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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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G. Bonheure,Association EURATOM-Etat Belge, ERM/KMS, Brussels, Belgium A. Murari, Association EURATOM-ENEA, Consorizio RFX, Padova, Italy M.Tsalas, Association EURATOM-Hellenic Republic, N.C.S.R. “Demokritos”,Greece S. Popovichev, Association EURATOM-UKAEA, Culham Science Centre, Abingdon, UK S. Conroy, Association EURATOM-VR, Uppsala University, SE-751 05 Uppsala, Sweden and JET EFDA contributors

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the recent ripple experiments [3]. The corresponding magnetic flux surfaces were uploaded from JET database of EFIT magnetic field reconstructions. These studies confirmed that the constraint is too restrictive for typical JET neutron emissivities which feature stronger radiation in the low field side. However, the Abel inversion can be used as a very sensitive indicator of assymmetries and/or as a tool for studies of symmetric neutron emissivities (e.g.

in low plasma density discharges).

A considerably more complicated but more fruitful constraint consists in introduction of an unisotropic smoothing into the standard 2D MFR algorithm. In this method, amplitude of reconstruction smoothing is set stronger in the direction tangent to the flux surface and weaker in the perpendicular direction. This new constraint was successfully implemented and preliminary Fig. 2. Tritium diffusion in results in transport analyses from the TTE campaign provide - for JET pulse 61372 (profile the first time - distinct quantitative values [4], see Fig. 2. In these evolution of the ratio of analyses, both D-D and D-T emissivities were reconstructed with the reconstructed D-T / D-D emissivities) shortly after newly implemented unisotropic smoothing, and the ratio of the two a tritium gas puff [4] emissivities – which reflects the fuel transport properties – was integrated in the poloidal direction in order to attenuate noise (statistical errors).

The MFR code is now ready for systematic studies of performance on simulated data, which will benchmark its features. Further minor improvements are foreseen, e.g. implementation of final width of viewing lines and of the up-down symmetry constraint.

MFR unfolding of the JET neutron spectra. The organic liquid scintillators known as NE213 provide relatively simple and robust neutron detection. Its pulse height spectrum carries also information on the energy of impacting neutrons, however, a proper detector calibration and unfolding technique is required. The MFR algorithm was successfully adapted to run the inversion process of the NE213 spectra unfolding with L-curve principle implementattion [5] and semi-automated application at JET.

In the last year, progress in the MFR unfolding was modest due to priorities given to the MFR tomography. The planned extension of the unfolding to the Stilbene scintillator detector was postponed, because the diagnostic was not ready for the spectral analyses: The n-gamma pulse separation was under development, and the detector response matrix was not known to the required precision. Instead, limited effort went to further extension of the MFR unfolding algorithm for the NE213 neutron detector, in particular the dependence of the photomultiplier gain factor on the neutron intensity was implemented.

References:

[1] G. Bonheure, et al., Nucl. Fus. 46 (2006) 725 [2] J. Mlynar, et al., 48th Annual Meeting of the DPP, Bull. Am. Phys. Soc. (2006) 196 [3] J. Mlynar, et al., 34th EPS Conference on Plasma Physics, Europhysics Conference Abstracts 31F (2007) P-2.129 [4] G. Bonheure, et al., to be presented at 35th EPS Conference on Plasma Physics (2008) [5] J. Mlynar, et al., FNDA 2006, Proceedings of Science PoS(FNDA2006)063 Part II - PHYSICS 2 Development of Plasma Auxiliary Systems

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Neutral beam injection system at COMPASS will present a new key element in the foreseen scientific programme, in particular with respect to the H-mode operation. Predictive simulations of the beam performance were focussed on the finalisation of the technical specifications for the system. Later in 2007, the specifications were agreed upon and implemented into detailed Call for tender for two neutral beam injectors for the COMPASS tokamak.

