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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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PROJECT NO. 24: CRONOS for COMPASS Objective: To adapt the CRONOS code for COMPASS simulations. CRONOS simulations of COMPASS operation with lower hybrid (LH) and neutral beam (NB) heating and current drive.

! CEA

Collaborating Associations:

PROJECT NO. 25: Core-edge code for COMPASS Objective: To develop a new modelling tool, a version of a self-consistent core-edge code for the COMPASS tokamak, 0D in the core and 1D in the SOL.

! IPPLM

Collaborating Associations:

PROJECT NO. 27: Nuclear data for IFMIF Objective: Measurements of activation cross sections at neutron energies below 35 MeV for nickel.

Neutron irradiation and cooling measurement of Ni samples, evaluation of data for activation crosssection. Comparison with IFMIF activation data file.

! FZK

Collaborating Associations:

PROJECT NO. 28: Laser induced removal Objective: Laser-induced removal of fuel and co-deposits from plasma facing comonents in tokamaks and characterisation of laser-irradiated surfaces. Study of samples influenced by laser ablation by various methods.

! IPPLM

Collaborating Associations:

PROJECT NO. 29: Transport and reinstallation of the COMPASS tokamak Objective: Transport of the COMPASS subsystems and parts. Design and test of the magnetic diagnostics and the feedback system for the COMPASS tokamak.

! UKAEA

Collaborating Associations:

PROJECT NO. 30: Turbulence studies Objective: Full Hamiltonian description of the interaction of particles with magnetic islands and edge plasma electrostatic turbulence. Anomalous diffusion of ions in the regimes of interchange instability and of the drift wabe turbulence.

! ÖAW

Collaborating Associations:

ANNUAL REPORT 2007 In the end of 2007 tokamak COMPASS was moved from Culham Science Centre to its new location in the Institute of Plasma Physics, Prague

–  –  –

II PHYSICS

The main areas of the research undertaken in the Association EURATOM/IPP.CR in 2007

were as follows:

1. Provision of Support to the Advancements of the ITER Physics Basis

2. Development of Plasma Auxiliary Systems

3. Development of Concept Improvements and Advances in Fundamental Understanding of Fusion Plasmas Here, the most important results, activities and achievements are briefly summarized; details are given after the list of publications. Notice that the major part of the Association activities in Physics relies on broad collaboration with other EURATOM Associations.

1. Provision of Support to the Advancements of the ITER Physics Basis

Measurements of the ion and electron temperature and of the plasma potential and diffusion coefficient in the edge plasma using the Katsumata type probe. The direct measurements of the plasma potential have been performed on ASDEX-U tokamak in June

2007. It has been used the design of the Ball-pen probe instead of the Katsumata-type probe due the better comparison with achieved results on CASTOR tokamak (2006, 2005). The Ball-pen probe head has been mounted on the reciprocating manipulator located at the midplane. The probe has been exposed in the edge plasma, a few millimeters outside the limiter shadow, for several times during the discharge. The measurements have been performed in the L-mode and H-mode. The data are analyzed to estimate also the electron temperature and the diffusion coefficient. It is not possible to obtain the values of the ion temperature from the I-V characteristics because this method is very sensitive to presence of the supra-thermal electrons, which modifies the shape of the measured I-V characteristics.

The Ball-pen probes has been also used for the direct measurements of the plasma potential and the electron temperature on toroidal device RFX in Padova (first test was in 2006). The Ball-pen probes measured the plasma potential in a low magnetic field Bt = 0.1 T, which is much lower than on ASDEX Upgrade (Bt = 2.5 T).The above mentioned experiments have shown that the Ball-pen probe is suitable for the direct measurements of the plasma potential and consequently of the electron temperature in different fusion facilities with wide range of the magnetic field.

ELM pace making. The ELM pacing experiments were realized on the ASDEX-Upgrade tokamak in several shots. As a biasing electrode, the midplane manipulator (MEM) with several carbon tips inserted into the massive carbon envelope was used. The first analysis of measured data shows a change of natural ELM frequency during a period of the biased MEM insertion and an occurrence of new smaller satellite ELMs. The data evaluation is in progress.

