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The COMPASS-U tokamak (R = 0.894 m, a = 0.27 m, Bt = 5 T, Ip = 2 MA) is a new medium-size device with fully metallic plasma facing components, currently under construction at the Institute of Plasma Physics of the Czech Academy of Sciences in Prague. It features a unique combination of parameters, such as a high temperature of the tokamak walls up to 500 ◦C allowing a high recycling regime, a high magnetic field connected with a high plasma density above 1020 m -3 and with a high heat flux (perpendicular to divertor targets) density at the outer strikepoint up to 90 MW/m2 in attached conditions. These parameters of the device pose strict constraints and requirements on the design of individual diagnostic systems. Strategy and present status of the development of the diagnostic systems for COMPASS-U are provided. Plans for a diagnostic set for the first plasma are reviewed. The review of the diagnostics systems involves the high-temperature compatible slow (up to 20 kHz) and fast (up to several MHz) inductive and non-inductive magnetic sensors (including Thick Printed Copper coils and Hall sensors), the sub-millimetre interferometer with an unambiguous channel, Electron Cyclotron Emission, the interlock and overview cameras, high resolution Thomson scattering, radiation diagnostics (neutron diagnostics, soft and hard X-ray diagnostics, bolometers, impurity monitors, effective ion charge), probe diagnostics (including rail probes) and manipulators.

COMPASS-U, a high magnetic field tokamak with hot walls, will be designed and built at IPP Prague. Unique features of this new device bring noticeable constraints and requirements on plasma diagnostics, which make their development highly demanding. In this paper, the main expected constraints influencing the conceptual design of diagnostic tools for COMPASS-U (high temperature of the vacuum vessel, high plasma density, high heat flux density, strong auxiliary plasma heating, spatial constraints, liquid metals in the divertor) are reviewed and possible solutions are indicated.

COMPASS Upgrade is a new medium size, high magnetic field tokamak (R = 0.9 m, Bt = 5 T, Ip = 2 MA) currently under design in the Czech Republic. It will provide unique capabilities for addressing some of the key challenges in plasma exhaust physics, advanced confinement modes and advanced plasma configurations as well as testing new plasma facing materials and liquid metal divertor concepts. This paper contains an overview of the preliminary engineering design of the main systems of the COMPASS Upgrade tokamak (vacuum vessel, central solenoid and poloidal field coils, toroidal field coils, support structure, cryostat, cryogenic system, power supply system and machine monitoring and protection system). The description of foreseen auxiliary plasma heating systems and plasma diagnostics is also provided as well as a summary of expected plasma performance and available plasma configurations.

The reduction of the incident heat flux onto the divertor will be a necessity for the future thermonuclear reactors. Impurity seeding is recognized as an efficient way to achieve the partial detachment regime, which allows to dissipate a large fraction of power flux by radiation. This paper presents a heat flux real-time feedback system (RTFS) based on impurity seeding controlled by a combined ball-pen and Langmuir probe divertor array in the COMPASS tokamak. A number of features of the system have been studied, such as the type of impurity, seeding location, constants used in the real-time controller and the diagnostic selections. A detailed description of the designed RTFS and the results of the implementation are presented. The findings confirm the applicability of the RTFS for reduction and control of the divertor heat fluxes. Another important implication of this research is the ability of installing such systems in next-step devices.

In this work we report on results of full size kinetic modelling of the COMPASS tokamak SOL. Presented simulations indicate, that i. kinetic effects are significant in the inner divertor (ID) plasma; ii. normalized power loads to the ID are above the classical values and are caused by non-Maxwellian super-thermal electrons; iii. different divertor current regimes do not influence overall SOL parameters, except those of the divertor sheath; iv. ions at the divertors plasma are colder than the electrons; v. parallel heat transport is strongly non-local. Modelling results are in a reasonable agreement with the experimental measurements.

Power handling experiments with a special liquid metal divertor module based on the capillary porous system technology were performed in the tokamak COMPASS. The performance of two metals (Li and LiSn alloy) were tested for the first time in a divertor under ELMy H-mode conditions. No damage of the capillary mesh and a good exhaust capability were observed for both metals in two separate experiments with up to 12 MW/m(2) of deposited perpendicular, inter-ELM steady-state heat flux and with ELMs of relative energy similar to 3% and a local peak energy fluence at the module similar to 15 kJ.m(-2). No droplets were directly ejected from the mesh top surface and for the LiSn experiment, no contamination of the core and SOL plasmas by Sn was observed. The elemental depth profile analysis of 14 stainless-steel samples located around the vacuum vessel for each experiment provides information about the migration of evaporated/redeposited liquid elements.

In the COMPASS tokamak, series of experiments were performed aimed at studying the impact of nitrogen, neon, and argon impurity seeding on the electron energy distribution function (EEDF) in the divertor region. The experiments were conducted in D-shaped, L-mode, deuterium plasmas. In order to obtain the radial distribution of the floating potential, ion saturation current, electron temperatures, and densities, the current-voltage characteristics were measured by Langmuir probes embedded in the COMPASS tokamak divertor. The properties of the plasma in the divertor region were measured before and during impurity seeding. Before the N2 seeding, the EEDF was bi-Maxwellian with a low-energy electron fraction with temperatures 3.5–5 eV, and a higher-energy one with temperatures in the range of 10 eV to 23 eV. During seeding with an increasing number of molecules per second, the EEDF changed from bi-Maxwellian to Maxwellian and the electron temperature decreased. The time-evolution was studied of the change in the EEDF during N2 seeding. When the seeding was carried out by a valve in the private flux region, the duration of the transition from a bi-Maxwellian to a Maxwellian EEDF was about 10–15 ms. When the N2 seeding took place through a low-field side valve, the transition from a bi-Maxwellian to a Maxwellian EEDF took longer −25–45 ms. The temporal evolution was also analyzed of the plasma parameters’ radial profiles when neon and argon were puffed using a valve in the divertor low-field side. The application is discussed of the probe measurements’ results to calculating the parallel heat-flux densities in the divertor region of the COMPASS tokamak.

Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened. Peer Reviewed Article escrit per 365 autors/autores: Labit, B.; Eich, T.; Harrer, G. F.; Wolfrum, E.; Bernert, M.; Dunne, M. G.; Frassinetti, L.; Hennequin, P.; Maurizio, R.; Merle, A.; Meyer, H.; Saarelma, S.; Sheikh, U.; Adamek, J.; Agostini, M.; Aguiam, D.; Akers, R.; Albanese, R.; Albert, C.; Alessi, E.; Ambrosino, R.; Andr be, Y.; Angioni, C.; Apruzzese, G.; Aradi, M.; Arnichand, H.; Auriemma, F.; Avdeeva, G.; Ayllon-Guerola, J. M.; Bagnato, F.; Bandaru, V. K.; Barnes, M.; Barrera-Orte, L.; Bettini, P.; Bilato, R.; Biletskyi, O.; Bilkova, P.; Bin, W.; Blanchard, P.; Blanken, T.; Bobkov, V.; Bock, A.; Boeyaert, D.; Bogar, K.; Bogar, O.; Bohm, P.; Bolzonella, T.; Bombarda, F.; Boncagni, L.; Bouquey, F.; Bowman, C.; Brezinsek, S.; Brida, D.; Brunetti, D.; Bucalossi, J.; Buchanan, J.; Buermans, J.; Bufferand, H.; Buller, S.; Buratti, P.; Burckhart, A.; Calabr, G.; Calacci, L.; Camenen, Y.; Cannas, B.; Cano Megías, P.; Carnevale, D.; Carpanese, F.; Carr, M.; Carralero, D.; Carraro, L.; Casolari, A.; Cathey, A.; Causa, F.; Cavedon, M.; Cecconello, M.; Ceccuzzi, S.; Cerovsky, J.; Chapman, S.; Chmielewski, P.; Choi, D.; Cianfarani, C.; Ciraolo, G.; Coda, S.; Coelho, R.; Colas, L.; Colette, D.; Cordaro, L.; Cordella, F.; Costea, S.; Coster, D.; Cruz Zabala, D. J.; Cseh, G.; Czarnecka, A.; Cziegler, I.; D’Arcangelo, O.; Dal Molin, A.; David, P.; De Carolis, G.; De Oliveira, H.; Decker, J.; Dejarnac, R.; Delogu, R.; Den Harder, N.; Dimitrova, M.; Dolizy, F.; Domínguez-Palacios Durán, J. J.; Douai, D.; Drenik, A.; Dreval, M.; Dudson, B.; Dunai, D.; Duval, B. P.; Dux, R.; Elmore, S.; Embréus, O.; Erds, B.; Fable, E.; Faitsch, M.; Fanni, A.; Farnik, M.; Faust, I.; Faustin, J.; Fedorczak, N.; Felici, F.; Feng, S.; Feng, X.; Ferreira, J.; Ferr, G.; Février, O.; Ficker, O.; Figini, L.; Figueiredo, A.; Fil, A.; Fontana, M.; Francesco, M.; Fuchs, C.; Futatani, S.; Gabellieri, L.; Gadariya, D.; Gahle, D.; Galassi, D.; Gałązka, K.; Galdon-Quiroga, J.; Galeani, S.; Gallart, D.; Gallo, A.; Galperti, C.; Garavaglia, S.; Garcia, J.; Garcia-Lopez, J.; Garcia-Mu oz, M.; Garzotti, L.; Gath, J.; Geiger, B.; Giacomelli, L.; Giannone, L.; Gibson, S.; Gil, L.; Giovannozzi, E.; Giruzzi, G.; Gobbin, M.; Gonzalez-Martin, J.; Goodman, T. P.; Gorini, G.; Gospodarczyk, M.; Granucci, G.; Grekov, D. 1; Grenfell, G.; Griener, M.; Groth, M.; Grover, O.; Gruca, M.; Gude, A.; Guimarais, L.; Gyergyek, T.; Hacek, P.; Hakola, A.; Ham, C.; Happel, T.; Harrison, J.; Havranek, A.; Hawke, J.; Henderson, S.; Hesslow, L.; Hitzler, F.; Hnat, B.; Hobirk, J.; Hoelzl, M.; Hogeweij, D.; Hopf, C.; Hoppe, M.; Horacek, J.; Hron, M.; Huang, Z.; Iantchenko, A.; Iglesias, D.; Igochine, V.; Innocente, P.; Ionita-Schrittwieser, C.; Isliker, H.; Ivanova-Stanik, I.; Jacobsen, A.; Jakubowski, M.; Janky, F.; Jardin, A.; Jaulmes, F.; Jensen, T.; Jonsson, T.; Kallenbach, A.; Kappatou, A.; Karpushov, A.; Kasilov, S.; Kazakov, Y.; Kazantzidis, P. V.; Keeling, D.; Kelemen, M.; Kendl, A.; Kernbichler, W.; Kirk, A.; Kocsis, G.; Komm, M.; Kong, M.; Korovin, V.; Koubiti, M.; Kovacic, J.; Krawczyk, N.; Krieger, K.; Kripner, L.; Křivská, A.; Kudlacek, O.; Kulyk, Y.; Kurki-Suonio, T.; Kwiatkowski, R.; Laggner, F.; Laguardia, L.; Lahtinen, A.; Lang, P.; Likonen, J.; Lipschultz, B.; Liu, F.; Lombroni, R.; Lorenzini, R.; Loschiavo, V. P.; Lunt, T.; MacUsova, E.; Madsen, J.; Maggiora, R.; Maljaars, B.; Manas, P.; Mantica, P.; Mantsinen, M. J.; Manz, P.; Maraschek, M.; Marchenko, V.; Marchetto, C.; Mariani, A.; Marini, C.; Markovic, T.; Marrelli, L.; Martin, P.; Martín Solís, J. R.; Martitsch, A.; Mastrostefano, S.; Matos, F.; Matthews, G.; Mayoral, M.-L.; Mazon, D.; Mazzotta, C.; Mc Carthy, P.; McClements, K.; McDermott, R.; McMillan, B.; Meineri, C.; Menkovski, V.; Meshcheriakov, D.; Messmer, M.; Micheletti, D.; Milanesio, D.; Militello, F.; Miron, I. G.; Mlynar, J.; Moiseenko, V.; Molina Cabrera, P. A.; Morales, J.; Moret, J.-M.; Moro, A.; Moulton, D.; Nabais, F.; Naulin, V.; Naydenkova, D.; Nem, R. D.; Nespoli, F.; Newton, S.; Nielsen, A. H.; Nielsen, S. K.; Nikolaeva, V.; Nocente, M.; Nowak, S.; Oberkofler, M.; Ochoukov, R.; Ollus, P.; Olsen, J.; Omotani, J.; Ongena, J.; Orain, F.; Orsitto, F. P.; Paccagnella, R.; Palha, A.; Panaccione, L.; Panek, R.; Panjan, M.; Papp, G.; Paradela Perez, I.; Parra, F.; Passeri, M.; Pau, A.; Pautasso, G.; Pavlichenko, R.; Perek, A.; Pericoli Radolfini, V.; Pesamosca, F.; Peterka, M.; Petrzilka, V.; Piergotti, V.; Pigatto, L.; Piovesan, P.; Piron, C.; Piron, L.; Plyusnin, V.; Pokol, G.; Poli, E.; Pölöskei, P.; Popov, T.; Popovic, Z.; Pór, G.; Porte, L.; Pucella, G.; Puiatti, M. E.; Pütterich, T.; Rabinski, M.; Juul Rasmussen, J.; Rasmussen, J.; Rattá, G. A.; Ratynskaia, S.; Ravensbergen, T.; Réfy, D.; Reich, M.; Reimerdes, H.; Reimold, F.; Reiser, D.; Reux, C.; Reznik, S.; Ricci, D.; Rispoli, N.; Rivero-Rodriguez, J. F.; Rocchi, G.; Rodriguez-Ramos, M.; Romano, A.; Rosato, J.; Rubinacci, G.; Rubino, G.; Ryan, D. A.; Salewski, M.; Salmi, A.; Samaddar, D.; Sanchis-Sanchez, L.; Santos, J.; Särkimäki, K.; Sassano, M.; Sauter, O.; Scannell, R.; Scheffer, M.; Schneider, B. S.; Schneider, P.; Schrittwieser, R.; Schubert, M.; Seidl, J.; Seliunin, E.; Sharapov, S.; Sheeba, R. R.; Sias, G.; Sieglin, B.; Silva, C.; Sipilä, S.; Smith, S.; Snicker, A.; Solano, E. R.; Hansen, S. K.; Soria-Hoyo, C.; Sorokovoy, E.; Sozzi, C.; Sperduti, A.; Spizzo, G.; Spolaore, M.; Stejner, M.; Stipani, L.; Stober, J.; Strand, P.; Sun, H.; Suttrop, W.; Sytnykov, D.; Szepesi, T.; Tál, B.; Tala, T.; Tardini, G.; Tardocchi, M.; Teplukhina, A.; Terranova, D.; Testa, D.; Theiler, C.; Thorén, E.; Thornton, A.; Tilia, B.; Tolias, P.; Tomes, M.; Toscano-Jimenez, M.; Tsironis, C.; Tsui, C.; Tudisco, O.; Urban, J.; Valisa, M.; Vallar, M.; Vallejos Olivares, P.; Valovic, M.; Van Vugt, D.; Vanovac, B.; Varje, J.; Varju, J.; Varoutis, S. 1; Vartanian, S.; Vasilovici, O.; Vega, J.; Verdoolaege, G.; Verhaegh, K.; Vermare, L.; Vianello, N.; Vicente, J.; Viezzer, E.; Villone, F.; Voitsekhovitch, I.; Voltolina, D.; Vondracek, P.; Vu, N. M. T.; Walkden, N.; Wauters, T.; Weiland, M.; Weinzettl, V.; Wensing, M.; Wiesen, S.; Wiesenberger, M.; Wilkie, G.; Willensdorfer, M.; Wischmeier, M.; Wu, K.; Xiang, L.; Zagorski, R.; Zaloga, D.; Zanca, P.; Zaplotnik, R.; Zebrowski, J.; Zhang, W.; Zisis, A.; Zoletnik, S.; Zuin, M.

