R. Dejarnac

Status of the COMPASS tokamak and characterization of the first H-mode

This paper summarizes the status of the COMPASS tokamak, its comprehensive diagnostic equipment and plasma scenarios as a baseline for the future studies. The former COMPASS-D tokamak was in operation at UKAEA Culham, UK in 1992–2002. Later, the device was transferred to the Institute of Plasma Physics of the Academy of Sciences of the Czech Republic (IPP AS CR), where it was installed during 2006–2011. Since 2012 the device has been in a full operation with Type-I and Type-III ELMy H-modes as a base scenario. This enables together with the ITER-like plasma shape and flexible NBI heating system (two injectors enabling co- or balanced injection) to perform ITER relevant studies in different parameter range to the other tokamaks (ASDEX-Upgrade, DIII-D, JET) and to contribute to the ITER scallings. In addition to the description of the device, current status and the main diagnostic equipment, the paper focuses on the characterization of the Ohmic as well as NBI-assisted H-modes. Moreover, Edge Localized Modes (ELMs) are categorized based on their frequency dependence on power density flowing across separatrix. The filamentary structure of ELMs is studied and the parallel heat flux in individual filaments is measured by probes on the outer mid-plane and in the divertor. The measurements are supported by observation of ELM and inter-ELM filaments by an ultra-fast camera.

Electron energy distribution function, plasma potential and electron density measured by Langmuir probe in tokamak edge plasma

The electron energy distribution function (EEDF) at different radial positions is derived from Langmuir probe measurements in the CASTOR tokamak edge plasma using the first derivative method. It is shown that the EEDFs are not Maxwellian but can be approximated as bi-Maxwellians with one dominant, low temperature electron population and one minority composed of hotter electrons. In the limiter shadow the measured EEDFs are Maxwellian. The values of the plasma potential and electron densities at different radial positions are also evaluated. The results presented in this paper demonstrate that the first derivative method allows one to acquire additional plasma parameters using the electron part of the current–voltage characteristics in strongly magnetized tokamak edge plasmas.

Supersonic gas injection on Tore Supra

Abstract A fueling system by supersonic pulsed gas injection has been installed on the high field side of Tore Supra. First results are encouraging, demonstrating a fueling efficiency four times higher than that of conventional gas puff. One-dimensional modeling shows that the increase of efficiency is linked to the short injection time and to the prompt cooling of the plasma edge consecutive to the massive injection of matter. Improvements of the system could lead to the formation of a high-β blob which could experience a drift down the magnetic field, analogously to pellet injection, thus further increasing the fueling efficiency of the method.

Advanced probes for edge plasma diagnostics on the CASTOR tokamak

Understanding of underlying physics in the edge plasma of tokamaks requires knowledge of the plasma density, potential, electron and ion temperature, ion flows and their fluctuations with a high spatial and temporal resolution. A family of electric probes, which have been designed and tested for this purpose in the CASTOR tokamak, is reviewed and examples of their performance are given. In particular, we focus on description of the 1D and 2D arrays of Langmuir probes for spatially resolved measurements of the edge turbulence, the Ball pen and emissive probes for direct measurements of the plasma potential, the optimized Gundestrup probe for measurements of parallel and perpendicular ion flow, and the tunnel probe for fast measurement of electron and ion temperatures. Additional information on individual diagnostics is available in the listed references. PACS 52.70.Ds

ELM-induced transient tungsten melting in the JET divertor

The original goals of the JET ITER-like wall included the study of the impact of an all W divertor on plasma operation (Coenen et al 2013 Nucl. Fusion 53 073043) and fuel retention (Brezinsek et al 2013 Nucl. Fusion 53 083023). ITER has recently decided to install a full-tungsten (W) divertor from the start of operations. One of the key inputs required in support of this decision was the study of the possibility of W melting and melt splashing during transients. Damage of this type can lead to modifications of surface topology which could lead to higher disruption frequency or compromise subsequent plasma operation. Although every effort will be made to avoid leading edges, ITER plasma stored energies are sufficient that transients can drive shallow melting on the top surfaces of components. JET is able to produce ELMs large enough to allow access to transient melting in a regime of relevance to ITER. Transient W melt experiments were performed in JET using a dedicated divertor module and a sequence of I-P = 3.0 MA/B-T = 2.9 T H-mode pulses with an input power of P-IN = 23 MW, a stored energy of similar to 6 MJ and regular type I ELMs at Delta W-ELM = 0.3 MJ and f(ELM) similar to 30 Hz. By moving the outer strike point onto a dedicated leading edge in the W divertor the base temperature was raised within similar to 1 s to a level allowing transient, ELM-driven melting during the subsequent 0.5 s. Such ELMs (delta W similar to 300 kJ per ELM) are comparable to mitigated ELMs expected in ITER (Pitts et al 2011 J. Nucl. Mater. 415 (Suppl.) S957-64). Although significant material losses in terms of ejections into the plasma were not observed, there is indirect evidence that some small droplets (similar to 80 mu m) were released. Almost 1 mm (similar to 6 mm(3)) of W was moved by similar to 150 ELMs within 7 subsequent discharges. The impact on the main plasma parameters was minor and no disruptions occurred. The W-melt gradually moved along the leading edge towards the high-field side, driven by j x B forces. The evaporation rate determined from spectroscopy is 100 times less than expected from steady state melting and is thus consistent only with transient melting during the individual ELMs. Analysis of IR data and spectroscopy together with modelling using the MEMOS code Bazylev et al 2009 J. Nucl. Mater. 390-391 810-13 point to transient melting as the main process. 3D MEMOS simulations on the consequences of multiple ELMs on damage of tungsten castellated armour have been performed. These experiments provide the first experimental evidence for the absence of significant melt splashing at transient events resembling mitigated ELMs on ITER and establish a key experimental benchmark for the MEMOS code.

