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«Association EURATOM / IPP.CR I N S T I T U T E O F P L A S M A P H Y S I C S, v.v. i. ACADEMY OF SCIENCES OF THE CZECH REPUBLIC ANNUAL REPORT ...»

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Development of advanced probe for edge tokamak plasmas - Emissive and tunnel probes. We have found that applicability of the strongly emitting probe technique for the estimation of the space potential depends on ratio Te/TeW and n, where Te is temperature of plasma electrons, TeW is temperature of emitted electrons and n is plasma density. If the Te and TeW are comparable the plasma potential determined by the strongly emitting probe can be overestimated. If TeW can be neglected with respect to Te (hot plasma), the plasma potential determined by the strongly emitting probe is underestimated by approximately one kTe/e. In addition, a relatively thorough study of the electron saturation current variations at varying probe heating was performed in the cylindrical magnetron (Prague) and DP-machine (Innsbruck) plasma. Magnitude of electron saturation current variations were characterized – variations were more pronounced in case of shorter probes. In the reference various possible processes leading to electron saturation current variations were discussed. New experimental data were recently obtained that give better understanding of studied phenomena. We concluded that strongly emitting probe technique is applicable in low temperature plasma with Te~2–3 eV and plasma density n~10e16 m-3. Observed variations of the electron saturation current at varying probe heating can be ascribed from the most part to the probe surface cleaning and contamination in laboratory argon discharge plasma.

Development of millimeter-wave reflectometry methods for the measurement of edge piedestal plasma in tokamak COMPASS. IST/CFN Lisbon was agreed in 2006 to be the main supplier of the reflectometry system with the technical support of IPP Prague. Five individual reflectometers are supposed to measure the density profile. The advanced reflectometers allow an arbitrary frequency sweeping modes and therefore the reflectometers can be used as well as an experimental diagnostics for studies of plasma turbulence.

The realization of the reflectometry system was supposed in a close collaboration with IST/CFN during 2006 – 2009. Unfortunately soon it was realized that IST/CFN has problem with the manpower for the construction, manufacturing and installation of the system.

The workplan agreed in 2006 was delayed and consequently cancelled. Therefore, because of no contribution of IST/CFN in 2006/2007, it was decided to construct the Ka-band (26.5 – 40 GHz) reflectometer in IPP. The design of the reflectometer was made in a similar way to the reflectometers developed in IST/CFN. In 2007 most of microwave electronics was constructed and tested in parts. Development of control unit and control SW is in progress.

This reflectometer will be able to work with a quasioptical band-combiners and antennas. The control unit and control SW has to be develope by our own way.

High Resolution Thomson Scattering for COMPASS. For the TS system, several different systems are considered and relevant parameters were calculated for both edge as well as core plasma. A large trade survey looking for new technologies was carried out in order to find some new possibilities for the edge detection system realization. With regards to the required measured range of electron temperature and density and requirements for the temporal and spatial resolution, two possibilities were chosen as the most suitable: a use of the Nd:YAG laser at the second harmonic frequency and the Littrow spectrometer with the ICCD camera, or a use of this laser at the first harmonic frequency and the Littrow spectrometer with the CMOS camera for the core, and the spectral filters with avalanche photodiodes for the edge.

ANNUAL REPORT 2007 ASSOCIATION EURATOM/IPP.CR A multi-pass system can be taken into account in order to increase the laser power effectively.

The use of the Nd:YAG laser at the second harmonic frequency was investigated in details and corresponding calculations of the scattered and detected photons were performed for this particular case. An implementation of a multi pass system as a future upgrade was discussed.

