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An international journal of news from the stellarator community
Editor: James A. Rome Issue 166 August 2019
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Report from the 19th Coordinated
Working Group Meeting
March 12–14, 2019,
Helmholtz Society, Berlin, Germany
Overview
Worldwide stellarator research has gained substantial
momentum: On Wendelstein 7-X (W7-X, Greifswald,
Germany), first campaigns with an uncooled divertor have
demonstrated the highest-ever fusion performance normalized
to the plasma volume and at the same time, the longest
pulse lengths. Deuterium experiments in the Large
Helical Device (LHD, Toki, Japan) showed confinement
at higher performance than gyro-Bohm scaling—differences
in terms of engineering parameters are less clear.
Smaller devices are enabling experimental insights for
physical understanding, such as turbulence studies in TJ-II
(Madrid, Spain), fueling studies in Heliotron-J (Kyoto,
Japan), and the examination of flows in HSX (Wisconsin,
USA). Progress in the refurbishment of Uragan 2-M
(Kharkiv, Ukraine) and new concept devices (CFQS,
Chengdu, China) complement the larger experiments, creating
new opportunities to assess aspects relevant to reactor-
like operation and stellarator optimization.
Equally important, stellarators benefit from the latest cutting-
edge developments in diagnostics, heating, and
plasma fueling. Coordinated programs in the EU, the US,
and Japan link innovative developments [e.g., integrated
systems for overload detection (EU consortium), x-ray
detectors (Princeton Plasma Physics Laboratory), superfast
surveillance cameras (EU around WIGNER Research
Centre for Physics, Hungary) and phase-contrast imaging
(MIT)] to the stellarator programs with obvious mutual
synergies: the pace of experiments is accelerated by
employing these unprecedented measuring capabilities.
More and more benefits for tokamaks are materializing:
experience gained in long-pulse stellarator experiments
contributes to large-scale devices such as JT-60SA and
ITER. A select example is the steady-state fueling pellet
injector from Oak Ridge National Laboratory (ORNL) that
is being brought to W7-X with additional support from
EUROfusion and NIFS, Japan, and which implements
technology planned for use on ITER.
The scientific community is excited by early theoretical
ideas to explain the latest results from stellarator experiments.
Worldwide groups, concentrated in research centers
as well as in universities, are increasingly applying
their expertise in the understanding of ground-breaking
physics questions, such as turbulence in three-dimensional
(3D) fields, impurity transport, fast-ion confinement, and
plasma flows and currents.
The above-mentioned aspects were the essence of a threeday
meeting held at the headquarters of the Helmholtz
Society, the funding agency of the German fusion programme
(see Fig. 1). The meeting was organized and
sponsored by the Max Planck Institute for Plasma Physics.
Roughly forty on-site and ten remote participants provided
reports on collaborations, grouped into seven topics. In an
informal workshop format, the participants discussed proposals
for joint action and experiments, taking advantage
of comparative studies in different devices. The CWGM is
not a scientific conference, but rather a interactive work-
In this issue . . .
Report from the 19th Coordinated Working
Group Meeting (CWGM)
This annual meeting was held at the headquarters of
the Helmholtz Society in Berlin March 12–14, 2019.
Roughly forty on-site and ten remote participants provided
reports on collaborations, grouped into seven
topics that reviewed progress during the past year,
and planned for the future. The coordinated working
group actions (CWGA) serve as a basis to follow up
joint actions and were agreed to be followed up in the
forthcoming meetings............................................... 1
Stellarator News -2- August 2019
shop wherein agreements on joint publications under the
auspices of the IEA Technology Collaboration Program on
Stellarators/Heliotrons are promoted to become the measurable
outcome of the CWGM. A session on the program
plans of the main contributors served to enable the
exchange of information, and the community was invited
to provide feedback to programmatic considerations.
China’s quickly developing stellarator program, with a
sound balance of sustainable build-up of know-how and
scientifically interesting new concepts, namely the outline
of a quasiaxially symmetric device, attracted great interest.
The sessions were led by colleagues serving as coordinators.
The remainder of this report gives brief summaries of
each session.
Divertor and Edge Physics in Stellarators
(M. Jakubowski)
Two major experimental campaigns were recently conducted
that aimed at investigating important aspects of
divertor physics in helical devices. At LHD, results of the
first deuterium campaigns were reported by Suguru Masuzaki.
There is no clear difference between hydrogen (H)
and deuterium (D) plasmas in terms of divertor physics.
