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The COMPASS tokamak (R = 0.56 m, a = 0.2 m, BT = 1.3 T, Ip ~ 300 kA, pulse duration 0.4 s) operates in ITER-like plasma shape in H-mode with Type-I ELMs. In 2019, we plan to install into the divertor a test target based on capillary porous system filled with liquid lithium/tin. This single target will be inclined toroidally in order to be exposed to ITER-relevant surface heat flux (20 MW/m2). Based on precisely measured actual heat fluxes, our simulations predict (for 45° inclination, without accounting for the lithium vapor shielding) the surface temperature rises up to 700°C within 120 ms of the standard ELMy H-mode heat flux with ELM filaments reaching hundreds MW/m2. Significant lithium vaporization is expected. The target surface will be observed by spectroscopy, fast visible and infrared cameras. The scientific program will be focused on operational issues (redeposition of the evaporated metal, ejection of droplets, if any) as well as on the effect on the plasma physics (improvement of plasma confinement, L-H power threshold, Zeff, etc.). After 2024, a closed liquid divertor may be installed into the planned COMPASS Upgrade tokamak (R = 0.84 m, a = 0.3 m, BT = 5 T, Ip = 2 MA, Pin = 8 MW, pulse duration ~2 s) with ITER-relevant heat fluxes loading the entire toroidal divertor. [ABSTRACT FROM AUTHOR]

Gas puff imaging (GPI) [S. J. Zweben, D. P. Stotler et al., Phys. Plasmas 9, 1981 (2002); R. J. Maqueda, G. A. Wurden et al., Rev. Sci. Instrum. 74, 2020 (2003)] is a powerful diagnostic that permits a two-dimensional measurement of turbulence in the edge region of a fusion plasma and is based on the observation of the local emission of a neutral gas, actively puffed into the periphery of the plasma. The developed in-vessel GPI telescope observes the emission from the puffed gas along local (at the puff) magnetic field lines. The GPI telescope is specially designed to operate in severe TEXTOR conditions and can be treated as a prototype for the GPI systems on next generation machines. Also, the gas puff nozzle is designed to have a lower divergence of the gas flow than previous GPI diagnostics. The resulting images show poloidally and radially propagating structures, which are associated with plasma blobs. We demonstrate that the local gas puff does not disturb plasma properties. Our results indicate also that the neutral gas emission intensity is more sensitive to the electron density than the electron temperature. Here, we present implementation details of the GPI system on TEXTOR and discuss some design and diagnostic issues related to the development of GPI systems in general. [ABSTRACT FROM AUTHOR]

Temporal characteristics of intermittent positive bursts of plasma density in the scrape-off layer of the CASTOR and Tore Supra tokamaks are investigated. A monotonic radial decrease of average burst rate together with an increase of average burst duration is observed in the Tore Supra tokamak [I. Nanobashvili et al., Czech. J. Phys. 56, 1339 (2006); I. Nanobashvili et al., J. Nucl. Mater. 363–365, 622 (2007)]. At the edge of the CASTOR tokamak the temporal characteristics stay nearly constant within a quite wide radial region. The reason is that radially elongated turbulent structures (streamers) govern the radial transport in CASTOR [P. Devynck et al., Phys. Plasmas 13, 102505 (2006)]. A comparative analysis allows to conclude that radial transport at the edge of the Tore Supra tokamak is governed by blob-like structures. Plasma biasing at the edge of CASTOR significantly modifies the burst characteristics, reduces plasma transport, and considerably improves the confinement. [ABSTRACT FROM AUTHOR]

On TEXTOR a new diagnostic was commissioned for the determination of the profile of the ion effective charge Zeff from bremsstrahlung emissivity measurements in the visible. The line-integrated bremsstrahlung emissivity is measured with high spatial resolution along 28 fiber optic channels, in one poloidal cross section of the tokamak. Twenty-four channels are coupled to a cooled fast charge-coupled device (CCD) camera and four to photomultiplier tubes for higher sampling rate. The design of the diagnostic is described, including a technique for coping with CCD image smearing, without the need for the calculation of the point spread function of the system. An in-vessel calibration was performed by means of an integrating sphere. A convenient alternative way is outlined for determining the relative calibration from plasma emissivity measurements only. Bremsstrahlung emissivity profiles can be reconstructed by Abel inversion of the line integrals measured with the camera. Overestimation of edge Zeff values due to nonbremsstrahlung contributions to the continuum radiation can be minimized via Tikhonov or maximum entropy regularization. The propagation of such edge continuum contributions towards the center of the reconstructed profile is illustrated in the case of ergodization of the edge plasma. [ABSTRACT FROM AUTHOR]

