Anda, G.

Bald, K.

Birkenmeier, G.

Galdón Quiroga, Joaquín

García Muñoz, Manuel

Olevskaia, V.

Rivero Rodriguez, Juan Francisco

Sochor, M.

Stroth, U.

Viezzer, Eleonora

English

idUS. Depósito de Investigación Universidad de Sevilla

An imaging heavy ion beam probe diagnostic for the ASDEX UpgradetokamakJ.Galdon-Quiroga1, G.Birkenmeier1,2, V.Olevskaia1,2, M.Sochor1, G.Anda3, K.Bald1,M.Dunne1, M.Garcia-Munoz4, A.Herrmann1, K.Kaunert1, D.Nagy3, J.F.Rivero-Rodriguez4,M.Rodriguez-Ramos 4, V.Rohde1, E.Strumberger1, J.J. Toledo 4, E.Viezzer4,E.Wolfrum1, S.Zoletnik3, U.Stroth1,2 and the ASDEX Upgrade4 Team1 Max Planck Institute for Plasma Physics, Garching, Germany2 Physics Department E28, Technical University of Munich, Garching, Germany3 Wigner RCP, EURATOM Association HAS, Budapest, Hungary4 Centro Nacional de Aceleradores (CNA), University of Seville, SpainIntroductionA new diagnostic - the imaging heavy ion beam probe (i-HIBP) [1, 2] - for the simultaneousmeasurement of plasma potential, magnetic field and density fluctuations is being developedat the ASDEX Upgrade (AUG) tokamak. The diagnostic uses an ion beam in order to extractinformation of the plasma as in classical heavy ion beam probes (HIBP) [3], but uses an injectortechnology similar to those of beam emission spectroscopy (BES) diagnostics and a scintillatorplate as particle detector like in fast-ion loss detectors (FILD) [4]. The measurement princi-ple is illustrated in Fig.1 of reference [1]. A neutral beam of heavy atoms (Cs in this case) islaunched into the plasma with an energy of 70 keV. As this neutral primary beam penetrates theplasma, it is ionized leading to the generation of a fan of charged secondary beams which aredeflected by the tokamak magnetic field. The fan of secondary beams is collected by a scintil-lator plate placed behind the limiter shadow of the machine. The selected scintillator materialis the so called TG-Green (SrGa2S4 : Eu2+), widely used in other diagnostics such as FILDs.The light pattern emitted by the scintillator, which is imaged by a fast camera, contains radiallyresolved information about the density, potential and magnetic field in the edge and scrape-offlayer regions of the plasma. The use of a neutral primary beam allows to reduce the probingbeam energy compared to standard HIBPs, while the use of an in-vessel scintillator as detectionsystem makes the system more compact and improves the radial resolution of the system.The plasma density can be recovered from the intensity measurement, while displacementsof the light pattern - in the following strike-pattern - provide information about plasma poten-tial and magnetic field fluctuations. Magnetic field fluctuations, such as those expected due tochanges in the current density profile, lead to a shift of the whole strike-pattern, while fluctu-ations of the plasma potential lead to local shifts of the strike-pattern. This way the displace-46th EPS Conference on Plasma Physics P1.1017ments due to magnetic field and potential perturbations can be discriminated. The difference inthe time-scales of the phenomena to be investigated can also be used for discrimination.Beam modellingThe trajectories of the secondary beams have been calculated with the full-orbit followingcode FIOS [1], which can handle 3D electric and magnetic fields. It was found that the toroidalfield ripple should be included in the simulations since it can lead to a shift of the strike-patternof up to ∼ 3.5 cm, if an unappropriate toroidal injection position relative to the toroidal fieldcoils is selected. In order to calculate the signal intensity at the scintillator plate, a model for thebeam attenuation has been implemented which takes into account electron impact ionizationand charge exchange reactions. The model is described in references [1] and [2]. It is foundthat the secondary ion current density reaching the scintillator strongly depends on the plasmadensity profile. For a scenario with a core line averaged density of n̄e = 2.6 ·1019 m−3, a currentdensity of the order of ∼ mA/m2 is expected at the scintillator plate, while for a scenario withn̄e = 7.8 ·1019 m−3, the current density expected at the scintillator plate is reduced to ∼ nA/m2.A number of simulations have been carried out to obtain the optimal parameters for the beaminjection and detection geometry. The optimization process aimed at achieving maximum signallevel at the scintillator plate as well as maximum strike-pattern displacements due to magneticfield and potential perturbations. At the same time, the optimization process was constrained byspace limitations, port availability and compatibility with currently existing diagnostics in thesame sector. The final injection parameters are for a baseline scenario with a plasma current Ipand magnetic field Bt of I p/Bt = 0.8MA/2.5T are: toroidal injection angle θtor = 5o; poloidalinjection angle θpol = 0o, injection energy E0 = 70 keV. The radial coverage of the diagnosticis expected to be from ρpol > 0.85.The expected range of operation of the diagnostic is expected to be wide enough to coverall the possible scenarios in forward field configuration. This is illustrated in Fig.1. In orderto estimate this operational range, we assume that the injection energy of the beam should bechanged for different Bt in order to keep the same effective Larmor radius of the secondarybeams. Therefore, the injection energy has to be