Radial and Parallel Transport Fast Camera Observation on LHD SOL Region

1. Radial and Parallel Transport Fast Camera Observation on LHD SOL Region D. Carralero1, M. Shoji2, E. de la Cal1, B. Ph. van Milligen1, J. L. de Pablos1, C. Hidalgo1, H.Yamada2 1Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Madrid, Spain 2National Institute for Fusion Science, Toki, 509-5292, Japan Author email: daniel.carralero@ciemat.es

2. Fast Camera Imaging in LHD: Context At the beginning of 2008, a series of contacts between LNF-CIEMAT and NIFS commenced, including a specific cooperation program in fast camera studies. By it, a CIEMAT´s Photron APX fast camera was installed in LHD, for joint operation during 2008 autumn campaign. Observations were made of a range of different phenomena, such as radiative plasma collapse, bremsstrahlung radiation observation of core structures or core density collapse phenomena (associated to ultra high dense core plasmas). Still, filament ejection and propagation in the edge of high b discharges was selected for further study. In this talk, the first results of these studies are highlighted. The scope of this work is to overcome the qualitative analysis (traditionally associated to fast imaging): produce quantitative results and compare them to those of other diagnostics. In February 2009, the Memorandum of Understanding on Academic and Scientific Cooperation between the NIFS and CIEMAT was signed to further encourage this kind of activities.

3. Experimental Setup Photron APX-RS (1024x1024 px, up to 600 kHz operation), coupled to a 75 mm objective by coherent OF bundle. Observation through 6T tangential port, around 25% of vacuum vessel covered. 10 and 50 kHz sampling rates employed, yielding 512x512/192x136 px spatial resolution (solid angles 20ºx20º/ 8x5.5º) Visible lines in LHD include H (X point), C (wall sputtering) [1] [1] M. Shoji et al., JNM, 337-339, 2005

4. High β Discharges Important achievements in LHD high b operation: values of 5% sustained many times tp. This is achieved mostly by optimizing the magnetic configuration and heating and fueling. Images from [2] Much work in progress covering this topic [2]: finite b effects reduce nested surfaces volume and shifts magnetic axis Core MHD activity is suppressed. Modes appear in the edge. m/n = 2/3 and m/n =1/1, more relevant ones, at r=1 and r=.9 respectively. [2] S. Sakakibara et al. PPCF, 50, 2008

5. Geometry I • When high b discharges are observed by fast camera, several features can be highlighted: • “Hollow” high temperature core • Bright X-point stripes • Helical impact region outside divertor • Structures traveling between them Structures not identifiable at 10 kHz sampling. 50 kHz and deaveraging of sequences reveal “comet”-like filaments traveling along and across the divertor leg channel

6. Geometry I

7. Geometry II Poincaré diagram data is represented in 3D environment for better insight of LHD magnetic geometry Two different divertor legs are represented in different colors. Several perspectives are provided for clarity. Finally, 3D points are projected into observed frames. Parallel/traverse directions can be observed for each divertor leg. Image from S. Masuzaki et al. PPCF, 44, 2002

8. Frequency Analysis: Signals Magnetic probe data provides information about MHD activity. Frequency analysis of several camera channels reveals strong similarity, suggesting m/n = 2/3 mode (2-3.5 kHz) could be responsible of these structures. A higher band of fluctuations (over 4-5 kHz) can not be directly related to MHD. Magnetic probe data courtesy of S. Sakakibara

9. Frequency Analysis: Movies Movies processed with passband filters: qualitative differences are found between the two bands. In the lower band (.5-3.5 kHz), stripes alternate while traveling along X-point and impact regions. Radial traveling waves In the higher band (4-25 kHz), comet-like filaments propagate both in the parallel and cross field direction (generally outwards, buy sometimes inwards) 0.5-3 kHz band filter 4-25 kHz band filter

10. Radial Channel Analysis A number of signals along a radial channel are surveyed: At low freq., the most notable feature is the oscillation at XP and SP regions. At high frequency, “secondary events” are found following main ejections. Main events at XP seem to be consistently followed by those at SP: particle ejection due to edge mode, followed after some delay by the particles hitting the wall.

11. Transport Deaveraged raw data allow a first approximation to structures transport. Not suitable for more serious analysis, but serves as order 0 reference. Radial transport already measured with radial channel survey (velocities around 3-6 km/s). Correlation between XP and SP signals confirms their relation and the velocity (delay 250 ms, channel length 0.8 m) Mean parallel velocity of the structures (high freq. band) is measured by windowed fft phase analysis. Apparent dependence with co/counter NBI power injection. Values similar to those found for edge ions in [3]. [3] M. Yoshinuma et al. PFR, 3, 2009

12. Rescaled Range (R/S) Analysis May f(t) be a experimental signal: g(t)=∫f(t)dt gets the “memory” of the signal while removing the short correlation noise. Range is defined as the deviation of g(t) from the mean tendency over a given interval t. When normalized with the std is know as “Rescaled Range” R/S(t) conveys information of memory effects of the signal: it evolves as a power of time. The exponent is the Hurst parameter, H. H gives a measure of the long term correlations in the signal: Gaussian, uncorrelated noise yields H = 0.5 1 > H > 0.5 means correlation or “persistence” 0.5 > H > 0 means anti-correlation or “antipersistence”

13. 10 40 60 70 R/S Analysis: Results R/S analysis is employed to observe the statistical behavior of fluctuations beyond the autocorrelation range into the mesoscale (i.e. delay > 100 ms) Several channels at inner radial positions show similar results: A first “persistent” region with high Hurst parameter values, followed by a “antipersistent” one with low H. Influence of m/n = 2/3 mode is observed around (2-3 kHz)-1 delays. This result suggests the presence of avalanch phenomena and is in good agreement with previous studies in several machines [4] 1/ Tmode Image from [4], B. Carreras et al. PoP, 6, 1999

14. Conclusions & Future Work • After first this first experiment series and subsequent analysis, a rather consistent picture of the observed phenomena is reached, and some preliminary conclusions can be stated: • Basic underlying mechanism for ejection of filaments has been identified (m/n = 2/3 edge mode) and a good description of the characteristics and transport of the structures has been provided. • - Apparent relation between toroidal velocity of the structures and co/counter • NBI injected power has been found • Finally, evidence of avalanch-like phenomena has been found by R/S analysis, and consistently related to similar analysis in other machines. • During next LHD campaign, a specific experimental program will be followed to gather more information about the relevant conditions in the SOL and confined edge, in order to deepen the analysis on MHD and/or SOC phenomena behind filament ejection mechanisms.

15. Thank you for your attention! Questions, comments and remarks…

17. Visible Camera Physics Visible Camera signal comes mainly from Ha radiation generated in a number of atomic reactions involving neutrals and electrons • Implications: • Range of Te allowing the existence of neutrals (inner, hot nuclei not observable) • - Source of neutrals (puffing, limiter,…) required to identify the origin of the signal • Unique high density, moderate temperature conditions in LHD core generate also VB radiation signal

18. High β Discharges