Predictive simulations of the planned neutral beam injection (NBI) system have been performed to support the final NBI design and other COMPASS modelling. ACCOME [1] is presently applied for predictive equilibrium calculations. The ACCOME Fokker-Planck NBI module was conceived for computational speed within ACCOME, which iteratively calculates a selfconsistent state between the tokamak MHD equilibrium and the inductive and non-inductivelydriven currents: bootstrap, lower hybrid, and neutral beam. The ACCOME MHD equilibrium module SELENE [2] solves the free-boundary Shafranov equation problem under very general boundary conditions. One specifies the dimensions, positions and turns of the poloidal field coils, and then selects a particular type of boundary problem. Either one can specify coil currents and find the resulting plasma shape, separatrix and possible X-points, or else one specifies a desired plasma shape, and calculates the resulting required coil currents. At the moment, we work with single X-point equilibria: SND and SNT (high triangularity) [3, 4]. These equilibria, determined by SELENE with or without NBI, are then also used for other COMPASS simulations (see the report of M. Stránský and V. Fuchs) and will be used for future, more detailed NBI simulations with FAFNER [5] or NUBEAM [6]. In turn, Astra [7] simulations provide temperature profiles for ACCOME and the stand-alone NB module FAFNER, and eventually also for NUBEAM.





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Toroidal current densities, computed by ACCOME for SND configuration with total plasma current Ip=170 kA, central toroidal magnetic field B0=1.2 T and co-injected NBI with total power 500 kW, are plotted in Fig. 1. It follows that NBI can drive a substantial portion of the total plasma current. However, ACCOME does not account for important loss channels – orbit losses, charge exchange losses. Moreover, the current drive calculation is considerably simplified. As a result, the NBI driven current can be significantly overestimated. This had been shown by calculations with the Monte Carlo code FAFNER [5], although for a slightly different equilibria, in [8]. Installation of an advanced Monte Carlo NBI code, e.g. FAFNER or NUBEAM, is envisaged to enable more detailed and realistic NBI simulations at IPP Prague.

Following the above modelling work, detailed technical specification of the NBI system for the COMPASS tokamak was finalised and agreed upon at the end of 2007. The 40 keV deuterium neutral beam with total power of up to 600 kW will be launched to plasma by two NBI systems.

The basic configuration – tangential injection in co-direction with respect to plasma current – is optimized for plasma heating. The aiming of both injectors can be shifted outside to achieve offaxis heating and current drive. For balanced injection both injectors will be located at the same port, aiming in co- and counter-current directions. Normal injection will be also possible, mainly for diagnostic purposes. Each injector must be able to operate at lower voltages (down to 20 keV), although at lower performance, and with hydrogen gas. Both units must be able to on/off modulate the beam with a minimum on-time of 20 ms and a minimum off-time of 20 ms using an external reference signal, with rise/fall time (from 10% to 90% beam power and viceversa) max. 2 ms. The longest time of continuous beam operation will be 300 ms. In order to minimise hydrogen leakage to the plasma the pressure at the beam duct to the COMPASS chamber must be kept below 10-2 Pa when the beam is operated, 2.10-4 Pa otherwise. Contents of heavy impurities must be kept below 1%, the beam energy and current accuracy has been specified to +/- 1% and 2%, respectively. The narrow vertical span of the COMPASS port in the tangential injection (70 mm) was pointed out as a major challenge so that performance tests with the port are required at the manufacturer’s site. Magnetic field parameters, available power supplies, cooling systems, vacuum connections and control system including the interlocks have been specified as well.

References:

[1] Laqua, H.P., et al., Bulletin of the American Physical Society, vol. 52, no. 16 (2007) 280 [2] Tani, K., Azumi, M., Devoto, R.S., Journal of Computational Physics 98 (1992) 332-341 [3] Bilykova, O., et al., Czechoslovak Journal of Physics 56 (2006) B24-B30 [4] Fuchs, V., et al., 33rd Annual European Physical Society Conference on Controlled Fusion and Plasma Physics (2006) P-1.103 [5] Lister, G.G., IPP Report 4/222 (1985) [6] Pankin, A., et al., Computer Physics Communications 159 (2004) 157-184 [7] Pereverzev, G.V. and Yushmanov, P.N., IPP Report 5/98 (2002) [8] Urban, J., et al., Czechoslovak Journal of Physics 56 (2006) B176-B181

In collaboration with:

V. Igochine, Association EURATOM IPP, IPP Garching, Germany M. Berta, A. Szappanos, Association EURATOM HAS, KFKI-RMKI Budapest, Hungary 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.