The resonant magnetic perturbation (RMP) has proven to be a useful way to suppress edge-localized modes that under certain conditions can damage the device by the large power fluxes carried from the bulk plasma to the wall. The effect of RMP on the L-mode plasma parameters in the divertor region of the COMPASS tokamak was studied using the array of 39 Langmuir probes embedded into the divertor target. The current-voltage (IV) probe characteristics were processed by the first-derivative probe technique to obtain the plasma potential and the electron energy distribution function (EEDF) which was approximated by a bi-Maxwellian EEDF with a low-energy (4-6 eV) fraction and a high-energy (11-35 eV) one, the both factions having similar electron density. Clear splitting was observed during the RMP pulse in the low-field-side scrape-off-layer profiles of the floating potential U fl and the ion saturation current density J sat; these two quantities were obtained both by direct continuous measurement and by evaluation of the IV characteristics of probes with swept bias. The negative peaks of U fl induced by RMP spatially overlaps with the local minima of J sat (and n e) rather than with its local maxima which is partly caused by the spatial variation of the plasma potential and partly by the changed shape of the EEDF. The effective temperature of the whole EEDF is not correlated with the negative peaks of U fl, and the profile of the parallel power flux density shows secondary maxima due to RMP which mimic those of J sat.

A new control module for the MAST-Upgrade Langmuir Probe system has been developed. It is based on a Xilinx Zynq FPGA, which allows for excellent configurability and ease of retrieving data. The module is capable of arbitrary bias voltage waveform generation, and digitises current and voltage readings from 16 probes. The probes are biased and measured one at a time in a time multiplexed fashion, with the multiplexing sequence completely configurable. In addition, simultaneous digitisation of the floating potential of all unbiased probes is possible. A suite of these modules, each coupled with a high voltage amplifier, enables biasing and digitisation of 640 Langmuir Probes in the MAST-Upgrade Super-X divertor. The system has been successfully tested on the York Linear Plasma Device and on the COMPASS tokamak. It will be installed on MAST-Upgrade ready for operations in 2018.

