Figueiredo, H.

Ionita, C.

Juul Rasmussen, Jens

Maszl, C.

Naulin, Volker

Schrittwieser, R.

Silva, C.

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Simultaneous measurements of fluctuations with cold and emissive probes in ISTTOKMaszl, C.; Ionita, C.; Silva, C.; Figueiredo, H.; Naulin, Volker; Rasmussen, Jens Juul; Schrittwieser, R.Published in:ProceedingsPublication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Maszl, C., Ionita, C., Silva, C., Figueiredo, H., Naulin, V., Juul Rasmussen, J., & Schrittwieser, R. (2009).Simultaneous measurements of fluctuations with cold and emissive probes in ISTTOK. In Proceedings (pp.P1.167). European Physical Society.  (Europhysics Conference Abstracts; No. 33E).Simultaneous measurements of fluctuations with cold and emissive probes in ISTTOK C. Maszl1, C. Ionita1, C. Silva2, H. Figueiredo2, V. Naulin3, J. Juul Rasmussen3,  R. Schrittwieser1 1Institute for Ion Physics and Applied Physics, Association Euratom-ÖAW,  University of Innsbruck, A-6020 Innsbruck, Austria 2Association EURATOM/IST, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal 3Association EURATOM - Risø DTU, Technical University of Denmark,  DK-4000 Roskilde, Denmark Abstract: An array of four heatable floating probes and one cylindrical probe staggered on the same poloidal meridian was used for fluctuation measurements in the ISTTOK edge plasma. The array permits the simultaneous measurement of density and poloidal electric field, and thereby of the radial turbulent flux. We have found that emissive probes deliver more reliable results than the cold probes.  The edge region of magnetically confined plasmas shows high fluctuation levels of den-sity and potential that lead to a large anomalous particle transport [1,2,3,4]. Of particular in-terest are the dynamics of edge and scrape off layer (SOL) transport of heat, particles and momentum for the understanding and control of edge properties in fusion machines.  ISTTOK (Instituto Superior Técnico TOKamak) is a large aspect ratio circular cross-section tokamak (major radius R = 46 cm, minor radius a = 85 mm, toroidal magnetic field BT = 0,5 T) with a fully poloidal graphite limiter at r = 85 mm. Typical values of the ISTTOK discharge parameters were: plasma current Ipl # 4,5 kA, discharge duration up to Ĳd # 100 ms, central electron density ne(0) # 3,51018 mí3, central electron temperature Te(0) # 150 – 200 eV and edge safety factor q(a) # 5.  The probe array consisted of four heatable probes and one cylindrical probe staggered on the same poloidal meridian above each other (see Fig. 1) with a distance of 2 mm between two adjacent probes. Each heatable probe consisted of a 8 mm long loop of 0,2 mm diameter tungsten wires. By "cold probe" we mean an unheated loop probe, while in case of an "emis-sive probe" the loop is heated to electron emission. The cylindrical probe was biased to ion saturation. The probe manipulator is mounted at the low field side mid-plane and allows mov-ing the probe radially through the plasma from r = 61 – 93 mm. In case of Fig. 1 two loop 36th EPS Conference on Plasma Phys. Sofia, June 29 - July 3, 2009 ECA Vol.33E, P-1.167  (2009)probes (#1 and #4) are shown in red as emissive whereas the other two (#2 and #5) are cold (blue) .  The floating potentials of the emissive probes were taken as reliable measures of the plasma potential )pl, whereas those of the cold probes were the conventional ones: Vfl = )pl – ĮTe, with Į = ln(Ies/Iis) # 2,5 in a magnetized hydrogen plasma [5]. Ies,is is the cold probe electron/ion saturation current. This array permits the simulta-neous measurement of density and poloidal electric field needed for deriving the radial turbulent flux. The poloidal electric field was de-termined, on one side, from the difference between the floating po-tentials of two cold probes and, on the other side, of two emissive probes, divided by the respective distances between the probes. As shown previously, emissive probes deliver more reliable measures than cold probes for the plasma potential )pl and thus also for the electric field [5]. First the floating potentials of all four heatable probes were compared with each other when two probes were heated and two not. The cross correlation between the signals of two emissive and two cold probes show qualitatively the same behaviour although the time lag is nearly double for the latter. Fig. 2 shows the correlation in case of two emissive probes (#1 and #4). The poloidal correlation is very high (Ȗ # 0,9) at the innermost probe position (r # 65 mm) and has a minimum at r # 74 mm (Ȗ # 0,7). Outside the limiter, the signal-to-noise ra-tio becomes very poor.  