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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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The 8 Hall sensors of A1322LUA type produced by Allegro MicroSystems, Inc. were mounted on a stainless steel ring symmetrically encircling the CASTOR plasma in poloidal direction 10 mm outside the limiter radius. The Hall sensors were oriented such that they measure the horizontal and vertical magnetic fields at four locations (top, bottom, high field side, and low field side). The special adjustable holders were used in order to ensure proper alignment and consequently to minimize the cross-talk from the toroidal magnetic field. The sensors have a nominal sensitivity of 31.25 mV/mT and dynamic range ±80 mT. The peak-to-peak noise level is below 1mT. The reasonably flat frequency response was achieved in the range DC-10 kHz. The operating temperature range is from -40°C to 150°C. A supply voltage of 5V is needed to drive each Hall sensor. More details on operational aspects of these Hall sensors can be found in [1].

We exploited the above described system of 8 Hall sensors to get further insight into vertical plasma position measurement on CASTOR. In previous years there was observed a systematic disagreement between CASTOR vertical plasma position measurements using the standard Fig. 2. Left panel: comparison of vertical plasma displacement evolutions deduced from: a coil pair (black line), Hall sensors pair (blue), Hall sensors pair corrected for presence of stray magnetic fields Bext (red), and rake of Langmuir probes (black +). Right panel: vertical plasma displacement as a function of hardware CASTOR feedback system switch Z obtained as the simple differential signal of a pair of Hall sensors (blue) versus the same differential signal but corrected for presence of Bext.

approach of a pair of magnetic coils placed at the top and at the bottom, inside the vacuum vessel and other available diagnostics (rakes of Langmuir probes, bolometers) [2]. The differential signal of the coils pair is used to drive the CASTOR vertical plasma position feed-back system [3]. The vertical plasma position deduced by this method is rather centred for most of the CASTOR operating regimes. In the contrary, measurement of separatrix position performed by a radial rake of Langmuir probes suggests significant downward shift of the plasma column. In these experiments the radial rake of Langmuir probes is introduced into the CASTOR edge plasmas from the top and the separatrix position is identified with the measured location of maximum in floating potential profile.

Explanation of this discrepancy was proposed, suggesting that the magnetic measurements have to be corrected for pick-up of stray magnetic fields induced by external tokamak windings. The present set-up of Hall sensors on the full poloidal ring allows measuring of these stray magnetic fields separately during plasma discharge. Consequently, it was possible to correct the evaluated signal of the vertical plasma position (see fig. 2). It is clearly seen in figure 2 (left panel) that a

Part II - PHYSICS

rather good agreement was achieved in determination of plasma vertical displacement between magnetic diagnostic (Hall sensors) and rake of Langmuir probes after elimination of signal proportional to stray magnetic fields. Figure 2 (right panel) presents example, how the dependence of the vertical plasma displacement on setting of the Z switch changes after application of above described correction. The Z switch (positions 0 - 12) is a knob on the CASTOR vertical plasma position control system used to pre-define the desired vertical plasma position before each CASTOR shot.

Installation of ex-vessel Hall probes on JET within JET EP2 enhancement project The project aims for enhancement of the existing JET ex-vessel magnetic diagnostics system, by installing new sensors, capable of measuring the magnetic field both directly via sets of 3D Hall sensors and also by integrating voltages of coils which are attached to all the Hall sensors. The main rationale behind this project from the JET point of view is to provide additional data to enhance the database for iron core modelling and to improve equilibrium reconstruction. From ITER point of view, it is extremely important to test performance of these ITER candidate sensors under fusion neutron spectrum and also to gather experience from operating them at the large tokamak facility like JET. The project is led by Ass. ENEA, the probes and electronics are provided by MSL, Lviv Polytechnic National University, Ukraine. The Association IPP.CR participates on testing, installation, and the high level commissioning of the system on JET including analysis and assessment of the measured signals.

Fig. 3. One of the JET EP2 ex-vessel Hall probes, developed by MSL Lviv Ukraine, in various stages of manufacturing process.

