Blob filament measurements with lithium beam emission spectroscopy - PowerPoint PPT Presentation

��������������������������������������������� Max-Planck-Institut für Plasmaphysik Blob filament measurements with lithium beam emission spectroscopy in the SOL of ASDEX Upgrade G. Birkenmeier, F. Laggner, E. Wolfrum, M. Willensdorfer, R. Fischer, P. Manz, D. Carralero, G. Fuchert, and the ASDEX Upgrade Team KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 1

Outline Part I: Introduction: What are blobs? Part II: Blob measurements with Li-BES Part III: Comparison with blob scaling laws KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 2

Introduction: What are blobs? The scrape-off layer is not steady! GEMR simulation: � Circular limiter plasma � Poloidal cross section Hot and dense blobs are permanently generated around the LCFS: � Main activity at the low-field side � Convective propagation towards the wall [M. Kocan et al ., PPCF (2012) ] KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 3

Introduction: What are blobs? Blobs are intermittently expelled density filaments in the scrape-off layer (SOL) � Low magnetic field component (in contrast to ELMs) � Occur in L mode or between ELMs in H mode � Can contribute significantly to main chamber wall degradation [B. Nold, PhD thesis] KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 4

Can we understand the blob dynamics? Blob theory focuses on velocity-size relation: � � [P. Manz et al ., Phys. Plasmas (2013)] Aims of our investigations: 1. Measure the size δ δ b of the blobs δ δ Measure the velocity v b of the blobs 2. δ b δ δ δ 3. Compare with theoretical scaling laws v b Physics-based prediction of perpendicular fluxes [T. Ribeiro et al ., PPCF (2008)] KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 5

Outline Part I: Introduction: What are blobs? Part II: Blob measurements with Li-BES Part III: Comparison with blob scaling laws KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 6

Lithium beam emission spectroscopy (Li-BES) ASDEX Upgrade: Major radius: 1.65 m Minor radius: 0.5 m 13 m 2 Volume: B-field (max): 3.9 T Plasma current (max): 1.4 MA Total heating: 27 MW [Schweinzer, PPCF 1992, Full tungsten machine Fischer, PPCF 2008] Recipe for Li-BES: � Extract Li ions from β -eucryptite ( LiAlSiO 4 ) emitter � Accelerate ions towards plasma � Neutralize ions to Li atoms by means of a Na neutralizer cell � Detect intensity of Li I line at 670.8 nm with optical system KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 7

Lithium beam emission spectroscopy (Li-BES) [Schweinzer, PPCF 1992, Fischer, PPCF 2008] Spectrum: Emission profile of Li I line: 35 radial channels KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 8

New optical head for fluctuation measurements New optical head (designed by HAS) � has a larger aperture � has a shorter distance � is new (no neutron degradation) New DAQ: time resolution 5 µ s [ M. Willensdorfer et al., PPCF 2014 ] KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 9

Density profiles from Li-BES � Detection of the Li I (2p → 2s) line radiation along the beam axis (z) Energy E [eV] � Occupation numbers N i (z) are modelled by the following rate equation system [E. Wolfrum et al., RSI 1993] Li I @ 670.8 nm � N i (z) depend on n e : Probabilistic data analysis to determine n e [ R. Fischer et al., PPCF 2008 ] measured calculated KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 10

Comparison to other diagnostics Equilibrium profiles agree: � Li-BES (old) � Li-BES (new) � Edge Thomson scattering � Core Thomson scattering � Sweeping reflectometer [M. Willensdorfer, PPCF 2014] Frequency spectra agree: Li-BES and sweeping reflectometer Li-BES and Langmuir probes KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 11

New optics for fluctuation measurements New LOS consist of 26 radially displaced channels: � Channel distance ~ 6 mm � Channel width ~ 5 mm (radial) and 11 mm (poloidal) We only can resolve structures larger than 5 mm (k < 12 cm -1 ) For poloidal velocities typical separatrix position 35 For n e profiles 3 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 25 15 … Li-BES channel … dead channel … Li-IXS channel (new) (old) KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 12

Li-BES raw data Blobs are visible in the rawdata: � Blob = signal exceeds 2.5 σ � Blobs propagate radially Blob amplitude � background � Separatrix Flux surfaces Li-BES x s 12 11 10 9 8 7 6 5 4 3 2 1 12 11 10 9 8 7 6 5 4 3 2 1 0 6 Magnetic axis Limiter KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 13

Conditional averaging of raw data Averaging procedure: � Save short time windows where signal exceeds 2.5 σ at reference channel � Average time windows � Analyze averaged data KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 14

Conditional averaging of raw data Averaging procedure: � Save short time windows where signal exceeds 2.5 σ at reference channel � Average time windows � Analyze averaged data KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 15

Results from conditional average Define center-of-mass coordinates and get: � Blob width (HWHM at ∆ t = 0) � Lifetime (FHWM at reference channel) � Blob amplitude � Blob frequency � Average radial velocity � Maximum radial velocity KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 16

Radial profiles of blob properties General blob properties: � Blob width: � Few cm � Increasing towards wall � Maximum radial velocity � Between 200 and 1000 m/s � Decreasing towards wall � Blob frequency: � Several hundreds per second KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 17

Outline Part I: Introduction: What are blobs? Part II: Blob measurements with Li-BES Part III: Comparison with blob scaling laws KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 18

Aim: large variation in velocity v b and size δ δ δ δ b Velocity-size relation for sheath-connected blobs: [P. Manz et al ., Phys. Plasmas (2013) ] B-field scan and variation in L || : Density constant: n e = 2.5 10 19 m -3 Ohmic heated L-mode KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 19

Comparison to theoretical scalings Size scaling: � Blob width does not well fulfill expected cold ion scaling ( τ i = T i /T e = 0) [S. I. Krasheninnikov et al. J. Plasma Physics (2008)] � BUT : agreement improves taking into account warm ions ( τ i = 3) � Additional factor [P. Manz et al ., Phys. Plasmas (2013)] KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 20

Comparison to theoretical scalings Velocity-size relation: � Decrease of velocity with size (agreement with sheath-connected scaling law!) KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 21

Comparison to theoretical scalings Velocity-size relation: � Decrease of velocity with size (agreement with sheath-connected scaling law!) � Absolute values in the same order of magnitude � Maximum velocities are relevant (Garcia et al., PoP 2006): � Warm ion scaling fits better [P. Manz et al ., Phys. Plasmas (2013)] KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 22

Summary and Outlook The new Li-BES system is well suited to measure radial profiles of � Blob width � Blob amplitude � Blob lifetimes � Blob velocities � Blob frequency From comparison with scaling laws we learned: � AUG blobs are well described by the sheath-connected scaling law (at least for low densities as expected) � Size and velocity scalings fit better to measurements if warm ion effects are taken into account Outlook: � Comparison with turbulence codes (GEMR) � Estimation of wall erosion induced by blobs � 2D measurement with poloidally displaced channels (poloidal velocities!) KSTAR conference, 24 th -26 th February 2014 G. Birkenmeier 23