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An international journal of news from the stellarator community
Editor: James A. Rome Issue 177 February 2022
E-Mail: James.Rome@ stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Report from the 21st Coordinated
Working Group Meeting
(Virtual Meeting Organized by
IPP Greifswald, Nov. 22–24,
2021)
Where do we stand with stellarators as a reactor concept?
About 130 people registered to attend a virtual workshop
focusing on the prospects for stellarator reactors. The
meeting was conducted within the framework of the
Coordinated Working Group Meeting (CWGM) under
auspices of the IEA Technology Collaboration Programme
(IEA TCP) on Stellarators and Heliotrons.
As with the 20th CWGM, the pandemic did not allow for
an in-person meeting originally planned to be held in
Kyoto. Instead, the attendees met on three consecutive
days in one-hour video conferences but because of the
many time zones involved, some attendees had to join in
the late evening or early morning. This compromise, however,
allowed the stellarator/heliotron community to take
part as much as possible from its world-wide distributed
locations. In order to further facilitate the scientific discussion,
the meeting sessions were recorded. Additionally,
presentation slides and notes were archived in an electronic
documentation system running the INDICO software
(https://event.ipp-hgw.mpg.de/event/67/). The
material is available for registered participants working in
laboratories that are members of the IEA TCP.
Content for each of the three sessions was provided by
introductory presentations addressing ‘Open questions for
a fast track to stellarator reactors’ (Allen Boozer, Columbia
University, USA), ‘What can we learn for the first W7-
X campaigns for a HELIAS reactor?’ (Robert Wolf, Max-
Planck-Institut für Plasmaphysik, Greifswald, Germany),
and ‘Multi-ion physics and isotope effects in helical
devices’ (Hiroshi Yamada, Tokyo University, Japan). Productive
outcome of the sessions was ensured by structured
discussions guided by expert Chairpersons [Arturo Alonso
(CIEMAT), Felix Warmer (IPP) and Friedrich Wagner
(IPP)].
Open Questions for a Fast Track to Stellarator
Reactors
Professor Allen Boozer presented arguments backing a
fast track to develop a demonstration fusion power plant
[1–3]. Estimates of the cost of the available solutions to
stop the use of CO2-producing fuels are in the 4 trillion
USD/year range. This is to be put in perspective of a proposed
investment of a few billion USD/year for a fusion
plant development program.
The stellarator concept is best placed for a fast development
with minimum risk. Compared to the more popular
tokamak line, a stellarator power plant would benefit from
much lesser requirements for active plasma control—the
failure of which could have serious consequences in a
tokamak reactor. Reliable computational design reduces
the number of generations of experiments needed to
develop a stellarator power plant. The design of the magnetic
configuration and coil set, Prof. Boozer argued,
should incorporate fundamental efficiency and “plasma
quality” properties of field distributions in an otherwise
high-dimensional search space.
In this issue . . .
Report from the 21st Coordinated Working
Group Meeting
This meeting was organized by IPP Greifswald and
held virtually Nov. 22–24, 2021. Content for each of
the three sessions was provided by introductory presentations
addressing ‘Open questions for a fast track
to stellarator reactors’ (Allen Boozer, Columbia University,
USA), ‘What can we learn for the first W7-X
campaigns for a HELIAS reactor?’ (Robert Wolf, Max-
Planck-Institut für Plasmaphysik, Greifswald, Germany),
and ‘Multi-ion physics and isotope effects in
helical devices’ (Hiroshi Yamada, Tokyo University,
Japan)...................................................................... 1
Stellarator News -2- February 2022
Stellarator reactors could operate in the 10 keV temperature
range, where the requirement of confinement
improvement over gyro-Bohm is smallest. A gyro-Bohm
scaling of the energy confinement time in stellarators forecasts
viable reactor design points. A number of technological
developments could help in realizing the promise of a
stellarator power plant. Coil technologies that are allowed
to have joints; improved, thinner blankets with higher
breeding ratios; and liquid-metal-based wall protection
layers were mentioned.