In this paper, the propagation of turbulent events along radial and poloidal probe arrays in the scrape-off layer (SOL) of the tokamak CASTOR [M. Hron et al., Czech. J. Phys. 49, 181 (1999)] is observed using a novel correlation technique. The typical turbulent structures appear as dipoles on the floating potential signals and as positive bursts on the density. It is found that both dipoles and density bursts propagate over long radial distances in the SOL while propagating poloidally. A comparison of the measured velocities of the potential and density fluctuations shows that the density structures move radially faster than the potential ones. This allows us to understand how the density is convected by the turbulence. The density moves radially along the potential valleys created by the poloidal dipoles without modifying them. In the framework of an interchange turbulence, this property indicates that charge exchange collisions with neutrals in the SOL of CASTOR lead to a high viscosity that damps the zonal flows. [ABSTRACT FROM AUTHOR]

The plasma potential Φ[sub pl] and its fluctuations &Φtilde;[sub pl] were measured by electron emissive probes in the edge plasma regions of two fusion experiments: the Instituto Superior Técnico Tokamak (ISTTOK) (Lisbon, Portugal), and the Czech Academy of Sciences Torus (CASTOR) tokamak (Prague, Czech Republic). Into ISTTOK, three emissive probes were inserted outside the last closed flux surface (LCFS) on different minor radii. In CASTOR, two emissive probes, poloidally separated, and two cold cylindrical probes, mounted on the same shaft, were used, which could be radially shifted outside and inside the LCFS. The advantages of a sufficiently emissive probe are that in principle Φ[sub pl] and &Φtilde;[sub pl] can be measured directly, without being affected by electron temperature fluctuations or drifting electrons. [ABSTRACT FROM AUTHOR]

Discusses the first evaluations of a prototype tunnel probe in the CASTOR tokamak. Measurements of electron temperature and parallel ion current density with high frequency at the same point in time in space; Possibility in precise kinetic simulations for calibration; Limitation to the use of tunnel probes.

The ion current collected by a probe in a magnetized plasma is sensitive to the angle between its surface and the flow streamlines. This intuitive concept is the basis of the Gundestrup probe, a polar array of planar collectors mounted around an insulating housing. Probe theory for measuring flows has been developed on two fronts: Recent kinetic and fluid models, reviewed here, give similar predictions for the collected current within the range of applicability of the model assumptions. A comparison with measurements by a rotating Mach probe in the CASTOR tokamak (Czech Academy of Sciences Torus) [J. Stöckel, J. Badalec, I. Duran et al., Plasma Phys. Controlled Fusion, 41, 577 (1999)] highlights the role of magnetization in ion collection at grazing angles of incidence between the probe surface and the magnetic field lines. [ABSTRACT FROM AUTHOR]

We have developed tunnel probes for localized measurements of the electron and ion temperature in the edge plasma region of smaller tokamaks and stellarators. A normal tunnel probe for Te measurements consists of a metallic tunnel of 5 mm diameter and 5 mm length, with the axis parallel to the magnetic field. One side is open, the other side is closed by a metallic backplate, isolated from the tunnel. If both electrodes are biased negatively, ions flow into the office, and their current is distributed between the tunnel and the backplate. The ratio of the two ion currents is a function of Te. With an additional diaphragm in front of the orifice, the probe becomes ion sensitive, since the electrons are prevented from the tunnel because of their smaller gyroradius, but ions can still reach it. In this way, the perpendicular ion temperature can be derived. By segmenting the tunnel axially into two parts, also an approximate measure for the parallel ion temperature can be found. [ABSTRACT FROM AUTHOR]

The turbulent transport of particles is investigated using rake probes at the top of the scrape-off layer (SOL) of circular ohmically heated L-mode plasmas in the Tore Supra tokamak [G. Giruzzi et al., Nucl. Fusion 49, 104010 (2009)]. Both radial and poloidal non-linear fluxes are estimated by means of two reciprocating arrays separated toroidally by 120°. The time average of the radial (poloidal) flux is positive (negative) through the whole SOL profile. The respective effective transport velocity are about r>t≈30m s-1 and θ>t≈-60m s-1 close to the last closed flux surface. Both components present a standard deviation about 10 times higher than their respective mean amplitude, and time-distributions highly skewed toward values of the same sign as their mean values. The existence of a nonlinear poloidal flux is associated with the local tilt of filament eddies due to electric and magnetic shear. At the last closed flux surface, where plasma filaments experience their early life, the orientation of the velocity field is consistent with structure dynamics which originate from the outboard midplane and spread along field lines toward the rest of the poloidal section. The localized tilt of the eddy structures corresponds to the effect of the magnetic shear. Further into the SOL, the orientation of the velocity field evolves along radius in agreement with a simple model of propagating filaments progressively tilted by an electric shear. The combined effects of electric and magnetic shear on the eddy tilting have potentially a crucial impact on the existence of a Reynolds stress rvθ>t component, which is strongly poloidally asymmetric at the edge of tokamak plasmas. [ABSTRACT FROM AUTHOR]