Transport of electrons in the tunnel of an ion sensitive probe

Ion sensitive probes serve to measure the ion temperature in magnetized plasma. Such a probe typically consists of a collector submerged inside a hollow tube, which is oriented perpendicularly to the magnetic field. The principle of the probe is based on geometrical shielding of the ion collector from plasma electrons. According to the basic theory, when the collector is retracted in the tube, electrons with their small Larmor radii should not be able to reach it and the collector becomes sensitive to ions. However, experimental results show that the electron shielding is in general inefficient, it only works in the case when the potential of the collector is the same as the potential of the inside surface of the tube.This problem is investigated using a full 3D particle-in-cell Cartesian code with a fast multigrid Poisson solver. We simulate the plasma behaviour in the vicinity of a model of the ion sensitive probe. A positive potential structure is formed at the entrance of the tube due to the space charge of ions that gyrate inside. This structure produces E × B drifts, which push electrons into the shielded space. A stream of electrons hitting the collector is observed for various potentials of the collector. Simulations revealed that electrons can penetrate inside the geometrical shadow in all studied cases; however, they do not reach the collector when the potential of the collector is equal to the potential of the tube.

Study of ion sheath expansion and anisotropy of the electron parallel energy distribution in the CASTOR tokamak

A novel diagnostic, the tunnel probe, is used to investigate the edge plasma of the CASTOR tokamak. Comparison with conventional small, cylindrical Langmuir probes (typical of those usually employed for turbulence measurements in magnetized plasmas) demonstrates the superiority of the tunnel probe. The collectors of the tunnel probe are concave, eliminating in theory all uncertainty of the effective collecting area, and thereby rendering the measurement of parallel ion current density, electron temperature, and floating potential more reliable. Two tunnel probes, mounted back-to-back in a Mach probe arrangement, are used to investigate directional asymmetries of the plasma parameters. The tunnel probes are used as standard Langmuir probes by applying voltage sweeps simultaneously to all their internal conductors. The measured electron temperature is higher on the electron side than on the ion side, and the floating potential is lower. The observed asymmetries, measured at low density and collisionality in CASTOR, could be consistent with a hot tail of non-thermal electrons flowing in the counter-current direction. The ratio of ion saturation currents to the internal conductors of the tunnel probe provides a second independent measurement of electron temperature whose directional asymmetry is less pronounced, in agreement with recent theoretical predictions that tunnel probes should be less sensitive to non-thermal electrons than Langmuir probes in certain conditions.

Numerical evaluation of heat flux and surface temperature on a misaligned JET divertor W lamella during ELMs

A series of experiments has been performed on JET to investigate the dynamics of transient melting due to edge localized modes (ELMs). The experiment employs a deliberately misaligned lamella in one module of the JET bulk tungsten outer divertor, allowing the combination of stationary power flux and ELMs to transiently melt the misaligned edge. During the design of the experiment a number of calculations were performed using 2D particle-in-cell simulations and a heat transfer code to investigate the influence on the deposited power flux of finite Larmor radius effects associated with the energetic ELM ions. This has been performed using parameter scans inside a range of pedestal temperatures and densities to scope different experimentally expected ELM energies. On the one hand, we observe optimistic results, with smoothing of the heat flux due to the Larmor gyration on the protruding side of the lamella which sees the direct parallel flux-the deposited power tends to be lower than the nominal value expected from geometric magnetic field line impact over a distance smaller than 2 Larmor radii, a finding which is always valid during ELMs for such a geometry. On the other hand, the fraction of the flux not reaching the directly wetted side is transferred and spread to the top surface of the lamella. The hottest point of the lamella (corner side/top) does not always benefit from the gain from the Larmor smoothing effect because of an enhanced power deposition from the second contribution.

Particle-in-cell simulations of plasma interaction with shaped and unshaped gaps in TEXTOR

This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps in castellated plasma-facing components (PFCs). The point of interest was the test limiter of the TEXTOR tokamak, a PFC designed for studies of plasma–wall interactions, in particular, related to impurity transport and fuel retention. Simulations were performed for various plasma conditions in the vicinity of the castellated surface, where the gaps can be either shaped or unshaped. It was observed that depending on plasma parameters the transport of plasma particles inside the gap can be either in potential- or geometry-dominated regimes. The mechanisms responsible for the formation of a potential peak inside the poloidal gap and its consequences on plasma deposition profiles are discussed. A study of gap shaping was performed in order to validate its effectiveness.

Sensitivity of electron temperature measurements with the tunnel probe to a fast electron component

The tunnel probe is a new kind of Langmuir probe for fast dc measurements of ion flux and electron temperature in the tokamak scrape-off layer. The probe is calibrated using two-dimensional kinetic analysis of the ion current distribution on the concave conductors. Though qualitative agreement with classical Langmuir probe measurements was found, the electron temperature given by the tunnel probe is several times lower. One possible explanation might be an overestimation of the electron temperature by the Langmuir probe, due to a non-Maxwellian electron velocity distribution which can be modelled in a first approach as a two-temperature distribution. Hence the possible influence on the tunnel probe of a small population of nonthermal electrons is investigated by means of the two-dimensional kinetic code XOOPIC. It is found that this influence can be seen as the result of two combined physical effects: nonthermal electrons will reach the back plate (BP) and the ion current distribution over tunnel and the BP will change. The resulting dependence on probe bias and parallel ion current density of the TP sensitivity to nonthermal electrons is not reflected in CASTOR measurement results. Thus nonthermal electrons on their own cannot fully explain the discrepancy between Langmuir and tunnel probe measurements.