In this paper, we report results of a dedicated experiment that gives the plasma penetration profiles inside a gap of a tokamak castellated plasma-facing component. A specially designed probe that recreates a gap between two tiles has been built for the purpose of this study. It allows to measure ion saturation profiles along the 2 sides and at the bottom of the gap for both poloidal and toroidal orientations. The novelty of such experiment is the real time measurement of the plasma flux inside the gap during a tokamak D-shaped discharge compared to previous experimental studies which were mainly post-mortem. This experiment was performed in the COMPASS tokamak and results are compared with particle-in-cell simulations. The plasma deposition is found to be asymmetric in both orientations with a stronger effect in poloidal gaps. The Larmor radius of the incoming ions plays a role in the plasma penetration only in poloidal gaps but seems to have little impact in toroidal gaps. Profiles are qualitatively well reproduced by simulations. Ion current is recorded at the bottom of a toroidal gap under certain conditions.

The First derivative probe technique for a correct evaluation of the plasma potential in the case of non-Maxwellian EEDF is presented and used to process experimental data from COMPASS tokamak. Results obtained from classical and first derivative techniques are compared and discussed. The first derivative probe technique provides values for the plasma potential in the scrape-off layer of tokamak plasmas with an accuracy of about ±10%. Classical probe technique can provide values of the plasma potential only, if the electron and ion temperatures are known as well as the coefficient of secondary electron emission. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper reports on probe measurements of the electron energy distribution function and plasma potential in the divertor region of the COMPASS tokamak during D-shaped plasmas. The probe data have been processed using the novel first-derivative technique. A comparison with the results obtained by processing the same data with the classical probe technique, which assumes Maxwellian electron energy distribution functions is presented and discussed. In the vicinity of the inner and outer strike points of the divertor the electron energy distribution function can be approximated by a bi-Maxwellian, with a dominating low-energy electron population (4-7 eV) and a minority of higher energy electrons (12-25 eV). In the private flux region between the two strike points the electron energy distribution function is found to be Maxwellian with temperatures in the range of 7-10 eV. The comparative analysis using both techniques has allowed a better insight into the underlying physical processes at the divertor region of the COMPASS tokamak. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Materials exposed to plasma may undergo various forms of surface modifications. Among the important phenomena for tungsten - as the prime candidate plasma-facing material for fusion devices – is a formation of helium-induced fibreform nanostructure, so-called tungsten fuzz. In this paper, we report direct observations of the interaction of the pre-prepared fuzzy tungsten surfaces with ELMy H-mode plasmas in the COMPASS tokamak as well as consequent ex-situ morphological analyses, with a particular focus on arcing as a potential erosion mechanism. Arcing events are documented from high-speed camera imaging. The sample surfaces are examined by scanning electron microscopy. Arc traces were observed on all samples, while their number was dependent on the sample position and orientation. Inside the arc traces, localized melting and densification of the original fuzz was observed, resulting in thickness reduction. The modified structure still retained some porosity and did not extend into the bulk.

The paper reports results from Langmuir probe current-voltage (IV) characteristic measurements in argon and oxygen low-gas-pressure magnetized plasma. The plasma was produced in a stainless steel discharge tube with length 1.5 m and diameter 0.17 m with a hot filaments cathode. The wall of the discharge tube was grounded. An axial magnetic field B was created by a solenoid. A platinum cylindrical Langmuir probe with radius R = 5· 10−5 m and length L = 5· 10−3 m was placed axially and radially at the center of the discharge tube in order to perform measurements along and across the magnetic field. Using the second derivatives of the measured IV probe characteristics, the EEDF in argon and oxygen was evaluated. The derivatives were taken numerically. Measurements in an argon (i.e., in the absence of negative ions) were performed to obtain results for comparison with the measurements in an oxygen gas discharge, where a high density of negative ions is expected. In the measurements of the second derivatives of the IV characteristics with the probe parallel to the magnetic field in oxygen, a peak appears close to the plasma potential due to the registration of negative ions. In the same time, the electron part of the second derivative was substantially suppressed relatively to the case with the probe oriented perpendicular to the magnetic field. Thus using the appropriate orientation of the probe and a low magnetic field, when the electron part of the second derivative is suppressed, since the negative ion part is not affected by the low magnetic field applied, we can evaluate precisely the negative ion density. The results from the plasma parameters evaluation (plasma potential, electron and negative ion densities and electron temperatures) are presented and discussed (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dust transport is among important issues for ITER and DEMO, where material erosion will be significant. One of possible mechanisms how material is eroded from plasma facing surfaces is the remobilization of dust particles linked to their lifetime there and to the formation of dust accumulation sites. On the COMPASS tokamak, dust remobilization experiments have been performed using a tungsten surface with well-defined seeded dust particles exposed to L-mode and ELMy H-mode discharges as well as to a disruption. On small flat tungsten blocks, a dust of the particle size up to 25 mm was deposited by a low speed gas gun. Dust particles were prepared as sub-millimetre spots positioned on the top and on the side surfaces of the block. Dust particle positions for all prepared spots were mapped using a scanning electron microscope (SEM). Then, the samples were mounted on a manipulator allowing insertion to a fixed position close to the divertor region and were exposed to a discharge of the COMPASS tokamak. After the removal of the blocks from the vacuum vessel, positions of the seeded tungsten dust particles were again mapped by SEM and compared with the original ones, deducing movements of individual particles as well as a pattern-like behaviour. Remobilization of dust particles were monitored by a fast visible light camera with a pixel resolution of 0.3 mm at 15-40 kfps. Heat flux conditions were derived from measurements of the divertor probes (0.02-0.5 MW/m22 in L-mode and inter-ELM periods of H-mode) and Langmuir and ball-pen probes (

The second-derivative Langmuir probe method for precise determination of the plasma potential, the electron energy distribution function (respectively the electron temperature,) and the electron density of gas discharge plasma at intermediate pressures (100-1000 Pa) is reviewed. Results of applying the procedure proposed to different kinds of gas discharges are presented. Factors affecting the accuracy of the plasma characteristics evaluated are discussed.

