V.A. Vershkov

Investigation of geodesic acoustic mode oscillations in the T-10 tokamak

Geodesic acoustic modes (GAMs) were investigated on the T-10 tokamak using heavy ion beam probe, correlation reflectometry and multipin Langmuir probe diagnostics. Regimes with Ohmic heating and with on- and off-axis ECRH were studied. It was shown that GAMs are mainly the potential oscillations. Typically, the power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with the contrast range 3–5. They are the manifestation of the torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. The frequency of GAMs changes in the region of observation and decreases towards the plasma edge. After ECRH switch-on, the frequency increases, correlating with growth in the electron temperature Te. The frequency of the GAMs depends on the local Te as , which is consistent with a theoretical scaling for GAM, where cs is the sound speed within a factor of unity. The GAMs on T-10 are found to have density limit, some magnetic components and an intermittent character. They tend to be more excited near low-q magnetic surfaces.

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.

A three-wave heterodyne correlation reflectometer developed in the T-10 tokamak

A three-wave heterodyne O-mode reflectometer was developed and tested in experiments on the T-10 tokamak. Three launched waves are obtained by frequency splitting due to amplitude modulation with a p-i-n diode. Source frequencies from 36 to 78 GHz were used in the experiments. The ability of the reflectometer to measure simultaneously density profile characteristics, such as the time evolution of the relative phase and the unambiguous time delay of the reflected wave together with small-scale density fluctuation characteristics such as the radial and poloidal correlation lengths, was demonstrated with T-10 experimental data. The reflectometer can provide important information about fine changes in the density profile during sawteeth and low-m magnetohydrodynamics oscillations. Poloidal correlation measurements make it possible to measure transverse turbulence velocities and velocity shear. It was shown that both types of correlation measurements enable one to distinguish different types of turbulence acco...

Poloidal SOL asymmetries and toroidal flow in DITE

Abstract In this paper an experimental investigation of asymmetries in the DITE plasma boundary will be presented for a limiter configuration consisting of a pumped limiter centred on the outside midplane and an inner bumper limiter displaced ~ 70 ° toroidally. A number of experiments have been performed in which the poloidal variation of the edge T e , n e and j sat has been monitored as a function, principally, of the direction of plasma current and toroidal magnetic field. Poloidal asymmetries in particle flux are observed with the measured flux increasing towards the outside midplane. The poloidal variation of T e is less marked. Considerable e-i side asymmetries are evident in the particle flux which, when interpreted as indicative of net plasma flow, give poloidally varying Mach numbers and flow directions dependent on the toroidal magnetic field direction and line-averaged central density. The persistence of the observed asymmetries when the magnetic field and plasma current directions are reversed show the effect to be related to plasma properties and not simply a consequence of varying connection lengths.

Plasma potential and turbulence dynamics in toroidal devices (survey of T-10 and TJ-II experiments)

A direct comparison of the electric potential and its fluctuations in the T-10 tokamak and the TJ-II stellarator is presented for similar plasma conditions in the two machines, using the heavy ion beam probe diagnostic. We observed the following similarities: (i) plasma potentials of several hundred volts, resulting in a radial electric field Er of several tens of V?cm?1; (ii) a negative sign for the plasma potential at central line-averaged electron densities larger than 1\times 10^{19}\,{\rm m}^{-3} SRC=http://ej.iop.org/images/0029-5515/51/8/083043/nf381326in001.gif/>, with comparable values in both machines, even when using different heating methods; (iii) with increasing electron density ne or energy confinement time ?E, the potential evolves in the negative direction; (iv) with electron cyclotron resonance heating and associated increase in the electron temperature Te, ?E degrades and the plasma potential evolves in the positive direction. We generally find that the more negative potential and Er values correspond to higher values of ?E. Modelling indicates that basic neoclassical mechanisms contribute significantly to the formation of the electric potential in the core. Broadband turbulence is suppressed at spontaneous and biased transitions to improved confinement regimes and is always accompanied by characteristic changes in plasma potential profiles. Various types of quasi-coherent potential oscillations are observed, among them geodesic acoustic modes in T-10 and Alfv?n eigenmodes in TJ-II.

Li experiments on T-11M and T-10 in support of a steady-state tokamak concept with Li closed loop circulation

This paper presents a review of the last T-11M and T-10 tokamak activity in the field of Li plasma facing component (PFC) investigation. Attention is mainly paid to the realization of the concept of closed loop lithium circulation as a solution of the PFC problem of a steady-state DT volumetric neutron source on a tokamak basis. Realization of the Li PFC concept demands the decision of three main tasks: lithium injection into the plasma, Li collection before its deposition on the vacuum vessel and the return of Li to the injection zone from the collector. This emitter?collector concept assumes that the main heat flux from a hot plasma to the PFC (limiters and divertor plates) can be dissipated on the entire vessel wall surface by non-coronal Li radiation, which will smoothen the local heat load PFC. A rail limiter on the basis of a capillary porous system manufactured from tungsten felt and provided with W wings was successfully tested in the last T-11M experiments as a prototype of steady-state Li emitter?collector. A witness-sample analysis showed that the lateral sides of the rail and ring limiters crossing the plasma scrape-off layer can collect a significant (~80%) part of Li, injected into the plasma during discharges. This can be used in the future for closing Li loop circulation. As was shown by Li pellet injection in T-10, the probability of Li penetration into the hot plasma core from its boundary is lower than that of deuterium by a factor of 5?10. This result can explain the effect of plasma cleaning (Zeff (0) ~ 1) during T-10 Li experiments. Some different schemes of future lithium circulation loops are discussed.

Experimental study of plasma fluxes in the shadow of a scoop limiter on T-10

Abstract Investigation of plasma fluxes measured by Langmuir probes on both sides of the T-10 scoop limiter shows the existence of large asymmetry, depending on discharge parameters. Two mechanisms can be responsible for such asymmetry. At low density, rotation of the plasma periphery in the direction of the plasma current causes the asymmetry. At high density it arises due to the preferential escape to the external side of the torus and its further spread along the magnetic field lines. The dependence of the neutral gas pressure under the limiter on the plasma influx to the channels was not linear. A pressure up to 30 mTorr has been observed.

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption. (Some figures in this article are in colour only in the electronic version)

Radial distributions and poloidal asymmetries of T-10 SOL parameters and turbulence

Abstract This work presents the results of T-10 SOL investigations with multipin Langmuir probes and correlation reflectometry. A special regime with a rail limiter deeply introduced from the bottom of the machine at SOL temperatures of up to 50 eV has been studied. Five types of plasma small scale turbulence were observed in the SOL, including the three previously found types of core turbulence. It was shown that the main SOL parameters are poloidally uniform. A high poloidal asymmetry of the fluctuations and the turbulent flux of particles, was found.

Edge plasma investigation on T-10

Abstract The results of studying the plasma scrape-off layer (SOL) structure in the T-10 tokamak are given in this paper. A set of Langmuir probes was used, including single probes located on the limiter and far from it and Mach probes. The presence of plasma flows along the upper and lower plasma column circumference is shown, their velocities and directions depend on plasma parameters and on main magnetic field and current directions. The fluxes correspond to co-current rotation of the plasma column periphery at high edge temperature and a plasma flow from the external circumference of the torus to the internal one at low temperature. A strong plasma asymmetry in the shadow of a circular limiter is shown. The results of a special experiment on propagation of a potential perturbation along the magnetic field lines are also given.