A reciprocating retarding field analyzer has been installed in the lower divertor of the WEST tokamak to measure the local ion temperature in ITER-like tungsten divertor conditions. The paper presents the design of the analyzer, the robust magnetically-driven reciprocating mechanism that is fully integrated into the actively-cooled tungsten divertor, and preliminary results obtained. It was found that the ion temperature can significantly exceed the electron temperature by a factor of up to four in the specific results shown here. These findings underscore the importance of direct ion temperature measurements in enabling accurate assessments of heat loads and material erosion in tokamak divertors.

This paper presents the conceptual design and construction of a DC gas-discharge tube combined with a magnetic field generated by a Helmholtz coil to study plasma parameters under magnetized conditions. The developed system consists of a seven-cathode configuration and a common anode that enables diagnostics with single electrical probes - parallel and perpendicular to the magnetic field lines. The maximum achievable magnetic field is 1000 G. The specially designed electronic system allows high-frequency and precise measurements of the current-voltage characteristics using the Langmuir probe method. The setup operates with different gasses (starting with argon) under controlled flow and low-pressure conditions. The modelling of the electron currents reveals significant differences in probe characteristics depending on their orientation to the magnetic field and on plasma parameters such as the temperature and density of the electrons. The developed device makes it possible to investigate the application of the probe methodology in plasma with magnetic field, in order to use it in various experimental devices - from magnetron discharges for advanced materials treatment technologies to fusion reactors.

The origin of the bi-Maxwellian electron energy distribution function (EEDF) observed in the scrape-off layer (SOL) of tokamak plasmas by means of Langmuir probes is still under discussion. It has been assumed that the ionization of hydrogen and deuterium neutrals by thermal electrons penetrating the SOL from the bulk plasma is the main reason for the appearance of a second Maxwellian. To validate this assumption, radial measurements of the electron temperatures and densities, or the plasma properties in helium plasmas in the GOLEM tokamak and the TJ-II stellarator were performed. The radial profiles of the low-temperature electron group densities follow the trend of the calculated radial profiles of the electron sources arising from the ionization of neutrals in both deuterium and helium plasmas in TJ-II. The difference in the radial location where the bi-Maxwellian EEDF appears can be explained by the difference in the rate coefficients for ionization of deuterium and helium. The results of probe measurements in GOLEM and the WEST tokamak divertor, at one radial location in the SOL, are compatible with the hypothesis concerning the ionization of neutral atoms and the type of the EEDF.

Electric current flowing onto the divertor of the COMPASS tokamak influences its heat loading. The current measured when a Langmuir probe is grounded to the divertor gives a local measurement of this heat loading, which, according to the classical theory of the Debye sheath, should be enhanced with respect to the case of locally ambipolar currents. The comparison of the calculated heat flux by probes with infrared thermography, when the influence of non-ambipolar currents is not considered, is grossly wrong; when the theoretical effect of non-ambipolar currents is included, however, the agreement is very good.

Abstract Experiments were conducted in deuterium plasma in the TJ-II stellarator by means of swept Langmuir probes mounted on reciprocating probes manipulators. The results were processed using the four-parameter fit, as well as the triple-probe and the first-derivative probe techniques. The parameters determined were the floating potential, the ion saturation current density, the electron temperature and density, and the plasma potential. The results were obtained for two plasma heating techniques – electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) heating. In the case of ECRH, employing the first-derivative probe technique resulted in finding that the electron-energy distribution function (EEDF) was not Maxwellian, but rather a bi-Maxwellian one with thermal (14-25 eV) and cold (4-5 eV) electrons. In comparison, during NBI heating we found a Maxwellian EEDF with the electron temperature being around 5 eV and slightly increasing in the confined plasma, but always remaining below 15 eV. We present a detailed analysis and discussion of the data for the plasma parameters as acquired by different techniques of using the reciprocating probe manipulator.