A computerized experimental set-up for second derivative Langmuir probe measurements is presented. As a signal generator for the differentiating signal a computer stereo sound card is used. The probe signal is detected using a selective amplifier - the lock-in amplifier combination, whose output is measured by an ADC card inside a computer. Special drivers working under Microsoft Windows98 Second Edition were created in Visual C++ 6.0 to control the operation of the probe system.

In a d.c. discharge tube with sectional cathodes and a common grid anode, second derivative Langmuir probe measurements were performed in the Faraday dark space in argon gas discharge at intermediate pressures. Experimental results for different radial probe positions and different distances from the cathode in axial direction are presented. It is shown that the electron energy distribution function is bi-Maxwellian. Taking into account the electron depletion caused by their sinking on the probe surface, an extension of the Druyvesteyn formula is applied for more accurate determination of the electron temperature value, T, the electron density, n, and the plasma potential, Upl, from the experimental results acquired.

We present a review of free electron and field diagnostics in gas discharges, giving a special emphasis to spatially resolved measurements. Probes and antennas are the kernel of this work and we mention refinements concerning their use as well as common sources of error.

The first-derivative probe technique was applied to derive data for plasma parameters from the IV Langmuir probe characteristics measured in the plasma boundary region in the COMPASS tokamak and in the TJ-II stellarator. It is shown that in the COMPASS tokamak in the vicinity of the last closed flux surface (LCFS) the electron energy distribution function (EEDF) is bi-Maxwellian with the low-temperature electron fraction predominating over the higher temperature one, whereas in the far scrape-off layer (SOL) the EEDF is Maxwellian. In the TJ-II stellarator during NBI heated plasma the EEDF in the confined plasma and close to the LCFS is bi-Maxwellian while in the far SOL the EEDF is Maxwellian. In contrast, during the ECR heating phase of the discharge both in the confined plasma and in the SOL the EEDF is bi-Maxwellian. The mechanism for the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is discussed. The comparison of the results from probe measurements with ASTRA package and EIRENE code calculations suggests that the main reason of the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is the ionization of the neutral atoms. Results for the electron temperatures and densities obtained by the first-derivative probe technique in the COMPASS tokamak and in the TJ-II stellarator were used to evaluate the radial distribution of the parallel power flux density. It is shown that in the SOL the radial distribution of the parallel power flux density is a double exponential. It is pointed out that in the calculations of the parallel power flux density at the LCFS the energy losses from ionization mechanisms must be taken into account.

A new system of probes was recently installed in the divertor of tokamak COMPASS in order to investigate the ELM energy density with high spatial and temporal resolution. The new system consists of two arrays of rooftop-shaped Langmuir probes (LPs) used to measure the floating potential or the ion saturation current density and one array of Ball-pen probes (BPPs) used to measure the plasma potential with a spatial resolution of ~3.5 mm. The combination of floating BPPs and LPs yields the electron temperature with microsecond temporal resolution. We report on the design of the new divertor probe arrays and first results of electron temperature profile measurements in ELMy H-mode and L-mode. We also present comparative measurements of the parallel heat flux using the new probe arrays and fast infrared termography (IR) data during L-mode with excellent agreement between both techniques using a heat power transmission coefficient γ = 7. The ELM energy density was measured during a set of NBI assisted ELMy H-mode discharges. The peak values of were compared with those predicted by model and with experimental data from JET, AUG and MAST with a good agreement.

The International Summer School on Vacuum, Electron and Ion Technologies (VEIT) has been organized biennially since 1977, when the series of VEIT School was launched by the Institute of Electronics, Bulgarian Academy of Sciences with the aim to act as a forum for exchange and dissemination of knowledge and ideas on the latest developments in electron-, ion-, and plasma-assisted technologies. The organizers of the 2013 edition of the event were the Institute of Electronics, Bulgarian Academy of Sciences, Sofia, Bulgaria and the the Dutch Institute for Fundamental Energy Research, Nieuwegein, The Netherlands. While the school has initially been providing a meeting place for researchers mainly from Eastern and Central European countries, its importance has grown issue by issue. The school is now a major scientific event and a meeting place for young scientists from Eastern and Western Europe involved in research and development associated with high-tech industries. Many former school participants have gone on to become leading scientists in research establishments and companies throughout the world. Leading international companies, such as High Voltage Engineering, Balzers, Varian, and Hauzer have used the VEIT forum to present their products through oral presentations, poster contributions or exhibits. The School Proceedings have been published in special issues of the international journals Vacuum, Plasma Processes and Polymers , and Journal of Physics: Conference Series . The eighteenth edition of VEIT was held in the Black Sea resort Sozopol, Bulgaria, on 7–11 October 2013. It was attended by 91 participants from 15 countries: Bulgaria, Czech Republic, France, Germany, Greece, The Netherlands, Portugal, Romania, Russia, Serbia, Slovakia, Slovenia, Sweden, Ukraine and the UK. Following the tradition of publishing the VEIT Proceedings, a selection of papers presented at the event is published in this special issue of Journal of Physics: Conference Series , under the originality and quality criteria of acceptance by the journal, including peer reviewing. The school comprised of fifteen plenary and three poster sessions. At the plenary sessions, 21 invited talks of general interest were presented by highly renowned international experts in the field, as well as 10 progress reports by young scientists. In total, 66 contributed papers were presented during the poster sessions. There were several scientific highlights covering fundamentals of interaction of fast particles with solids and challenging practical applications ranging from novel techniques for creating hard coatings, optical/protective layers, biocompatible materials to nanosized structures produced by evaporation, sputtering or external irradiation. Latest results were presented on ion-beam synthesis and modification in both low-energy (deposition and film growth) and high-energy (sputtering, implantation) regimes, and processing of solid materials aiming at patterning the surface or at creating nanophase systems for electronic or tribological/wear resistant applications. Despite the busy scientific program, the atmosphere was relaxed and informal. The early afternoons of most conference days were free to stimulate both scientific and social interaction between participants, which often took place on the beach. The social program included a welcome party, an official dinner, and an outing to historical landmarks in old Sozopol. VEIT 2013 owes its success to many people. The International Advisory Committee shaped the scientific program and ensured high-quality plenary presentations by careful selection of invited speakers. The Local Committee bore the brunt of the organization both at the conference site and in dealing with correspondence, abstracts, and manuscripts for these proceedings. We are grateful to our sponsors, the Dutch Institute for Fundamental Energy Research, Nieuwegein, The Netherlands, the companies AllData and Astel, and the Ministry of Education and Science of Republic of Bulgaria for their generosity, which enabled us to support the attendance of students and provided support to deal with mailing, printing, renting the conference site, etc. We are also grateful to the members of the International Advisory Committee — Th Czerwiec, M Dimitrova, G Dinescu, A Ehiasarian, N Guerassimov, I Katardjiev, H Kersten, D Mataras, W Möller, I Petrov, Z Petrovic, B Rauschenbach, M C M (Richard) van de Sanden and M Ürgen — for their continuous concern about the high scientific level and the future of the event, as well as for their contribution to the formation of a new generation of scientists and experts in the field. And, last but not least, we would also like to thank all authors for their valuable contributions to these proceedings and to the school, and all reviewers for their hard and tedious, but very important, work. The next conference in the series will be held in September 2015. M C M van de Sanden, Miglena Dimitrova and Chavdar Ghelev Guest Editors ##IMG## [http://ej.iop.org/images/1742-6596/514/1/011001/veit.jpg] {Conference photograph}

