Behler, K.

Herrmann, A.

Höhnle, H.

Kasparek, W.

Mlynek, A.

Neu, R.

Reich, M.

Stober, J.

Stroth, U.

Treutterer, W.

English

MPG.PuRe

Electron Cyclotron Resonance Heating in the second harmonic ordinary Mode at ASDEX Upgrade

Electron Cyclotron Resonance Heating in the Second Harmonic OrdinaryMode at ASDEX UpgradeH. Höhnle1, J. Stober2, K. Behler2, A. Herrmann2, W. Kasparek1, R. Neu2, A. Mlynek2,M. Reich2, U. Stroth1,2, W. Treutterer2 and the ASDEX Upgrade Team21 Institut für Plasmaforschung, Universität Stuttgart, D-70569 Stuttgart, Germany2 Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85748 Garching,GermanyIntroductionIn the last years, the first wall of ASDEX Upgrade was changed from carbon to tungsten-coated carbon tiles in order to demonstrate the lower erosion and hydrogen co-deposition withtungsten. However, tungsten as high-Z material exhibits strong radiation at fusion relevantplasma parameters. In H-modes with good confinement and reduced gas puff at ASDEX Up-grade, the tungsten tends to accumulate in the plasma centre, ventually leading to a radiationcollapse and the premature end of the discharge. Central electron cyclotron resonance heatingwas found to be a good candidate to reduce the tungsten concentratio by changing the electrontemperature gradient in the centre and increase the outwardransport of the impurities [1, 2].In the past, the usage of ECRH was limited due to the cutoff forcentral densities of 1.2 ·1020 m−3 of the typically used second harmonic extraordinary (X2) mode at 140 GHz for amagnetic field of 2.5 T. This limits the safety factors toq95 > 4.5 for high confinement dis-charges near the Greenwald limit (nGW ∝ IP).To enlarge the operational space of the ECRH to higher densiti s and lower safety factors,the polarisation can be changed to the ordinary mode (O2), having twice the cutoff density ofthe X2 mode.O2-Mode Heating ScenarioAt typical plasma parameter of ASDEX Upgrade (ne< 2·1020 m−3; Te< 5 keV) the O2 modeis afflicted with incomplete absorption; therefore the shine-through power and subsequent strayradiation can destroy components or diagnostics, which aresensitive on microwave radiation,like the electron cyclotron emission or the bolometer diagnostic at ASDEX Upgrade.Therefore, a scenario was developed to increase the absorption of the O2 mode at ASDEXUpgrade in order to minimize the stray radiation and to maximize the central deposition (r/a <0.2), having the possibility to control the tungsten concentration also in high density plasmas.A simulation with the beam-tracing code TORBEAM [3] for plasma parameters ofne(0) =38th EPS Conference on Plasma Physics (2011) P2.0951.4 ·1020 m−3 andTe(0) = 3.5 keV yields a single pass absorption of≈ 70 % [4]. This meansthat≈ 30 % are not absorbed, which is too high in the view of machine saf ty. However, witha second pass through the plasma centre an absorption of > 90 %can be achieved. Thereforetwo mirrors (for two 1 MW gyrotrons) at the central column of ASDEX Upgrade are needed.These mirrors have to be conformal to the wall, to avoid erosion by the plasma or disturbing theplasma. Simultaneously, the reflection has to be optimized for high absorption for the secondpass in the plasma centre. These conditions are fulfilled forphase-reconstructing holographicreflectors, which were optimized for good efficiency in trade-off some manufacture limits [4].Localisation of the BeamTo ensure the focusing of the beam on the holographic mirror,a new beam-localisation systemwas installed. The system is based on the Ohmic losses of the O2-mode beam on the surface ofthe tungsten coated holographic mirrors and the measurement of the resulting local temperatureincrease with NiCr-Ni thermocouples (diameter 0.25 mm and afew ms response time). In a firstversion with 4 thermocouples in each mirror the localisation p ssibility of system was tested onhigh power microwave and it was found, that a monitoring of the location of the beam on thesurface (357 cm2) is hardly possible with only 4 thermocouples [4]. Thereforthe number ofthe thermocouples in the mirrors was enlarged to 8 per mirror(see also Fig. 1) in this campaign.During the installation process, the thermocouples were tested on local heating of the holo-graphic mirrors with a hot air gun and a similar response of the thermocouples was demon-strated. Unfortunately, after the mounting of mirrors at the inner column of ASDEX Upgradeand tests of the thermocouples on high power microwaves during plasma discharges, differencesof the temperature response were observed. Such discrepanci s can be caused by local varia-tions of the Ohmic losses on the mirror surface or by small differences of the location of thethermocouple tip in respect to the mirror surface. For the next campaign the new version of thebeam localisation system has to be tested additionally on local heating by strong microwaves.Feedback Control MechanismIn an O2-mode discharge in the 2009 campaign a non-expected density rise led to a change inthe refraction index and to a movement of the beam to the bottom and finally out of the mirror,and therefore to the loss of the second absorption. Unfortunately, the bottom thermocouple wasbroken so we did not see the movement of the beam at all. The loss of 200 kW central ECRH(only one new gyrotron in O2-mode was available during this time) could have triggered a risingtungsten concentration leading finally to a radiation collapse [4].38th EPS Conference on Plasma Physics (2011) P2.095Figure 1: Circuit for the real-time feedback control of the O2-mode heating scenario.171819202122232425pol. injection angle (°)-18.0-17.0-16.0-15.0-14.0-13.0-12.0-11.00.