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An international journal of news from the stellarator community
Editor: James A. Rome Issue 168 January 2020
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Report from the 2019 ISHW
Coordinated Working Group
Meeting (September 24, 2019)
Overview
Following a successful Coordinated Working Group
Meeting (CWGM) in March 2019, a second meeting was
held to report on progress since that meeting. This took
place as an evening session during the 2019 International
Stellarator and Heliotron Workshop (ISHW) held in Madison,
Wisconsin, USA. Reports from the session organizers
of the previous meeting were presented, detailing status
and progress since the meeting in March. A special discussion
was held to explore the role of stellarators in the
International Tokamak Physics Activity (ITPA), led by
Carlos Hidalgo. Finally, the location of the next meeting of
the CWGM was announced: Kyoto University, Japan.
Divertor and Edge Physics in Stellarators
(M. Jakubowski)
The ongoing comparison between helical (the Large Helical
Device, LHD) and island divertors (Wendelstein 7-X,
W7-X) is yielding preliminary results. See Figs. 1 and 2 to
compare these two geometries. Joint experiments exploring
detachment, the role of particle drifts, the effect of
finite beta, and migration of material are all aiding to better
highlight commonalities and differences among divertors.
The role of low- vs high-recycling impurities in
detached divertor states has been explored in both devices.
The injection of low-recycling impurities results in edge
localized radiative behavior, while injection of high-recycling
impurities has a more global effect on radiative cooling
of the plasma. Stable detachment is found in both
devices; it appears that the helical divertor requires perturbative
fields to achieve this state, but this is not necessary
for the island divertor. In both devices drift effects play a
role in the highly three-dimensional (3D) structure of the
divertor footprints. Experimental evidence suggests that
plasma parameters also play a significant role in the divertor
footprints of both devices. The lack of a high-recycling
regime in LHD is attributed to counterflows on neighboring
surfaces. As the comparison proceeds, each individual
task will have a corresponding task coordinator. At the
moment, Masahiro Kobayashi will coordinate seeding and
detachment experiments between W7-X and LHD, and
Ken Hammond will compare edge drifts between LHD
and W7-X.
Fig. 1. Poincaré plot showing the LHD flux surfaces, ergodic
layer, and divertor legs. Source: Y. Feng et al., Nucl.
Fusion 49 (2009) 095002.
In this issue . . .
Report from the 2019 ISHW Coordinated Working
Group Meeting (September 24, 2019)
The meeting took place during the 2019 International
Stellarator and Heliotron Workshop (ISHW) held in
Madison, Wisconsin, USA. Reports from the session
organizers of the previous meeting were presented,
detailing status and progress since the last meeting in
March. A special discussion was held to explore the
role of stellarator in the International Tokamak Physics
Activity (ITPA). ........................................................ 1
New Joint German-US Stellarator Project
The Max Planck Institute for Plasma Physics (IPP) in
Greifswald and the University of Wisconsin-Madison
have founded a joint research project to investigate
the power exhaust from a hot stellarator plasma. For
this purpose, IPP in Greifswald and the University of
Wisconsin-Madison have founded the Helmholtz International
Laboratory for Optimized Advanced Divertors
in Stellarators (HILOADS)........................................ 3
Stellarator News -2- January 2020
Scaling and Operational Limits (E. Ascasibar
on behalf of G. Fuchert)
Concerning scalings and operational limits, in LHD neutral
beam injection (NBI)-heated plasmas with mixed H,D
fueling have been studied and follow the mass-dependent
energy confinement time scaling presented at the last
CWGM. Helium plasmas are a work in progress. For W7-
X, a paper summarizing the operational boundaries of the
first divertor campaign is in preparation. Investigations of
MHD-related instabilities are making progress. Two possible
instabilities are being discussed in an upcoming
paper. Further work will clarify how to distinguish these
experimentally. Finally, initial studies have been started at
the tokamak ASDEX Upgrade in order to compare the
density limit of stellarators and tokamaks.
3D Fast Ion Physics (S. Lazerson)
A activity has begun to create a stellarator fast-ion database.
This database will place discharges in LHD, TJ-II,
Heliotron-J, and W7-X in the context of existing reactor
designs. Such a database will allow us to better understand
the role of fast ions in a reactor system and allow crossmachine
comparisons to proceed. A set of machine parameters
has been assembled, and collection of stereotypical
discharge parameters from multiple machines has begun.
Additional data from discontinued devices (e.g., ATF) is
also being sought. A collaboration on the neutral beam
modeling of HSX was also discussed. Finally, it was
reported that through participation in the previous CWGM
meeting a collaboration between the St. Petersburg Polytechnic
University and the National Institute for Fusion
Science (NIFS) has begun. This highlights the value of the
CWGM in connecting researchers globally and broadening
our scientific endeavors.
Fueling Pellets and Impurity Injection
(N. Tamura)
Comparative studies of pellet fueling and impurity injection
are being prepared for upcoming campaigns on multiple
devices. Simulations with the HPI-2 pellet injection
code have been performed for W7-X, TJ-II, and Heliotron-
J. Benchmarking analysis is under way with special attention
being applied to Heliotron-J. Explorations of the use
of pellets to exceed the Sudo density limit are being conducted
on LHD, TJ-II, and Heliotron-J. Additional data
from TJ-II and Heliotron-J are needed to complete the
analysis. Development of more detailed TESPEL (hydrocarbon
pellet) ablation models is being supported by data
from LHD, W7-X, and TJ-II. Isotope studies examining
ratios of deuterium to hydrogen have been conducted in
LHD using pellet injection. This work is being expanded
to other device.
