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Published by Oak Ridge National Laboratory
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Editor: James A. Rome Issue 143 April 2014
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
Systems code approach for
burning plasma stellarator
devices
With ITER now under construction as the first fusion
device to study the physics of a burning plasma in detail,
an increased focus is being placed on a demonstration
power plant (DEMO).
This has been emphasized by the IAEA, which organized
the “Second IAEA DEMO Programme Workshop” [1] in
Vienna last December. This workshop was organized to
facilitate the discussion of current physics and engineering
status and issues for a DEMO fusion device. One of the
main topics discussed was the fusion systems code
approach illustrated in Fig. 1.
Systems codes, also known as design codes, are comprehensive
yet simplified models of a complete fusion facility.
Since they combine physics, engineering, and
economic aspects, they are used to develop conceptual
design points and to conduct sensitivity studies.
One commonly employed instrument is the systems code
PROCESS [2] maintained by the Culham Centre for
Fusion Energy (CCFE), which is used in the EU to study
and optimize tokamak-based DEMO design concepts. For
the heliotron line, the well-developed systems code
HELIOSCOPE [3] maintained by the National Institute for
Fusion Science (NIFS), is used for purposes such as
design window analysis of the force-free helical reactor
[4]. Up to now, no such tool was available for the helical
advanced stellarator (HELIAS) line. Therefore a HELIAS
module has been developed by the Max Planck Institute
for Plasma Physics (IPP) and implemented into the framework
of PROCESS by CCFE.
The advantage of developing a stellarator module for
PROCESS is that common routines for non-device-specific
systems such as plant power balance or routines for
optimization are already available and have gained maturity
through many applications. Moreover, this common
framework allows direct comparative studies of tokamak
and stellarator design concepts.
In order to incorporate a stellarator module into PROCESS,
stellarator-specific models are required that reflect
the specific properties of the stellarator. These models
include
􀃖 a geometry model based on Fourier coefficients that
Fig. 1. Concept of systems codes and their interaction with
detailed simulations and experiments.
In this issue . . .
Systems code approach for burning plasma
stellarator devices
A stellarator-specific (HELIAS) module has been
developed and implemented in the systems code
PROCESS. This approach is investigated to allow for
detailed design studies of burning plasma HELIAS
devices, and to facilitate a direct comparison of tokamak
and stellarator power plant concepts. ............. 1
Coordinated Working Group Meeting
(CWGM13) for Stellarator-Heliotron Research
Minutes of the 13th Coordinated Working Group Meeting
(CWGM13) held 26–28 February 2014 at the Uji
Campus of Kyoto University. .................................. 3
Stellarator News -2- April 2014
can represent the complex three-dimensional (3D)
plasma shape,
􀃖 a basic island divertor model that assumes diffusive
cross-field transport and high radiation at the X-point
[5],
􀃖 a coil model which combines scaling aspects based on
the Helias 5-B design [6] in combination with analytic
inductance and field calculations, and
􀃖 a transport model that employs a predictive confinement
time scaling derived from 1D neoclassical [7]
and 3D turbulence [8] simulations.
One requirement of this development is to retain low calculation
times without compromising the necessary accuracy
and complexity of the 3D stellarator-specific
properties.
As an example, in Table 1 results for the coil module are
compared to actual values for Wendelstein 7-X (W7-X).
Only the mass of the support structure and the mass of the
winding pack (WP) differ by a small degree. Since the
mass of support structure was not a costing factor for
W7-X, it was not optimized in this regard. Also, the winding
pack (WP) of W7-X includes additional materials,
explaining the higher mass compared to the calculation. If,
in contrast, HELIAS burning plasma devices are considered,
the structure certainly needs to be optimized, which
means that the calculations will be more valid for larger
devices.
Conclusions
A stellarator module has been developed and implemented
in the systems code PROCESS and benchmarked against
W7-X, showing good agreement.