Prof. Boozer appealed to the stellarator community to
instill a sense of thrill and opportunity by bringing a stellarator
fusion reactor to contribute to a medium-term solution
for the great environmental challenges that lie ahead
of us in the 21st century.
What can we learn for the first W7-X
campaigns for a HELIAS reactor?
Professor Robert Wolf summarized the status of experimental
findings from Wendelstein 7-X (W7-X) in terms of
stellarator optimization. Demonstrating Prof. Boozer’s
argument about the potential of computational design
improvements, W7-X is a proof of concept as the first
magnetic device derived from comprehensive optimization
procedures based on physics criteria.
Substantial milestones have been attained in the first campaigns
of W7-X: even without full cooling of plasma-facing
components and without the full envisaged heating,
fueling, and exhaust capabilities, W7-X could demonstrate
that a magnetic field with very small, noncritical field
errors could be created with superconducting coils [4].
Small and controllable plasma currents were demonstrated
[5], supporting the HELIAS concept goal: to avoid difficulties
for exhaust and plasma stability. Experiments with
core pellet fueling and peaked density profiles have
already demonstrated reduced neoclassical energy transport
[6]. Expectations for higher plasma beta, however,
were not yet attainable: MHD stability and equilibrium as
well as the impact on fast-ion confinement will need a successive
increase of heating power that is now underway.
Furthermore, an integration of highest plasma performance
with feasible divertor operation [7] needs wider
exploration and requires demonstration in discharges
much longer than the maximum pulse durations achieved
thus far. The successful reduction of neoclassical transport
in stellarators, draws attention to the understanding of turbulence
in helical geometries.
For an assessment of reactor prospects, the discussion
revealed the necessity for confirmation of neoclassical
optimization at high plasma beta. Swiftly attaining beta
values up to ‹β› ~ 4% would add beneficial support for the
development of the stellarator line. MHD stability, equilibrium
stiffness, fast particle confinement, and electromagnetic
effects in plasma transport, e.g., turbulence
stemming from kinetic ballooning modes, were confirmed
as high-priority topics. Achievable high-beta operation is
apparently restricted by heating power and plasma confinement;
uncertainties in performance predictions need to
be overcome to define required heating power levels. On a
short time scale, two options appear to offer ways forward:
a considerable improvement in confinement (H-mode,
potentially related to a power threshold that can be beneficially
affected by deuterium operation), and low-field
operation (1.7 T using the full available ECRH power but
in X3-mode).
Findings from the W7-X campaigns underlined the roles
of both particle transport and impurity transport. The
observed flat or even peaked density profiles in W7-X
cannot be understood by neoclassical theory. Signatures
for an inward (turbulent) particle pinch effect were suggested
as a research focus. The discussion brought up the
question of wether it would make sense to aggressively
pursue the development of a high-velocity pellet injector
(low investment, high reward).
A lack of theoretical investigations of 3D turbulent particle
transport was pointed out. It was agreed that a clear
understanding of impurity transport is necessary for reactor
scenarios—a balance of improved thermal transport by
a suppression of turbulence may be required to avoid
impurity accumulation.
With respect to heat and particle exhaust, very reproducible,
reliable, and stable detachment was achieved, but
needs to be qualified for longer discharges. At the end of
this path, demonstration of integrated core/divertor scenarios
needs to combine good plasma performance in the core
with feasible heat and particle exhaust with detached
divertor plasmas. This litmus test is considered to be one
of the most important qualification milestones for future
reactor proposals. Moreover, the effect of metallic plasmafacing
components in stellarator geometry appears to be
largely unknown and needs broader assessments, building
on experience gained with tokamak operation. How
exactly an integrated reactor scenario would look in existing
devices, and expected insights and limitations, concluded
the discussion.
Multi-ion physics and isotope effects in helical
devices
Professor Hiroshi Yamada addressed two related topics
highly relevant to burning helical plasmas: the isotope
effect on confinement and the observation of plasmas with
different ion density profiles, so-called nonmixing plasmas.
Evidence is provided by recent experiments conducted
on the Large Helical Device (LHD).