This paper presents an experimental characterization of the local plasma parameters in the COMPASS open divertor in deuterium, L-mode plasmas by means of Langmuir probes during neutral beam injection (NBI) heating. The effect of NBI heating of the core plasma on the local divertor plasma parameters is investigated under different plasma configurations and different directions of the magnetic field and plasma current for different line-averaged electron densities. At low line-averaged densities below 6 × 1019 m-3, a low NBI heating power level does not lead to any significant changes in the plasma parameters in the divertor region. It is also found that for a reversed magnetic field the influence of NBI in the divertor is stronger at higher line-averaged electron density and plasma current. In the divertor, the impact of the NBI heating is more pronounced at high delivered powers (above P NBI = 320 kW) and line-averaged densities (above 7 × 1019 m-3). The electron temperature and the floating potential at the outer target increases respectively with 10–50% and 100%, while the plasma potential in the inner divertor increases twofold. At a higher line-averaged density, the NBI heating affects the edge plasma, namely, the electron temperature in the midplane scrape-off layer and the heating moves toward the divertor. Thus, a twice as high electron temperature is observed in the divertor and a bi-Maxwellian electron energy distribution function arises.

This paper presents the results from swept probe measurements in the divertor region of the COMPASS tokamak in D-shaped, L-mode discharges, with toroidal magnetic field BT = 1.15 T, plasma current Ip = 180 kA and line-average electron densities varying from 2 to 8×1019 m−3. Using neutral beam injection heating, the electron energy distribution function is studied before and during the application of the beam. The current-voltage characteristics data are processed using the first-derivative probe technique. This technique allows one to evaluate the plasma potential and the real electron energy distribution function (respectively, the electron temperatures and densities). At the low average electron density of 2×1019 m−3, the electron energy distribution function is bi-Maxwellian with a low-energy electron population with temperatures 4-6 eV and a high-energy electron group 12-25 eV. As the line-average electron density is increased, the electron temperatures decrease. At line-average electron densities above 7×1019 m−3, the electron energy distribution function is found to be Maxwellian with a temperature of 6-8.5 eV. The effect of the neutral beam injection heating power in the divertor region is also studied.

In this work, the advantages are presented and discussed of the first-derivative probe technique over the three- and the four-parameter conventional probe techniques for diagnostics of fusion plasmas. The conventional probe techniques for estimation of the plasma potential and the electron temperature and density can only be used when the electron energy distribution function (EEDF) is Maxwellian. The first-derivative probe technique can provide reliable results for the plasma potential and the real EEDF when the latter deviates from Maxwellian. To exemplify the application of the results obtained by different techniques, results on the parallel power-flux density distribution in the divertor region of the COMPASS tokamak, IPP.CR, are presented and discussed.

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 Ufl and the ion saturation current density Jsat; 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 Ufl induced by RMP spatially overlaps with the local minima of Jsat (and ne) 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 Ufl, and the profile of the parallel power flux density shows secondary maxima due to RMP which mimic those of Jsat.

The radial distributions of the main plasma parameters in the scrape-off-layer of the COMPASS tokamak are measured during L-mode and H-mode regimes by using both Langmuir and ball-pen probes mounted on a horizontal reciprocating manipulator. The radial profile of the plasma potential derived previously from Langmuir probes data by using the first derivative probe technique is compared with data derived using ball-pen probes. A good agreement can be seen between the data acquired by the two techniques during the L-mode discharge and during the H-mode regime within the inter-ELM periods. In contrast with the first derivative probe technique, the ball-pen probe technique does not require a swept voltage and, therefore, the temporal resolution is only limited by the data acquisition system. In the electron temperature evaluation, in the far scrape-off layer and in the limiter shadow, where the electron energy distribution is Maxwellian, the results from both techniques match well. In the vicinity of the last closed flux surface, where the electron energy distribution function is bi-Maxwellian, the ball-pen probe technique results are in agreement with the high-temperature components of the electron distribution only. We also discuss the application of relatively large Langmuir probes placed in parallel and perpendicularly to the magnetic field lines to studying the main plasma parameters. The results obtained by the two types of the large probes agree well. They are compared with Thomson scattering data for electron temperatures and densities. The results for the electron densities are compared also with the results from ASTRA code calculation of the electron source due to the ionization of the neutrals by fast electrons and the origin of the bi-Maxwellian electron energy distribution function is briefly discussed.