The asymmetries in divertor particle fluxes, that arise due
to edge drifts look identical in both plasmas. The same is
true for divertor load patterns. Additional non-evaporative
getter pumps installed under the dome structure of the
helical divertor allowed for increasing of the pump speed
by about 25%. Stable detachment was achieved using
superimposed seeding of krypton and neon in low-density
discharges. In the seeding experiments, the edge electron
temperature decreased about 50%, while the radiated
power was doubled compared with neon seeding.
At W7-X, a high-density campaign with an island divertor
was conducted. As reported by Marcin Jakubowski, this
led to the first achievement in stellarators of a high recycling
regime, where downstream densities are significantly
higher than upstream densities. This allowed
reaching stable detachment of the plasma with plasma
duration of up to 30 s. It is possible to reduce divertor
power loads with the help of impurity seeding. Florian
Effenberg showed that with neon as the seed, global reduction
of divertor loads by a factor of 4 could be achieved at
W7-X. Seeding with N2 leads to more subtle influence on
power loads. Nitrogen seeding results in lower recycling
and requires a long puff duration in order to establish a
significant enhancement of radiated power. Nitrogen is
therefore a promising candidate for radiated power control
using a feedback system.
As is the case in LHD, edge transport is observed to be
affected by drift effects in W7-X, e.g., EB drifts. Ken
Hammond presented results from experiments with positive
and negative directions of the main magnetic field.
Victoria Winters discussed experiments on the influence
of carbon impurities on radiation patterns. During methane
puffs, the percentage increase of C-VI radiation is the
same at different density levels, which probably means
Fig. 1. Participants of the 19th CWGM in Berlin (March 12–14, 2019).
Stellarator News -3- August 2019
that transport processes are similar. Grzegorz Pełka presented
first results for modeling of W7-X limiter plasmas
with a new code FINDIFF. It solves fluid equations for
main ions and neutrals in curvilinear coordinates. The
code is still not in good enough shape to be able to reproduce
experimental data; nevertheless, significant progress
has been made in the last year.
Several areas in which joint research could be performed
at LHD and W7-X were identified. They include the
investigation of stable detachment, the role of plasma
drifts at the edge, changes in divertor structures due to
plasma dynamics, and 3D migration patterns in helical
devices. More specific actions will be defined in the
course of 2019.
Scaling and operation limits (G. Fuchert)
Knowledge about operational limits and scalings of global
confinement parameters is crucial for efficient scenario
development in stellarators. In this session, three main topics
were discussed: The scaling of the energy confinement
time, MHD and reconnection instabilities, and the radiative
density limit.
Concerning the energy confinement time scaling, Yamada
presented experimental data on isotope effects in LHD. In
plasmas heated using neutral beam injection (NBI), no
obvious difference in the global energy confinement time
is observed. It was stressed, however, that the lack of degradation
in deuterium violates gyro-Bohm scaling and is in
that sense an isotope effect. Participants discussed how the
isotope dependence is reflected in the current International
Stellarator-Heliotron Database. The extension of available
data led to a proposal for a new version of the empirical
energy confinement time scaling in stellarators. However,
no deuterium data will be available from W7-X in the near
future to provide more data points.
Fuchert presented operational limits found in W7-X plasmas
heated by electron cyclotron resonance heating
(ECRH) so far and showed experimental data indicating
that the global energy confinement time may be affected
close to those limits. This should also be taken into
account for future scaling efforts, since many fusion-relevant
scenarios are in fact close to operational limits.
Suzuki and Zocco reported the latest progress in the theoretical
and numerical description of MHD and reconnection-
related instabilities in LHD and W7-X. Such
instabilities can lead to severe confinement degradations
or plasma termination. An instability observed in W7-X is
likely caused by magnetic reconnection, mediated by electron
inertia with ions in the (gyro) kinetic regime. Important
questions are: How general are these observations, and
which magnetic field properties are needed to prevent such
Instabilities?
Furthermore, Gates presented a brief comparison of the
density limit in stellarators and tokamaks and introduced
fundamental differences and similarities. It was stressed
that there was seemingly no interest in the tokamak community
to tackle this issue at the moment, and it was proposed
that the stellarator community should lead an effort
to verify the presented models experimentally.
Joint actions agreed to in this session are:
 A comparison of operational limits for different stellarators
including, but not limited to, the three largest
ones: LHD, TJ-II, and W7-X.
 An investigation on whether identified instabilities
are generic to stellarators or depend on the specific
details of a particular magnetic configuration.
 An assessment of the density limit with special focus
on a comparison with tokamaks.