The production of metal hydrides in vapour phase is one of the problems which makes their spectroscopic investigation at high resolution difficult. The molecular densities are usually low and the absorption is often increased by the use of multipass cells or intracavity setups. In this contribution a discharge tube with coaxial geometry is investigated, which is able to produce relatively high densities of NiH (≈10(12) cm(-3)). Additional advantage of the present geometry is that the densities are very homogeneous along the discharge length, 250 mm in our case, which can be made in principle arbitrary long. As a result, reliable absorption was detected even in a single pass experiment. We also present the results of a numerical model which explains the general properties of the plasma in the tube. Based on this understanding, we discuss possible improvements and other applications of this discharge geometry.

This paper reports experimental data obtained on the COMPASS tokamak by an array of Langmuir probes embedded in the divertor tiles. The measured current-voltage probe characteristics were processed by the recently published first-derivative probe technique for precise determination of the plasma potential and the electron energy distribution function (EEDF). The measurements were performed during L-mode hydrogen and deuterium plasma with a toroidal magnetic field Bt = 1.15 T, plasma current Ip = 180 kA and average electron density ne = 8*1019 m-3. The spatial profile of the electron temperatures shows that in the vicinity of the inner and outer strike points in hydrogen plasma the EEDF can be approximated by a bi-Maxwellian distribution, with a dominating low-energy electron population (4 - 7 eV) and a minority of electrons with higher energies (12 - 18 eV). In the private flux region between the two strike points, the EEDF is found to be Maxwellian with temperatures in the range of 7 - 9 eV. In the case of deuterium plasma under similar discharge conditions, the EEDFs in the vicinity of the inner and outer strike points, as well as in the private flux region between the two strike points, are found to be bi-Maxwellian.

A novel construction of gas discharge tube has been tested for production of high densities of metal hydrydes. Its performance turned out to be comparable with the existing sources of the same type and even better. First results of the tests on NiH are reported and critically analysed. Plans for future modifiaction of the construction and application of the tube are discussed.

This paper reports experimental results on low-pressure argon capacitive RF discharge (parallel-plate capacitively-coupled plasma - CCP) under different conditions, namely, gas pressure in the range 3 -r- 30 Pa and RF power in the range 10 - 100 W. The IV characteristics measured were processed by two different second-derivative probe techniques for determination of the plasma parameters and the electron energy distribution function. The radial profiles of the main plasma parameters are presented.

The plasma parameters during an L-mode hydrogen discharge in the COMPASS tokamak with a toroidal magnetic field BT =1.15 T, line-averaged electron density ne = 6×1019 m-3 and a plasma current variation from 209 kA to 100 kA were studied in the divertor region. The electron energy distribution function for 209 kA at the high-field side and the private region is Maxwellian with a temperature in the range of 5 -- 9 eV, while around the outer strike point and the low-field side it is bi-Maxwellian with a low-energy electron group (4 -- 5 eV) and higher energy electrons (10 -- 20 eV). As the plasma current decreases, the appearance of the bi-Maxwellian EEDF is shifted towards the low-field side; at plasma current of 100 kA, the EEDF is Maxwellian in the whole divertor region.

The scrape-off-layer (SOL) parameters in the COMPASS tokamak are studied by using a Langmuir probe mounted on a horizontal reciprocating manipulator. The radial profiles of the plasma potential, the electron energy distribution function and the electron densities are derived from the measured current-voltage probe characteristics by applying the firstderivative probe technique (FDPT). It is shown that close to the tokamak wall the electron energy distribution function is Maxwellian, while in the SOL, in the vicinity of the last closed flux surface and inside the confined plasma, the electron energy distribution function is bi-Maxwellian with a low-energy electron fraction dominating over a higher energy one. The radial profiles of the electron pressure and the parallel electron power flux density in COMPASS are also presented.

The first derivative probe technique was applied to study the ISTTOK tokamak plasma. This technique employs the electron part of the Langmuir probe current-voltage (IV) characteristic and yields information on the plasma potential and the electron energy distribution function (EEDF). The IV characteristic was measured with new electrical probes mounted on a horizontal manipulator, one oriented in parallel and the other perpendicularly to the magnetic field lines. Using the first-derivative probe technique, the plasma potential and the EEDF at different radial positions were acquired. We show that, in the vicinity of the last close flux surface (LCFS), the EEDF is non-Maxwellian and can be approximated by a bi-Maxwellian one with a dominant cold electron population and a minority group of hot electrons. In the limiter shadow, the EEDF obtained is Maxwellian. The comparison of the plasma parameters evaluated using the first-derivative probe technique for both probes shows a satisfactory agreement.