The isotope effect manifests in tokamaks as an increase of
the energy confinement time τE with the ion mass M. A
Stellarator News -3- February 2022
full understanding is lacking but a positive M-scaling does
match, e.g., with gyro-Bohm scaling of confinement
( ). Gyro-Bohm behavior is also found in helical
devices [8]. For tokamaks, the isotope effect is ubiquitous
and can control energy, particle, impurity, and
momentum transport. A critical aspect for ITER operation
is the power threshold for transitions to H-mode (Pthr ~
M−1). The isotopic effect is favorable for reaching the D-T
fusion reactor targets.
It appears to be self-evident that any isotope effect is also
relevant to helical systems. In consequence, readiness for
deuterium operation in large stellarators is crucial to
develop plasma scenarios with enhanced confinement
(e.g., H-mode confinement with edge pedestals) or discharges
with self-induced density profile peaking.
While LHD could make substantial progress once it has
been commissioned for deuterium and mixed plasma operation,
smaller devices contribute as well. W7-AS, TJ-II,
Heliotron-E, and CHS did not show any noteworthy M
scaling. LHD confirms this finding with its own isotope
scaling for the confinement time τE ~M−0.07. On the other
hand, similar energy confinement times for hydrogen and
deuterium imply an improvement with M with respect to
the gyro-Bohm expectation [9]. Excellently matched,
dimensionally similar plasmas in LHD show a gyro-Bohm
like turbulence level and spectrum but nevertheless also a
distinct isotope effect as . It is concluded
that helical systems are also governed by an isotopic
energy confinement dependence, possibly smaller than
that in tokamaks.
The control of isotope ratios is important for the efficacy
of fusion in burning plasmas. Deviations of the individual
isotopic shapes of ion density profiles with respect to the
electron density profile appear to be possible for multi-ion
plasmas so long as the ambipolarity condition is maintained.
New diagnostic capabilities recently allowed the
study of ion density profiles from different isotopes on
LHD. Studies of the evolution of H+ and D+ density profiles
in mixed plasmas showed two different states. An
enthralling observation was plasmas with nonmixing
states, in contrast to mixing states for which all density
profiles have the same shape. Nonmixing states were
found at low densities(~3 101 9m and transiently in
fueling experiments [10]. Evidence was found that the
density source location is decisive where shape deviations
occur. Linear gyro-kinetic calculations and turbulence
measurements provide indications that the destabilization
of trapped-electron modes favors nonmixing deviations
whereas ion-temperature gradient (ITG) mode turbulence
ends up in isotope mixing.
The particle sources in multi-ion plasmas at low collisionality
that are expected in the nuclear phase of ITER are
also affected by anomalous inward pinches. In contrast,
hollow profiles in stellarators may occur due to a neoclassical
temperature-gradient-driven outward flow.
The discussion revealed the importance of the topic.
Action needs to be taken for a better understanding of
isotope-dependent particle transport. The forthcoming
experimental campaign on LHD will make it possible to
address some of resulting open questions. Systematic differences
in the Alfvenic turbulence between H+ and D+
due to differences in fast-ion velocity and Alfvenic speed
have not yet been studied but will be investigated in the
future. The possible effect of beta on multi-ion transport
physics in the interplay of trapped electron mode (TEM)
and ITG (when interchange modes become relevant) has
not yet been studied, but it may not be possible to explore
this question with sufficient quality on LHD. The transition
to H-mode in LHD does not lead to a strong increase
in temperature but rather to an increase in density. There is
only a small increase in confinement and no distinct pedestal
is formed. In order to study He transport, He-NBI
injection has been started at LHD. The strong recycling of
He did not yet allow conclusive results, and a He pump is
needed. Helium transport will be an important topic for
multi-species plasmas in LHD.
The interesting question of whether the physics of
reversed shear tokamak configurations, which are known
to not show an isotopic confinement effect, could play a
role in stellarators could not yet be answered.
The pioneering studies with deuterium operation in LHD
provided important new insights, and thus the study of isotopic
effects in W7-X was deemed to be of highest interest.