3D Fast Ion Physics (S. Lazerson)
The study of fast ions in stellarators speaks to a fundamental
issue of nuclear fusion, specifically the nuclear fusion
reaction of hydrogen isotopes not only to generate neutrons
but also to sustain the plasma parameters necessary
for the reaction by heating the plasma with the fusion-produced
alpha particles While to date no deuterium-tritium
stellarator experiments have been performed, a wide range
of devices spanning both device size and plasma parameters
use heating mechanisms which produce particles with
energies significantly above those of the background
plasma species.
The status of energetic particle confinement in LHD was
reported by Ogawa and Nuga. Äkäslompolo reported on
first experiments with NBI in W7-X. Cappa reported on
the interplay between ECRH and fast ions in TJ-II experiments.
These reports indicate that there is an outstanding
task to place this data in the context of fusion alphas.
Doing so will allow the identification of future experiments
to clarify the important fast-particle physics for a
burning plasma reactor. Scaling to alpha particles will also
clarify the role ITER plays in development of a stellarator
reactor. Goncharov presented work highlighting the role
that non-thermonuclear fast ion populations can play in DD
and D-T neutron production. An outcome of this work
was the conclusion that auxiliary heating systems which
drive such populations can result in neutrons with energies
higher than 14.1 MeV. This observation has important
implications for first wall materials. Bader reported on
optimization and design work focused on improving stellarator
energetic particle confinement through tailoring of
magnetic fields. These collisionless simulations show that
it is possible to use magnetic field shaping to improve the
confinement of fusion alphas in a collisionless sense. Placing
this work in the context of collisional simulations and
mode activity is an open task.
Stellarator News -4- August 2019
Joint actions were agreed to be conducted on:
 Identification of a set of dimensionless parameters
relevant to energetic particle physics in stellarators
 Determination of reactor-relevant values for said
parameters to better place experimental results in the
context of a reactor
Fueling Pellets and Impurity injection
(N. Tamura)
The injection of hydrogen-isotope pellets and impurities is
a highly important technique in magnetic fusion research
for both the tokamak and the stellarator concept. Pellet
injection is still a principal tool for fueling steady-state
fusion reactors and ITER. Impurities may also be injected
as actuators for plasma performance and diagnostics.
G. Motojima (NIFS) reported hydrogen pellet experiments
in plasmas with NBI and NBI+ECH in Heliotron J. A
deeper pellet penetration was observed in the NBI+ECH
plasma, which might be explained by the effect of fast
electrons or ions. K. J. McCarthy (CIEMAT) reported
plans and progress for a tracer-encapsulated solid pellet
(TESPEL) preparation laboratory at CIEMAT. The integrity
of TESPELs made at CIEMAT will be checked in the
TJ-II experiments. A first TESPEL batch is expected
around the end of 2020, before the OP2 of W7-X. N.
Panadero (CIEMAT) reported recent results of pellet ablation
analysis. For TJ-II, the effect of fast electrons on the
plasmoid drift was considered to explain a higher fueling
efficiency. For W7-X, new simulation results for campaign
OP1.2b suggest that the predicted fueling efficiencies
are close to the experimental results. N. Tamura
(NIFS) reported first TESPEL injection experiments on
W7-X. In the OP1.2b of W7-X, the latest TESPELs (e.g.,
multi-tracers) were injected, which allowed performance
of a detailed study of impurity transport in W7-X. Transient
effects (e.g., increment of electron temperature) of
TESPEL injection on W7-X plasmas were also observed.
T. Wegner (IPP) reported recent results from laser blowoff
(LBO) experiments in W7-X. In OP1.2b, a variety of
impurities was injected by LBO, which contributes to the
Z-dependence study of impurity transport. The impurity
amount scan experiment by LBO shows that the LBO can
be also a plasma killer. L. Baylor (ORNL) reported on the
current status of the continuous pellet injector for W7-X.
The injector was designed based on twin-screw extruder
(TSE) and gas gun technology. Now a prototype TSE is
under fabrication. E. Gilson (PPPL) reported recent results
and plans for an impurity power dropper (IPD). The IPD
showed many beneficial effects (e.g., ELM mitigation,
power exhaust) in previous tokamak experiments. In stellarators,
a probe-mounted powder injector (PMPI) was
successfully deployed in the OP1.2b of W7-X, and the
IPD will be commissioned in the JFY2019 campaign.
These reports clearly suggested that it is almost time for
comparative studies of pellet fueling and the impact of
impurity injection in stellarators.
