Belapure, J.

Chlechowitz, E.

Glaubitz, M.

Hasinger, G.

Huber, B.

Krychowiak, M.

Laqua, H.

Marsen, S.

Müller, E.

Otte, M.

Schlosser, D.

Stange, T.

Wagner, F.

Weller, A.

Wolf, R.

Zhang, D.

English

MPG.PuRe

Diagnostic Development for Wendelstein 7-X at the WEGA StellaratorM. Otte, J. Belapure1, E. Chlechowitz, M. Glaubitz, G. Hasinger1, B. Huber1,M. Krychowiak, H.P. Laqua, S. Marsen, E. Müller, D.M. Schlosser2, T. Stange,F. Wagner, A. Weller1, R. Wolf, and D. ZhangMax-Planck-Institut für Plasmaphysik, EURATOM Ass., D-17491 Greifswald, Germany1 Max-Planck-Institut für Plasmaphysik, EURATOM Ass., D-85748 Garching, Germany2 PNSensor GmbH, Otto-Hahn-Ring 6, D-81739, München, GermanyIntroduction WEGA is a classical five period, l = 2 stellarator with a major radius of R =72cm, a minor radius of the vessel of r = 19cm and a maximum plasma radius of a ≈ 11cm[1]. The magnetic field coil system allows quasi steady state operation at 0.5 T for about20 s. The plasmas are generated in helium, argon and hydrogen at 0.5 T operation utilizinga 28 GHz ECRH system (10 kW, cw). Additionally, a transformer for Ohmic heating andtwo 2.45 GHz magnetrons (26 kW in total, cw) are available. Maximum electron densitiesof ne = 1.4× 1019 m−3 and bulk electron temperatures of a few ten eV are routinely obtained.However, during Bernstein wave and lower hybrid heating experiments also a supra-thermalelectron component with an energy in the keV range and currents of a few hundred A can bedetected [2]. Within the last years an increasing set of contact-less diagnostics has been installedat WEGA in preparation for Wendelstein 7-X. In this paper an overview of these diagnosticsincluding selected results is presented.Microwave Emission Diagnostics A start-up diagnostic of W7-X will be the Electron Cy-clotron Emission (ECE) diagnostic to determine the electron temperature. The diagnostic willbase on a heterodyne system [3] where the second harmonic extraordinary (X2) cyclotron emis-sion from the plasma is mixed down to an intermediate frequency (IF) of 2 to 40 GHz. Forfrequency resolved measurements the IF is split into continuous frequency bands by two ra-diometer banks. One of them in the range of 18-40 GHz is used in combination with a low noisepreamplifier as standalone system to detect the cyclotron emission at WEGA [4]. Depending onthe discharge scenario the microwave emission spectrum is collected by two different horn an-tennas. The first one is aligned perpendicular to the magnetic field lines for considering the X2-emission being optically thin for WEGA plasmas. The second horn antenna is used at electrondensities greater than 0.97× 1019 m−3 i.e. above the corresponding second harmonic ordinary(O2) wave cut-off). At this over-dense plasma operation which is also planned to be realized at37th EPS Conference on Plasma Physics P5.125W7-X there is no X2-emission. Therefore, a determination of the electron temperature by thestandard ECE diagnostic will be impossible. However, the electrostatic Bernstein wave basedon a coherent motion of electrons around the magnetic field lines has no upper density limit andthus is able to propagate. By a two-step conversion process (BXO) at the plasma edge Bernsteinwaves are converted into a slow X-mode and then into an elliptically polarized O-Mode whichcan be detected from outside the plasma usable for electron temperature measurements. Sucha system is currently under development at WEGA. Since the conversion efficiency dependson the emission angle with respect to magnetic field vector the emission of the resulting O-mode shows a maximum under a certain angle which is 55◦ in the case of WEGA. For futureinvestigations of the angle dependency of XO-conversion and the determination of the conver-sion efficiency an optimized quasi-optical mirror system with astigmatic elliptic mirrors wasdesigned.Supersonic Helium Beam Diagnostic A supersonic helium beam diagnostic was setup al-lowing the determination of the central electron temperature and density from line ratio mea-surements. The helium neutrals are excited by collisional interactions with the electrons in theplasma. Due to the different density and temperature dependencies of the excitation and de-excitation rates some line intensity ratios can be utilized for the derivation of the plasma param-eters. For the temperature measurement the singlet transition 31S→ 21P (λ = 728.12 nm) andthe triplet transition 33S→ 23P (λ = 706.52 nm) were considered. For the density measure-ment additionally the mentioned singlet transition and a second singlet transition 31D→ 21P(λ = 667.82 nm) were chosen. Two different approaches have been applied to calculate theline intensity ratios using the collisional-radiative model for atomic helium and to determinethe electron temperature and density. In the steady-state approach the line intensity ratios arecalculated locally for a given set of temperature and density. In the time-dependent approach themovement of the beam through the plasma is additionally taken into account. The helium beamis injected via a solenoid valve (orifice diameter of 0.8 mm) in vertical direction through thecenter of the plasma with a velocity of v = 1700 m/s and a pulse length of 200µs. A skimmerwith 2 mm in diameter generates a collimated neutral beam with a on-axis particle density ofnHe = 4×1018 m−3. With the help of the diagnostic the intensities of the mentioned lines couldbe detected in helium plasma applying a X2 heating scenarios with the 28 GHz ECRH. Thecalculated temperature was about 30 eV at the plasma edge and about 80 eV in the