With such a tool available, direct comparative studies of
tokamak and stellarator reactors can be prepared. Also
systems studies of HELIAS burning plasma devices can be
carried out to find consistent design points.
This tool can be used not only for reactor-sized facilities
but also for an intermediate-step stellarator. A direct step
from W7-X to a HELIAS reactor would be substantial and
therefore poses certain risks. An intermediate step, an
ITER-like stellarator, may thus be desirable to study collective
particle behavior in a burning plasma in 3D geometry.
The systems code approach will be a valuable tool in
designing and optimizing such a device.
References
[1] http://www-naweb.iaea.org/napc/physics/meetings/
TM45256.html
[2] D. Ward, Fusion Sci. Technol. 56 (2009) 581.
[3] T. Goto et al., Nucl. Fusion 51 (2011) 083045.
[4] T. Goto et al., Plasma Fusion Res. 7 (2012) 2405084.
[5] Y. Feng et al., Nucl. Fusion 45 (2005) 1684.
[6] F. Schauer et al., Fusion Eng. Des. 88 (2013) 1619.
[7] Y. Turkin et al., Phys. Plasmas 18 (2011) 022505.
[8] P. Xanthopoulos et al., Phys. Rev. Lett. 99 (2007)
035002.
F. Warmer1, C.D. Beidler1, A. Dinklage1, Y. Feng1, J. Geiger1, R.
Kemp2, P. Knight2, F. Schauer1, Y. Turkin1, D. Ward2, R. Wolf1,
and P. Xanthopoulos1
1Max Planck Institute for Plasma Physics, Wendelsteinstr. 1,
17491 Greifswald, Germany
2Culham Centre for Fusion Energy, Abingdon, OX143 DB,
Oxfordshire, United Kingdom
Table 1. Comparison of the PROCESS stellarator coil
model with the actual values from W7-X.
Stellarator News -3- April 2014
Coordinated Working Group
Meeting (CWGM13) for
Stellarator-Heliotron Research
The 13th Coordinated Working Group Meeting
(CWGM13) was held 26–28 February 2014 at the Uji
Campus of Kyoto University. The materials presented at
this meeting are available at http://ishcdb.nifs.ac.jp/ and
http://fusionwiki.ciemat.es/wiki/
Coordinated_Working_Group (􀃆 CWGM13). A brief
summary of the meeting is provided here.
Three-dimensional (3D) transport in divertors
In response to a proposal made at the last CWGM
(CWGM12 in Padova), recent progress on the experimental
identification and physics interpretation of the 3D
effects of magnetic field geometry/topology on divertor
transport was reviewed. This information has passed the
domestic (Japan) selection process for presentation at the
25th IAEA Fusion Energy Conference. Identification of
key parameters for 3D effects should open new perspectives
on divertor optimization for future reactors. Interactions
between the 3D structure of the magnetic field,
current in the scrape-off layer (SOL)/stochastic layer, and
parallel and perpendicular electric fields should be systematically
clarified through diagnostics and modeling for a
range of magnetic field configurations. Issues in formulating
joint experiments, potential and current measurements,
and 2D temperature and density measurements were discussed
among the HSX, LHD and TJ-II teams. Comparison
with linear devices “without 3D effects” is also
recommended to elucidate 3D effects in a comparative
manner.
Impurities
Impurity issues have been raised at several CWGM meetings,
but unfortunately there had been no impurity sessions.
This time, an impurity session was formed to
reactivate discussions in the framework of CWGM.
Experiments on impurity transport were reported from
LHD for both intrinsic and tracer-encapsulated solid pellet
(TESPEL)-induced impurities and from TJ-II for a broad
range of materials (LiF to W). In TJ-II, TESPEL injection
is now under consideration with a newly installed pellet
injector. In-surface variation of electrostatic potential
caused by ion drift-kinetic dynamics has been proposed as
potentially important for radial impurity transport. TJ-II
has identified asymmetries in C6+ impurity density and
floating potential, for which comparisons with the
EUTERPE code result have commenced. The importance
of the impurity issues calls for coordinated action both in
experiments (Heliotron J, LHD, and TJ-II) and simulations
(FORTEC-3D, EUTERPE, fluid codes, ...) to assess the
existence of in-surface potential variation and its contribution
to radial impurity flux. R. Burhenn et al. published a
joint paper on impurity issues in 2009 [“On impurity handling
in high performance stellarator/heliotron plasmas,”
Nucl. Fusion 49 (2009) 065005]. Follow-up joint papers
(documenting subsequent developments) can be formulated
by reactivating joint activities in CWGM.