Determination of reflection characteristics of small hydrocarbon ions of low incident energies in collisions with room-temperature and heated carbon and tungsten surfaces.

Collisions of simple hydrocarbon ions CD3+, CD4+, and CD5+ with room-temperature carbon (HOPG) surfaces were investigated at low incident energies of 3–10 eV. Mass spectra, angular and translational energy distributions of the product ions were determined. From Part II - PHYSICS these data, information on processes at surfaces, absolute ion survival probabilities, scattering diagrams, and effective mass of the surface involved in the collisions was determined.





Incident ions CD3+ and CD5+ showed inelastic non-dissociative (CD3+) or non-dissociative and dissociative (CD5+) scattering, the radical cation CD4+ exhibited both inelastic, dissociative, and reactive scattering, namely occurrence of H-atom transfer and C-chain build-up in reactions with hydrocarbons present on the room-temperature carbon surface. The absolute survival probability, at 10 eV and the incident angle of 300 (with respect to the surface), was about 12 % for CD5+, and 0.3-0.4 % for CD3+ and CD4+•. It decreased towards zero at lower incident energies. Estimation of the effective surface mass involved in the collisional process led to m(S)eff corresponding roughly to the mass of one or several CH3- (or C2H5-) terminal units of surface hydrocarbons (accepted for publication).

Fragmentation of polyatomic cations and dications (C7Hn+/2+, n=6,7,8) with hydrocarboncovered surface was investigated over the incident energy range 5-50 eV to estimate the role of the projectile charge on energy partitioning in surface collisions (see the reference).

Surfaces relevant to fusion devices (plasma-sprayed tungsten, carbon fiber composite, Be) were bombarded by deuterated hydrocarbon ions of energies up to 100 eV. Sticking coefficients to the surface of one of the collision products, deuterium molecules, were determined using the nuclear reaction analysis combined with the Rutherford back-scattering method. Upon bombardment with CD3+ ions, the sticking coefficient of D2 to plasmasprayed tungsten (PSW) was found to be about 0.2, and 0.2-0.3 on carbon fiber composite (CFC), in both cases little dependent on the incident energy between a few and 100 eV.

Experiments and modeling of fast particle generation at LH and ICRF heating. A retarding field analyzer (RFA) was used during lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) experiments in the Tore Supra tokamak to measure the flux of supra-thermal particles emanating from the near field region in front of the antennas. The fast electron energy distribution of accelerated particles in the fast electron beam was determined.

The fast electron beam generated by the parasitic LH wave absorption extends for high PLH up to the LCFS and is up to 5 cm or even more radially wide, with possible corresponding radially broader heat loads in case that the beam impacts on the wall. The observed existence of the wide beam needs to be explained by theory, as the current theory predicts the radial width of the beam of several mm. The fast electron beam intensity very near to the LCFS decreases more quickly with decreasing PLH than the fast beam intensity near the LH grill mouth. As the comparison of the 1st and 2nd beam shows, the mean collector current (and therefore similar unwanted heat loads) in the two beams is comparable at higher PLH.

Modeling of direct LH wave ionization, development of EDGE2D code to 3D. We performed numerical modeling of near Lower Hybrid (LH) grill Scrape-off-Layer (SOL) plasma density variations as a function of gas puff and LH power with the fluid code EDGE2D. The gas puff ionization is difficult to model because it is a 3D problem, the antenna, first wall components and gas pipe have finite dimensions in the toroidal direction, but an amended 2D poloidal – radial model can provide useful insights. The code includes direct SOL ionization by the LH wave. The modeling shows that both puff and heating/ionization are important in raising the density in the far SOL, what is important for LH wave coupling in ITER like discharges. Although OMP (Outer midplane) seems to be the best location for gas puffing, the other two puff locations (near RCP – reciprocating probe, and at the top) also give an increase in far SOL density with heating. This is again an important information for ITER, where top and not OMP gas puffing is assumed. The modeling shows the flattening of the far SOL density profile, which is observed in experiments.

