Characterization of the ion temperature spatial distribution in the island divertor of W7-X

Matt Kriete¹

V. Perseo², D. Gradic², D.A. Ennis¹, R. König², D.A. Maurer¹, and the W7-X Team²

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¹Auburn University

²Max Planck Institute for Plasma Physics

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Stellarator Seminar Series

March 7, 2025

Outline

• Overview of the W7-X island divertor scrape-off layer

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• Coherence imaging spectroscopy measurements of impurity ion temperatures near the divertor in attached conditions

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• Collisional-radiative modelling to relate impurity and main ion temperatures

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• Temperature evolution approaching detachment

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D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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W7-X uses the island divertor concept to exhaust heat and particles

• 5 islands in standard magnetic configuration
• Carbon plasma facing components (PFCs)
• Parallel and perpendicular transport carry heat/particles from upstream region (near last closed flux surface) to downstream region (near divertor)

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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W7-X scrape-off layer (SOL) consists of magnetic islands intersected by divertor targets

core plasma

magnetic islands

Measuring ion temperature near the divertor is crucial to develop the island divertor physics basis

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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P.C. Stangeby, PPCF 60 044022 (2018)

C²⁺ impurity ion temperature measured using high-resolution Doppler spectroscopy

D. Gradic et al., NF 61 106041 (2021)

Stellarators SOLs are fully 3D → need diagnostics with high spatial information to fully characterize edge plasma

Coherence imaging spectroscopy is a camera-based diagnostic that simultaneously measures multiple spectroscopic quantities

• Coherence imaging spectroscopy (CIS): camera-based polarization interferometer that can measure line-integrated parameters using emission from well-isolated impurity emission lines
• CIS well-established for impurity radiation and flow velocity measurements^1,2
• Recently advanced to measure impurity ion temperature³

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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¹V. Perseo et al, RSI 91 013501 (2020)

²V. Perseo et al, NF 59 124003 (2019)

³D.M. Kriete et al, RSI 95 073503 (2024)

CIS technique has recently been advanced to measure impurity ion temperatures in the scrape-off layer

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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CIS C III intensity image

¹D.M. Kriete et al, RSI 95 073503 (2024)

C²⁺ ion temperature image

Multiple carbon radiation bands exist in the divertor SOL

• Standard magnetic configuration typically exhibits three radiation bands near divertor:
• Primary strike line on horizontal target
• “Secondary” strike line on horizontal target
• Vertical target primary strike line

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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How do C²⁺ temperatures in each radiation band compare and vary across edge plasma parameter space?

20230215.056

t = 1.0–3.5 s

20230215.056

t = 1.0–3.5 s

C²⁺ divertor temperature distribution measured across W7-X standard configuration parameter space

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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¹C. Killer et al, PPCF 61 125014 (2019)

²Y. Gao et al, NF 59 106015 (2019)

How do C²⁺ temperatures measured by CIS reflect the main ion temperature?

Coupling between measured impurity ion temperatures and main ion temperature depends on plasma parameters near divertor

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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Temperature of any carbon charge state governed by two processes [J.D. Hey et al., J. Phys. B 35 1525 (2002)]:

• Collisional heating by main ions → thermalization
• Ionization to next charge state → terminates heating

Calculated with ColRadPy:

C.A. Johnson et al., NME 20 100579 (2019)

C²⁺ lifetime

C²⁺ temperature is coupled to main ion temperature up to ≈25 eV

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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C²⁺ temperature relative to main ion (hydrogen) temperature

long C²⁺ lifetime

rapid C²⁺ heating

High temperature solution typically excluded by target Langmuir probe and He beam spectroscopy measurements

C²⁺ temperature measurements are a decent proxy for main ion temperature over most of parameter space

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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High temperature solution typically excluded by target Langmuir probe and He beam spectroscopy measurements

CIS measurements during density ramp to detachment in high-mirror configuration

• Strongest C III radiation near vertical target strike line
• Gradual decrease of C²⁺ temperature throughout density ramp
• High temperature structure pops up during detached phase
• Diagnostic artifact or real?

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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C²⁺ temperature evolution averaged over bright C III bands (high-mirror density ramp to detachment)

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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Summary and outlook

D.M. Kriete | Stellarator Seminar Series | March 7, 2025

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This work has been supported by US Department of Energy grant DE-SC0014529