A newly developed Langmuir probe system for measurements of current-voltage (IV) characteristics in the tokamak divertor area is presented and discussed. The system is partially controlled by a computer allowing simultaneous and independent feeding and registration of signals. The system is mounted in the COMPASS tokamak, Institute of Plasma Physics, Academy of Sciences of the Czech Republic. The new electronic circuit boards include also active low-pass filters which smooth the signal before recording by the data acquisition system (DAQ). The signal is thus less noisy and the data processing is much easier. We also designed and built a microcontroller-driven waveform generator with resolution of 1 Ms/s. The power supply is linear and uses a transformer. We avoided the use of a switching power supply because of the noise that it could generate. Examples of measurements of the IV characteristics by divertor probes in the COMPASS tokamak and evaluation of the EEDF are presented.

The International Summer School on Vacuum, Electron and Ion Technologies (VEIT) has been organized biennially since 1977, when the VEIT Summer School series was launched by the Institute of Electronics, Bulgarian Academy of Sciences. The aim was to act as a forum for the exchange and dissemination of knowledge and ideas on the latest developments in electron-, ion- and plasma-assisted technologies. The organizers of the 2011 edition of the event were the Institute of Electronics, Bulgarian Academy of Sciences, Sofia, Bulgaria and the Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands. Whilst the school initially provided a meeting place for researchers mainly from Eastern and Central European countries, its importance has grown issue by issue. The school is now a major scientific event and a meeting place for young scientists from Eastern and Western Europe involved in research and development associated with high-tech industries. Many former school participants have gone on to become leading scientists in research establishments and companies throughout the world. Leading international companies, such as High Voltage Engineering, Balzers, Varian, and Hauzer have used the VEIT forum to present their products through oral presentations, poster contributions and exhibits. The School Proceedings have been published in special issues of the international journals Vacuum, Plasma Processes and Polymers and Journal of Physics: Conference Series. The Seventeenth edition of VEIT was held in the Black Sea resort of Sunny Beach, Bulgaria on 19?23 September 2011. It was attended by 96 participants from 18 countries: Belgium, Brazil, Bulgaria, Czech Republic, Denmark, France, Germany, Greece, The Netherlands, Romania, Russia, Serbia, Sweden, Switzerland, Turkey, Ukraine, UK and USA. Following the tradition of publishing the VEIT Proceedings, a selection of papers presented at the event is published in this special issue of Journal of Physics: Conference Series, under the originality and quality criteria of acceptance by the journal, including peer reviewing. The school comprised thirteen plenary and three poster sessions. At the plenary sessions, 21 invited talks of general interest were presented by highly-renowned international experts in the field, as well as thirteen progress reports by young scientists. In total, 60 contributed papers were presented during the poster sessions. There were several scientific highlights, covering the fundamentals of interaction of fast particles with solids, and challenging practical applications. These ranged from novel techniques for creating hard coatings, optical/protective layers and biocompatible materials, to nanosized structures produced by evaporation, sputtering or external irradiation. The latest results were presented on ion-beam synthesis and modification in both low-energy (deposition and film growth) and high-energy (sputtering, implantation) regimes, the processing of solid materials aimed at surface patterning, and the creation of nanophase systems for electronic or tribological/wear resistant applications. Despite the busy scientific program, the atmosphere was relaxed and informal. The early afternoons of most conference days were free to stimulate both scientific and social interaction between participants, which often took place on the beach. The social program included a welcome party, an official dinner, and an outing to historical landmarks in the vicinity of Nessebar. VEIT 2011 owes its success to many people. The International Advisory Committee shaped the scientific program and ensured high-quality plenary presentations by their careful selection of the invited speakers. The Local Committee bore the brunt of the organization, both at the conference site and in dealing with correspondence, abstracts, and manuscripts for these proceedings. We are grateful to our sponsor, the Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands, for its generosity that enabled us to support the attendance of students, and provided support dealing with mailing, printing, renting the conference site, etc. We are also grateful to the members of the International Advisory Committee: G Dinescu, A Ehiasarian, N Guerassimov, D Mataras, W M?ller, I Petrov, Z Petrovic, B Rauschenbach, M C M (Richard) van de Sanden and M ?rgen, for their continuous concern for the high scientific level and the future of the event, and for their contribution to the development of a new generation of scientists and experts in the field. And, last but not least, we would also like to thank all authors for their valuable contributions to these proceedings and to the school, and all reviewers for their hard and tedious, but very important, work. The next conference in the series will be held in September 2013. M C M van de Sanden, Miglena Dimitrova and Chavdar Ghelev Guest Editors

We report results of the evaluation of the plasma potential, the electron temperature and the electron density during the current shots in the COMPASS tokamak, (IPP.CR, Prague, Czech Republic). The comparisons with results from model calculations based on an extended formula for the electron probe current show a satisfactory agreement. The results presented demonstrate that the procedure proposed allows one to acquire additional plasma parameters using the electron part of the current-voltage Langmuire probe (LP) characteristics in tokamak edge plasma probe measurements.

In this work, methods for using Langmuir probes (LPs) in magnetized plasmas are presented. The electron part of the current–voltage probe characteristics is used to obtain the plasma potential, the electron energy distribution function (EEDF), the electron temperature and the electron density. The application of LPs to EEDF evaluation in the presence of magnetic fields in the range 0.01–0.1 T is investigated and discussed based on kinetic theory in a non-local approach. Data for EEDFs in magnetic fields in the range 0.015–0.079 T are acquired using current–voltage characteristics measured in low pressure Ar and He dc gas discharges. It is also shown that the EEDFs are Maxwellian up to the energy of the first excited states of argon and helium. The values of the plasma potential, electron temperature and density are evaluated. Comparison of the results obtained with probes perpendicular and parallel to the magnetic field results in satisfactory agreement.The results presented demonstrate that the procedures proposed allow one to acquire the main plasma parameters using the electron part of the current–voltage LP characteristics in magnetized plasmas.

The applicability of the first derivative Langmuir probe method in strongly magnetized tokamak plasmas is discussed. The method is used for processing the electron part of the current-voltage (IV) probe characteristics measured in the CASTOR tokamak edge plasma (Institute of Plasma Physics, Association EURATOM-IPP, Prague, Czech Republic). The comparison of the results obtained with the results given by the Stangeby method yields a satisfactory agreement.