Hall probes on Tore Supra High temperature resistant (200°C) Hall probe developed by MSL Lviv, Ukraine in collaboration with IPP Prague, was installed in-vessel on Tore Supra tokamak behind the poloidal antennas protecting limiter. Special electronics driving this probe allows precise measurement of magnetic fields in the frequency band 0-250 kHz.

References:

[1] G. Van Oost et al., Nuclear Fusion, 47 (2007), 378-386 [2] J. Sentkerestiová et al., Czechoslovak Journal of Physics, 56, D2 (2006) [3] M. Valovič, Czechoslovak Journal of Physics, B38 (1988)

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In collaboration with:

R. Schrittwieser, C. Ionita, P. Balan, Innsbruck University, Association EURATOM-ÖAW

Emissive probe was in focus of the investigations in 2007. Two subjects were investigated:

a) applicability of strongly emitting probe technique in the low temperature plasma; b) variations of the electron saturation current collected by probe at varying probe heating. Applicability of the strongly emitting probe technique depends on ratio of temperature of the emitted electrons and plasma electrons in case of low temperature plasma. Experimental data were compared with theoretical model.

Construction of the emissive probe used in experiments is depicted in Fig. 1. The probe wire was made from tungsten or thoriated tungsten with diameter typically d = 150 µm in present experiments. Electrical contact between the probe wire and feeding line was realised by fine copper wires that were tightly bounded around the probe wire. This construction ensured excellent electrical contact and relatively easy probe preparation. Probe was placed into the double bored Degusit tube. Experiments were performed on the cylindrical magnetron system described e.g. in [1,2].

We have found that applicability of the strongly emitting probe technique for the estimation of the space potential depends on 3695 K - melting point of tungsten ratio Te/TeW and n, where Te is temperature of 5 plasma electrons, TeW is temperature of emitted electrons and n is plasma density. This is 4

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In collaboration with:

M. Manso, A. Silva, P. Varela, L. Cupido, Association EURATOM- Instituto Superior Technico / Centro de Fusão Nuclear, Lisbon This is for the Activity Report 2007, part of the construction of the microwave reflectometry system for Compass tokamak. IST/CFN Lisbon was agreed in 2006 to be the main supplier of the reflectometry system with the technical support of IPP Prague [1]. The reflectometry system for Compass will be mainly designed to perform relevant plasma density profile measurements in the pedestal region. 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 [2].

The realization of the reflectometry system was supposed in a close collaboration with IST/CFN during 2006 – 2009 [1]. In the new timetable it is supposed that all parts of the reflectometry system will be constructed, assembled and tested by IST/CFN in Lisboa in 2008 - 2010, with the support of microwave engineers from IPP.CR.

In November 2007 the detailed design of the reflectometry system was agreed in Lisbon including the new timetable and cost estimation. The horizontal 8/9 Compass vessel port will be dedicated for the reflectometry system. The port has the inner diameter 150 mm and length of about 200 mm. The port is too small for the placing of a set of horn antennas into the vacuum vessel. Moreover one of NBI beam is crossing this port. Therefore the external antenna system for the reflectometry is more suitable. Then antennas are placed in front of the port. The port is equiped with a proper vacuum window transparent for microwaves (e.g. quartz window). The frequency combiners and quasi-optical antennas are necessary to transmit and receive all channels (O-mode K, Ka, U and E frequency bands and one X-mode Ka frequency band) in a tokamak port. A band-combiner combines the four O-mode beams and on X-mode beam into the one transmitting beam. The same combiner splits them from the receiving beam. Similar system has been developed for JET [4]. Due to the complexity of such a device, a suitable provider will be seeked.

Transmiters and receivers for the all bands are individual. Only the Ka-band the O-mode and Xmode signal will be splitted from a common source. The block scheme of the electronics for the K band is in Fig. 1. Other bands have the same scheme in principle, only different voltagecontrolled oscilators and frequency multiplication.

The original workplan of the collaboration agreed for 2006 was delayed. Therefore, the workplan for IPP was reformulated and the construction of the Ka-band (26.5 – 40 GHz) reflectometer was started at the beginning of 2007 in IPP. The design was made in a similar way to the reflectometers developed recently in IST [2,3]. Most of microwave electronics of this reflectometer was constructed and tested in parts in 2007. Development of control unit and Part II - PHYSICS control SW is in progress. This reflectometer will be able to work with the suggested quasioptical band-combiners and antennas. The control unit and control SW is developed by our own way.

