[Paper Review] High flux expansion divertor studies in NSTX

V. Soukhanovskii, R. Maingi+11 more|ArXiv.org|Dec 21, 2009

Magnetic confinement fusion research1 references3 citations

TL;DR

This study investigates high flux expansion divertor configurations in the National Spherical Torus Experiment (NSTX) by varying the X-point height ($h_X$) to reduce peak heat flux on the divertor target. By increasing poloidal flux expansion from 6 to 26, the peak heat flux dropped from 7–8 MW/m² to 1–2 MW/m², demonstrating that flux expansion is the dominant mechanism for heat flux reduction, with the divertor approaching detachment at low $h_X$. The results support the use of high flux expansion in compact spherical tokamaks to mitigate heat loads and improve plasma performance.

ABSTRACT

High flux expansion divertor studies have been carried out in the National Spherical Torus Experiment using steady-state X-point height variations from 22 to 5-6 cm. Small-ELM H-mode confinement was maintained at all X-point heights. Divertor flux expansions from 6 to 26-28 were obtained, with associated reduction in X-point connection length from 5-6 m to 2 m. Peak divertor heat flux was reduced from 7-8 MW/m$^2$ to 1-2 MW/m$^2$. In low X-point configuration, outer strike point became nearly detached. Among factors affecting deposition of parallel heat flux in the divertor, the flux expansion factor appeared to be dominant

Motivation & Objective

• To investigate the impact of divertor geometry, specifically X-point height ($h_X$), on heat and particle fluxes in spherical tokamaks.
• To assess whether high flux expansion can reduce peak heat flux on the divertor target to acceptable levels in compact, high-performance H-mode plasmas.
• To evaluate the role of flux expansion versus other loss mechanisms (e.g., conduction, radiation) in divertor heat flux mitigation.
• To determine the conditions under which outer strike point detachment is achieved and particle recycling is improved.

Proposed method

• Systematically varied the X-point height ($h_X$) from 5–6 cm to 18–22 cm in lower single null H-mode discharges at 1 MA and 6 MW neutral beam heating.
• Maintained steady-state configurations by adjusting the divertor PF1A coil current (10–18 kA), enabling shot-to-shot control of plasma vertical position.
• Measured divertor parameters using infrared thermography, spectrally filtered cameras, UV-visible spectrometry, and pressure gauges to assess $T_e$, $n_e$, $q_{pk}$, $D_\alpha$ brightness, and neutral pressure.
• Calculated flux expansion factor $f_{exp} \simeq (B_\theta/B_{tot})^{MP}/(B_\theta/B_{tot})^{SP}$ and connection lengths $L_X$ and $L_c$ to quantify geometric effects.
• Mapped peak heat flux to midplane using $q_{\parallel} = q_{pk}/\sin\gamma$, where $\gamma$ is the field line incidence angle.
• Analyzed the relative contributions of flux expansion, conduction, and radiation losses to heat flux reduction using measured $q_{pk}$, $P_{div}$, and $T_e$ profiles.

Experimental results

Research questions

• RQ1 How does varying the X-point height ($h_X$) affect the peak heat flux on the divertor target in high triangularity spherical tokamak plasmas?
• RQ2 To what extent does flux expansion dominate over other loss mechanisms (e.g., conduction, radiation) in reducing divertor heat flux?
• RQ3 Does decreasing $h_X$ lead to outer strike point detachment, and what evidence supports this?
• RQ4 How does divertor geometry influence neutral pressure, particle recycling, and deuterium compression in the divertor region?
• RQ5 What is the relationship between $h_X$, flux expansion factor $f_{exp}$, and connection length $L_X$ in determining divertor performance?

Key findings

• Peak divertor heat flux decreased from 7–8 MW/m² at high $h_X$ (18–22 cm) to 1–2 MW/m² at low $h_X$ (5–6 cm), demonstrating a strong inverse correlation with $h_X$.
• The flux expansion factor $f_{exp}$ increased from ~6 to ~26 as $h_X$ decreased from 23 cm to 7 cm, indicating significant geometric flux spreading.
• At low $h_X$, divertor $T_e$ decreased and $n_e$ increased, with elevated recombination rates and $D_\alpha$ brightness, indicating the outer strike point was approaching detachment.
• The divertor compression factor $P_{div}/P_{mid}$ increased nearly linearly with decreasing $h_X$, suggesting improved neutral entrapment and reduced recycling efficiency.
• The X-point to target connection length $L_X$ increased from 2 m to 5–6 m as $h_X$ decreased, while the midplane-to-target length $L_c$ remained nearly constant at 15±2 m.
• The peak divertor power $Q_{div}$ varied weakly between 1.5–2 MW, indicating that increased radiated power at low $h_X$ contributed to heat flux reduction.