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An international journal of news from the stellarator community
Editor: James A. Rome Issue 158 August 2017
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Coordinated Working Group
Meeting (CWGM16) for
Stellarator-Heliotron Research
The 16th Coordinated Working Group Meeting
(CWGM16) was held 18–20 January 2017 in Madrid,
Spain, under the auspices of the IEA Energy Technology
Network (IEA Technology Collaboration Programme for
Cooperation in Development of the Stellarator-Heliotron
Concept).
Figure 1 shows the on-site workshop participants, representing
CIEMAT (Madrid), IPP Greifswald and IPP Garching
(Germany), IPPLM (Poland), IPT Kharkov
(Ukraine), Kyoto University and NIFS (Japan), and
ORNL, PPPL, and the University of Wisconsin-Madison
(USA). Staff from the host institution, CIEMAT, attended
many of the sessions, which brought the total number of
those engaging in the meeting to about 30.
This was the largest CWGM participation in recent years,
and it may have resulted from the great interest generated
by the then imminent operational phase OP1.2 campaign
at Wendelstein 7-X (W7-X) and the first deuterium plasma
campaign at the Large Helical Device (LHD). Discussion
of physics problems that could be addressed by these new
experiments had a prominent place in all the sessions.
As in previous editions, a group of coordinators (Ascasibar,
Dinklage, Gates, and Yokoyama) prepared the meeting
by identifying topics for international cooperations.
Colleagues from CIEMAT (Velasco, García-Regaña,
Ascasíbar) took care of local organization. The list of topics
and session coordinators was:
1. Core electron root plasmas (Felix Warmer).
2. Fueling; pellet injection (Kieran J. McCarthy).
3. Impurity transport (Novimir Pablant).
4. Turbulence, isotope effect (Teresa Estrada).
5. Divertor physics (Jeremy Lore).
6. Plasma wall interaction (Suguru Masuzaki / Ana
Belén Martín-Rojo).
The materials presented at the 16th CWGM are available
at http://fusionsites.ciemat.es/cwgm16/ and http://ishcdb.
nifs.ac.jp/. Here we provide a summary of each session
of the meeting.
Fueling; pellet injection (Kieran J. McCarthy)
Three presentations were made by members of the Particle
Transport and Fuelling Group. In the first report, Kieran J.
McCarthy (CIEMAT) discussed recent pellet fueling efficiency
measurements made on TJ-II. A significant database
has now been built up, in particular for hydrogen
pellets containing between 5 1018 and 1 1019 hydrogen
Fig. 1. On-site CWGM16 participants.
In this issue . . .
Coordinated Working Group Meeting
(CWGM16) for Stellarator-Heliotron Research
The 16th Coordinated Working Group Meeting
(CWGM16) was held 18–20 January 2017 in Madrid,
Spain. Physics problems that could be addressed in
upcoming campaigns of LHD and W7-X were the center
of discussions. ................................................... 1
Stellarator News -2- August 2017
atoms injected from the low-field side (LFS) into plasmas
heated with electron cyclotron resonant heating (ECRH)
and/or neutral beam injection (NBI) in the standard magnetic
configuration. The low efficiencies achieved with
ECRH may reflect a strong outward drift of pellet particles.
This needs further study and may be confirmed once
the HPI2 simulation code is working for TJ-II.
A brief summary was made of Heavy Ion Beam Probe
(HIBP) measurements on TJ-II during pellet injection in
which low-frequency fluctuations (≤ 20 kHz) are seen to
significantly decrease in the core immediately after an
injection before recovering again.