Highlights in experiments and invitation to
joint experiment
Recent experiments in the Heliotron J device were
reviewed. Plasma startup; the plasma parallel flow measurement
and its comparison with neoclassical prediction;
fast-ion driven MHD and related particle flux studies by
using several probe systems; the external control of energetic-
ion-driven MHD instabilities by ECCD; the fast ion
distributions in ICRF experiments; high density operation
through high intensity gas puff (HIGP) fueling and supersonic
molecular beam injection (SMBI); transition to
improved confinement in such a high-density regime; etc.
were emphasized to trigger proposal and discussions for
joint experiments. From LHD, steady progress in plasma
parameters (ion temperature, simultaneous high temperatures,
and steady-state operation) was reported. New diagnostics
enabling high dynamic-range spectroscopic
measurement of the Balmer- lines, have facilitated quantitative
understandings of the impacts of discharge cleaning
on producing high ion temperature plasmas. RMP
experiments have fertilized 3D physics, such as magnetic
island dynamics (growth/healing), and observation of
peaked pressure profiles inside the magnetic island after
pellet deposition. A tentative schedule of deuterium experiment
was also mentioned, along with planned upgrades of
heating and diagnostics capability. MyView (advanced
data viewer) and TASK3D-a (integrated transport analysis
suite) were introduced, which should facilitate joint experiments.
Invitations to the 18th LHD campaign (in 2014)
were presented.
Reactor/systems code
After the kick-off session at CWGM10 (2012, Greifswald),
interaction between systems codes (HELIOSCOPE
and PROCESS) and physics models has been successfully
enhanced. Plasma operation control scenario consideration
has been progressing for FFHR-d1, in which a transport
model based on LHD experiment has been employed.
Coupling with TASK3D (an integrated transport analysis
suite) is in preparation for consistency checks (time evolution
of equilibrium, heating along with plasma profiles).
As for PROCESS (see the first article in this issue), modules
for plasma geometry, modular coils, and an island
divertor have been developed and implemented. An ITG
transport model, deduced on the basis of GENE simulation
Stellarator News -4- April 2014
results, will also be included. In this way, common development
for these two reactor design activities is in progress
to implement further physics models. Benchmarking
activities between these developments were proposed.
However, it is too difficult at this moment because many
modules depend on the individual design. Mutual information
exchange and closer links to physics models are anticipated.
Flows and viscosity, transport
Flows and viscosity strongly depend on magnetic configurations,
collisionality and radial electric field etc. A variety
of magnetic configurations, either within a single device
or covering several devices, provide a wide variety of
research subjects to be challenged in experiment, theories,
and simulations. In terms of neoclassical transport, the
neoclassical poloidal viscosity analyses for LHD biasing
plasmas, the validation of stellarator optimization via
extended neoclassical simulations and dedicated experiment,
and parallel flow in Heliotron J NBI plasmas were
reported. Benchmarking of a suite of neoclassical transport
codes both with (FORTEC-3D) and without nonlocal
effects has been progressing through the use of experimental
data. Setting a standard case (such as the “Cyclone
DIII-D base case” [C.M. Greenfield et al., Nucl. Fusion 37
(1997) 1215]) for this joint activity was proposed, utilizing
the International Stellarator-Heliotron (ISH) Profile Database.
Uploading the simulation results was also suggested.