center. Thederived plasma density was about ne = 1018 m−3. This in satisfying correspondence with resultsfrom Langmuir probes and microwave emission measurements.37th EPS Conference on Plasma Physics P5.125Magnetic Diagnostics With respect to investigations of plasma energy and plasma currentsa set of magnetic diagnostic was build up at WEGA. In order to measure the plasma currentboth an internal and external Rogowski coil were put into operation. An absolute calibrationhas been performed by means of a conductor carrying a current with known amplitude insidethe torus. The plasma energy can be determined by a system consisting of a compensation coiland a diamagnetic coil installed at a poloidal plane inside the torus. Furthermore, a two loopsystem, comprised of two additional diamagnetic loops of different diameter, was developedand integrated in the torus. Here, the diamagnetic effect can be determined from a change inthe ratio of both signals. Thus, it was possible to verify plasma energy measurements by usingtwo different and independent diagnostics. To perform the temporal integration of the signalsthe digital integrator designed for W7-X, was used. The integrator bases on a chopped inputstage and a digital integration [5]. Due to the polarity change of the input signal the drift of thefollowing electric components averages out when the signal is reconstructed. Thus, it does notcontribute to the integral signal anymore. The system allows precise measurements on a multi-minute time scale. In WEGA it was possible to measure plasma currents with a resolution of1 A for discharges of several seconds and plasma energies below 1 J.Bolometry A 12-channel bolometer and a fast 16-channel Absolute Extreme Ultra Violet(AXUV) diode array with a resolution of below 30 µs are routinely used. The lines of sight arecovering the whole poloidal plasma cross-section, hence the diagnostic is used to determine thetemporal development of radiation profiles and the total emitted power. Especially the AXUVsystem is a useful tool for the investigation of transient events and transport parameter. Withthe help of this diagnostic the plasma startup, the propagation of heat waves or the detection ofpower deposition zone are investigated. However, it was found that the AXUV diodes showeda strong decrease in the sensitivity in UV range below 400 nm hence the determination of theabsolute emitted power is an issue. The system was hardened against microwave stray radiationand shows now an improved electromagnetic compatibility.Soft and Hard X-Ray Diagnostics Although the bulk temperature of the electrons in WEGAdischarges is of the order of a few 10 eV a small component of supra-thermal electrons inthe keV-range could be detected at 0.5 T operation during OXB-mode conversion experimentswith the 28 GHz ECRH and lower hybrid heating scenarios with the additional non-resonant2.45 GHz heating system. For the detection of the supra-thermal electron component two newSilicon Drift Detectors (SDD), manufactured by PNSensor at the semiconductor laboratory of37th EPS Conference on Plasma Physics P5.125the Max-Planck-Institutes, were successfully applied. The first sensor SDD1 has a thickness of450µm and a on-chip collimator of 3.2 mm in diameter. It has a 8µm beryllium and 30 nm alu-minum layer on top to avoid visible light detection. The sensor is sensitive in the range between0.8 keV and 30 keV with a detector resolution of 139 eV at an energy of 5.9 keV (Mn Kα ). Asmentioned before, during the OXB heating scenarios in the helium plasma, a soft X-ray spec-trum in the range of 1.5−15 keV could be detected. For the analysis the detector response wastaken into consideration. The detector response as a function of energy was obtained using thedetector efficiency as well as the filters used in front. The continuum bremsstrahlung emissionmodel, folded with the above detector response function, is fitted to the observed spectra byexcluding the region of line emissions. The lines were fitted separately with single Gaussianson top of the continuum. The temperature of about 10.8 keV is derived from the slope of thecontinuum. This value is good agreement from the Bernstein emission experiments where alsoa radiation temperature in the keV-range during OXB heated plasma was detected. Out of the5 peaks 3 could be identified as Kα lines form argon, chromium and iron, respectively. Whileargon is a residual gas from previous experiments, iron and chromium are plasma impuritiesfrom the plasma vessel and the ECRH antenna system. Furthermore, a second sensor SDD3with an anti reflection coating for visible light, coupled to a CsI(Tl) scintillator crystal was usedfor hard X- and gamma ray detection. This detector is sensitive up to energies of a few 100 keV.During lower hybrid heating scenarios in helium plasma discharges an energy of up to 250 keVwas detected. The energy spectrum below 100 keV showed a strong decrease of the intensitycaused by the thickness of the stainless steel plasma vessel of 3 cm.References[1] M. Otte et al., AIP Conf. Proc. 993, 1 (2008)[2] H.P. Laqua et al., 18th Topical Conf. on Radio Frequency Power in Plasmas I8 (2009)[3] S. Schmuck et al., Fusion Engineering and Design 84 1739 (2009)[4] T. Stange et al., Stellarator News 120 3 (2009)[5] A. Werner, Rev. Sci. Instrum. 77, 10E307 (2006)37th EPS Conference on Plasma Physics P5.125

Diagnostic Development for Wendelstein 7-X at the WEGA Stellarator

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Diagnostic Development for Wendelstein 7-X at the WEGA Stellarator

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