In the discussion, the better understanding of the role
of TEM and other turbulence mechanisms on mixed ion
species discharges in optimized configurations was identified
as a need for reactor predictions. In this context, the
role of theory and modeling was underlined. Specific
points were the question of the impact of neoclassical
transport on the global confinement time and its isotopic
effect. Moreover, modeling of electromagnetic turbulence
is limited by the lack of precise simulation codes. The
topic is flagged as a priority for theory developments.
Conclusion
Recent LHD and W7-X experiments have delivered substantial
scientific progress highly important to a stellarator
reactor concept. Developments in stellarator theory and
increasing maturity of computational tools for optimized
stellarator designs open the door for rapid concept developments.
The meeting identified open questions and provided
ways to address them. No showstopper for a
stellarator reactor has been revealed. It is concluded that
the time scale for scientific progress is dominantly set by
the progress and available resources in the experimental
programs, lead times for critical technologies (e.g., metal-
Ei  *–3
Ei  M0.94*–3.02
Stellarator News -4- February 2022
lic divertor targets), and decision-making periods, which
are not within the control of scientific planners. The documented
progress in the scientific exploitation of the facilities
allows one to define specifically required steps for an
assessment of the stellarator line as a reactor concept.
Smaller devices that can focus more flexibly on dedicated
objectives (e.g., qualification of techniques like wall conditioning
or turbulence studies) play a vital role in this
process. Open questions for helical reactors are identified
and deliver specific objectives for forthcoming experiments,
such as operation at high plasma beta and its
impact on stability, neoclassical, and turbulent transport.
Particle transport, fueling, and exhaust define the development
path to exploit the paramount advantage of stellarators
for steady-state operation. First glimpses into multiion
plasma reveal how much isotopic effects may affect
the operation at reactor relevant conditions. A comparative
discussion with upcoming JET results and the perspective
of fusion plasmas in ITER give rise to the
anticipation that multi-ion plasmas in 3D will get more
and more into the focus of stellarator research. It appears
to be of utmost interest to study multi-ion plasmas also in
optimized plasmas as in W7-X—a potential benefit could
be a lower threshold for improved confinement regimes.
The next CWGM is planned to be in 2022 in Japan. This
virtual meeting indicated strong interest and enthusiasm in
the community, providing strong motivation to keep the
CWGM as accessible as in the 21st edition. The stellarator
community is invited to subscribe to the CWGM mailing
list by sending an e-mail to cwgm-subscribe@ipp.mpg.de.
The authors would like to thank Ms. Maria Radau, Mr.
Peter Kurz, and Ms. Andrea Kleiber for their continuous
support.
Andreas Dinklage, Arturo Alonso, Enrique Ascasibar, Allen
Boozer, David Gates, Yasuhiro Suzuki, Friedrich Wagner,
Felix Warmer, Robert Wolf, and Hiroshi Yamada
On behalf of the 21st CWGM
References
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important, Nucl. Fusion 60, 65001 (2020).
[2] Boozer, AH, Stellarators as a fast path to fusion, Nucl.
Fusion 61, 096024 (2021)
[3] Boozer, AH, Carbon dioxide, fusion, and stellarators,
Stellarator News issue 176, https://arxiv.org/pdf/
2111.04213.pdf (2021).
[4] Sunn Pedersen, T, et al., Confirmation of the topology
of the Wendelstein 7-X magnetic field to better than
1:100,000, Nature Comm. 7, 13493 (2016).
[5] Dinklage, A, et al., Magnetic configuration effects on
the Wendelstein 7-X stellarator, Nature Phys. 14, 855
(2018).
[6] Beidler, CD, et al., Demonstration of reduced neoclassical
energy transport in Wendelstein 7-X, Nature 596,
221 (2021).
[7] Jakubowski, , et al., Overview of the results from divertor
experiments with attached and detached plasmas at
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[8] Yamada, H, et al, Characterization of energy confinement
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[9] Yamada, H, et al., Isotope Effect on Energy Confinement
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