Dedicated experiments have been carried out at the TEXTOR tokamak in biasing-induced improved confinement regimes using Langmuir probe and GPI measurements. The results show the first evidence for the intimate interaction among sheared flows, eddy structures, Reynolds stress, zonal flows, and ambient fluctuations during a transition to an improved confinement. The sheared flows impose dynamic effects on turbulence structures, resulting in generation of Reynolds stress and zonal flows and eventually the suppression of background fluctuation levels via nonlinear energy transfer processes. The latter may therefore trigger a transition to the H mode. [ABSTRACT FROM AUTHOR]

Abstract: The plasma wall-interaction in ITER is facing several challenges including surface erosion, material redeposition and tritium retention. In order to assess some of the possible issues, samples from TEXTOR (Forschungszentrum Jülich Tokamak, Germany) have been studied in this work. A smooth gradient in the form of a dimple should be established to enlarge the view of the depositions in depth before analyzing them with X-ray photoelectron spectroscopy (XPS). The dimple depth was optimized in order to cover the deposition thickness. Profilometer measurements with a confocal microscope were carried out to retrieve information on the profile and to ensure the optimum size/form of the dimples. Scanning electron microscopy measurements were carried out with a JSM 6310 microscope in order to obtain the morphology of the dimple area to verify if the deposition was not put under stress. The chemical composition and distribution of the elements along the dimples and on the surface of the samples was detected with an energy dispersive X-ray spectroscopy (EDS). [Copyright &y& Elsevier]

Abstract: The objective of this work was to examine the structure, composition and properties of co-deposited films from the TEXTOR tokamak. Hydrogenated films formed on the toroidal belt pump limiter (ALT-II), and on the ICRF antenna grill were studied using a set of material analysis techniques. Plasma edge diagnostics were used to assess the parameters influencing the film formation during discharges auxiliary heated by ICRF. The essential results are summarized as follows: (i) the distribution of plasma impurities co-deposited in the films is non-uniform and (ii) the surface topography, crystallographic structure, fuel retention and composition (i.e., content of re-deposited plasma impurities) of the films show significant diversity depending on the location where they were formed. These differences are associated with the local geometry, the tokamak operation scenarios and the resulting plasma edge properties. [Copyright &y& Elsevier]

Abstract: Changes on transport and radiation properties of intrinsic carbon under the influence of the Dynamic Ergodic Divertor depend on the modal composition of the induced error field. For 12/4 mode operation, significant carbon screening is not observed up to half of the nominal DED maximum current achieved so far. On the contrary, for 3/1 DED operation the very efficient mechanism of expulsion of highly ionized carbon ions, observed to occur locally at the foot prints of the 3/1 helical structure and explained in terms of frictional forces, leads to a global carbon decontamination of the central plasma. This result has been established using a new multi-chord diagnostic, which allows to distinguish global from local carbon behavior. [Copyright &y& Elsevier]

Abstract: This contribution reports on isotope exchange studies with both Ion Cyclotron Wall Conditioning (ICWC) and Glow Discharge Conditioning (GDC) in TEXTOR and TORE SUPRA. The discharges have been carried out in H2, D2 (ICWC and GDC) and He/H2 mixtures (ICWC). The higher reionization probability in ICWC compared to GDC, following from the 3 to 4 orders of magnitude higher electron density, leads to a lower pumping efficiency of wall desorbed species. GDC has in this analysis (5–10) times higher removal rates of wall desorbed species than ICWC, although the wall release rate is 10 times higher in ICWC. Also the measured high retention during ICWC can be understood as an effect of the high reionization probability. The use of short RF pulses (∼1s) followed by a larger pumping time significantly improves the ratio of implanted over recovered particles, without severely lowering the total amount of removed particles. [Copyright &y& Elsevier]