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Impurity flow measurements with Coherence Imaging Spectroscopy at Wendelstein 7-X

Valeria Perseo

12.02.2021, PPPL Stellarator Seminars

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Teaser: studies of SOL particle flows

valeria.perseo@ipp.mpg.de

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12.02.2021

CIS measurement

EMC3-EIRENE modelling

From C III emission

[V. Perseo, NF 59 (2019) 124003]

Spectroscopic studies of SOL flow by detecting visible light from impurity spectral emission

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The Wendelstein 7-X experiment

valeria.perseo@ipp.mpg.de

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Non-planar coils

ι

5/5

5/4

5/6

Planar coils

R_maj = 5.5 m

r_min ≈ 0.5 m

B_axis = 2.5 T

Poloidal turns

Toroidal turns

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SOL concept for W7-X

Magnetic island twist around the central plasma

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Example of

standard configuration

SOL with 5

independent islands

and connect the different divertor modules

W7-X divertor:

island divertor

10 units

made of fine graphite

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Particle dynamics in the SOL – parallel flows

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distance from LCFS

pressure

LCFS

Example: SOL width

sink

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Parallel flows – expectations for W7-X

Each island is cut by divertor units

🡪 every magnetic field line would show a similar behaviour to 1D model

In one island: different regions are connected to different divertor modules

🡪 For predicting correctly the particle flows: need of fully 3D simulations

🡪 EMC3-EIRENE: 3D SOL physics simulations (Monte-Carlo method)

valeria.perseo@ipp.mpg.de

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Simulated

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Teaser: studies of SOL flows

valeria.perseo@ipp.mpg.de

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12.02.2021

CIS measurement

EMC3-EIRENE modelling

From C III emission

[V. Perseo, NF 59 (2019) 124003]

Spectroscopic studies of SOL flow by detecting visible light from impurity spectral emission

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SOL flows 2D pictures: Coherence Imaging Spectroscopy

Complex 3D SOL of W7-X requires measurements with good spatial coverage

🡪 Hard with conventional flow diagnostics (Mach probes, dispersive spectroscopy)

valeria.perseo@ipp.mpg.de

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12.02.2021

Spectral line: C III

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Parallel flows in the SOL – impurity case

valeria.perseo@ipp.mpg.de

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12.02.2021

Ion friction force

Drag of the main ion on the impurity, towards the target

Ion thermal force

From collisionality dependence on temperature, away from the target

If friction force dominates 🡪 impurity good proxy for main ions

For investigated W7-X scenarios: friction force regime 🡪 CIS can give info on main ions

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CIS diagnostic – basic components

CIS polarization interferometer:

2 beam interferometer where the beams are the polarizations of the incoming light

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12.02.2021

Fiber bundle

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CIS – the interferometer cell

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Polarizer (a):

Linear polarization of incoming light

🡪 two polarizations with equal amplitude on birefringent crystal

Polarizer (b):�Linear polarization of out-coming light

🡪 two polarizations back on the same� plane and with equal amplitude

🡪 Alignment of optical axes determines the effectiveness of the interferometer

🡪 Birefringent crystals determine the resolution of CIS in measuring the spectroscopic quantities

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The CIS interferogram

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From one image 🡪 3 parameters

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The CIS interferogram – phase and velocity

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CIS at W7-X and its calibration

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Major upgrade at W7-X: calibration source
Continuous-wave emission tunable laser

Sub-pm precision for wavelength �in 450-650 nm
Fine wavelength scans possible

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Importance of precise calibration

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12.02.2021

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W7-X CIS – steps forwards in the diagnostic field

valeria.perseo@ipp.mpg.de

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[V. Perseo, RSI 91 (2020) 013501]

[D. Gradic FED 146 (2019) 995-998]

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CIS views in W7-X

Two views on lower divertor in module 3 with different crystal configurations optimized for flow velocity measurements

Studies of behavior of C²⁺ 🡪 C III filter used
C²⁺ max photoemission coefficients for �T_e = 10-20 eV 🡪 C III radiation mostly �localized in the SOL

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CIS reliability: comparison with other diagnostics

Other flow diagnostics: Mach probes, dispersive spectroscopy

CIS system shares lines of sight with high wavelength resolution spectrometer (1.2 pm/pixel)

🡪 Direct impurity flow velocities comparison between the two diagnostics (C III spectral line)

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Comparison with Mach probes in attachments

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Diagnostic comparison – dispersive spectroscopy

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42

20181016.19

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CIS validation recap and CIS exploitation

First visualization of counter-streaming flow

Successful comparison with EMC3-EIRENE (clear correspondence in structure and similar order of magnitude)

Successful comparison with other flow diagnostics

Dispersive spectroscopy: similar velocity range, same clear changes in flows

🡪 Next: CIS exploitation

Exploitation of diagnostic capabilities: 2D spatial coverage and high time resolution

Forward and reversed field experiments
Different magnetic configurations
Impurity seeding
Increase of net toroidal current
Detachment

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CIS exploitation: forward and reversed field experiments

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forward

reversed

Forward

Reversed

[V. Perseo, NF 59 (2019) 124003]

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CIS exploitation: magnetic topology effects on flows

W7-X: possible to change the edge magnetic topology with planar coils� 🡪 change in number of magnetic islands, followed by flow structure

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Standard: 5 islands

High iota: 4 islands

Low iota: 6 islands

[V. Perseo, PhD thesis (2020)]

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CIS exploitation: impurity seeding

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Ne puffs

[F. Effenberg, NF 59 (2019) 106020]

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CIS exploitation: increase of toroidal current

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mapping time traces to plasma current

20181010.16

[C. Killer, PPCF 61 (2019) 125014]

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CIS exploitation: detachment

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CIS exploitation: fast and clear transition to detachment

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Domain for analysis

[V. Perseo for IAEA FEC 2020-2021]

Decrease in velocity could be related to:

Decrease in parallel convective heat transport
Decrease of SOL pressure
Radiation movement in region characterized by lower velocities

🡪 Phenomenon currently under investigation

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Summary

Coherence imaging spectroscopy: powerful and reliable 2D interferometric diagnostic

Fully characterized thanks to new calibration source and methods
Successfully optimized for flow velocity measurements in the SOL of W7-X, sustained by comparison with Mach probes and dispersive spectroscopy

New insights on W7-X SOL:

First direct visualization of counter-streaming flows

🡪 successful comparison with EMC3-EIRENE synthetic camera

First measurements of counter-streaming parallel flows in forward and reversed field experiments

🡪 evidence of importance of drifts for future modelling

Identification of the influence of the magnetic island number and position on the flows� 🡪 successful analysis in different magnetic configuration and with increasing plasma current
Identification of decrease of heat loads with decrease in flow velocities� 🡪 first flow velocity measurements during detachment and impurity seeding experiments

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