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An international journal of news from the stellarator community
Editor: James A. Rome Issue 182 February 2023
E-Mail: James.Rome@ stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Can we sustain a high ion temperature
with low Te/Ti ratio in
a burning plasma?
Among the crucial issues in a burning plasma are how to
heat ions using alpha particles and whether a high ion temperature
plasma can be sustained using electron heating. A
high core-ion temperature is usually associated with the
ion transport barrier, and the electron temperature is lower
than the ion temperature (Te/Ti < 1). However, most of the
energy of the alpha particles produced by the nuclear
fusion reaction contributes to electron heating rather than
ion heating because the energy of the alphas is extremely
high compared to the plasma temperature. An important
question is whether the ion transport barrier can be
achieved in the plasmas in which Te/Ti ≥ 1. The threshold
value of Te/Ti is a key parameter for predicting the feasibility
of an ion transport barrier and an ion temperature
high enough to sustain nuclear fusion reactions in a burning
plasma with alpha particle electron heating.
A recent LHD experiment shows the disappearance of the
ion transport barrier (a drop of normalized ion temperature
gradient) with Te/Ti > 0.75, as seen in Fig. 1 [1]. This
experimental result implies that the formation of an ion
transport barrier is impossible with electron heating by
alpha particles, where the Te/Ti ratio cannot be lower than
unity through the electron-ion collision process, which can
take several hundred milliseconds or more. Therefore,
direct ion heating without the electron-ion collision process
is desirable to achieve a low Te/Ti ratio and good ion
confinement (due to the ion transport barrier) as a step
towards realizing a compact fusion reactor.
The MHD wave excited by alpha particles enhances the
energetic particle loss and degrades the confinement.
However, it is speculated that the wave energy could
instead be passed on to fuel ions, through a process known
as alpha-channeling. The key question is to what extent
these effects (energetic particle loss and alpha-channeling)
can be achieved simultaneously.
vv
Fig. 1. The normalized ion temperature gradient as a function
of the ratio of electron temperature (Te) to ion temperature
(Ti). Here R is major radius, and LTi is the scale length
of the ion temperature gradient. (Adapted from Fig.1(b) in
Ref. [1].)
In this issue . . .
Can we sustain a high ion temperature with low
Te/Ti ratio in a burning plasma?
Alpha particles from fusion reactions are much more
energetic than the bulk fusing plasma, and thus slow
down and transfer their energy primarily to the plasma
electrons. But good ion confinement requires an ion
transport barrier, which implies Te/Ti < 1. LHD has
measured direct energy transfer from neutral beam
injected ions to thermal and impurity ions associated
with MHD activity that might be due to Landau or
inverse Landau damping. This mechanism could be
used to enable alphas to directly heat the thermal ions
in a fusing plasma. ......................................................1
The 24th LHD experiment campaign in JFY2022
was finished successfully
The last experiment campaign using deuterium plasmas
was successfully finished. Outputs from the LHD
experiments have been published in important
papers. Although the LHD project has officially ended,
operation of the LHD will be continued for the next
three fiscal years for academic research. The next
campaign will be in the spring of 2024. ................... 3
Stellarator News -2- February 2023
Alpha-channeling is collisionless energy transport from
alpha particles to bulk ions through wave-particle interactions,
known as Landau and inverse Landau damping. The
alpha-channeling can be a solution to achieve plasmas
with lower Te/Ti and high ion temperature with an ion
transport barrier in the burning plasma. The question is:
How much Landau damping and inverse Landau damping?
These well-known basic physics processes have been
observed in magnetosphere and laboratory plasmas. However,
collisionless energy transfer from energetic particle
to bulk ions through these processes has not been directly
observed in a laboratory plasma. Very recently, the deformation
of the ion velocity distribution from Maxwellian
associated with MHD burst events have been observed via
measurements of the ion velocity distribution function
with fast charge-exchange spectroscopy in LHD, as seen
in Fig. 2 [2].
The energy gain due to the collisionless energy transfer is
a 7% increase for bulk ions and a 20% increase for carbon
impurity ions. The energy gain for carbon impurity ions is
three times that for bulk ions. The higher energy gain is
obtained when the thermal velocity is close to the resonance
velocity due to the larger gradient in velocity space.
