Assmus, D.

Kasparek, W.

Keil, D.

Laqua, H.

Lischtschenko, O.

Marsen, S.

Otte, M.

Schubert, M.

Wagner, F.

Warr, G.

English

MPG.PuRe

New 28 GHz Plasma ECR Heating System for the WEGA Stellarator

New 28 GHz Plasma ECR Heating System for the WEGA StellaratorG. B. Warr1, H. P. Laqua1, D. Assmus1, W. Kasparek2, D. Keil1,O. Lischtschenko1, S. Marsen1, M. Otte1, M. Schubert1, F. Wagner11 Max-Planck-Institut für Plasmaphysik, EURATOM Association,TI Greifswald, D-17491 Greifswald, Germany2 Institut für Plasmaforschung, Universität Stuttgart,Pfaffenwaldring 31, D-70569 Stuttgart, GermanyIntroduction15kW 28 GHz HeatingInterferometerFast reciprocating Langmuir probeCoherence Imaging spectrometerHelical coilsToroidal field coilsWEGA – Top ViewFigure 1: Location of the heating systemand diagnostics on the WEGA stellarator.WEGA is a medium sized classical l = 2, m = 5stellarator operating at IPP Greifswald since 2001[1]. A 28 GHz Electron Cyclotron ResonanceHeating (ECRH) System has recently been in-stalled on WEGA for plasma production and heat-ing studies at 0.5 T magnetic fields. The ECRHsystem generates centrally peaked plasma densityand temperature profiles that are required for basicplasma physics studies and for evaluation of a newHeavy Ion Beam Probe diagnostic.In water load tests, the Gyrotron has produced up to ∼ 15 kW RF output power. The systemhas since been set up for initial second harmonic extraordinary (X2) mode heating of theWEGA28 GHz Gyrotron (CPI VGA-8028)0.1›10 kW CW15 kW 10s20 kW 1s80 dBDirectionalCouplerTE02-TE01ConverterDC Break & Mode FilterTE01-TE11ConverterHE11 WaveguideHE11UptaperTE11-HE11ConverterVacuumWindowWEGAVacuumVesselArcDetectorArcDetectorCorrugatedHE11 AntennaForwardPowerDetectorReflectedPowerDetector4.3 mSmoothWaveguideTE02 TE01TE11TE01UptaperTE11 HE11HE11 HE11Ø41 mmØ63.5 mmØ44.45 mmØ63.5 mmØ32.6 mmØ32.6 mm Ø32.6 mmFigure 2: Schematic layout of 28 GHz ECRH system for initial experiments. Measurements ofthe waveguide mode profiles are also shown.33rd EPS Conference on Plasma Phys. Rome, 19 - 23 June 2006 ECA Vol.30I, P-2.122  (2006)plasma with an expanding Gaussian beam launched from an HE11 antenna, as shown in Fig. 2.First results of these experiments are presented.Results and DiscussionInitial Ar discharges were made with the magnitude of the magnetic field on axis |B0|= 0.5 T,rotational transform ι/2pi = 0.2 and gas fill pressure pfill = 3.0×10−6 mbar.T i(eV)Ar+Intensity (au)Z (cm)Z (cm)Figure 3: Ar+ 488.0 nm light emission in-tensity and ion temperature Ti during Gy-rotron plasma heating for |B0|= 0.5 T.Ion temperature and emission profiles, mea-sured with the Coherent Imaging interferenceSpectrometer [2], are shown in Fig. 3. Note thehigh values of Ti at the edge of the plasma are notrealistic as the ion emission is too low for reliablecalculations to be made.Profiles of electron density (ne), temperatureof thermal electron population (Tes) and temper-ature of fast electron population (Te f ) determinedfrom Langmuir probe measurements are shown inFig. 4. The electron density profile is normalisedto match the line-integrated density measured bya 3.7 mm wavelength interferometer that probesthrough the centre of the plasma. The Langmuirprobe is swept from−100 V to+50 V in 256 stepswith a dwell time of 60 µs per step, while the probe is moved into the plasma over a ∼ 3 s pe-riod. Individual points in the density and temperature profiles are determined by fitting a twotemperature model [3] to the average of five measured Langmuir probe characteristic curves.The Ti profile is centrally peaked, with Tmaxi = 5 eV, before the fast reciprocating Langmuirprobe enters the plasma at 15.5 s. The change in the light intensity at 18 s is due to the probeholder entering the edge of the plasma. When the probe is removed from the plasma at 18.5 sthe Ti profile recovers its centrally peaked profile.Heating the electrons with the Gyrotron RF