Joint activities for the evaluation of expected potential
variation on a flux surface ( 1) based on EUTERPE and
FORTEC-3D are planned in relation to the issue of impurity
transport. Neoclassical transport in tokamaks with 3D
magnetic perturbations has also been progressing through
collaborations among NIFS, PPPL, and the Japan Atomic
Energy Agency.
In addition to neoclassical transport, correlations among
flow, turbulence, and transport were discussed, stimulated
by presentations from HSX (core density turbulence and
plasma flows) and LHD (toroidal flows and turbulence,
electromagnetic gyrokinetic simulation in finite-beta plasmas).
Because such issues have been rapidly evolving
among several stellarator-heliotron devices, establishing
of a new session or identifying a new key person was proposed
to further activate collaborations on this topic.
Plasma startup
In stellarator-heliotron devices, current-free plasmas have
been produced by 1st/2nd harmonic ECH or (in LHD) tangential
NBI. No successful plasma startup by NBI only
has occurred in medium-size devices such as Heliotron J
and TJ-II. Successful startup is required in W7-X. Reliable
startup at low toroidal electric field is an important issue in
superconducting tokamaks. These issues have been programmatically
investigated in the ITPA Integration Operation
Scenario (IOS) topical group, and Heliotron J and TJII
have contributed to it. Multi-device experiments and
analyses have been made for detailed characterization of
plasma startup, yielding better understanding for modeling.
As the kick-off of this plasma startup session, (1)
issues of plasma startup in stellarator-heliotron plasmas,
(2) 2nd harmonic ECH breakdown in Heliotron J, LHD,
WEGA, and prediction for W7-X, (3) modeling of NBI
startup in LHD and W7-AS and the possibility of plasma
startup by NBI in W7-X, and (4) effects of ohmic induced
toroidal electric field were reported, as were modeling
efforts. A joint paper on ECH breakdown is in preparation.
Energetic particles, Alfvén eigenmodes
Database activity on Alfvén eigenmodes has been developing
among H-1NF, Heliotron J, and LHD. The Data
mining tool has been upgraded (updated clustering).
Effects of ECH/ECCD on Alfvén eigenmodes have been
programmatically investigated among Heliotron J, LHD,
and TJ-II. In Heliotron J, the global Alfvén eigenmode
(GAE) has been targeted for comparison with TJ-II
results, and the findings have passed the domestic (Japan)
selection process for the 25th IAEA Fusion Energy Conference
as a joint paper. Anomalous transport and loss of
energetic particles by MHD instabilities are also a leading
topic (3 devices mentioned above). The STELLGAP code
has been upgraded to consider the coupling between shear
Alfvén waves and acoustic waves. Beta-induced Alfvén
eigenmodes (BAE) including EGAM (energetic particledriven
GAM), which are observed in many devices, will
be examined by STELLGAP from the viewpoints of lowfrequency
modes in the gap caused by Alfvén and acoustic
waves.
3D equilibrium
The programmatic validation and cross-benchmarking initiative
for 3D equilibrium calculations (involving 11 codes
from 6 institutions) was introduced. Stellarator symmetric
tokamak equilibrium with small nonaxisymmetric perturbations
(ELM suppression experiments in DIII-D, shot
146058) allows participation of a wide range of equilibrium
codes. Calculations have found disagreement
between VMEC and linearized tokamak codes, and the
source of disagreement has not yet been resolved. A dedicated
run day for scanning key plasma parameters and Icoil
spectral scans is being planned to widen the database
for guiding validation and cross-benchmarking. A joint
activity utilizing stellarator-heliotron experiments was recommended.
The big impacts of the toroidal current on
magnetic topology especially in low magnetic shear configurations
were pointed out, and this issue has been systematically
investigated in W7-X (VMEC/EXTENDER,
HINT2) and TJ-II (HINT2). Some of these investigations
will be reported in the plenary talk at the coming EPS
Stellarator News -5- April 2014
(June 2014 in Berlin) by J. Geiger (IPP-Greifswald). Comparative
studies on Heliotron J should be started. The helical
core in RFPs and 3D displacement for tokamaks are
also emerging as collaborative topics in 3D equilibrium.