Langmuir probes are known for their ability to provide local measurements of very important plasma parameters, namely the plasma potential, the density of the charged particles and the electron energy distribution function (EEDF). The correctness of using the probes under adverse conditions, such as presence of magnetic fields or high plasma temperature is still being questioned. In this paper we report the application of the first-derivative Langmuir probe method for processing the electron part of the volt-ampere (IV) characteristics measured in the CASTOR tokamak plasma. First results of EEDFs at different radial positions in the edge plasma are presented and the values of the plasma potential, electron temperature and electron densities are estimated. The applicability of the first-derivative Langmuir probe method in strongly magnetized, high temperature tokamak plasmas is discussed.

The COMPASS tokamak at IPP Prague is a small-size device with an ITER-relevant plasma geometry and operating in both the Ohmic as well as neutral beam assisted H-modes since 2012. A basic set of diagnostics installed at the beginning of the COMPASS operation has been gradually broadened in type of diagnostics, extended in number of detectors and collected channels and improved by an increased data acquisition speed. In recent years, a significant progress in diagnostic development has been motivated by the improved COMPASS plasma performance and broadening of its scientific programme (L-H transition and pedestal scaling studies, magnetic perturbations, runaway electron control and mitigation, plasma-surface interaction and corresponding heat fluxes, Alfvenic and edge localized mode observations, disruptions, etc.). In this contribution, we describe major upgrades of a broad spectrum of the COMPASS diagnostics and discuss their potential for physical studies. In particular, scrape-off layer plasma diagnostics will be represented by a new concept for microsecond electron temperature and heat flux measurements - we introduce a new set of divertor Langmuir and ball-pen probe arrays, newly constructed probe heads for reciprocating manipulators as well as several types of standalone probes. Among optical tools, an upgraded high-resolution edge Thomson scattering diagnostic for pedestal studies and a set of new visible light and infrared (plasma-surface interaction investigations) cameras will be described. Particle and beam diagnostics will be covered by a neutral particle analyzer, diagnostics on a lithium beam, Cherenkov detectors (for a direct detection of runaway electrons) and neutron detectors. We also present new modifications of the microwave reflectometer for fast edge density profile measurements.

Advanced Langmuir probe techniques for evaluating the plasma potential and electron-energy distribution function (EEDF) in magnetized plasma are reviewed. It is shown that when the magnetic field applied is very weak and the electrons reach the probe without collisions in the probe sheath the second-derivative Druyvesteyn formula can be used for EEDF evaluation. At low values of the magnetic field, an extended second-derivative Druyvesteyn formula yields reliable results, while at higher values of the magnetic field, the first-derivative probe technique is applicable for precise evaluation of the plasma potential and the EEDF. There is an interval of intermediate values of the magnetic field when both techniques—the extended second-derivative and the first-derivative one—can be used. Experimental results from probe measurements in different ranges of magnetic field are reviewed and discussed: low-pressure argon gas discharges in the presence of a magnetic field in the range from 0.01 to 0.08 T, probe measurements in circular hydrogen plasmas for high-temperature fusion (magnetic fields from 0.45 T to 1.3 T) in small ISTTOK and CASTOR tokamaks, D-shape COMPASS tokamak plasmas, as well as in the TJ-II stellarator. In the vicinity of the last closed flux surface (LCFS) in tokamaks and in the TJ-II stellarator, the EEDF obtained is found to be bi-Maxwellian, while close to the tokamak chamber wall it is Maxwellian. The mechanism of the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is discussed. Comparison of the results from probe measurements with those obtained from calculations using the ASTRA and EIRENE codes shows that the main reason for the appearance of a bi-Maxwellian EEDF in the vicinity of the LCFS is the ionization of the neutral atoms.

При общо 76 деца, 36 с улцерозен колит, 27 с болест на Крои и 13 е неспецифичен колит, на средна възраст от 13,54±3,13 г. (между две и 17 г.), хоспитализирани между 1.1.2008 г. и 31.VII.2020 г., са проведени образни изследвания и са изследвани някои клинични характеристики на тези три заболявания, констелация от строго подбрани хематологични и биохимични показатели, модерни биологически маркери и за първи път в България - експресията на 11 микрорибонуклеинови киселини при болните с улцерозен колит и болест на Крон и при здрави деца. Измененията на показателите на анемията (брой на еритроцитите и концентрации на хемоглобина и серумното желязо) и възпалението (брой на левкоцитите и концентрации на С- реактивния протеин, фибриногена, фекалния калпротектин, p-ANCA и c-ANCA) имат определена диагностична и диференциално-диагностична стойност при тези заболявания. Находките от фиброколоноскопията, абдоминалната ултрасонография, компютърната томография, ендоскопията, биопсията и фиброгастроскопията допринасят съществено за прецизиране на диагнозата. Експресията на всички тези микрорибонуклеинови киселини е по-висока при болните, отколкото при здравите деца. Статистически достоверните корелационни зависимости между конкретните изследвани показатели в рамките на тази констелация доказват клиничната им приложимост в детската гастроентерологична практика., In a total of 76 children, 36 with ulcerative colitis, 27 with Crohn’s disease and 13 with unspecific colitis, at a mean age of 13,54±3,13 years (range, two tol7 years), hospitalized between January 1, 2008 and July 31,2020, imaging examinations are performed and several clinical characteristics of these three diseases, the constellation of strictly selected hematological and biochemical parameters, contemporary biological markers and for the first time Bulgaria - the expression of 11 microribonucleic acids in the patients with ulcerative colitis and Crohn’s disease and in healthy children as well are studied. The changes of the parameters of the anemia such as erythrocyte number and concentrations of hemoglobin and serum iron as well as of the inflammation such as leukocyte number and concentrations of C-reactive protein, fibrinogen, fecal calprotectin, p-ANCA and c-ANCA possess a definite diagnostic and differential-diagnostic value in these diseases. The findings from fibrocolonoscopy, abdominal ultrasonography, computed tomography, endoscopy and fibrogastroscopy substantially contribute to the precise diagnosis. The expression of all these microribonucleic acids is higher in the patients than in the healthy children. The statistically significant correlation dependences between the concrete parameters within this constellation prove their clinical applicability in pediatric gastroenterological practice.