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References:

[1] Framework Agreement for Scientific Collaboration between Institute of Plasma Physics Association Euratom/IPP.CR and Centro de Fusão Nuclear, Instituto Superior Técnico Association Euratom/IST [2] L.Cupido et al., Frequency hopping millimeter wave reflectometer, Review of Sci. Instr., 75 (10), 2004 [3] A.Silva at al., Ultrafast broadband FM-CW reflectometry system to measure density profiles on Asdex-U, Review of Sci. Instr., 67 (12), 1996 [4] L.Cupido et al., New millimeter-wave access for JET reflectometry and ECE, Fusion Engineering and Design 74 (2005) 707–713

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In collaboration with:

M Petrov, Atomic Physics Department A.F.Ioffe Physical-Technical Institute, St Petersburg Fast neutral atoms escaping from the COMPASS plasma will be analyzed by means of the neutral particle analyzer, which allows simultaneous analysis of hydrogen and deuterium atoms in the energy range 0.25-100 keV. Current status of this diagnostics is reported.

The energy spectra of fast atoms will be measured by the neutral particle analyzer (NPA) ACORD 12. This analyzer was constructed by the Atomic Physics Department of the A.F.Ioffe Physical-Technical Institute in St. Petersburg, Russia and exploited for measurement of the ion temperature on COMPASS in Culham. The principal scheme of the analyzer is shown in Fig. 1 together with main elements. The picture of the NPA taken in Culham is shown in Fig. 2.

Fig. 1 Major components Of the NPA: (L) vacuum shatter; (Sent) “step like” entrance slit;

(Co) “cleaning” electrostatic condenser, removing charge particles from the flux; (G) stripping cell chamber; (Sstr) stripping chamber slits; (M) electromagnet; (C) analysing electrostatic

condenser;(D1 - D24) two rows of detectors;(IS) Fig. 2. Some of the major elements labeled are:

auxiliary ion source for NPA testing; (H0, D0) (A) NPA; (B) vacuum pump; (C) flight line atoms emitted by plasma; (H+, D+) secondary connecting NPA with tokamak vacuum vessel ions.

Some characteristics of the NPA are shown in Tab 1.

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Fig. 3. Pictures of some elements of the NPE (from left to right): Detector unit, view of the electrostatic analyzer, the analyzing magnet.

It was concluded that the status is satisfactory and ready for measurements soon after having a stable and well defined plasma. Just small refurbishment and connection to the central CODAC is required. It was decided that the NPA will be exploited for analyses of fast neutral atoms after installation of the neutral beam injection (NBI) for ion heating, which is scheduled for the first half of the year 2010. Therefore, installation of the NPA is planned on beginning of 2010.

Future collaboration with the A.F.Ioffe Physical-Technical Institute in the re-installation of the NPA on COMPASS is envisaged. Our colleagues will assist in the calibration of the NPA by means of the beam of potassium ions. Furthermore, the numerical code MC-DOUBLE, developed at Ioffe Institute St. Petersburg, will be purchased and modified to COMPASS geometry. This code is required for detail interpretation of experimental data.

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In collaboration with:

team of the Department of Optical Diagnostics, IPP AS CR, Turnov.

The use of the Nd:YAG laser at the second harmonic frequency was investigated and corresponding calculations of the scattered and detected photons were performed for this particular case. Calculations of the parameters of the detection system are being performed.

Calculations of photon number A key parameter for the design of the TS detection system is the number of scattered photons. For the edge TS diagnostic this parameter is essential. Software for estimation of this value for arbitrary radial profiles of Te and ne was developed. As an input parameter for current calculations, the radial profiles of Te and ne measured on the COMPASS tokamak in the Culham laboratory (edge TS) are used. Number of collected photons in each spatial and wavelength channel for the edge TS is shown in figure 1.

Number of scattered photons in each spatial and wavelength channel

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