The minimum in the correlation at r # 74 mm can be explained by a de-correlating shear layer. Further indications for this assumption is the change of the radial electric field direction for r # 74 mm, seen from the radial plasma potential pro-file (Fig. 3, red line). Fig. 3 shows also the profile of the cold floating potential (blue line). The dif-ference between the two profiles is proportional to the electron temperature [5]. Fig. 4 shows the ra-dial profile of the averaged radial turbulent flux with a maximum at r # 74 mm. From this figure is also obvious that the averaged flux is un-Fig. 2: Cross correlation between thetwo emissive probes shown in red in Fig.1 as function of the minor radius.  Fig. 1: Schematic ofthe probe array. Four heatable wire loopprobes and one coldprobe are mounted in acase of boron nitride.  36th EPS 2009; C.Maszl et al. : Simultaneous measurements of fluctuations with cold and emissive probes in I... 2 of 4derestimated when measured by cold Langmuir probes. Furthermore both types of probes see an inward flux for r # 64 mm. The change of the sign and the non-vanishing divergence of the fluxes seem to indicate that ion sources play an important role in this region. This will be sub-ject of a forthcoming publication.  Investigations on intermittent events exhibit clear differences in the floating potentials meas-ured by emissive and cold probes. Fig. 5 presents the temporal evolutions of the floating potentials of emissive probe #1 and cold probe #2, and of the ion saturation current for r = 85 mm. Thus probe #1 was measuring )pl(t) while #2 was measuring Vfl(t). Blob-like transport events are better visible and finer structured in the emissive probe signals, suggesting that temperature perturbations associ-ated with blobs cannot be neglected [5]. Obviously emissive probes reveal more details of fluctuations in fusion plasmas than cold probes. This is also visible in Fig. 6 which shows a close-up of one blob event, marked in Fig. 5 by two red vertical Fig. 5: Temporal evolution of )pl meas-ured by probe #1 (red), of Vfl measuredby probe #2 (blue) and of the ion satura-tion current to probe #3 (black) at r = 85 mm. The two red vertical bars indi-cate the time interval shown in Fig. 6.  Fig. 3: Radial profile of the plasma po-tential (red squares), measured by probe#4 as emissive one, and of the cold float-ing potential (blue circles), measured byprobe #4, when cold.  Fig. 4: Radial profiles of the averaged radial turbulent flux, once measured with probes #2 and #5 as emissive probes (red squares), once with the same probes as cold probes (blue circles).  36th EPS 2009; C.Maszl et al. : Simultaneous measurements of fluctuations with cold and emissive probes in I... 3 of 4bars. We point out the three peaks in the ion satu-ration current (black line) which obviously stem from the same event visible in the emissive probe signal (red line), however, smeared out in the sig-nal of the cold probe (blue line). The temporal shift between the red peaks and the black ones is also an indication of the poloidal propagation of the blob from top to down. The time delay is about 1,5 Ps and the distance between probe #1 and #3 is 4 mm, so that the poloidal velocity is approxi-mately 4103 m/s.  Summarizing we have investigated radial turbulent transport in ISTTOK with emissive and cold probes and found evidence for blob-like transport events. Once again we found that emis-sive probes are better suited than cold probes for the diagnostic of plasma potential and electric fields in fusion experiments. Acknowledgements This work, supported by the European Communities under the Contracts of Associations be-tween EURATOM and ÖAW, IST and RISØ, was carried out within the framework of the EFDA. The content of the publication is the sole responsibility of its authors and it does not necessarily represent the views of the Commission or its services. This work was also sup-ported by projects L302-N02 and P19901 of the Austrian Science Fund. References [1] B.A. Carreras, C. Hidalgo, E. Sánchez, M.A. Pedrosa, R. Balbín, I. García-Cortés, B. Van Milligen, D.E. Newman, V.E. Lynch, Phys. Plasmas 3, 2664 (1996).  [2] V. Naulin, New J. Phys 4, 28.1-28.18 (2002); also J. Nucl.Mat. 363-365, 24 (2007).  [3] O.E. Garcia, V. Naulin, A.H. Nielsen, J. Juul Rasmussen, Phys. Plasmas 12, 062309 (2005).  [4]  S.J. Zweben, J.A. Boedo, O. Grulke, C. Hidalgo, B. LaBombard, R.J. Maqueda, P. Scarin, J.L. Terry, Plasma Phys. Control. Fusion 49, S1 (2007). [5]  R. Schrittwieser, C. Ionita, P. Balan, C. Silva, H. Figueiredo, C.A.F. Varandas, J. Juul Rasmussen, V. Naulin, Plasma Phys. Control. Fusion 50, 055004 (2008).  Fig. 6: Close-up of signals of emissiveprobe #1 (red), cold probe #2 (blue) andof the cylindrical probe #3 (black), indi-cated in Fig. 5 by two red bars. The ar-rows show a blob event first appearingin )pl(t) and then in Iis(t).  36th EPS 2009; C.Maszl et al. : Simultaneous measurements of fluctuations with cold and emissive probes in I... 4 of 4

Simultaneous measurements of fluctuations with cold and emissive probes in ISTTOK

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Simultaneous measurements of fluctuations with cold and emissive probes in ISTTOK

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