7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

High neutral particle pressure in the divertor section by low temperature modein LHD
G. Motojima1, 2, U. Wenzel3, M. Kobayashi1, 2, M. Goto1, 2, Y. Hayashi1, 2, S. Masuzaki1, 2, G.
Kawamura1, 2, B. Jagielski3 and V. Haak3
1 National Institute for Fusion Science (NIFS), 2 SOKENDAI (The Graduate University for
Advanced Studies), 3 Max Planck Institute for Plasma Physics
e-mail (speaker): motojima.gen@nifs.ac.jp
In the LHD (Large Helical Device) helical divertor,
extremely high neutral particle pressure has beenobserved at a specific magnetic configuration [1]. A
pressure up to a maximum of 1.4 Pa has been achieved,
which is equivalent to the pressure achieved in thepoloidal divertor in tokamaks. This high neutral particle
pressure is considered to be a result of very low
temperature within the divertor plasma with low
temperature from the various measurements and shows
strong dependence on the magnetic configuration.
Divertor detachment is crucial for reducing the thermalload in the divertor, but it also leads to a decrease inparticle flux to the divertor. Maintaining high neutralpressure within the divertor during detachment isimportant for efficient particle exhaust, and this study
indicates the potential of achieving high neutral particle
pressure during detachment using the helical divertor.
Figure 1 shows the variation of the neutral particle
pressure with different magnetic configurations alongwith typical plasma parameters. In these shots theplasma density was ramped up to about 15 × 19 m−3.
The plasma was heated with tangential andperpendicular neutral beams (traces a and f). In both
plasmas an energy of about 800 kJ was stored,decreasing toward the end of the shot due to the highdensity at constant heating power. Fig. 1 shows also thesub-divertor pressures and the ion saturation currents of
the Langmuir probes. The sub-divertor pressures weremeasured by fast ionization gauges which enable thedirect measurement in the sub-divertor [2], one locatedin the 6I section and another one in the 8I section (forthe geometry of the set-up see Fig. 2 in [2]). The letter“I” stands for inboard side and the number stands forthe toroidal position in the LHD. The ion saturation
currents were measured with two probe arrays mounted
in the target plates in section 8I. Fig. 1(e) and (j) show
the sums over the left array, the sums over the rightarray, and the total ion saturation current giving anapproximate measure of the particle fluxes to thetargets.
In LHD, different magnetic configurations can beachieved by shifting the magnetic axis position (Rax),
which was found to influence the particle flux
distribution [3]. The study demonstrated that the highneutral particle pressure in the divertor section wasachieved at a relatively inner magnetic axis position ofRax = 3.55 m, corresponding to a pressure approximately
five times higher than in the magnetic configuration withthe magnetic axis shifted 5 cm outward (Rax = 3.60 m).
Measurements such as divertor spectroscopy wereconducted to investigate the cause of the high neutral
particle pressure in the divertor section, leading to theestimation of temperatures around the divertor section at
Rax = 3.55 m in the range of 0.25-0.42 eV, indicating atemperature range in that volume recombination occurs.
In this presentation, we will further discuss themechanism behind the observed high neutral pressure.
The data supporting the findings of this study areavailable in the LHD experiment data repository at
https://doi.org/10.57451/lhd.analyzed-data.

Figure 1: Time traces of two plasma shots with different
positions of the magnetic axis: Rax = 3.6 and 3.55 m. (a,f)
heating powers, (b,g) stored energy, (c,h) line-averaged
densities, (d,i) sub-divertor neutral pressures, and (e,j) ion
saturation currents of the target mounted Langmuir probearrays.
References[1] U. Wenzel et al., submitted to PRL 2023.
[2] G. Motojima et al.,, Physica Scripta, 97(3):035601,2022.
[3] T. Morisaki et al.,e Nucl. Fusion 53 063014 (2013).