In the second contribution, Nerea Panadero (CIEMAT)
reported on her work at IPP-Greifswald, where she has
successfully implemented the HPI2 code for W7-X. She
reported on simulations made for OP1.2a, when a refurbished
pellet injector will be used to make the first pellet
injections. This pellet injector is limited to low velocities,
≤ 250 m/s; hence simulations were made for both LFS and
high-field side (HFS) injection. It is predicted that higher
fueling efficiencies will be achieved with HFS injections
from AEK41. She also reported on simulations for injections
with a new high-velocity pellet injector. In this case,
only LFS injection is possible. Finally she reported on her
efforts to implement HPI2 for TJ-II. This requires additional
work due to difficulties arising because the number
of particles in the pellet is similar to the number of particles
in the plasma, etc.
In the third presentation, Gen Motojima (NIFS) reported
on the new pellet injection system for Heliotron J. Pellets
of ~1.1 mm diam will be produced for injection at a speed
of several hundreds of meters per second. He informed the
meeting about commissioning tests and about first injections.
He reported that after a pellet injection, the density
in the plasma peripheral region is suppressed to the same
level as prior to injection. On the other hand, a steep density
gradient (3 times higher than before the pellet injection)
is formed in the plasma core. Further studies will be
undertaken. A short contribution was also made by R.
Sakamoto (NIFS) concerning the cryogenic pellet injection
experiments planned for the 2017 deuterium campaign
in LHD.
On the final day of the meeting, K. J. McCarthy presented
a summary of the three presentations and of discussions.
Given the successful implementation of the HPI2 code on
W7-X, it was considered that the code should also be fully
implemented on TJ-II and on Heliotron J so that comparative
studies could be made. (The code has been implemented
already on LHD.) It was mentioned that plasmoid
drift is strongly affected by magnetic configuration, hence
the need for significant effort to adapt the HPI2 to each
device. it was also mentioned that A. Mischenko is looking
into pellet ablation and cloud dynamics theory. However,
the current status of this work is unknown.
Finally several proposals for joint actions were made.
These are summarized as:
7. Penetration depth database (IPADBASE): Request
data for all devices to create a model as a function of
Te, Ne, Vp, and Mp. Create an intermachine comparison
with NGS scaling model. Can we determine what
additional factor(s) may be needed for stellarators? A
basic comparison could act as a seed!
8. HPI2 code: Currently implemented for W7-X and
LHD. It may be possible to benchmark W7-X version
on TJ-II (to be done in 2017). Could HPI2 be adapted
for Heliotron J?
9. Influence of fast electrons on ablation: Proposal for
next campaign on W7-X (open to external collaborators).
Will attempt to generate fast electrons in TJ-II
ECRH plasmas. LHD sees fast electron ablation.
10. 19th LHD experimental campaign: Joint research proposal
to study discharges with hollow Ne profiles
(pronounced central heating, strong edge fueling, no
central particle sources). A single pellet (or series of
pellets) will be injected. Monitor Ne profile during
and after injection to determine if such hollow profiles
can be mitigated with single injections or a series
of injections, and to what degree slow diffusive particle
transport and fast transport mechanisms are
important.
11. Intermachine comparison studies: Similar sizes and
parameters (Heliotron and TJ-II; LHD and W7-X).
Exchange of ideas for such studies between machines:
Broadcast calls for proposals with due dates.
12. Transport of deposited material: Evolution of density
profile after deposition; Transport studies by J. L.
Velasco.
Impurity transport (Novimir Pablant)
Six presentations were made in the impurity session.
José Manuel García Regaña (CIEMAT) and Albert Mollén
(IPP Greifswald) discussed several theoretical aspects of
neoclassical impurity transport usually neglected in standard
calculations: JMGR focused on the effect on radial
impurity transport of the variations of the electrostatic
potential on the flux surface, while AM commented on the
effect of interspecies collisions.
Naoki Tamura (NIFS) presented ideas for the comparison
study of impurity transport in helical plasmas. Thomas
Wegner (IPP Garching) presented impurity transport studies
using the laser blow-off system on ASDEX Upgrade
and near-future plans for W7-X.