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 COMPASS tokamak is one of the present devices operating with an ITER-like plasma shape. Its flexibility due to its small size combined to an extensive set of edge diagnostics and NBI heating allow to address a broad range of key areas in support of the worldwide fusion programme such as H-mode physics, MHD, runaways electrons, disruption studies, plasma-wall interactions. The recent results obtained in COMPASS addressing these key issues are reviewed here.

The radial distribution of the main plasma parameters in the scrape-off-layer of the COMPASS tokamak is measured in Land H-mode regimes using both Langmuir and ballpen probes mounted on a horizontal reciprocating manipulator. The radial profiles of the plasma potential and the electron energy distribution functions, i.e., the electron temperatures and densities, are derived from the measured Langmuir probe current-voltage characteristics by applying the first-derivative probe technique. The ball-pen probe measurements provide direct evaluation of the plasma potential with a high temporal resolution. The results for the plasma potential as derived from Langmuir probes and ball-pen probes measurements are compared and found to be in a quantitative agreement.

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 first-derivative 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 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. ((c) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

At the present time, the thermoassociative block copolymers usually consist of a favorable soluble fragment and a temperature-responsive one. In this study, the ceric ion redox method and the atom transfer radical polymerization (ATRP) were applied to synthesize novel thermally sensitive ABA block copolymers of poly(N-isopropylacrylamide) (PNIPAAm,) and polypropylene glycol (PPO) by using PPO with molecular weight of 2000 g/mol as a precursor. Due to the different lower critical solution temperatures (LCST) of PNIPAAm and PPO, we were able to obtain thermoassociative block copolymers, which have PNIPAAm segments with higher transition temperature and PPO segments of lower transition temperature. As a result, our copolymers exhibited a two-stage phase transition in aqueous solution. This attractive behavior offers an opportunity to obtain nanoparticles or microaggregates. The aggregation of the block copolymers in aqueous solutions was monitored by using the dye technique. The size of the copolymer nanoaggregates was determined with the aid of the dynamic light scattering (DLS). The particles obtained from aqueous solution were visualized by means of scanning force microscopy (SFM) and scanning electron microscopy (SEM) methods.

Novel, water-soluble thermoassociative graft copolymers based on high molecular weight (HMW) poly(ethylene oxide-co-glycidol) backbone and relatively short grafts of poly-N-isopropyl acrylamide (NIPAAm) were prepared. The copolymer precursors with two architectures (block and graft) were synthesized using Ca-amide-alkoxide initiators. The OH groups in the copolymer precursors have been utilized for grafting NIPAAm using ceric ion (Ce 4+ ) redox initiation. The idea was to imprint the smart properties of PNIPAAm grafts into common HMW poly(ethylene oxide). The sensitive moieties undergo reversible association transitions by changing the temperature of dilute and semidilute aqueous solutions of the copolymers. Associative properties were studied by viscosity and rheology measurements. Two types of interactions, induced by heating, depending on the copolymer concentration namely intra- and intermolecular association were observed.

Novel high molecular weight copolymers of ethylene oxide (EO) and glycidol or ethoxy ethyl glycidyl ether (EEGE) were obtained via suspension anionic coordination polymerization using calcium amide-alkoxide initiating system. H-1 and C-13 NMR spectroscopy were used for the structural characterization of the copolymers as well as for the determination of the molar content (up to 2 mot%) of the functional monomers incorporated into the PEO backbone. SEC measurements of the copolymers determined (M) over bar (n). in the range of (2-11) x 10(5) g/mol. Surprisingly low polydispersity values of average (M) over bar (w)/(M) over bar (n) = 1.5 were obtained for the EO/EEGE copolymers. The calcium amide-alkoxide initiated polymerization of EEGE resulted in high molecular weight polymers with relatively broad polydispersity values ((M) over bar (n) = (0.8-1.5) x 10(5) g/mol, (M) over bar (w)/(M) over bar (n) less than or equal to 4.2). EO/glycidol copolymers and polyglycidol were obtained after cleavage of the acetal protecting group of the EEGE units.Polymers with hydrophilic backbone bearing hydrophobic stearyl moieties were obtained after modification of EO/glycidol copolymers with stearic acid. Their associating properties in aqueous solution were studied. Increasing either temperature or polymer concentration induces aggregate formation. (C) 2002 Elsevier Science Ltd. All rights reserved.