M. Tendler

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.

Radial current in a tokamak caused by a biased electrode

It is demonstrated that measurements of radial electric currents excited from biased electrodes in TUMAN 3 and other tokamaks provide a crucial test in validating different models for L-H transitions. The results are assessed from the viewpoint of a previously developed theory, which is briefly described. There is evidence from the voltage-current characteristics in TUMAN 3 which corroborates features of this theory. A spontaneously occurring Ohmic H-mode is switched off when a substantial positive biasing voltage is applied to the electrode

Plasma based waste treatment and energy production

During the past centuries, industrial processes and energy conversion plants have shown no or little care for environmental quality. The result is a huge accumulation of pollution and hazardous by-products, left as a heritage for the present and future generations. Recuperation of by-products or thermal energy is not only motivated by cost saving, but also by resource saving considerations. Environmental awareness is more than staying within the lines of the existing regulations.By the application of a plasma based system to a wide range of possible feedstocks which are CO2 neutral, a clean syngas of high caloric value is produced from the organic substances simultaneously with a non-leachable vitrified lava from the inorganic substances. The results will provide the advanced technology for the environmentally friendly treatment of hazardous wastes, biomass and low grade fuel. The driving force behind the task is to give priority to environmental quality at affordable costs. Thus, the investigation of ways to increase the efficiency of the process is very important. A plasma based remediation system is the only technology that prevents undesired pollution in the by-products and end product (such as syngas or other gases). The problem to be solved is twofold: recuperate clean energy from waste and renewables without pollution at affordable costs. Such a technique fulfils the objectives of sustainable development.Today, one of the main reasons that restricts the use of plasma based methods is the cost of electrical energy. The crucial element is the plasma torch performance. Hence, the physics of modern plasma torches is addressed in detail. The optimistic scenario holds the promise to provide 10?15% of the energy needs for the European Union (EU). Thus, the investigation of ways to increase the efficiency of the process is very important.

The impact of a biasing radial electric field on the scrape‐off layer in a divertor tokamak

The issue of transport on open field lines is addressed using a fluid approach invoking anomalous inertia and viscosity. The emphasis of the model is on the transport of the toroidal momentum, which is notoriously anomalous. The model suggests that tokamaks with divertors may benefit from a significant amplification of the electric field. It is shown that biasing of the scrape‐off layer (SOL) with respect to the grounded first wall results in fundamental phenomena, affecting the performance of a divertor. The theoretical model appears to be consistent with experimental results obtained on Tokamak de Varennes [Phys. Plasmas 1, 1485 (1994)].

Interpretation of the observed radial electric field inversion in the TUMAN-3M tokamak during MHD activity

Interpretation of the observed radial electric field inversion in tuman-3M tokamak during MHD-activity

H mode in the TUMAN‐3 tokamak triggered by edge plasma perturbations*

Several types of edge plasma perturbations in the TUMAN‐3 tokamak [Proceedings of the 13th International Atomic Energy Agency Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 509] have been demonstrated to trigger the Ohmic H‐mode transition. It is shown that three different methods, (1) radial electric field of either sign imposed by an electrode biased up to 500 V, (2) perturbation of the edge plasma density by strong gas puffing, and (3) LiD pellet injection, bring about the Ohmic H mode. In biasing experiments, the degree of improvement of particle and energy confinement depends on the polarity of the electric field and is higher for negative biasing. The evaporation of a LiD pellet (V∼150 m/sec, size ∼0.3 mm, density perturbation∼50%) in the peripheral region of the plasma column can also lead to the H‐mode transition. Experimental results are shown to be in reasonable agreement with the theory of radial electric fields in tokamaks.

Alfvén wave forces, affecting the tokamak edge plasma in the presence of impurities or dust

Dust particles appear on the edge of tokamak plasmas as a result of disruptions and plasma interactions with divertor plates, limiters, or blankets, as well as of pellet injections. It is shown that the presence of the dust impurities can lead to an effective decrease of the slow Alfven wave absorption in the region. These waves are used for heating, current drive, and control of the low-to-high (L–H) confinement transition. Radio-frequency forces can also be strongly changed in the presence of dust particles.

H-mode studies on TUMAN-3 and TUMAN-3M

The focus of the TUMAN-3 and TUMAN-3M tokamaks programme is on issues of improved confinement. The transition from an ordinary ohmic regime into improved confinement mode has been found in circular limiter configuration in a vessel with all-metallic walls and limiters. The signatures of the H-mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the radial electric field was found in experiments with internal probe biasing. Other techniques were demonstrated to trigger H-mode: short increase of the working gas puffing rate, minor radius magnetic compression and pellet injection. The scaling of the energy confinement time in ohmic H-mode was obtained, which differs dramatically from the scaling for the ordinary ohmic regime. A strong dependence of on plasma current was found. The scaling for the ohmic H-mode is consistent with the scaling proposed for devices with powerful auxiliary heating (JET/DIII-D H-mode scaling). The result shows that H-mode physics is universal in tokamaks with different geometries and heating methods. In 1994 a new vacuum vessel was installed in the TUMAN-3 tokamak. The modified device, TUMAN-3M, is able to produce higher and , up to 2 T and 0.2 MA, respectively. During the first operational period a plasma current of 0.15 MA was achieved at T, which corresponded to . The impact of the quality of wall coating on confinement was asserted. The longest energy confinement time (30 ms) was observed under the conditions of best boronization.

Effects of velocity-space loss regions on the alpha-particle distribution function and collisional alpha-particle losses in tokamak reactors

The steady-state distribution function for alpha-particles in the presence of loss regions in velocity space is derived in an analytical form on the basis of the Fokker-Planck equation. The particle losses and the associated energy loss of alpha-particles due to scattering into the loss region are estimated for parameters characteristic of a tokamak reactor.

Pattern recognition in probability spaces for visualization and identification of plasma confinement regimes and confinement time scaling

Pattern recognition is becoming an increasingly important tool for making inferences from the massive amounts of data produced in fusion experiments. The purpose is to contribute to physics studies and plasma control. In this work, we address the visualization of plasma confinement data, the (real-time) identification of confinement regimes and the establishment of a scaling law for the energy confinement time. We take an intrinsically probabilistic approach, modeling data from the International Global H-mode Confinement Database with Gaussian distributions. We show that pattern recognition operations working in the associated probability space are considerably more powerful than their counterparts in a Euclidean data space. This opens up new possibilities for analyzing confinement data and for fusion data processing in general. We hence advocate the essential role played by measurement uncertainty for data interpretation in fusion experiments.