[en] Full text: Partial detachment is the desired regime for the baseline burning plasma scenario in ITER and next-step devices, as it allows converting the majority of the energy carried by charged particles through the scrape-off-layer (SOL) into isotropic radiation and thus avoids localized heat flux deposition in the divertor region. In order to maintain relevance to ITER and DEMO, a concentrated effort has been initiated at the COMPASS tokamak to achieve detached operation by means of impurity seeding. Series of experiments with impurity injection in the range of 2-9 x 10²⁰ molecules per second at different locations in the divertor were performed with the aim to cool the plasma and influence the particle and heat transport in the divertor region and provoke partial detachment. Previously reported results were largely extended by injection of nitrogen at the outer divertor target and also by attempts to seed the plasma with neon. The effects on SOL and divertor plasma conditions were monitored by means of horizontal reciprocating probe manipulator located at the outer midplane and by arrays of divertor Langmuir and Ball-pen probes. The radiation in the edge plasma was observed by AXUV bolometers and fast visible cameras. Experiments in L-mode discharges with nitrogen injected at the outer divertor target have shown that the presence of radiating impurity leads to drop of pressure in the divertor. Depending on the magnitude of the seeding, the upstream pressure can be also affected, suggesting possible penetration of nitrogen into the confined plasma region. The target pressure, however, drops at faster rate than upstream, which allows reaching the regime of partial detachment. Similar results were obtained by the HFS nitrogen injection, however, the change in divertor pressure was more generally more abrupt and was less sensitive to the amount of injected nitrogen. (author)

[en] The complexity of making magnetic confinement fusion a suitable source of energy for mankind imposes long lasting investigations. To accompany these efforts, new generations of physicists have to be trained over the years to ensure continuity of the research. These scientists must have very versatile profiles as fusion inquiries are multiphysics (magnetohydrodynamic, plasma-wall interactions, cutting edge technologies...) and require skills in both experimental and theoretical physics. The Institute of Plasma Physics IPP Prague of the Czech Academy of Sciences (IPP Prague) has contributed to the education and training of a non-negligible number of students in this area, one reason being the good accessibility of its fusion facility, the other being its strong connection with universities. This article shows the views and achievements of IPP Prague on education. In particular, we insist on the fact that to keep students in the field they have to be acquainted with practical plasma physics as early as possible (on the bachelor level if possible). We also show that a strong integration of PhD candidates within the scientific staff is an important factor for their training and serves them to be autonomous. Last, we present two experimental 2 weeks training courses, called SUMTRAIC and EMTRAIC, that are annually organized by IPP Prague and that have acquainted more than 300 participants to the practical physics of tokamaks. (paper)

[en] In this Note, it is pointed out that emissive probes cannot be used to directly and reliably measure plasma potential fluctuations. An experimentally validated model demonstrates indeed that the floating potential fluctuations of an emissive probe which floats at the mean plasma potential depend not only on the plasma potential fluctuations but also on electron density and temperature fluctuations

[en] Ball-pen probes have been used in fusion devices for direct measurements of the plasma potential. Their application in low-temperature magnetized plasma devices is still subject to studies. In this context, a ball-pen probe has been recently implemented on the linear plasma device Mirabelle. Produced by a thermionic discharge, the plasma is characterized by a low electron temperature and a low density. Plasma confinement is provided by an axial magnetic field that goes up to 100 mT. The principle of the ball-pen probe is to adjust the saturation current ratio to 1 by reducing the electron current contribution. In that case, the floating potential of the probe is close to the plasma potential. A thorough study of the ball-pen probe operation is performed for different designs of the probe over a large set of plasma conditions. Comparisons between ball-pen, Langmuir, and emissive probes are conducted in the same plasma conditions. The ball-pen probe is successfully measuring the plasma potential in these specific plasma conditions only if an adapted electronics and an adapted probe size to the plasma characteristic lengths (λ_D, ρ_ce) are used.

[en] The ratio of electron and ion saturation currents measured by probes embedded in the plasma-facing components (PFCs) in magnetised plasmas is typically lower than predicted by Langmuir theory. In the past, several works targeted various effects influencing the ion saturation current, which can be larger than expected, especially if the magnetic field is at a grazing angle with respect to the probe surface. In this contribution, we focus on the electron current, which can be reduced when the magnetic field line intercepting the probe passes through the magnetic pre-sheath of the nearby PFC. In such a case an effective potential well forms and repels a fraction of the electrons incoming from the plasma. Experimental results obtained by a tiltable limiter at the DITE tokamak are revisited and reproduced by means of 2D3V particle-in-cell simulations. The reduction of the electron current is indeed observed both in simulations and in experiment. This mechanism may also explain why some divertor biasing experiments did not produce flows in the divertor region. (paper)

[en] Microturbulence has been implicated in anomalous transport at the exit of the Hall thruster, and recent simulations have shown the presence of an azimuthal wave which is believed to contribute to the electron axial mobility. In this paper, the 3D dispersion relation of this E×B electron drift instability is numerically solved. The mode is found to resemble an ion acoustic mode for low values of the magnetic field, as long as a non-vanishing component of the wave vector along the magnetic field is considered, and as long as the drift velocity is small compared to the electron thermal velocity. In these conditions, an analytical model of the dispersion relation for the instability is obtained and is shown to adequately describe the mode obtained numerically. This model is then fitted on the experimental dispersion relation obtained from the plasma of a Hall thruster by the collective light scattering diagnostic. The observed frequency-wave vector dependences are found to be similar to the dispersion relation of linear theory, and the fit provides a non-invasive measurement of the electron temperature and density

[en] The first results of particle-in-cell simulations of the electrostatic sheath and magnetic pre-sheath of thermionically emitting planar tungsten surfaces in fusion plasmas are presented. Plasma conditions during edge localized modes (ELMs) and during inter-ELM periods have been considered for various inclinations of the magnetic field and for selected surface temperatures. All runs have been performed under two assumptions for the sheath potential drop; fixed or floating. The primary focus lies on the evaluation of the escaping thermionic current and the quantification of the suppression due to the combined effects of space-charge and Larmor gyration. When applicable, the results are compared with the predictions of analytical models. The heat balance in the presence of thermionic emission as well as the contribution of the escaping thermionic current to surface cooling are also investigated. Regimes are identified where emission needs to be considered in the energy budget. (paper)

[en] The effect of the collective light scattering diagnostic transfer function is considered in the context of the dispersion relation of the unstable E×B mode previously reported. This transfer function is found to have a contribution to the measured frequencies and mode amplitudes which is more or less significant depending on the measurement wavenumbers and angles. After deconvolution, the experimental data are found to be possibly compatible with the idea that the mode frequency in the jet frame (after subtraction of the Doppler effect due to the plasma motion along the thruster axis) is independent of the orientation of the wave vector in the plane orthogonal to the local magnetic field.

[en] Microsecond probe measurements of the electron temperature during the tokamak edge localised mode (ELM) instability show that the peak values significantly exceed those obtained by conventional techniques. The temperatures measured at the plasma facing component (divertor) are around 80% of the initial value (at the pedestal). This challenges the current understanding, where only several percent of the pedestal value are measured at the divertor. Our results imply a negligible energy transfer from the electrons to the ions during the ELM instability, and therefore no associated increase of the ion power loads on the divertor. This observation is supported by the simple analytic free-streaming model, as well as by full kinetic simulations. The energetic ELM ion loads are expected to be one of the main divertor damaging factors; therefore, the obtained results give an optimistic prediction for next generation fusion devices. (paper)

[en] 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.