Stellarator News -3- August 2017
Aaron Bader (University of Wisconsin-Madison) commented
on the impurity program at Wisconsin. Finally,
Motoki Nakata (NIFS) showed neoclassical and gyrokinetic
(GK) simulations of impurity transport and comparison
with the experiment for LHD plasmas.
Several joint proposals were identified. In the first one
(IT.1, Tamura), it was proposed to study the Z-dependence
of impurity transport and impurity accumulation. The second
one (IT.2 ,Velasco) is intended to develop an understanding
of the physics underlying the impurity hole. The
third one (IT.3, García-Regaña) focuses on investigation
of the variation of the electrostatic potential on the flux
surface and its effects on impurity transport. Finally, a
fourth task (IT.4) would be the development of a generalpurpose
3D stellarator impurity deposition/ionization/
transport tool.
Turbulence, isotope effect (Teresa Estrada)
A session on isotope effect and turbulence was held for the
first time in the CWGM series. GK simulations and experimental
results were presented addressing the effect of the
isotope mass on plasma turbulence and zonal flows. Isotope
dependence was found in the GK simulations for
LHD plasmas with a zonal flow enhancement in D plasmas
as compared to H plasmas. These results will be verified
in the LHD D–D experiments. Experimental results
from Heliotron-J and TJ-II were also discussed. While no
impact of the isotope mass on the L-H transition is found
in TJ-II plasmas, an increase in the zonal flow amplitude is
detected as the D concentration becomes dominant in
Heliotron-J. This effect, however, depends on the magnetic
configuration. Turbulence simulations for HSX plasmas
were also presented with a detailed plan for
experimental validation.
Several joint actions were discussed in order to study,
numerically and experimentally, the effect of the isotope
mass and magnetic configuration on zonal flow development
and confinement.
Plasma-wall interaction (Suguru Masuzaki/
Ana Belén Martín-Rojo)
In this session, plasma-wall interaction studies in LHD,
TJ-II, Uragan devices, and W7-X were presented. Material
migration in vacuum vessel, liquid metal experiments,
wall conditioning, in situ monitoring of wall condition,
postmortem analyses of plasma-facing components, comparison
between experimental observation and simulation,
and other topics were discussed.
For a joint action, “material migration in helical devices”
was proposed. In the action, long-term material probes on
first wall and divertor and/or limiter tiles will be analyzed
in helical devices. Isotope gas injection was proposed as a
tool for studying material migration study. The effects of
various magnetic configurations in a device should be considered.
To understand the observed material migration,
simulation is essential.
Divertor physics (Jeremy Lore)
Edge and divertor physics in 3D systems were discussed,
with presentations including results from LHD, W7-X,
and HSX. A common theme was a focus on simulation
and validation using the EMC3-EIRENE code, including
the need to compare the results to simple models based on
field line following. Specific aspects that can be explored
through experiments and modeling across various devices
include simultaneous matching of upstream and downstream
parameters, characterizing impurity transport and
radiation in boundary islands, the transition to high-recycling
and detached divertor conditions, and up-down flux
asymmetry. A desire was expressed for advanced code
capabilities such as additional atomic and molecular physics
including volume recombination and cross-field drifts
to better capture these effects.
The upcoming W7-X OP1.2 campaign with island divertor
configurations and the deuterium campaign at LHD offer
excellent opportunities to investigate physics critical to
future 3D reactors, with a wide range of collaborative
areas identified. Divertor physics–related goals of the
OP1.2 campaign include characterization of the power
loads (e.g., heat flux width), heat transport in stochastic
regions, impurity transport, and access to the high-recycling
regime. Multi-machine comparative studies in these
areas are of high importance, with several topics identified
as active projects, in particular detachment, impurity
screening, and stochastic boundary transport. Many of the
listed problems cut across topical areas, as exemplified by
the scraper element divertor components to be installed
during OP1.2. These components are designed to address
an issue that arises because the toroidal current evolution
in certain long-pulse scenarios is predicted to modify the
edge topology, resulting in a shift in the heat flux on the
order of centimeters.
