ASDEX Upgrade Team

Becoulet, M.

Blackman, T.

Bobkov, V.

Brzozowski, J.

Challis, C.

Gerasimov, S.

JET-EFDA contributors

Lamalle, P.

Maraschek, M.

Mayoral, M.

Monakhov, I.

Noterdaeme, J.

Saibene, G.

Walden, A.

Wouters, P.

English

MPG.PuRe

Studies of ELM toroidal asymmetry using ICRFantennas at JET and ASDEX UpgradeVl.V. Bobkov1, M. Becoulet2, T. Blackman3, J. Brzozowski4, C. Challis3,S. Gerasimov3, P.U. Lamalle5, M. Maraschek1, M.-L. Mayoral3, I. Monakhov3,J.-M. Noterdaeme1 6, G. Saibene7, A. Walden3, P. Wouters5, ASDEX Upgrade Teamand JET-EFDA contributors1-Max-Planck-Institut für Plasmaphysik, EURATOM-Association, Boltzmannstr. 2,D-85748 Garching, Germany2-Association Euratom-CEA, CEA Cadarache, F-13108 St. Paul-lez-Durance, France3-EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB,UK4-Association EURATOM-VR, Stockholm, Sweden5-LPP-ERM/KMS, Association EURATOM-Belgian State, TEC, Royal Military Academy,B-1000 Brussels, Belgium6-Gent University, EESA Department, B-9000 Gent, Belgium7-EFDA Close Support Unit, Boltzmannstr. 2, D-85748 Garching, GermanyIntroductionEdge Localized Modes (ELMs) are known to affect dramatically the coupling of theIon Cyclotron Range of Frequencies (ICRF) antennas [1, 2]. In order to investigate theinfluence of transient effects on the operation of the ICRF systems, data acquisitionsystems with high time resolution are used on JET and ASDEX Upgrade. It wasfound on both machines, that the response of the four toroidally distributed antennasto ELMs is not simultaneous. The observations indicate constraints for design of anefficiently working ELM-resilient ICRF system [3, 4]. The observed asymmetry canalso provide additional information on ELM physics [2, 5, 6, 7]. This paper presentsstudies of the delays in appearance of the perturbation on RF signals due to type IELMs for different ICRF antennas on JET and ASDEX Upgrade.Experiment and measurementsTwo transmission lines (out of 4) for each A2 antenna [8] on JET have fast measure-ments of reflected voltages with 10 µs time resolution. All transmission lines of theantenna straps on ASDEX Upgrade [9] were equipped with measurements of RF cur-rent with time resolution of 1 µs. This provided 8 measurement points distributedtoroidally both for JET (see Fig. 1) and ASDEX Upgrade.The ICRF antennas respond sensitively to changes of the plasma density in frontof the antennas. The RF signals change rapidly at the rising edge of ELMs. However,the antennas do not provide poloidal and radial resolution of the change in densitydistribution during ELMs. A semi-automatic software has been used to deduce thetimes between responses on ELMs for the antennas (straps) based on the followingapproach, which assigns a time of reaction of an antenna to the maximum of the 1stderivative of the RF signal (the most dynamical point):131st EPS Conference on Plasma Phys. London, 28 June - 2 July 2004 ECA Vol.28G, P-1.141 (2004)1OCTANT2345678T007T008T009T010" (P802)T001T002T003T004T005" (P402)T006RF signalsT001T002T003T004T005" (P402)T006T007T008T009T010" (P802)Time, [s]19.5744 19.5745 19.5746 19.5747400 µ sMagnetics (CATS)initial perturbation initial perturbationA3A4B3B4C3C4D319.5743 19.5744 19.5745 19.5746400 µ sD4ABCDShot 58742BtIpTime, [s]main perturbationFigure 1: Example of the toroidal propagation of an ELM perturbation seen on magnetic coilsand reflected voltages on the antenna transmission lines in JET. Vertical lines on the RFmeasurements indicate the software evaluated times of reaction. Straps D3 and D4 measurethe ”initial perturbation” in this case.1) ELM is detected using Dα signal;2) RF signals are smoothed, the 1st derivative is taken and smoothed and the 2ndderivative is taken;3) two time points