile control, as well as real-time plasma transport simulations. This paper presents the architecture of the new control system and its integration into the TCV plant.

A distributed system for fast timing and event management on the mast experiment

This paper describes an expandable and distributed system that produces the timing signals for the correct operation of the MAST diagnostic and data acquisition systems and performs the broadcast, processing and recording of the occurrence time of externally generated events for real-time control purposes. The hardware will be implemented using the VME and CAMAC standards. The software control interface will be incorporated in the MAST control and data acquisition system, allowing for an easy database access of the system timing parameters.

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.