Development of magnetic diagnostics and the feedback system for the COMPASS tokamak. Magnetic diagnostics: The dismantling of magnetic diagnostics on COMPASS was carried out during two missions of IPP staff in the Culham Science Centre. The existing documentation of magnetic diagnostics was checked and completed. The magnetic sensors position was identified and documented. Cables from magnetic sensors to corresponding

cubicles were disconnected. All equipment was transported to IPP Prague. Feed-back system:

Test of the integrators and the other electronic equipment (Waveform Generators, PID controllers) were done. The existing documentation was checked and completed. As the result, the existing analogue feedback system was found to be obsolete and not appropriate for our tasks. Therefore, it was decided not to restore it. Digital system under contract collaboration with the IST Lisbon, which is experienced in building similar systems for other Associations, was selected. Algorithms for feedback control will be generated by the MAXFEA code, developed by P.Barabaschi. Geometry, mesh, material, and circuits files were done for COMPASS. Hardware and control of the the digital system will be based on the ATCA technology standard; MIMO controllers, developed in IST, will be used. Three identical fast amplifiers are being built in the IPP for the feedback stabilization of the horizontal and vertical plasma positions. The circuit safety is designed for overcurrent, overvoltage and optical isolation protection inside the energizer that determines the amplifiers inputs.





COMPASS Control and data acquisition system design. An efficient operation of COMPASS in Prague requires a new CODAC (Control, Data Acquisition, and Communication) system, which is being developed jointly by the Associations IPP.CR and IST. The tokamak control involves several areas, each of different levels: 24 hours a day control of building infrastructure, interlock, central experiment control, and real time control during the experiment run. Each of the systems has different purposes and diverse features.

3. Development of Concept Improvements and Advances in Fundamental Understanding of Fusion Plasmas EEDF Measurements in the CASTOR Tokamak Using the First Derivative Langmuir Probe Method. Langmuir probes (LP) are widely used to provide local measurements of important plasma parameters like the plasma potential, the density of the charged particles or the electron energy distribution function (EEDF). The accuracy of LPs under adverse conditions, such as the presence of magnetic fields or high plasma temperature, is still being questioned. We report here the application of a new technique, the first-derivative method, for processing the electron part of the I-V characteristics measured in the CASTOR tokamak.

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. In the confined plasma we find the EEDF to be bi-Maxwellian. The results obtained are in good agreement with classical method (second derivative) usually used for LP data processing. Results from different methods of differentiating the I-V characteristics are also discussed.

Modelling of ITER Plasma Facing Component Damage and Consequences for Plasma Evolution Following ELMs and Disruptions. We have developed a unique tool to measure the plasma deposition into a gap between tiles. The experimental data confirm the Part II - PHYSICS numerical predictions we made with our 2D self-consistent numerical code. In the case of toroidal gaps, we have a quantitative and qualitative agreement. However, in the case of poloidal gaps, it is only qualitatively acceptable. The 2-sided deposition is confirmed, with the good order of magnitude. This set of experiments confirms nevertheless the understanding of the plasma deposition in tile gaps presented in [26].

Measurement of plasma flows into tile gaps. A two-dimensional cylindrical, self-consistent kinetic code for the tunnel probe calibration has been developed. For this purpose, we used an existing code in Cartesian coordinates. After benchmarking, we used the code to perform a set of simulations that cover the range of expected density and temperature in tokamak edge plasmas for characterizing the plasma deposition inside the tunnel probe. The electron temperature depends on the ratio of the current collected by the tunnel over the back plate and calibration curves has been performed.

Edge turbulence studies. Robustness of the combined magnetic probe for TJ-II, containing set of Hall probes and coils was enhanced preparing for its installation on a new TJ-II reciprocating probe drive.

Simulation of EC waves from overdense plasma. We have adapted our EBW code to obtain a crude estimate of the profile of the current driven by 2.45 GHz wave in the WEGA stellarator. We investigated the dependence of the current profile on the central magnetic field and the temperature of the suprathermal component present in the WEGA plasma. Significant effects of the suprathermal electron population on EBW propagation have been demonstrated.

Even a modes fraction of suprathermal electrons enables very efficient absorption. The parallel wave vector direction of the EBWs can be reversed during the propagation through the plasma. The driven current direction has to be reversed analogously. Such behaviour has been observed experimentally. The current density profile was, for the first time, measured by a small Rogowski coil. EFIT2006, a modern EFIT implementation in C++ and Fortran 95, has been successfully parallelized. Boost MPI libraries have been applied to enable convenient transmission of the complex C++ objects used in EFIT2006. Linear scaling of the computational power has been demonstrated. EFIT2006 has been installed to IPP Prague computers and benchmarked using MAST data, which were read remotely from UKAEA server through IDAM interface. Equilibrium reconstructions have been performed for COMPASS, applying experimental data from UKAEA database and predicted equilibria with ACCOME.