of zero 2nd derivative are taken, with the values of the 1st derivativeat least 20% of the absolute value of the maximal 1st derivative;4) if two points are taken at step (3), the first zero of the 2nd time-derivative is takenas the ”initial perturbation” and the second zero is taken as the ”main perturbation”(see Fig. 1, antenna D); if one point is taken at step (3), it is the ”main perturbation”;5) the response time of the single antenna to an ELM is determined by averaging the”main perturbation” times of two adjacent antenna straps;6) velocity of propagation of the ELM perturbation is calculated by averaging the ve-locities of the propagation between antennas (5), the plasma radius at which ELMspropagate is assumed Rp =3.7 m for JET and Rp =2.15 m for ASDEX Upgrade.ResultsOn both machines, the ICRF antennas response to the majority of type I ELMs in-dicates a toroidal propagation of ELM perturbation in the counter-current direction.The direction of the propagation is the same as for the electron diamagnetic drift.Fig. 1 shows an example of the fast signals collected on the JET toroidal arrayof magnetic coils and on 8 transmission lines of the four A2 antennas, at the risingedge of a type I ELM in JET. Both magnetic and RF measurements indicate an initialperturbation starting in octant 6 (magnetic coil T007 and antenna D) and propagatingcounter-clockwise, until the ELM perturbation has spread over the whole torus. Thestarting location of the propagation differs from ELM to ELM and typically has nopreferred point. The time delays between the antennas are typically larger for the an-231st EPS 2004; V.L.V.Bobkov et al. : Studies of ELM toroidal asymmetry using ICRF antennas at JET and A... 2 of 4Line averaged density, [x 1019, m-2]Propagation velocity, [km/s]a)14001200100080060040020004.6 4.8 5.0 5.2 5.4 5.6 5.8 6.01400120010008006004002000Line averaged density, [x 1019, m-3]5 6 7 8 9 10800 kA1000 kAb)Propagation velocity, [km/s]Figure 2: Velocities of propagation of the ELM perturbation as measured by ICRF antennas,depending on central line averaged densities for JET (a) and ASDEX Upgrade (b).tennas which the ”main perturbation” passes at first. This indicates, that the toroidalvelocity of the ELM perturbation increases with the development of the perturbation.The shape of the RF time traces at the rising edge of ELMs is very similar foradjacent straps, but is not always similar for different antennas. In the followinganalysis, only cases with similar RF signals shape have been taken into account.A first attempt to find a correlation between evaluated velocities of propagationof the ELM perturbation from the antenna delays and basic plasma parameters hasbeen made. The velocities are plotted in Fig.2 vs. the central line-of-sight densitymeasured by interferometry. Fig.2a includes ELMs from 3 JET discharges (Bt = 2.45T, Iplasma = 2 MA, fICRH = 42 MHz), Fig.2b includes ELMs from 29 ASDEX Upgradedischarges (Bt = 2.0 T, fICRH = 30 MHz). ASDEX Upgrade discharges with 800 kAand 1 MA are treated separately. One observes the highest velocities of propagationof the ELM perturbation at lower densities, however no monotonic dependence on thedensity is observed.1400120010008006004002000Propagation velocity, [km/s]0 10 20 30 40∆ Wmhd, [kJ]800 kA1000 kAFigure 3: Velocity of propagation of the ELMperturbation vs. change of diamagnetic energyfor ASDEX Upgrade.The average velocity of propagation ofthe ELM perturbation for JET is about200 km/s which corresponds to ≈ 120 µstime for a full toroidal turn. The av-erage velocity in ASDEX Upgrade 220km/s corresponds to a toroidal turn timeof 60 µs. Such delays for one toroidalturn have marginal effect on the opera-tion of ELM-resilient ICRF systems