ASDEX Upgrade Team

Conway, G.

Fuchs, C.

Kurzan, B.

Manso, M.

Müller, H. W.

Suttrop, W.

Varela, P.

English

Müller, H.

MPG.PuRe

Density pedestal measurements with microwavereflectometry on ASDEX UpgradeP. Varelaa), and M. MansoAssociação EURATOM/IST, Centro de Fusão Nuclear, Instituto Superior Técnico1049-001 Lisboa, PortugalG. Conway, C. Fuchs, B. Kurzan, H. W. Müller, W. Suttrop, and ASDEX Upgrade TeamMax-Planck-Institüt für Plasmaphysik, EURATOM AssociationD-85748 Garching, GermanyThe understanding of confinement improvement through the formation andsustainment of localized transport barriers requires diagnostics with high temporaland spatial resolutions. One diagnostic that can meet such high requirements isbroadband microwave reflectometry. In particular, it can measure the density profilewith better than 30  s temporal resolution and millimetre spatial resolution. The maindifficulty has always been the evaluation of the density profile in the presence ofplasma turbulence. Recent advances in the data analysis techniques and the highperformance of the reflectometry diagnostic enabled the automatic and routineevaluation of density profiles in all plasma regimes of ASDEX Upgrade. The largenumber of profiles measured during a discharge makes it possible to obtain thedetailed evolution of plasma parameters crucial for transport analysis, such as theedge pedestal density, even under moderate levels of plasma fluctuations. Here a newmethod is presented that automatically extracts the temporal evolution of the edgepedestal density from the measured group delays. To reduce the effect of plasmafluctuations and to improve the accuracy of the pedestal detection, the method can useburst sequences of closely spaced reflectometry samples. The automatic evaluation ofthe pedestal was applied to ASDEX Upgrade standard H-mode plasmas. The resultspresented show that reflectometry can accurately detect the formation of the edgepedestal at the L to H transition, and also that the detailed evolution of the densityprofile can be resolved, in particular during ELMs (edge localized modes).a) Email: pvarela@cfn.ist.utl.ptI. INTRODUCTIONThe H-mode plasma regime with type I ELMs is considered to be the reference scenariofor ITER. One parameter that is crucial to understand H-mode performance is the density ofthe edge pedestal, due to the impact of this plasma region in the global energy confinementand stability and also on the divertor plates life-time, due to the effect of ELMs. It is thereforecrucial to have measurements with high temporal and spatial resolutions to be able to resolvethe density profile changes at the edge and the associated fluctuations levels. Existentmeasurements do not have the required temporal resolution and/or lack spatial coverage ofthe edge pedestal region.In ASDEX Upgrade, around 3000 density profiles with 30  s temporal resolution andmillimetre spatial resolution (under good propagation conditions) are obtained withbroadband reflectometry in each discharge at each side of the plasma, covering the densityrange 19 30.4 15 10 m−− × . Thus, broadband reflectometry meets the requirements for edgemeasurements and is able to contribute substancially to the study of edge plasma physics, inparticular in the pedestal region, where it can complement the measurements performed bythe Thomson scattering, interferometer, and Li-beam diagnostics.Due to advanced data analysis techniques that have been developed, reflectometry densityprofiles are now automatically evaluated and are routinely available shortly after eachASDEX Upgrade discharge. A new method has also been developed to automatically extractthe density of the edge pedestal from the measured group delays. The application of this newmethod to ASDEX Upgrade standard H-mode discharges is reported here. The resultsobtained demonstrate that reflectometry can resolve the detailed evolution of the densityprofile during the L to H transition and during the occurrence of ELMs. In particular, it isshown that reflectometry can follow the evolution of the density of the edge pedestal since itsformation immediately before the L to H transition.The remainder of the paper is organized has follows. In Sec. II, the a brief description ofthe profile evaluation techniques is given. Section III describes the method developed toautomatically detect the edge pedestal density. In Section IV the experimental results arepresented. Section V discusses the results and gives some hints concerning future work.II. PROFILE EVALUATIONThe ASDEX Upgrade broadband reflectometer features ultra-fast sweeping and the uniquecapability of probing from both magnetic field sides simultaneously.1,2 Profile measurementscan be performed in three modes: (i) equally spaced sweeps covering the complete discharge,(ii) in bursts of closely spaced sweeps with a larger interval between the bursts, and (iii) inmixed mode, where parts of the discharge are covered with equally spaced measurements andothers with bursts. Burst-mode measurements offer the possibility to obtain average profilesin plasma regimes where single measurements would fail such as the ones characterized byhigh levels of plasma fluctuations. The profile evaluation system is based on the best-pathalgorithm.3 This method uses the spectrogram of the reflected signals that shows how thereflected energy is distributed in the time-frequency domain.In ASDEX Upgrade, the results obtained so far show that the signal-to-noise ratio (S/N) ofthe reflected signals plays an important role in the evaluation of the profiles. If it is high theinversion of the profile from single sweep data presents no problems, which is normally thecase in plasma regimes with low levels of plasma turbulence, such as H-mode plasmas. Asthe level of turbulence increases, the S/N ratio may become very low, which prevents theaccurate inversion of the density profile.The solution is usually adopted is to average over several consecutive sweeps to improveaccuracy. We adopted a different approach that takes advantage of both the ultra-fastsweeping capability of the diagnostic and of the large number of measurements that can beperformed in a single discharge. In