PIC simulations of the SOL in the presence of non-Maxwellian and/or supra-thermal particles. In collaboration with J. P. Gunn at CEA Cadarache, a quasineutral particle-in-cell code (QPIC) was developed for simulating ion and electron transport along magnetic field lines in the Scrape off Layer (SOL) of the tokamak plasma edge. The code has been recently validated by comparison of its results with a known solution of a kinetic problem, namely that of the Mach probe in a strongly magnetized plasma. [67]. Last year's effort on QPIC included an extension of the code to a SOL configuration with non-floating walls allowing a non-zero SOL current. First results were reported at the 2007 EPS conference [68].

Simulation of processes in high-temperature plasma toward better interpretation of experimental data. The work in 2007 covered two main areas: (i) Numerical investigations of plasma parameters in COMPASS tokamak and (ii) Preparation of 3D particle code of magnetized plasmas. In frame of the first part numerical investigation of plasma parameters in COMPASS tokamak was performed and presented in the 34th EPS Plasma Conference on Plasma Physics in Warsaw, Poland. The plasma parameters in the device were analyzed in the ANNUAL REPORT 2007 ASSOCIATION EURATOM/IPP.CR frame of the self-consistent model of central plasma and edge region. The possibility of achieving high recycling and detached regimes in COMPASS divertor was discussed.

In frame of the task preparation of 3D particle code of magnetized plasmas there was developed during 2007 the self-consistent fully 3D Particle-In Cell code for modeling of plasma-solid interaction with new Poisson solver based on direct LU decomposition combined with multigrid approach. The model was tested on a cylindrical cavity geometry, i.e. on a hollow cylindrical chamber opened to the plasma, in both collisionless and collisional plasmas (see Fig. 2). The basic studied feature was the influence of non-axial orientation of magnetic field – this parameter is important for the analysis of experimental data from the Katsumata probe. Earlier version has been accepted for publication in the Vacuum journal.

PIC simulations of the ball-pen probe. The first systematic PIC modeling (using the XOOPIC code) of the Ball-pen probe has shown that the floating potential of the probe is varying with respect to the ration R of the electron and ion saturation currents. This is in agreement with the Langmuir probe theory and the systematic measurements on CASTOR tokamak (2005, 2006). Moreover, the simulation also confirmed that the Ball-pen probe can reach R to be equal to one. Then, the corresponding probe potential is assumed as the plasma potential, which is expected from the theory. However, that probe potential was not equal to the value obtained directly in 3D potential profile in PIC simulation. This discrepancy might be caused by wrong determination of the unperturbed plasma potential in PIC simulation.

The PIC modeling did not provide any explanation why the strongly magnetized (B = 1 T) electrons are detected inside the ceramic shielding tube in the measurements with Ball-pen probe.

Adaptation of CRONOS for COMPASS simulations. Currently, transport modeling of the tokamak COMPASS is performed using the ASTRA and CRONOS codes. The ASTRA and CRONOS codes solve coupled diffusion equations for heat, matter, and magnetic field (current). These partial differential equations are coupled to a magnetic field equilibrium code and modules for LH and NB injection. Such codes thus allow fully self-consistent calculations of tokamak operation, and are highly desirable for the study of COMPASS operation with the planned LH and NB systems. During the mobility stay of M. Stránský at CEA Cadarache in April 2007, the CRONOS transport code was acquired and adapted for the COMPASS tokamak, and installed in Prague by the CEA staff in November 2007. In addition, during the visit of Irina Voitsekhovitch in September 2007 a robust transport model for COMPASS was developed for the ASTRA code, including the possibility of external heating, so now transport simulations and comparisons are possible on two transport codes. In December 2007, Y. Peysson of the CEA staff has also made available to us the “Starwars” suite of lower hybrid (LH) toroidal ray-tracing (C3PO) and 3-D Fokker-Planck (LUKE) codes, which can simulate LH heating and current drive in a stationary state.



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