The topic of designing future devices with optimization
criteria related to divertor and edge physics was also discussed.
Calculations intended to quantify divertor resilience
were presented, with residency defined as the
capability of a magnetic configuration to have divertor
loads that are insensitive to plasma current and beta evolution.
Developing cost functions related to divertor physics
suitable for introduction into an optimizer such as STELLOPT
remains an open research area.
Core electron root plasmas (Felix Warmer)
With the start of W7-X and extended diagnostic capabilities
in smaller devices like Heliotron J, interest in the coreStellarator
News -4- August 2017
electron-root-confinement (CERC) regime has been
revived after being dormant for several years.
Because W7-X was heated in the first operation phase
solely by ECRH at low densities, a clear CERC regime
was observed with peaked, high electron temperatures and
flat, low ion temperatures. The detailed investigation of
the CERC regime in W7-X is currently ongoing, including
comparison to neoclassical predications, in particular for
the critically important radial electric field. Further, a
magnetic configuration scan was carried out in OP1.1 and
the impact on the CERC regime is being studied.
W7-X and LHD have about the same plasma volume,
magnetic field strength and ECRH capabilities, so it is
intriguing to compare the CERC features in both devices
and the behavior and impact of the positive radial electric
field. A preliminary look at W7-X and LHD shows that
very similar electron temperature profiles (also in magnitude)
develop at comparable density and heating power. A
more detailed study is planned once W7-X data has been
analyzed more thoroughly.
In this context, the combined CERC investigations in W7-
X and LHD are a prime example of the international collaboration
activities in the frame of the CWGM. For both
experimental devices, many proposals have been submitted
for the next experimental campaigns (W7-X OP1.2
and LHD deuterium campaign) to study specific aspects of
the CERC regime in a comparative manner, including
impurity transport, particle transport, bootstrap current,
and the role of the radial electric field, to name only a few.
Further, a multi-machine scaling shall be derived for the
CERC threshold in the frame of the ISHCDB. As reported
at the last CWGM, the ISHCDB is to be integrated in a
common EUROfusion infrastructure. The new CERC data
should serve as a test case for the implementation in this
system in the near future.
Concerning the CERC regime, not only are the large stellarators
of interest, but also the helical devices which have
been operated for longer times and thus have accumulated
extensive experience. In particular, electron internal transport
barriers (eITB) have been observed recently at Heliotron
J and previously at TJ-II. The degree to which eITB
and CERC are different (or similar) transport regimes has
been a topic of discussion. It was pointed out that at TJ-II a
transition from CERC to an eITB with a stronger positive
electric field has been observed. Future work is needed to
clarify the degree to which these regimes exhibit different
features and how they are established. This is especially of
importance for the planned CERC threshold scaling.
In that context it was reported that the magnetic configuration
has a strong impact on the transition phase from ionroot
to electron-root. In particular, in Heliotron J it was
observed that a higher effective helical ripple can reduce
the density threshold at fixed power. It is hypothesized that
an emerging core magnetic island may be responsible for
this behavior due to the faster loss of electrons. Similar
observations have in the past been made in TJ-II, but more
work is required to understand these observations.
Beyond a brief follow-up meeting within the next International
Stellarator/Heliotron Workshop in Kyoto (Fall
2017), it was agreed to continue the Coordinated Working
Group Meetings in Spring 2018 at PPPL in Princeton.
Acknowledgments
The organizers are grateful to the host institution, Laboratorio
Nacional de Fusión, CIEMAT, and EUROfusion. In
particular, Ms. Loly Romero, Ms. Carolina Perales, and
Dr. Rodrigo Castro are appreciated for their kind support.
José Luis Velasco, E. Ascasibar, A. Dinklage, T. Estrada, J. M.
García-Regaña, D. Gates, J. Lore, S. Masuzaki, K. J. McCarthy,
N. Pablant, M. Yokoyama, F. Warmer, and the participants of
16th CWGM