power generates a fast electron population.Through collisions, this generates the slow thermal electron population and heats the ions. Thecentrally peaked ion temperature profile indicates the electron density profile is also centrallypeaked, as the electrons are the only source of heating. As shown in the time trace of the ion tem-perature profile, the Langmuir probe causes a flattening of the profile when it is inserted into theplasma for measurements. Thus the electron density and temperature profiles determined withthe Langmuir probe are flat or hollow.33rd EPS 2006; G.B.Warr et al. : New 28 GHz Plasma ECR Heating System for the WEGA Stellarator 2 of 4700 720 740 760 780 8000.05.0x10161.0x10171.5x10172.0x10172.5x10173.0x10173.5x10174.0x10174.5x10175.0x1017ne (m-3 )Major Radius R (mm)Magnetic Axis Limiter700 720 740 760 780 8000246810T es (eV)Major Radius R (mm)Magnetic Axis Limiter700 720 740 760 780 800050100150200250300350T ef (eV)Major Radius R (mm)Magnetic Axis LimiterFigure 4: Profiles of electron density (ne),temperature of thermal electron popula-tion (Tes) and temperature of fast electronpopulation (Te f ) determined from Lang-muir probe measurements of the Gyrotronplasma.The magnitude of the magnetic field calculatedin the Gyrotron polodial plane for |B0| = 0.5 Tand ι0/2pi = 0.2 is shown in Fig. 5. The fieldvaries by at least 20% over the radial extent ofthe closed flux surfaces. Fig. 6 shows the reso-nance region for optimum heating of the electronsis rather broad, as can be expected from the largeplasma size and broad ECRH launching beam.Future plansA lens will be used to focus the Gaussian beamemanating from the end of the waveguide into thecentre of the plasma where, using ISS95 [4], av-erage electron temperatures < Te >= 50 eV andelectron densities ne < 5×1018 m−3 are expected.The system will be used to generate supra-thermal particles for fast particle confinementstudies in different magnetic configurations (var-ious ι) in the stellarator geometry. It can also beused for such studies in stellarator-tokamak hy-brids, when a power supply for the WEGA ohmicheating coils is installed.The ECRH system has been designed with po-tential future use for overdense OXB mode heat-ing of the WEGA plasma in mind, where anoblique launch arrangement is required. Here, us-ing ISS95, average electron temperatures< Te >=25 eV and electron densities ne > 1019m−3 are an-ticipated. This is ideal for wave physics studiesand to test W7-X divertor diagnostics.AcknowledgementsIt is a pleasure to acknowledge the skilful work of R. Gerhardt, N. Paschkowski and the IPPtechnical staff on the project. We are grateful to CIEMAT Madrid for the loan of transmission33rd EPS 2006; G.B.Warr et al. : New 28 GHz Plasma ECR Heating System for the WEGA Stellarator 3 of 4Figure 5: Magnitude of the magnetic fieldin the Gyrotron polodial plane for |B0| =0.5 T and ι0/2pi = 0.2. The colour bar scaleis in Tesla and indicative flux surfaces areshown.|B0| (T)¶Te(r=0) (arbitrary units)Figure 6: Variation of the electron tempera-ture in the centre of the plasma with chang-ing on-axis magnetic field magnitude. Thefunction is calculated from the square of theratio of the ion saturation current (from aLangmuir probe in the centre of the plasma)to the interferometer line integrated density.line components and to IPF Stuttgart for the design of new transmission line components.References[1] J. Lingertat et. al., 30th EPS Conference on Plasma Phys. & Contr. Fusion, St. Petersburg,7–11 July 2003, P-1.10 (2003)[2] J. Chung et. al., Plasma Phys. Control. Fusion 47 919–940 (2005)[3] K. Horvath et. al., Contrib. Plasma Phys. 44, 650–655 (2004)[4] U. Stroth et. al., Nucl. Fusion 36 1063–77 (1996)33rd EPS 2006; G.B.Warr et al. : New 28 GHz Plasma ECR Heating System for the WEGA Stellarator 4 of 4

New 28 GHz Plasma ECR Heating System for the WEGA Stellarator

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