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

Influence of dynamic pressure induced by transient recycled neutral flux onreduction of pulsed particle load in the linear plasma device Magnum-PSI
Y. Hayashi1,2, H. Tanaka3, N. Ohno3, S. Kajita4, T.W. Morgan5, H.J. van der Meiden5, J. Scholten5,J.W.M. Vernimmen5 H. Natsume6, K. Sawada7, S. Masuda7
1National Institute for Fusion Science, 2SOKENDAI, 3Nagoya University,
4
The University of Tokyo, 5Dutch Institute for Fundamental Energy Research,
6
National Institutes for Quantum Science and Technology, 7Shinshu University
e-mail (speaker): hayashi.yuki@nifs.ac.jp
Detached divertor operation is an essential scenario toreduce the divertor heat load and to protect plasma
facing components in fusion devices. Because inconventional tokamaks transient heat loads are released
from the confinement region due to edge localized mode(ELM), both steady-state and transient heat loads arerequired to be handled in scrape-off layer (SOL) anddivertor region. The interaction between the detachedplasma and the transient heat load should be investigatedunder the high density and high particle flux condition
which is similar to the divertor plasma in ITER. The
linear plasma device Magnum-PSI enables thisexperiment because of its high performance for steadystate plasma generation with particle flux up to 1025m-2s-1[1] and pulse plasma production. In the present
study, the impacts of a pulsed plasma input into thedetached plasma on the particle load were investigatedand discussed from the experiments and modeling inMagnum-PSI.
Figure 1 schematically shows the experimental
configuration used in the present study. Steady-state
plasma is generated by a cascaded arc source. In parallelwith this, a capacitor bank system transiently increased
the discharge power for ~ 1 ms, leading to increase in theelectron temperature, Te, and electron density, ne, in theplasma source region [2]. The steady-state and transientpulsed plasmas were measured using a Langmuir probe
and target plate. Laser Thomson scattering (TS)measurements and optical emission spectroscopy (OES)
were performed during the steady-state phase. The
dynamic response of the steady state plasma in front of
the target plate was measured by the ion saturation
current, Isat, with the Langmuir probe and target current,Itarget, with a negatively biased target plate.
Figure 2 shows the time evolutions Itarget in various
neutral pressure, pn, conditions. The low and high pncases are separately shown on the figures 2(a) and (b),respectively. The trigger timing of the pulse
corresponded to a time, t, of 0 ms. The time width of thepulsed plasma should be ~ 0.8 ms because waveforms ofthe discharge power show that under the all pn conditions.
However, as shown in figure 2(a), the particle flux was
not maintained for ~ 0.8 ms at the target plate. The
waveforms of Itarget indicate that the pulse duration
detected at the target decreased with a drop in pn. Similartendency was not observed by the Langmuir probe
located at 200 mm upstream. Because the first parts ofthe pulsed plasma were observed, their last ones wereconsidered to be partially suppressed in front of thetarget plate. In high pn cases, on the other hand, Itarget in
all cases were maintained at least for ~ 0.8 ms as shownin figure 2(b). The waveforms of Itarget were mitigated byincreasing pn. The reduction in Itarget in the high neutralpressure cases might be due to the enhancement of aplasma-neutral interaction.
Regarding the low pn cases, because the mean-free
path of the charge exchange was shorter than that of then-n elastic scattering, the impact of the recycled neutral
flux on the pulsed plasma suppression by the transientenhancement of the plasma-neutral interaction wasspeculated. Modeling was performed by coupling theLINDA (LINear Divertor Analysis) code [3] with aneutral transport code [4]. The modeling results showedthat the dynamic pressure induced by the transient
recycled neutral flux in the opposite direction from thetarget caused a sufficient momentum loss to stagnate thepulsed plasma.

Figure 1. Schematic of experiments in Magnum-PSI.

Figure 2. Time traces of Itarget during the pulsed plasma.
References[1] H.J.N. van Eck, et al., Fusion Eng. Des. 142 (2019)26.
[2] J.J. Zielinski, et al., Plasma Sources Sci. Technol. 21(2012) 065003.
[3] T. Furuta, et al., Fusion Sci. Technol. 63 (2013) 411.
[4] K. Sawada, et al., Contrib. Plasma Phys. 60 (2020)
e201900153.