Speaker
Tomohiro Morisaki
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O3.101.pdf
Effect of pumped closed helical divertor on edge plasma behavior in LHD
T. Morisaki1,2, G. Motojima11,2, T. Murase1, H. Tanaka3, S. Masuzaki1,2, M. Shoji1,
S. Oliver4, and LHD Experiment Group
1
National Institute for Fusion Science, Toki 509-5292, Japan
2
SOKENDAI, The Graduate University for Advanced Studies, Toki 509-5292, Japan
3
Nagoya University, Nagoya 464-8603, Japan
3
University of Wisconsin, Madison, WI, USA
The closed helical divertor (CHD) with cryogenic pump was installed in LHD, aiming for the
confinement improvement through the effective edge plasma control [1]. Due to the strong
nonuniformity of helical divertor flux distribution, quite large pumping capability is
available only with partial installation of CHD modules along the divertor striking trails. The
measured pumping speed was 675 m3/s which is identical to ~ 20 % of the LHD main
evacuation system, and the pumping capacity was 58,000 Pa m3, which is equivalent to
20,000 deuterium pellets or 20 days of gas amounts of high density experiments [2].
The CHD experiment has started since the last experimental campaign which was the first
deuterium experiment in LHD, and initial results to demonstrate the CHD performance were
obtained. In the experiment, the plasma was produced and maintained by NBI with moderate
power of 10 – 20 MW. Fuelling was performed with pellet injection and/or gas puffing. It
was found that about half of the fuelled gas was efficiently evacuated with pumped CHD,
thus quite low recycling state was obtained, where effective particle confinement time is one
order smaller than that without CHD pumping. This strong controllability for neutral
particles also affects the formation of electron density and temperature profiles. In the
repetitive pellet fuelled plasma, a peaked temperature profile was realized, and higher central
temperature was obtained, compared to the reference discharge without pumping in the same
fuelling condition. However, a hollow profile of electron density was formed, although
continuous pellet fuelling was performed. This result was brought about by the combination
of some physical processes, e.g., heat and particle transport, particle recycling, energy
balance, pellet ablation, etc. At the conference, effects of the pumped CHD on profile
formation are discussed, taking those process into consideration.
[1] T. Murase et al., Plasma. Fusion Research 11, (2016) 1205030.
[2] G. Motojima et al., 2018 Nucl. Fusion 58 014005.
