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An international journal of news from the stellarator community
Editor: James A. Rome Issue 174 August 2021
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Continuous cryogenic pellet
fueling system for the W7-X
stellarator
Toroidal magnetic fusion reactors such as ITER will be
fueled by cryogenic deuterium-tritium pellets (15 K)
accelerated to high velocity (200–1000 m/s) into the hot
plasma. Over the last 40 years, Oak Ridge National Laboratory
(ORNL) and its partners have developed the physics
and technology of pellet fueling and demonstrated its use
in large tokamak and stellarator plasma confinement
experiments around the world.
These experiments used plasmas lasting some seconds, but
power reactors will require continuous fueling. The next
stage of pellet fueling experiments will be carried out
using the Wendelstein 7-X (W7-X), a large (major radius =
5.5 m) stellarator device at the Max Planck Institute for
Plasma Physics (IPP), in Greifswald, Germany. W7-X has
superconducting magnetic coils and is designed to sustain
high-performance plasmas for up to 30 minutes.
Exploratory experiments on W7-X in 2018 [1] showed
that the injection of 50 2-mm H2 pellets at a velocity of
200 m/s could sustain a high-density (peak ~ 1020 m-3)
plasma heated with 5 MW of microwave power with electrons
and ions at temperatures ~ 3 keV for about a second.
Physics analysis shows that these pellet-fueled plasmas
have reduced turbulent transport, which improves the
plasma energy confinement [2].
An international team is now constructing a continuous,
high-speed pellet system to fuel W7-X plasmas in quasi
steady-state conditions. The team includes researchers
from ORNL, IPP, Princeton Plasma Physics Laboratory
(PPPL) and the National Institute for Fusion Science
(NIFS) in Japan.
The W7-X continuous pellet fueling system will inject
3-mm H2 or D2 pellets into W7-X at speeds of up to
1000 m/s with a repetition rate of up to 10 pellets/s. The
pellet size and repetition rate will be controlled in real
time to maintain optimum plasma profiles for confinement.
The major challenge for the design of the pellet system
was extrusion under screw pressure of a ribbon of solid
hydrogen while maintaining a constant temperature of
15 K using cryocoolers for cooling (Fig. 1).
In this issue . . .
Continuous cryogenic pellet fueling system for
the W7-X stellarator
ORNL and its partners are constructing a continuous
pellet injector for use in Wendelstein 7-X. It will inject
3-mm H2 or D2 pellets into W7-X at speeds of up to
1000 m/s with a repetition rate of up to 10 pellets/s. It
will be tested this autumn and shipped to Greifswald in
early 2022. ............................................................... 1
FY2021 LHD Research Forum
The FY2021 Large Helical Device Research Forum
was held 12–16 July 2021 via Zoom. This forum is
intended to allow the proponents to present and discuss
ideas for experiments to be conducted in the
upcoming 23rd LHD experiment campaign. 192 experiment
idea proposals were submitted; 47 were from
overseas institutions. .............................................. 2
Fig. 1. Continuously extruded H2 ribbon, 3-mm diameter.
Stellarator News -2- August 2021
The continuous pellet system is now being assembled at
ORNL (see drawing in Fig. 2) and will be fully tested in
autumn 2021 before shipment to Germany in early 2022
and integration into the W7-X facility in 2022, with
plasma experiments starting late that year.
References
[1] J. Baldzuhn et al., Plasma Phys. Control. Fusion 61,
095012 (2019).
[2] P. Xanthopoulos et al., Phys. Rev. Lett. 125, 075001
(2020).
[3] S. J. Meitner et al., IEEE Trans. Plasma Sci., 48, 1585
(2019).
L. R. Baylor,1 J. Baldzuhn,2 S. J. Meitner,1 J. H. Harris,1 G. H.
Neilson,, R. Sakamoto,4 D. A. Gates3
1Oak Ridge National Laboratory, Oak Ridge, TN, USA
2Max Planck Institute for Plasma Physics, Greifswald, Germany
3Princeton Plasma Physics Laboratory, Princeton, NJ, USA
4National Institute for Fusion Science, Toki, Japan
FY2021 LHD Research Forum
The FY2021 Large Helical Device (LHD) Research
Forum was held 12–16 July 2021 via Zoom. This forum is
intended to allow proponents to present and discuss ideas
for experiments to be conducted in the upcoming 23rd
LHD experiment campaign, which will last from the middle
of October 2021 until the middle of February 2022.
Previously, meetings such as this forum have been held for
each topical group. However, we determined to hold the
LHD Research Forum for proponents and co-proponents
because we believe that we can conduct experiments more
efficiently and effectively if all the proponents and co-proponents
know and understand all the experiment ideas.
Because of the COVID-19 pandemic, the LHD Research
Forum was held via Zoom.
For the upcoming campaign, 192 experiment idea proposals
were submitted. Of these, 47 (about 1/4) were from
overseas institutions. The topical groups have been
renewed for the remaining two (23rd and 24th) LHD
experiment campaigns. The areas addressed by the
renewed topical groups are as follows: (1) multi-ion, (2)
turbulence, (3) spectroscopy, and $4) instability. The
breakdown of experiment idea proposals by topical group
was as follows: 58 in the multi-ion plasma topical group,
39 in the turbulence topical group, 41 in the spectroscopy
topical group, and 54 in the instability topical group.
Almost all proposals require experiment time allocation in
the 23rd experiment campaign, and thus those needed to
be explained and discussed in the forum. The experiment
idea proposals were submitted by 111 proponents, 40 of
whom (about 1/3 of the total) are affiliated with overseas
institutions. The number of the experiment idea proposals
from overseas is significantly higher than the previous
campaign. In total, 111 proponents and 187 co-proponents
participated in the LHD Research Forum.
In consideration of the time differences, the first morning
sessions of the forum were scheduled to accommodate
proponents in the US, and the late afternoon sessions of
the forum were scheduled to accommodate proponents in
the EU. Typically, 8–9 experiment idea proposals were
presented in each session. Sessions were typically
attended by about 30–50 participants, who discussed
experiment idea proposals explained by the proponents.
All experiment idea proposals will be reviewed by the topical
group leaders. The review will take into account each
proposal’s scientific quality and consistency with the
research priorities of the 23rd LHD experiment campaign,
based on the explanations and discussions at the LHD
Research Forum and the content of the experiment idea
proposals. The detailed program of the 23rd LHD experiment
campaign will be announced to all the proponents
and co-proponents around the beginning of September.
Fig. 2. Continuous pellet fueling system for W7-X.
Stellarator News -3- August 2021
Finally, we would like to thank all the participants in the
LHD Research Forum. Some EU and US colleagues
joined the forum in the very early morning or very late
night in their time zone. Especially, we would like to thank
once again all those who made such a great effort to participate
in the forum. We believe that the 23rd LHD experiment
campaign will be the most fruitful experiment
campaign based on the beneficial discussions in the LHD
Research Forum.
Naoki Tamura (Secretary of the Forum)
E-mail: tamura.naoki@nifs.ac.jp
National Institute for Fusion Science, Japan
Fig.1. Screenshots (group photos) of participants of
FY2021 LHD Research Forum on Zoom.