onJET and ASDEX Upgrade, if the strapsfrom one toroidal octant are connected tothe compensation network.For ASDEX Upgrade data, a qualita-tive dependence of the propagation velocities on absolute losses of diamagnetic energyof the plasma ∆Wmhd during ELMs has been found. The smaller propagation velocities(both maximal and minimal values) are correlated with the larger losses of the plasmadiamagnetic energy. To estimate ∆Wmhd, a difference between energy 1 ms before ELM(detected as described above) and the minimal value between the detected ELM andthe subsequently detected ELM is calculated. The dependence of the velocities on the331st EPS 2004; V.L.V.Bobkov et al. : Studies of ELM toroidal asymmetry using ICRF antennas at JET and A... 3 of 4relative losses of the diamagnetic energy ∆Wmhd/Wmhd has no good correlation.For the described set of data for ASDEX Upgrade, the dependence of ∆Wmhd (es-timated as above) on the density has a non-monotonic character, as ∆Wmhd increaseswhen density is increased in low density range and decreases with the density in highdensity range.ConclusionsBoth on JET and ASDEX Upgrade, the perturbations associated with type I ELM riseare observed on ICRF antennas. The perturbations move in electron diamagnetic driftdirection. Experiments on JET have shown that RF data is in agreement with magneticmeasurements which also see the ELM perturbation propagation. Typical velocities oftoroidal propagation of 200 km/s (lying in general in 50-1200 km/s range) are observedfor JET and 220 km/s (lying in general in 50-1400 km/s range), corresponding to 120µs and 60 µs toroidal turn times for JET and ASDEX Upgrade respectively. Thehighest measured velocities belong to low density cases. Therefore for most of theobserved ELMs, the delays between the antenna straps situated in the same toroidaloctant have marginal, or no effect on the operation of the compensation networks forthe ELM-resilient ICRF systems.Measurements on ASDEX Upgrade show a decrease of the propagation velocitieswhen the absolute losses of the plasma diamagnetic energy are increased. However thenature of the toroidal rotation of the ELM perturbation is still not understood. As anext step, comparative analysis of the toroidal propagation velocities delivered fromthe RF signals and electron diamagnetic velocity from the pedestal parameters [10] isunderway.References[1] I. Monakhov et al., 15th Topical Conference on Radiofrequency Power in Plasmas,Moran, AIP Conference Proceedings 694 (2003) 146-149.[2] J.-M. Noterdaeme et al. 21th EPS Conference on Plasma Physics and Controlled Fusion,Montpellier, ECA Vol. 18B (1994) 842-845.[3] G. Bosia, Fusion Science and Technology 43 (2003) 153-159.[4] P.U. Lamalle et al., 15th Topical Conference on Radiofrequency Power in Plasmas,Moran, AIP Conference Proceedings 694 (2003) 118-121.[5] M. Becoulet et al., Plasma Phys. Control. Fusion 45 (2003) A93-A113.[6] Vl. V. Bobkov et al., 16th International Conference on Plasma Surface Interactions inFusion Devices, Portland, Maine USA, P2-50.[7] T. Eich et al., 16th International Conference on Plasma Surface Interactions in FusionDevices, Portland, Maine USA, R-3.[8] A. Kaye et al., Fusion Eng. Des. 24 (1994) 1-21.[9] J.-M. Noterdaeme et al. Fusion Eng. Des. 24 (1994) 65-74.[10] H. Urano et al. Plasma Phys. Control. Fusion 45 (2003) 1571-1596.431st EPS 2004; V.L.V.Bobkov et al. : Studies of ELM toroidal asymmetry using ICRF antennas at JET and A... 4 of 4

Studies of ELM toroidal asymmetry using ICRF antennas at JET and ASDEX Upgrade

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Studies of ELM toroidal asymmetry using ICRF antennas at JET and ASDEX Upgrade

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