As joint actions/experiments, the following topics were
discussed:
 A benchmark activity for the HPI2 code, especially in
terms of magnetic configuration. This action can be
performed on LHD, W7-X, TJ-II and Heliotron J, and
can be one of the bridges between CWGM and ITPA
through a comparison of penetration depth in tokamaks
and stellarators in the reactor-relevant regime.
 Optimization of the fueling scheme to achieve the
highest density beyond the Sudo limit, and to realize
the peaked density profile during on-axis ECRH heating.
This activity can be pursued on LHD, W7-X, TJII
and Heliotron J.
 Development of the accurate TESPEL ablation model
can be performed with LHD, TJ-II and W7-X. This
activity will be beneficial to the comparative studies
regarding impurity transport modeling among those
devices using the STRAHL code (this is related to the
joint task in the impurity transport session).
 A reactor-oriented mixed-species particle control.
This is a demanding task in a fusion reactor, but has
not been developed in stellarators.
 Z-dependence studies of impurity transport using
multi-impurities-embedded TESPELs. The optimization
of the impurities embedded in the TESPEL is
included in this activity. The application of TEPSELs
to tokamak experiments was also discussed.
Equilibrium (Y. Suzuki)
In the Equilibrium Session Six contributions were
reported. Those contributions can be categorized into three
areas: (i) equilibrium reconstruction, (ii) full field model
to the fast ion and edge transport simulations, and (iii)
developments of new theory and modeling. In category (i),
Schmitt (Auburn) and Lazerson (PPPL) reported recent
progress in equilibrium reconstruction using V3FIT and
STELLOPT. Both codes well work to reconstruct equilibria
of the W7-X experiment. From these results, two joint
actions were planned:
 Cross-checking of codes using specific targeting shots
or synthetic data.
 Application of V3FIT and STELLOPT to LHD and
comparisons of reconstruction codes and predictive
modeling.
In category (ii), Suzuki (NIFS) reported recent progress on
full field calculation including net toroidal current to interpret
experimental observation of IR camera measurements.
Here, two more joint actions were discussed
 Validation of full field models by fast ion and edge
transport simulations. (Suzuki, Lazerson)
Stellarator News -5- August 2019
 Validation and verification of codes by cross benchmarking.
Responsible persons were assigned to each joint action,
and progress will be reported in the next CWGM.
In category (iii), Loizu (EPFL) and Landreman (U. Maryland)
reported progresses of new theoretical and modeling
work. SPEC was applied to model the saturated tearing
mode, and analytical theory is developing to construct a
quasisymmetric configuration directly. A possible idea for
future collaboration is under consideration: extension of
analytical models to include higher order terms (triangularity
and plasma beta) and integration with stellarator
optimization. Discussion of these ideas will continue.
Finally, Moiseenko (KIPT) reported on ultra-short repetitive
pulse wall conditioning discharges. This technique
was considered as a possible area for future collaboration,
but applicability of ultra-short pulses to other devices
should be considered carefully.
3D Turbulence (M. Nakata)
The 3D turbulence session was the largest session, covering
a full day of the CWGM. There were many contributions
(14 talks) from experimental and theoretical aspects,
where the main outcomes are summarized as follows:
 Identification of microinstability and turbulent fluctuations,
i.e., TEM and ITG characteristics including
the surface-global effects, the isotope effects, the geometric
dependence, and the verification of stellarator
optimizations.
 Impact of Er and Er-shear on turbulence and zonal
flows, i.e., electron- to ion-root transition, intrinsic
coupling of neoclassical and turbulence dynamics,
and turbulence suppression/spreading in the edge-
SOL region.
 Validation and extension of gyrokinetic (GK) turbulence
simulations, i.e., full-f global model development,
validations with flux tube simulations, and the
stability map for electromagnetic turbulence observations.
Based on the above results, several research targets for
joint experiments and/or cooperative simulation studies
have been specified:
 Validation activity on fluctuations, zonal flows (ZFs),
and Er, in addition to transport levels. This can be
addressed by joint experiments and cooperative simulation
studies with local and/or global GK codes and
neoclassical codes.
 Joint studies on nonlinear energy transfer and saturation
mechanisms in multiple devices.
 Continued joint activity on constructing the “stellarator
base case” for verification platform.
A collaboration framework for these topics will be organized.
Impurity Transport (N. Pablant)
A comprehensive range of topics was covered as part of
the impurity transport session through nine presentations.
Significant progress has been made on all of the Impurity
Transport Joint Tasks, and this activity has led to a number
of joint papers in 2018/2019.
Updates on research related to the effect of potential asymmetries
(1) on impurity transport were discussed by S.