burst-mode, the average profile is not obtained fromsimple averaging over the ensemble of group delays, as it would be too sensitive to strongperturbations in the individual samples, but using the burst-mode data processing method.4Typical measurments have bursts of eight closely spaced (10µs) ultra-fast sweeps (20µs) toobtain an average profile every 230µs. The burst-mode spectrogram still retains all featurescommon to the different sweeps and simultaeously reduces the perturbations of the individualgroup delays, therefore giving a more accurate and reliable burst profile.III. AUTOMATIC PEDESTAL DETECTIONIn ASDEX Upgrade, density measurements are also performed by the Li-beam diagnostic,at the edge, by the Thomson scattering, and by the interferometer. A density profile has beenroutinely provided that combines Li-beam and interferometry data. Reflectometry is expectedto improve the resulting profile firstly by providing the density at which the pedestal occurs.In order to provide the evolution of the pedestal as soon as the density profiles are availablewe developed a method which automatically determines the density of the pedestal from thecomputed group delays.Figure 1 shows a schematic represen-tation of the method used. The turningpoint, which results from the gradientchange at the pedestal, is determined as theintersection of two lines: the first one isobtained through a linear fitting performedover the frequency range 18 – 35 Ghz; thesecond one results from another linearfitting this time in the range defined by thefrequency above which the group delaydeviates from the previous linear fittingmore than a pre-defined value, and the lastmeasured frequency. The pedestal density isobtained from the frequency f , where bothlines intersect, through the relation2 / 81epn f≅ .Figure 2 shows the results obtained inASDEX Upgrade standard H-modedischarge #14313. The upper part of fig. 2shows the evolution of the average density(DCN), the flux of particles at the divertor(D!), and the neutral beam injection power(PNI), along the discharge. The lower partof fig. 2 shows the evolution of the pedestaldensity obtained with the method describedabove in the time interval corresponding tothe dashed region A in the upper part of thefigure, going from the L phase up to the Lto H transition. The results show that thedensity of the pedestal starts to increasebefore the transition occurs. After thetransition, the profile peaks and the pedestalis lost as the density increases. Figure 3shows the density profiles right before andafter the L to H transition. As shown,reflectometry can follow the abrupt peakingof the density profile typical of the L to Htransition. These results clearly demonstratethe capabilities of reflectometry to accura-tely follow the profile changes bothtemporaly and spatially.IV. ELM MEASUREMENTSDue to its good temporal and spatial resolutions, reflectometry can greatly contribute tofurther the study about the impact of ELMs in the density profile.FIG. 1 – Schematic representation of the auto-matic detection of the pedestal density.0 2 4 6 824#14313PNI[MW]time [s]24Dα[a.u.]246810A DCN[x1019m-3]1.38 1.44 1.50 1.56 1.62 1.681.92.02.12.22.32.42.5Pedestaldensity[x1019m-3]time [s]AFIG. 2 – Automatic detection of the pedestaldensity: upper part – evolution of the averagedensity (DCN), flux of particles at the divertor(D!), and neutral beam injection power (PNI),along the discharge; lower part – evolution of thepedestal density. The gray curve was obtaineddirectly from the group delays while the blackcurve results from a smoothing using a 15–pointwindow.Figure 4 shows the profile changes measured atthe high-field side during an ELM. The upper partof fig. 4 shows the D! signal during the ELM. Thelower part shows the evolution of the decay lengthin the density ranges 19 30.8 2.0 10 m−− × and19 32.0 6.0 10 m−− × . The decay length at the innerlayer is shown to follow the time scale of theELM. However, the outer layer remains peakedand only flattens at the outset of the ELM.Figure 5 shows the density profiles measuredsimultaneously at the low- and high-field sides ofthe plasma right before, during, and after the ELMdepicted in fig. 4. The characteristic flattening ofthe profile due to the ELM can be clearly seen aswell as the reduced level of fluctuations on thehigh-field side.V. DISCUSSIONThe results presented in this paper show thatreflectometry is able to play a major role in thestudy of the edge plasma. The major advances inthe data processing system of the ASDEXUpgrade broadband reflectometer, which impro-ved the accuracy and reliability of the densityprofiles, allow reflectometry to make a majorcontribution to the improvement of the standarddensity profile of ASDEX Upgrade as well as tothe understanding of edge plasma phenomena, likeELMs. The method presented here to automati-cally evaluate the pedestal density is nowundergoing extensive testing before we make thepedestal density evolution routinely available.ACKNOWLEDGMENTSThis work has been carried out within theframework of the Contract of Association betweenthe European Atomic Energy Community and“Instituto Superior Técnico”. Financial supportfrom “Fundação para a Ciência e Tecnologia” e“Praxis XXI” was also received.1. Silva et al., Fusion Technology, edited by C. Ferro, M.Gasparotto, and H. Knoepfel, (North-Holland, Amsterdam,1992), Vol. 1, p. 747.2. Silva et al., Rev. Sci. Instrum. 72, 307 (2001).3. P. Varela et al., Rev. Sci. Instrum. 70, 1060 (1999).4. P. Varela et al., Rev. Sci. Instrum. 72, 315 (2001).Dα4.888 4.890 4.892 4.89434562.0-6.0x1019m-30.8-2.0x1019m-3decaylength[cm]time [s]FIG. 4 – Decay length evolution in twodistinct profile regions, showing that theinner region flattens and peaks in the ELMtime scale, while the outer one flattens onlyat the outset of the ELM.1.0 1.1 1.2 1.3 1.401234HLn e[x1019m-3]R [m]FIG. 3 – Density profiles measured at thehigh-field side right before and after theL to H transition.1.0 1.2 1.4 2.0 2.2012345678H-modeELMn e[x1019m-3]R [m]FIG. 5 – Density profiles measuredsimultaneously at the low- and high-fieldside right before, during and after an

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