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6tor. injection angle (°)ne(0) (1020 m-3)Figure 2: Poloidal and toroidalinjection angle for central depo-sition and central hit of O2-modebeam onto the holographic mir-ror versus the central density.Because of these results in 2009 a feedback control sys-tem based on the thermocouples was developed. In Fig. 1 thefeedback circuit is sketched. With TORBEAM the poloidaland toroidal injection angles for central deposition and cen-tral hit of the O2-mode beam onto the holographic mirror atthe expected plasma parameter and magnetic configurationare calculated offline and adjusted at the ECRH launcher.Density variations lead to a movement of the beam on themirror during the discharge and to different temperatures onthe mirror surface, which are measured with the thermocou-ples (green, violet and orange crosses in Fig. 1). The data ac-quisition (DAQ) transfers the measured data to a PC wherethe data are offset corrected, filtered and transmitted via thereal-time network to the discharge control system (DCS).Here a controller calculates the new poloidal injection an-gle and transmits it to the launcher. A real-time toroidal angle adjustment is not possible withthe ECRH launcher but luckily it is also not needed. This can be seen in Fig. 2, where thepoloidal and toroidal injection angles, to center the beam onto the holographic mirror, werecalculated with TORBEAM for a variation in the central density. The red and green curves be-long to different density shapes (green peaked profile and red flat profile) and the blue curve tomagnetic configuration with higher triangularity. It can beseen that the variation of the poloidalinjection angle is much larger (∆θ ≈ 6◦) than that for the toroidal injection angle (∆φ ≈ 0.7◦).38th EPS Conference on Plasma Physics (2011) P2.095The black dots in Fig. 2 show the poloidal (∆θHM ≈ 3◦) and toroidal (∆φHM ≈ 5◦) dimensionof the holographic mirror for the flat profile with a central density of 1.4 ·1020 m−3. Therefore,it is sufficient to adjust the poloidal injection angle with the response of the thermocouples inthe top (orange) and in the bottom (violet) of the holographic m rrors.Additionally, an interlock is realized for all thermocouples (orange, violet and green crosses)if the temperature of the holographic mirror is getting too high during a discharge.First Experiments with the Feedback ControlFirst tests with O2-mode injection during a feed-forward angle scan in a plasma dischargeshow a different response of the thermocouples on local heating. The bottom thermocouple ofone holographic mirror shows a higher response to the local Ohmic losses of the beam thanthe top one, therefore the control algorithm has to be adjusted to the behaviour of each singlethermocouple. In first tests of the response of the algorithmto a manually misaligned beam, theamplification factor and other control parameters were tuned until the feedback loop shows theexpected behaviour.SummaryIn the 2009 campaign the first discharges with the O2 mode was demonstrated without thefeedback algorithm [4]. Here the problem was the limited ECRH power and the loss of thesecond absorption due to density increase and the outwards movement of the beam off theholographic mirror.The feedback control based on thermocouples in the holographic mirror is scheduled to reacton these density changes. In first experiments, with a manually misaligned injection angle, theexpected behaviour of the feedback algorithm could be demonstrated; also the interlock wastested successfully.Heating experiments in discharges with high plasma currentand a safety factory ofq95 = 3.5are planned at the end of this campaign.References[1] C. Angioni, R. Dux, E. Fable, A. G. Peeters and the ASDEX Upgrade Team; PlasmaPhysics and Controlled Fusion 49 (2007) 2027[2] D.H.Wagner, J.K. Stoberet al; Plasma Science, IEEE Transactions on 37 (2009) 395[3] E. Poli, A. G. Peeters, G. V. Pereverzev; Computer Physics Communications136 (2001)90[4] H. Höhnle, J. Stoberet al; accepted for publication in Nucl. Fusion38th EPS Conference on Plasma Physics (2011) P2.095

Electron Cyclotron Resonance Heating in the Second Harmonic Ordinary Mode at ASDEX Upgrade

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Electron Cyclotron Resonance Heating in the Second Harmonic Ordinary Mode at ASDEX Upgrade

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