3D Turbulence (M. Nakata)
Studies to validate models of turbulent transport, zonal
flows, radial electric fields, and fluctuations are being supported
by interdevice comparative studies. Optimization
of stellarators for turbulent transport has been identified as
an emerging topic in this group. Such work will be highlighted
at future meetings of the CWGM. Work is moving
forward on formulating a stellarator base case turbulent
transport simulation. This base case will be the analogue
for the cyclone base case, which was previously developed
for turbulence simulations. General features of this benchmark
have been identified, and responsible persons are
being identified for the majority of stellarator turbulence
codes.
Impurity Transport (N. Pablant)
Work continues on five joint tasks in the impurity transport
group. Assessment of the role of impurity charge
number on impurity transport is ongoing as TESPEL data
from W7-X is being processed. This activity will be
extended through “super-multiple” TESPEL injection on
LHD in the upcoming campaign. Radially global multispecies
simulations are informing investigations into the
LHD impurity hole. These simulations are being combined
with analysis of impurity transport to further clarify
the role of the neoclassical radial electric field. Simulations
with the SFINCS, EUTERPE, FORTEC-3D, and
KNOSOS codes including the effect of potential variation
on a flux surface have been conducted. These simulations
are awaiting experimental data to validate their models.
Work is ongoing to incorporate the STRAHL impurity
transport code into a W7-X data analysis workflow. A
boron dropper installed on LHD will provide useful data to
the impurity transport group in the upcoming experimental
campaign. Initial data from W7-X is suggesting that impurity
transport is being dominated by turbulent processes.
Work is under way to model and assess these plasmas.
Fig. 2. Poincaré plots of the Wendelstein 7-X line of island
divertors. Source: IPP-Greifswald.
Stellarator News -3- January 2020
Conclusions
The sessions highlighted ongoing activities in the CWGM
and continued to track progress on various activities. A
general topic discussed among groups was the need for a
clear link to the ITPA. Each group was tasked with identifying
a representative from the CWGM who also participates
in related ITPA activities. It was also suggested that
the next meeting of the CWGM place a focus on contributions
to upcoming IAEA Fusion Energy Conference.
New Joint German-US
Stellarator Project
The Max Planck Institute for Plasma Physics (IPP) in
Greifswald and the University of Wisconsin-Madison in
the United States have founded a joint research project to
investigate the power exhaust from a hot stellarator
plasma. The Helmholtz International Laboratory for Optimized
Advanced Divertors in Stellarators (HILOADS), in
which Forschungszentrum Jülich and Auburn University
in Alabama also participate, is financially supported by the
Helmholtz Association of German Research Centres.
Fusion systems of the stellarator type promise highperformance
plasmas in continuous operation. Accordingly,
heat and particles from the hot plasma permanently
stress the vessel walls. It is the task of the so-called divertor—
a system of specially equipped baffle plates to which
the particles from the edge of the plasma are magnetically
directed—to regulate the interaction between plasma and
wall. The structure of the magnetic field and the choice of
material for the plates determine how well the divertor can
perform this task and how well the plasma can be thermally
insulated. The divertor design for new stellarators is
therefore highly demanding in terms of both plasma physics
and technology and requires extensive experimental
and theoretical investigations.
To address these challenges, IPP in Greifswald and the
University of Wisconsin-Madison have founded
HILOADS. HILOADS offers a framework to intensify the
successful cooperation of the University of Wisconsin-
Madison as the central institution with IPP in Greifswald,
Forschungszentrum Jülich, and other American universities.
The scientists involved will optimize and coordinate
divertor designs, materials, and plasma confinement.
Fig. 1. The Wendelstein 7-X stellarator at Greifswald.
Source: IPP, Volker Steger.
Stellarator News -4- January 2020
For the needed experiments, both Wendelstein 7-X (Fig.
1) in Greifswald, the world’s largest stellarator, and the
much smaller but very flexible HSX (Helical Symmetric
Experiment) in Madison (Fig. 2) are available. The two
devices differ not only in size, but also in their completely
different concepts for the divertor and for optimizing
plasma confinement. The small CTH (Compact Toroidal
Hybrid) device at Auburn University (Auburn, Alabama)
is another resource. In addition to these three stellarators,
two linear plasma systems will be used for investigations
of materials and wall conditioning, as well as for the
development of measuring instruments: PSI-2 in Jülich
and MARIA in Madison. Equipped in this way, HILOADS
will promote the development of the next generation of
optimized stellarators and, in particular, support the development
of a concept for a new medium-sized stellarator
experiment in Madison.
With the funding program of Helmholtz International
Labs, the Helmholtz Association, to which the IPP is affiliated
as an associated institute, aims to expand international
cooperation with excellent research institutions and
create visible research activities of the Association at locations
abroad. The Helmholtz Association will provide 24
percent of the €6.125 million estimated for HILOADS
over the next five years. The universities in Madison and
Auburn will contribute 35 and 15 percent, respectively;
IPP and Forschungszentrum Jülich, 18 and 8 percent,
respectively. HILOADS is scheduled to start in spring
2020.
Isabella Milch
Max Planck Institute for Plasma Physics
Greifswald, Germany
Fig. 2. The Helical Symmetric Experiment (HSX) at Madison,
Wisconsin. Source: University of Wisconsin-Madison.