A poloidal section of the diagnostic ports of the COMPASS tokamak suitable for fast spectroscopic detectors such as fast AXUV-based bolometers, semiconductor SXR detectors and visible radiation monitors was chosen with respect to a presence of the two NBI heating systems in the vacuum vessel and a possibility to realize tomographic reconstructions of the selected radiation ranges. The small size of the allocated ports, a large number of the detectors inside them and requirements for cooling during a vessel baking led to an optimization in a choice of the detectors and to an integrated port design. A high temporal resolution in a combination with a good spatial resolution, namely in the pedestal region, will be reached by the AXUV20-ELM (IRD Inc.) based arrays of bolometers and LD35-5T-JET-Windowless (Centronic) arrays of the SXR detectors located inside the diagnostic ports, see Fig.1. Visible light observations will be realized using a pinhole camera connected to optical fibers and a spectrometer/photomultipliers located in a diagnostic room.

Fig.1: On the left, there are the six AXUV20-ELM (IRD Inc.) based 20-channel arrays of fast bolometers equipped with the ceramic sockets. On the right, there are the four LD35-5T-JETWindowless (Centronic) 35-channel arrays of the soft X-ray detectors.

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A cooperation with HAS Association on the implementation of the fast visible camera on COMPASS has been started and the first model of the EDICAM camera with an exposition up to 20 microseconds was tested on the CASTOR tokamak in 2007, see Fig.2.

Fig.2: Sequences of the 1280x1024 pixel frames taken by the EDICAM camera during CASTOR tokamak shots with 500 µs (upper frames, original colors) and 20 µs (lower frames, rescaled colormap) exposition time.

Dynamically rescaled colormaps are used to visualize details, namely for exposition time shorter than 100 µs.

In collaboration with:

I. Bolshakova, R. Holyaka, V. Erashok, MSL, Lviv Polytechnic National University, Ukraine A. Quercia, Association EURATOM-ENEA CREATE P. Moreau, Association EURATOM-CEA 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.

This technique is well established for many applications in experimental physics as well as industry, although it is rarely implemented in the fusion plasma physics. Therefore, besides the tests of radiation hardness of the ITER candidate Hall sensors, the experience with their use in tokamak environment must be gathered. Although, principally aimed for steady state applications, Hall sensors offer some advantages over magnetic coils also for present pulsed fusion devices. It is mainly their smaller size and direct relation of the measured signal to the magnetic field. The frequency response is typically limited to few tens of kilohertz. Several experiments dedicated to testing of various types of Hall probes were done and are being prepared on CASTOR, JET, and Tore Supra.

Poloidal ring of integrated Hall sensors on CASTOR Advancements in semiconductor technology hand in hand with a broad spectrum of industrial applications have driven development of new types of Hall sensors for magnetic measurements in recent years. A particular advancement is the availability of ‘integrated’ Hall transducers, where the sensing element together with the complex electronic circuitry is integrated on a single small chip with characteristic dimension of a few millimeters. The on-chip integrated circuits provide Fig. 1. The ring of up to 16 Hall sensors, 16 coils and 96 Langmuir probes before its installation on CASTOR tokamak (left panel). Detailed view of the Hall sensors attachment system with a single Hall sensor shown before its installation on the ring (left panel).

stabilization of the supply voltage, output voltage amplification, signal conditioning in order to suppress the high frequency noise, and elimination of temperature dependence of the sensor’s output. Because of the widespread industrial use of such sensors, their cost is rather low (of the ANNUAL REPORT 2007 ASSOCIATION EURATOM/IPP.CR order of 1 Euro/piece). We have performed the first tests of this type of Hall sensors in a tokamak in-vessel environment on CASTOR tokamak (R/a=0.4m/0.085m, Ip=10kA, BT=1T, ne=1019m-3).



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