Description
The scrape-off-layer (SOL) heat-flux width, λq, is a key parameter for plasma exhaust and power handling, but its scaling in stellarators remains poorly understood [1]. In tokamaks, λq scales inversely with the poloidal magnetic field, with weak dependence on heating power or machine size (Eich-Goldstein law [2]). The present experiment aims to determine the dependence of λq on magnetic field strength B and heating power Ph in the TJ-II stellarator [3] using radial Langmuir probe (LP) measurements.
In addition to the scaling study, this radial characterization will further contribute to determining effective perpendicular transport coefficients in reduced SOL physics-models. We do this by quantitative comparison between experimental measurements and the EMC3_lite code (using an anisotropic heat diffusion model) [4], as the transport parameters in the latter are varied.
TJ-II operates at nominal and half nominal B, heated by ECRH (2x300kW) [5] and NBI (2x450kW) [6]. Power heating modulation is also performed. Reciprocating multi-pin LPs [7] will measure ρ>0.85, with simultaneous multi-point acquisition. TJ-II operates in a limiter configuration; the probes access the plasma from the D4-sector top port, separated from the limiter region. Swept measurements (100 kH, –150 V--150 V) will be recorded at ≥2 MHz to reconstruct the I–V characteristics.
Density, electron temperature, and potential will be derived from I–V curves using standard fitting [8]. Radial electric fields will be calculated from the plasma potential gradient. Complementary diagnostics (Thomson scattering, interferometry, and He-beam [9]) will provide calibration and verification. Data will be averaged over the sweep period, justified by separation between the probe sweep frequencies and higher-frequency turbulence present in the SOL [10].
This campaign provides the first systematic measurements of the SOL λq in TJ-II as a function of B and Ph. Comparison with tokamak scalings clarifies the role of 3D magnetic geometry in stellarator SOL transport. The data enable evaluation of effective transport coefficients for SOL modeling, supporting reactor-relevant stellarator design and future multi-machine validation.
[1] J.A. Alonso et al., Nucl. Fusion, 62 036024 (2022)
[2] T. Eich et al., Nucl. Fusion, 53 093031 (2013)
[3] J.A. Alonso et al., Nucl. Fusion, 64 112018 (2024)
[4] Y. Feng et al., Plasma Phys. Control. Fusion 64 125012 (2022)
[5] J. Martínez-Fernández et al., Fusion Eng. And Design, 161 112065 (2020)
[6] M. Liniers et al., Fusion Eng. And Design, 88 960963 (2013)
[7] C. Killer et al., J. Instrum. 17 P03018 (2022)
[8] R. B. Lobbia and B. E. Beal, J. Propul. Power 33 566–581 (2017)
[9] A. González Ganzábal et al., Fusion Eng. Des., 195, 115613 (2025)
[10] G. Grenfell et al., Nucl. Fusion 59 016018 (2019)