Buller (Chalmers) and Regaña (CIEMAT). Bueller presented
updates on theoretical advances in understanding
the effect of fast ions, along with a discussion on optimization
of 1 to achieve desired impurity transport properties.
Regaña showed simulations from EUTERPE, SFINCS,
and KNOSOS of impurity fluxes in the mixed-collisionality
regime with 1, along with simulations of expected D
and V profiles in W7-X with and without the consideration
of 1. Related to these discussions, S. Kumar (U. Wisc)
presented theoretical modeling of impurity transport in
mixed collisionality regimes for HSX-like plasmas. This
set of talks led to an extended discussion on how to
develop joint experiments that could be used for validation
of these theoretical findings.
The next topic discussed was the impact of pressure
anisotropy on impurity transport, which was presented by
I. Calvo (CIEMAT). Calvo showed a theoretical presentation
of how anisotropy can affect impurity transport, along
with an expression for when this consideration is important.
A new code is being developed to study the effect of
anisotropy in realistic geometries, which can then be compared
with other codes and experimental results.
The effect of turbulence on impurity transport was discussed
by M. Nunami (NIFS) and J. Alcusón (IPP). These
presentations discussed the state of the art in using GK
simulations to study turbulent transport of impurities.
Nunami showed GK simulations of LHD impurity hole
plasmas which indicate that turbulent transport cannot
account for the observed impurity fluxes; however, a study
of the effect of externally applied torque on the neoclassical
fluxes may provide a possible explanation.
The next set of talks was focused on experimental results
from HSX, presented by S. Kumar (Wisconsin) and from
W7-X presented by M. Kubkowska (IPPLM) and A. Langenberg
(IPP). The presentation on HSX showed some initial
first results from a recently installed LBO system. The
W7-X talks by Kubkowska and Langenberg highlighted a
wide range of experimental results, including topics of Zdependence,
1D transport modeling, the effect of turbulent
transport, and observed scaling laws. These talks highlight
the fact that impurity transport experiments from W7-X
are now sufficiently mature to start cross-machine comparisons
and advanced theoretical validation exercises.
Stellarator News -6- August 2019
The final presentation in this session was on the use of an
IPD for impurity injection by E. Gilson (PPPL). Gilson
presented capabilities of this system on W7-X and LHD
and showed some first W7-X results. An extended discussion
on the possible use of the IPD for high-Z impurity
injections studies followed the presentation.
Several additional topics related to impurity transport were
discussed in the dedicated session on fueling pellets and
impurity injection, as summarized by N. Tamura.
At the conclusion of the session it was decided that the
five Joint Tasks in impurity transport identified in the
prior CWGM are still representative of the major open
research questions and are serving well to organize collaborative
research around these topics. The Impurity Transport
Joint Tasks are:
 Z dependence of impurity transport and impurity
accumulation.
 Investigation of impurity holes.
 Investigation of potential asymmetries (1) on impurity
transport.
 Development of general-purpose 3D stellarator impurity
deposition/ionization/transport tools.
 Turbulent modeling for impurity transport.
Conclusion
Ongoing collaborations on plasma terminating events and
electron root confinement studies were reported and
agreed to be continued.
Given the broadness of the aforementioned topics, the
CWGM effectively tracked progress in the most active
fields aiming at collaborative studies. Initiated in the 2018
Princeton edition of the CWGM, coordinated working
group actions (CWGA) serve as a basis to follow up joint
actions and will be followed up in forthcoming meetings.
Information on forthcoming CWGMs will be provided to
subscribers of the mailing list. Sign up at:
cwgm@ipp.mpg.de
The next CWGM activity will be a brief meeting—the
22nd International Stellarator and Heliotron Workshop
2019 in Madison, Wisconsin (September 23–27, 2019).
Acknowledgements
The authors are indebted to Ms. Maria Radau for her
excellent support in organizing the meeting. The participants
and organizers are grateful for support by Max-
Planck-Institut für Plasmaphysik and EUROfusion.
A. Dinklage,1 E. Ascasibar,2 D. A. Gates,3 Y. Suzuki,4 M.
Jakubowski,1 G. Fuchert,1 N. Tamura,4 S. Lazerson,3 M.
Nakata,4 N. Pablant,3 F. Warmer1
1Max-Planck-Institut für Plasmaphysik, Greifswald, Germany
2CIEMAT, Madrid, Spain
3Princeton Plasma Physics Laboratory, Princeton, NJ, USA
4National Institute for Fusion Science, Toki, Japan