scaled as Eb = ( BB0 )2 ·E0, where the subindex 0refers to the baseline setup. On the other hand, we assume that variations of the plasma currentlead to a global displacement of the strike-line of 1 cm / 100 kA, as inferred from the simu-lations discussed above. The validity of this estimation has been assessed by the strike-patterncalculation at specific scenarios, indicated by the colored diamonds in Fig.1, spanning differentBt and Ip values. The beam energy was scaled accordingly as described previously. In all casesthe strike-pattern was found to lay within the scintillator limits.46th EPS Conference on Plasma Physics P1.1017Figure 1: Estimated op-erational range for the i-HIBP diagnostic at AUG.The contour lines indicatethe injection energy of thebeam. The shaded regionindicates the operationalspace accesible to the di-agnostic, without takinginto account the beam de-flection plates.A sensitivity study has been carried out to determine whichphysics phenomena can be investigated with the i-HIBP at AUG[5]. To this end, the displacement of the strike-pattern on the scin-tillator plate due to magnetic field and plasma potential fluctuationshas been evaluated for different phenomena such as: current den-sity evolution during an edge localized mode (ELM) cycle, applica-tion of externally applied magnetic perturbations (MP) or geodesicacoustic modes (GAMs) among others. The expected strike-patterndisplacements and sensitivity are in agreement with those describedin [1], ranging from ∼ 0.2mm local shifts due to potential perturba-tions up to ∼ 3mm global shifts due to magnetic field perturbations.Hardware arrangement: out vesselThe main out-vessel component of the diagnostic is the beaminjector. The injector will be placed in sector 13 of AUG, wherethe injection is done through a port located slightly above the mid-plane. The injector is oriented 5o toroidally towards sector 14, ac-cordingly to the baseline injection parameters found in the previ-ous section. The injector is based on the design described in [6]and has been provided by the Wigner Institute. The main elementsare: a high-voltage cage that contains the ion source, the emitter(∼ 70kV ) and extractor (∼ 60kV ) electrodes; an electron suppres-sion ring (∼−0.5kV ); a pair of deflection plates for beam poloidaland toroidal steering (±0.5kV ), as well as for fast beam chopping(up to 250 kHz); a neutralizer chamber; and a diagnostic chamber,which contains a Faraday cup and 2 cameras for beam monitoring, and which has been re-designed to include a set of collimators to reduce the size of the beam. An additional Faradaycup can be placed between the deflection plates and the neutralizer, and a mirror can be placedbetween the HV box and the deflection plates for observation of the ion source.Hardware arrangement: in vesselThe main in-vessel components of the diagnostic are an optical head and an image guide.The optical head, shown in Fig.2 contains the scintillator plate (16.9 cm height x 6 cm width)for the measurement of the secondary beams, a thermocouple to monitor the temperature of theplate, a current measurement and a lamp. The optical head is protected by a tile on the plasmafacing side, which is 2.5 cm behind the shadow of the limiter surface. The optical head has an46th EPS Conference on Plasma Physics P1.1017Figure 2: (a) Optical head of the i-HIBP diagnostic. (b) Picture of the scintillator powder de-posited on the stainless steel plate.opening of approximately ∼ 5x15 cm in the top side, in order to allow the secondary beamtrajectories into the scintillator. A shutter that closes this opening is also installed to avoid thecoating of the scintillator during boronizations. The light emitted by the scintillator is imagedby an objective consisting of 6 lenses which projects the image of the scintillator onto a 1.8 mlong SCHOTT image guide, consisting of 1700x700 glass fibers, which is then imaged fromoutside the vacuum window with a high speed camera.SummaryAn imaging heavy ion beam probe has been designed and will be installed in the AUG toka-mak. Modelling of the beam trajectories has been carried out with a full orbit following codewhich can handle 3D magnetic and electric fields, while the attenuation of the beam is calcu-lated taking into account electron impact ionization and charge exchange reactions. Based onthese simulations, the injection and detection geometries have been optimized while respectingthe spatial restrictions, and the in- and out- vessel components have been designed accordingly.Laboratory experiments are currently ongoing to characterize the beam and the scintillator re-sponse to Cs irradiation, while the manufacturing of all components of the diagnostic is beingfinalized. The i-HIBP at AUG is foreseen to go in operation by the end of 2019.References[1] J.Galdon-Quiroga, Journal of Instr. 12, C08023 (2017)[2] G.Birkenmeier et al, Journal of Instr. (submitted)[3] A.V. Melnikov et al, Nucl. Fusion 57, 072004 (2017)[4] M.Garcia-Munoz et al, Rev. Sci. Instrum. 80, 053503 (2009)[5] V.Olevskaia et al, DPG Conference (2019)[6] G.Anda et al, Rev. Sci. Instrum. 89, 013503 (2018)46th EPS Conference on Plasma Physics P1.1017

An imaging heavy ion beam probe diagnostic for the ASDEX Upgrade tokamak

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An imaging heavy ion beam probe diagnostic for the ASDEX Upgrade tokamak

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