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

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

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Max-Planck-Institut fr Plasmaphysik Blob filament measurements with lithium beam emission spectroscopy in

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SLIDE 1
  • G. Birkenmeier

1

KSTAR conference, 24th-26th February 2014

Max-Planck-Institut für Plasmaphysik

  • Blob filament measurements with lithium

beam emission spectroscopy in the SOL

  • f ASDEX Upgrade
  • G. Birkenmeier,
  • F. Laggner, E. Wolfrum, M. Willensdorfer, R. Fischer,
  • P. Manz, D. Carralero, G. Fuchert,

and the ASDEX Upgrade Team

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SLIDE 2
  • G. Birkenmeier

2

KSTAR conference, 24th-26th February 2014

Outline Part I: Introduction: What are blobs? Part II: Blob measurements with Li-BES Part III: Comparison with blob scaling laws

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SLIDE 3
  • G. Birkenmeier

3

KSTAR conference, 24th-26th February 2014

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) ]

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SLIDE 4
  • G. Birkenmeier

4

KSTAR conference, 24th-26th February 2014

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]

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SLIDE 5
  • G. Birkenmeier

5

KSTAR conference, 24th-26th February 2014

Blob theory focuses on velocity-size relation:

  • Aims of our investigations:

1. Measure the size δ δ δ δbof the blobs 2. Measure the velocity vb of the blobs 3. Compare with theoretical scaling laws Physics-based prediction of perpendicular fluxes

Can we understand the blob dynamics?

[P. Manz et al., Phys. Plasmas (2013)] [T. Ribeiro et al., PPCF (2008)]

vb

δ δ δ δb

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SLIDE 6
  • G. Birkenmeier

6

KSTAR conference, 24th-26th February 2014

Outline Part I: Introduction: What are blobs? Part II: Blob measurements with Li-BES Part III: Comparison with blob scaling laws

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SLIDE 7
  • G. Birkenmeier

7

KSTAR conference, 24th-26th February 2014

Lithium beam emission spectroscopy (Li-BES)

Recipe for Li-BES:

  • Extract Li ions from β-eucryptite (LiAlSiO4) 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

[Schweinzer, PPCF 1992, Fischer, PPCF 2008]

ASDEX Upgrade: Major radius: 1.65 m Minor radius: 0.5 m Volume: 13 m2 B-field (max): 3.9 T Plasma current (max): 1.4 MA Total heating: 27 MW Full tungsten machine

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SLIDE 8
  • G. Birkenmeier

8

KSTAR conference, 24th-26th February 2014

Lithium beam emission spectroscopy (Li-BES)

[Schweinzer, PPCF 1992, Fischer, PPCF 2008]

Emission profile of Li I line: Spectrum:

35 radial channels

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SLIDE 9
  • G. Birkenmeier

9

KSTAR conference, 24th-26th February 2014

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 µs

[M. Willensdorfer et al., PPCF 2014]

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SLIDE 10
  • G. Birkenmeier

10

KSTAR conference, 24th-26th February 2014

Density profiles from Li-BES

  • Detection of the Li I (2p→2s) line radiation along

the beam axis (z)

  • Occupation numbers Ni(z) are modelled by the

following rate equation system

  • Ni(z) depend on ne:

[E. Wolfrum et al., RSI 1993]

Li I @ 670.8 nm Energy E [eV]

[R. Fischer et al., PPCF 2008] measured calculated Probabilistic data analysis to determine ne

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SLIDE 11
  • G. Birkenmeier

11

KSTAR conference, 24th-26th February 2014

Comparison to other diagnostics

Equilibrium profiles agree:

  • Li-BES (old)
  • Li-BES (new)
  • Edge Thomson scattering
  • Core Thomson scattering
  • Sweeping reflectometer

Frequency spectra agree: Li-BES and sweeping reflectometer Li-BES and Langmuir probes

[M. Willensdorfer, PPCF 2014]

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SLIDE 12
  • G. Birkenmeier

12

KSTAR conference, 24th-26th February 2014

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)

New optics for fluctuation measurements

typical separatrix position

8 1 5 9 14 18 22 26 25 24 23 4 3 2 21 16 12 10 20 19 17 15 13 11 7 6

3 15 35

… dead channel … Li-BES channel (new) … Li-IXS channel (old)

25

For ne profiles For poloidal velocities

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SLIDE 13
  • G. Birkenmeier

13

KSTAR conference, 24th-26th February 2014

Li-BES raw data

Blobs are visible in the rawdata:

  • Blob = signal exceeds 2.5σ
  • Blobs propagate radially
  • Blob amplitude background
8 1 5 9 4 3 2 12 10 11 7 6 8 1 5 9 4 3 2 12 10 11 7 6

xs 6 Separatrix Flux surfaces Limiter Magnetic axis Li-BES

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SLIDE 14
  • G. Birkenmeier

14

KSTAR conference, 24th-26th February 2014

Conditional averaging of raw data

Averaging procedure:

  • Save short time windows where signal

exceeds 2.5σ at reference channel

  • Average time windows
  • Analyze averaged data
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SLIDE 15
  • G. Birkenmeier

15

KSTAR conference, 24th-26th February 2014

Conditional averaging of raw data

Averaging procedure:

  • Save short time windows where signal

exceeds 2.5σ at reference channel

  • Average time windows
  • Analyze averaged data
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SLIDE 16
  • G. Birkenmeier

16

KSTAR conference, 24th-26th February 2014

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
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SLIDE 17
  • G. Birkenmeier

17

KSTAR conference, 24th-26th February 2014

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
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SLIDE 18
  • G. Birkenmeier

18

KSTAR conference, 24th-26th February 2014

Outline Part I: Introduction: What are blobs? Part II: Blob measurements with Li-BES Part III: Comparison with blob scaling laws

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SLIDE 19
  • G. Birkenmeier

19

KSTAR conference, 24th-26th February 2014

Velocity-size relation for sheath-connected blobs: B-field scan and variation in L||: Density constant: ne = 2.5 1019 m-3 Ohmic heated L-mode

Aim: large variation in velocity vb and size δ δ δ δb

[P. Manz et al., Phys. Plasmas (2013) ]

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SLIDE 20
  • G. Birkenmeier

20

KSTAR conference, 24th-26th February 2014

Size scaling:

  • Blob width does not well fulfill expected

cold ion scaling (τi = Ti/T

e = 0)

  • BUT: agreement improves taking into

account warm ions (τi = 3)

  • Additional factor

Comparison to theoretical scalings

[S. I. Krasheninnikov et al. J. Plasma Physics (2008)] [P. Manz et al., Phys. Plasmas (2013)]

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SLIDE 21
  • G. Birkenmeier

21

KSTAR conference, 24th-26th February 2014

Velocity-size relation:

  • Decrease of velocity with size

(agreement with sheath-connected scaling law!)

Comparison to theoretical scalings

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SLIDE 22
  • G. Birkenmeier

22

KSTAR conference, 24th-26th February 2014

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

Comparison to theoretical scalings

[P. Manz et al., Phys. Plasmas (2013)]

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SLIDE 23
  • G. Birkenmeier

23

KSTAR conference, 24th-26th February 2014

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!)