International Stellarator-Heliotron Confinement
and Profile Database (ISH-CDB, PDB)
The long history of the ISH databases was reviewed, in
terms of the evolution of global energy confinement scalings
(ISS95, ISS04), and from CDB to PDB with equilibrium
database. Reexamination of the iota dependence in
ISS04 (~ 0.41) was proposed, exploiting the extended TJII
data (a wide scan of  is available) for reducing collinearity
of  with geometrical parameters such as the aspect
ratio. Extension of HSX data (recent 1-T operation) was
also proposed. Recent TASK3D extension in LHD has
enabled sequential production of 0D data in CDB, so that
registrations from LHD are foreseen. A. Kus (IPP-Greifswald),
who has devotedly contributed to CDB and PDB,
will retire this September. He has used the statistical analysis
software, JMP. At this occasion, he prepares his
scripts to be widely available as a basis for future extension.
These will be uploaded in the ISH-DB web page. A
trial of the statistical approach for deducing ion and electron
heat diffusivities for a wide-range of LHD plasmas
(for example the “LHD profile database”) was also introduced.
Framework of collaborations
The restructuring of fusion activities in Europe, EUROFUSION,
was explained along with the EFDA road map
to the realization of fusion energy (http://www.efda.org/
wpcms/wp-content/uploads/2013/01/JG12.356-web.pdf).
Stellarators are part of this road map, as an alternative
approach and to mitigate risks such as steady-state operation
and plasma startup. Focus is put on the HELIAS line.
Work on other helical concepts (heliotrons, compact stellarators)
will continue as a part of international collaborations.
In this regard, CWGM is highly relevant to in the
EUROFUSION activity. TJ-II experimental plans were
introduced to emphasize that they are very much aligned
with EUROFUSION work programs. The Steady State
Operation Coordination Group (SSOCG) activity was also
introduced. This activity has been formulated by coordinating
the participation of related International Energy
Agency (IEA) Implementing Agreements and national
laboratories. Among 7 work packages, there is a “road
map to steady-state operation,” to which Stellarator-Heliotron
efforts should make substantial contributions. It was
agreed to hold a brainstorming meeting (video-conference)
led by T. Mutoh (NIFS) and A. Dinklage (IPP) along
with interested colleagues (such as the steady-state operation
theme group at LHD). The outcome from that meeting
will be reported at the next CWGM.
Miscellaneous
Setting up a “steering group” for CWGM activity was proposed
to facilitate organizational discussions such as the
prioritization of topics, session organization, etc.
M. Yokoyama (NIFS) was appointed to initiate discussions
on this issue. It was also recommended to report the
creation of the steering group to the Executive Committee
of the IEA Implementing Agreement on Cooperation in
Development of the Stellarator-Heliotron Concept.
At the end of the meeting, A. Dinklage (IPP-Greifswald)
proposed to hold the 14th CWGM in Europe. The CWGM
activity has been recognized in EUROFUSION (see,
“Framework of collaborations”) as one of the international
collaborative frameworks. It recommends holding two
meetings, one abroad and one in Europe, in a year. Based
on recent increased interest in the stellarator concept in
Hungary and Poland, he will contact these two countries to
inquire about their interest in hosting the next CWGM.
Acknowledgements
We sincerely appreciate all the efforts made by the local
organizers (Heliotron J group, Kyoto University). In particular,
Ms. A. Murata is appreciated for her sincere kindness
to make CWGM13 comfortable and run smoothly.
We are indebted to Mr. R. Klatt (IPP-Greifswald) for videoconference
support through EFDA-TV. The 13th
CWGM is partly supported by NIFS (National Institute for
Fusion Science)/NINS (National Institutes of Natural Sciences)
under the project, “Promotion of the International
Collaborative Research Network Formation.”
M. Yokoyama (NIFS) on behalf of all participants in the 13th
CWGM.