This observation implies that a significant amount of neutral
beam injection energy can be transferred to the bulk
and impurity ions associated with the MHD burst. Massdependent
collisionless energy transfer from energetic particles
to bulk ions and impurity ions is thus directly
observed for the first time. One-third of the beam energy
loss is transferred to the bulk ions, while two-thirds of the
beam energy loss is due to enhanced particle diffusion
associated with the MHD burst. This experiment suggests
that alpha-channeling is comparable to the particle diffusion
loss and can be a possible scenario for ion heating in
the burning plasma.
Reference
[1] M.Osakabe et. al., Nucl. Fusion 62 (2022) 042019.
[2] K.Ida et. al., Commun. Phys. 5 (2022) 228.
Katsumi Ida
National Institute for Fusion Science
e-mail: ida.katsumi@nifs.ac.jp
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Fig. 2. Ion velocity distribution function before and after the
onset of an MHD burst event for (a) carbon impurity ions
and (b) bulk ions dominated by deuterium. Here τ = 0 is the
onset time of the MHD burst. (c) Increase of kinetic energy
(energy gain) during the MHD burst for carbon impurity
ions and bulk ions. (Adapted from Fig. 6 in Ref. [2].)
Stellarator News -3- February 2023
The 24th LHD experiment campaign
in JFY2022 was finished
successfully
The 24th Large Helical Device (LHD) experiment campaign
in JFY2022 was successfully finished in December
2022. Due to a dramatical increase in fuel costs in Japan,
the 24th experiment campaign, originally scheduled from
the end of September 2022 until the beginning of February
2023, was substantially shortened. However, during the 14
weeks of the campaign until the end of December 2022,
LHD operated for 53 days, and 8,091 discharges were
accomplished. No significant trouble with LHD, heating
or diagnostics occurred during the campaign. During the
past two years, we have learned how to deal with COVID-
19. Consequently, overseas visits to Japan were again
available, albeit with restrictions, so many overseas collaborators
could participate in LHD experiments. And we
continued to conduct remote experiments utilizing the
Zoom and Microsoft Teams applications to expand opportunities
for participation in LHD experiments.
We continued our mission of acquiring scientific knowledge
that will deepen our understanding of magnetically
confined toroidal plasmas. One of the significant outcomes
of this campaign was the first observation of the
process of waves carrying plasma heat, which was published
in Communications Physics (also see preceding article).
Plasma heating via the interaction with the
electromagnetic waves generated in the plasma is a critical
process to maintain ignited fusion plasmas and also causes
particle acceleration in the Earth’s magnetosphere.
And, of course, the LHD experiment contributed to fusion
plasma research. For example, A. Matsuyama (QST) published
a paper in Physical Review Letters describing how
deeper penetration of the hydrogen pellets into the hightemperature
plasmas can be achieved. This result will contribute
to the establishment of plasma control technologies
for future fusion reactors, including ITER in France. More
information on these and other significant scientific results
obtained in the LHD experiment campaign can be found
here. LHD/NIFS has also made significant contributions
to open science. For example, any researcher can access
LHD experimental data through the LHD Experiment
Data Repository. We very much hope that researchers
unable to participate in the 24th LHD experiment campaign
can use this repository and publish exciting papers
based on their original ideas.
The 24th LHD experimental campaign was the last experiment
campaign in which deuterium could be used. With
this experimental campaign, the LHD project also ended.
Fortunately, however, we were allowed to continue the
operation of the LHD, as an academic research platform,
for the next three fiscal years. We will continue to conduct
experiments that do not use deuterium and do not produce
neutrons in LHD. We hope that people in various research
fields, not only those in plasma and fusion research, will
use this fortunate three-year opportunity. The next LHD
experiment campaign will start in the spring of 2024, and
thus experimental proposals will be collected through the
website in 2023.
We look forward to receiving your interesting and meaningful
proposals for the next campaign.
Katsumi Ida
(Executive Director on Science, Large Helical Device Project)
Naoki Tamura
(Leader of Topical Group “Muti-ion Plasma”)
for the LHD Experiment Group
National Institute for Fusion Science, Japan
E-mail: tamura.naoki@nifs.ac.jp