Cherenkov detectors for fast electron measurements. Measurements indicate that ANNUAL REPORT 2007 ASSOCIATION EURATOM/IPP.CR introduced modifications of the Cherenkov detection system enabled an electromagnetic interference to be reduced and a direct hard radiation to be eliminated completely. It was confirmed that the recorded Cherenkov signals were induced just by fast electrons ( 50 keV). On the CASTOR tokamak, the dependences of the fast electron signals on the radial position of the Cherenkov detector, as well as on the plasma density, plasma current and toroidal magnetic field value were investigated and explained. A connection between the Cherenkov emission and the hard X-rays, which were measured at a close vicinity of the Cherenkov detector, was found and discussed. The results of the statistical approach using a single-count analysis of the time-resolved Cherenkov data indicates possible transport mechanisms of fast electrons – the fast burst losses in combination with a slow diffusion. A question on an unclear origin of the hard radiation of energy approximately 3.9 MeV reaching the shielded detection system only during the tokamak discharges stays still unanswered.

Neutron data analysis on JET. The Minimum Fisher Regularisation (MFR) algorithm, adapted for tomography analyses of the JET neutron profile monitor, was further extended by the Abel inversion where neutron emissivities are constrained to constants on flux surfaces.

The upgraded algorithm was applied to studies of both fuel transport in TTE and recent data from JET campaigns C15-C19, see [65]. These studies confirmed that the Abel inversion constraint is not relevant for typical JET neutron emissivities with stronger radiation from the low field side. However, the Abel inversion can be used as a sensitive indicator of assymmetries. Following these results, the 2D MFR algorithm with unisotropic smoothing constrained by magnetic flux surfaces was implemented, tested on JET Neutron profile monitor data and optimised to the expected use in the fuel transport studies. This upgrade was used in quantitative analyses of Tritium transport, with results that have been submitted for publication. Concerning the MFR unfolding of the NE213 neutron detector data, the algorithm was slightly upgraded, in particular the dependence of the photomultiplier gain factor on the neutron intensity was taken into account.

2. Development of Plasma Auxiliary Systems

NBI for COMPASS-D. Predictive simulations of the beam performance were in 2007 focussed on the finalisation of the technical specifications for the system. Code Astra has been setup to use its internal NBI package. Besides that, it can use external outputs from the ACCOME code. Based on the extensive modelling, technical specifications were agreed upon at the end of theyear and implemented into detailed Call for tender for two neutral beam injectors for the COMPASS tokamak.

Spectroscopic diagnostics for COMPASS. Spectroscopic diagnostics on the COMPASS tokamak will cover a wide spectral range of the core and edge plasma emission aiming to realize a fast tomography at microsecond time scales (fast bolometry, soft X-rays, visible range). Additionally, the fast visible camera will be implemented on COMPASS in a cooperation with the HAS Association.

Performance analysis of ITER candidate Hall sensors in tokamak environment. Use of various configurations of flux loops for measurement of magnetic field in fusion devices is inherently limited by the pulsed operation of these machines. A principally new diagnostic method must be developed to complement the magnetic measurements in true steady state regime of operation of fusion reactor. One of the options is the use of diagnostics based on Hall sensors. Several experiments dedicated to testing of various types of Hall probes were done or they are being prepared on CASTOR, JET, and Tore Supra. On CASTOR, the Part II - PHYSICS measurements with a ring of integrated Hall sensors were further analyzed including the modeling of CASTOR vacuum field configuration [4, 13, 92]. Previous, in-vessel measurements with 3D Hall probe on CASTOR were published in [93]. Collaboration on preparation of a set of ex-vessel Hall probes for JET continued leading to delivery of the full set of probes to JET in the end of 2007 by MSL Lviv Ukraine [73,76,91]. A new high temperature resistant (up to 200 degC) Hall probe with measurement bandwidth DC-250 kHz was installed in in-vessel environment of Tore Supra in the end of 2007.



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