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Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy.
Published by Fusion Energy Division, Oak Ridge National Laboratory
Building CR 5600 P.O. Box 2008 Oak Ridge, TN 37831-6169, USA
Editor: James A. Rome Issue 138 August 2012
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
Wendelstein 7-X News
U.S. trim coils
On 26 June, a key milestone was reached when the first
“trim coil” for Wendelstein 7-X (W7-X) was delivered to
Greifswald (Fig. 1). The 3 year, $7.5 million project to
design and manufacture a set of 5 trim coils for W7-X was
launched in 2011 in cooperation with Princeton Plasma
Physics Laboratory (PPPL), Oak Ridge National Laboratory
(ORNL), and Los Alamos National Laboratory
(LANL). The participating U.S. institutions will become
partners in the research program at the Max-Planck-
Institut für Plasmaphysik (IPP).
Scientists and engineers from PPPL started the design of
the coils in January 2011. The construction order was
placed with Everson Tesla, Inc. (ETI), of Nazareth, PA, in
October. ETI is one of the larger U.S. magnet manufacturers
and has collaborated with PPPL for more than 20 years
on different projects. Holding to schedule in spite of the
large distance and 6 hour time difference separating the
partners is a noteworthy achievement. Constructive cooperation
between PPPL, ETI, and IPP was facilitated by
numerous video conferences and also site visits.
Experiments that will engage U.S. researchers are planned
to answer a key question about the suitability of a stellarator
for operation as a power plant: Is it possible to generate
a stable and at the same time spatially precise magnetic
field in such a way that particles and energy can be
removed from the plasma while maintaining acceptable
loads on the divertor? The key to answering this question
lies in the symmetry of the main magnetic field, its accuracy,
and the ability to influence the particle and energy
fluxes with the help of smaller correction fields. Beside
these practical considerations, the trim coils provide an
additional experimental tool for investigating the complex
interaction of plasmas and fields, studying for instance the
influence of particle drifts and shielding effects.
Fig. 1. Staff members standing at the first trim coil at Everson
Tesla (top) and at IPP Greifswald.
:
la
Photo: IPP, Anja Richter Ullmann Photo: Everson Tesla, Tom Stenulis
In this issue . . .
Wendelstein 7-X News
The collaboration with the U.S. has resulted in delivery
of the first trim coil. At Greifswald, the five modules of
W7-X are in their final places on the machine foundation
and are being connected. Internal bolts and supports
are being installed robotically. ....................... 1
CoordinatedWorking Group Meeting (CWGM10)
for Stellarator-Heliotron Research
A summary of the meeting is presented. ................ 3
Stellarator News -2- August 2012
One trim coil will be installed on each of the five Wendelstein
7-X modules (Fig. 2). In contrast to the superconducting
main coils, the trim coils are normal conducting
copper coils with integrated cooling channels. This is
acceptable because the trim coils generate only a small
correction field. Four of the five coils are identical in size
and shape. Due to limited space on the outer vessel, the
fifth trim coil is smaller. Each trim coil will be fed by an
independent power supply unit designed for continuous
operation. The power supplies provide each coil with
almost 2000 A.
The first coil with its overall dimensions of 3.5  3.3 m
will be prepared for assembly during the next weeks. First
the coil will be measured precisely. Next points must be
marked where the supports have to be fixed to attach the
coil to the outer vessel. With a budget of $4.3 million this
largest contribution of the German-American cooperation
project will be finished with the delivery of the last coil in
2013.
Other contributions of the U.S. partners have also made
good progress. With ORNL in charge, a design is being
developed for a special divertor target element that can
absorb high heat fluxes from the plasma and thus protect
more sensitive structures from overload.
Status of W7-X construction
The five modules are in their final position on the
machine’s foundation. Three modules are already joined:
the central rings have been screwed together and the
plasma and outer vessel module planes have been welded.
This process was again verified by laser measurements.
The port assembly for the last module is near completion,
while the assembly of the in-vessel components has just
started. In every module about 1200 bolts and supports for
fixing the in-vessel components to the plasma vessel will
have to be welded in place with the help of a positioning
robot (Fig. 3).
Beate Kemnitz
Max-Planck-Institut für Plasmaphysik Greifswald
Wendelsteinstraße 1
17491 Greifswald, Germany
E-mail: Beate.Kemnitz@ipp.mpg.de
Fig. 2. The trim coils (blue) are shown at their locations on
W7-X.
Fig. 3. Positioning robot inside the vacuum vessel.
Photo: FANUC Robotics Europe S.A.
Stellarator News -3- August 2012
CoordinatedWorking Group
Meeting (CWGM10) for
Stellarator-Heliotron Research
The 10th Coordinated Working Group Meeting
(CWGM10) was held 6–8 June 2012 at the Max-Planck-
Institut für Plasmaphysik in Greifswald, Germany. More
than 50 experts (in person and via video conference) participated.
More detailed materials presented at the 10th CWGM are
available at http://ishcdb.nifs.ac.jp/cwgm10.html. This
paper is an overall summary of the meeting.
At this 10th meeting of the CWGM, H. Yamada (NIFS)
reflected on the original intent of the working group,
“sharing goals, and acceleration of the output-outcome
wheel,” referring to material from an informal meeting in
2003 (Greifswald) on the Stellarator-Heliotron Database
and the IEA Stellarator Executive Committee meeting in
2004 (Villamoura). Since then, the CWGM activity has
embodied these ideas by promoting a comprehensive and
exact understanding of complex physics through the Stellarator
Database, joint experiments, benchmarking, and
joint papers. It was also pointed out that the CWGM
become a gateway to global/domestic programs such as
the International Tokamak Physics Activity [(ITPA) http://
www.iter.org/org/team/fst/itpa] and power plant design
activities.
Emerging from CWGM activities, many contributions to
the three-dimensional (3D) physics aspects of ITPA have
already been made by the stellarator-heliotron (S-H) community.
In addition, a reactor session was set up at this
10th CWGM to promote joint efforts in this direction. To
further promote joint experiments among S-H devices, the
status of the devices LHD, TJ-II, and Heliotron J (H-J)
was reviewed. New resources that will soon become available
include higher electron (ECH) and ion (ICH) heating
power and closed divertor (cryo-pumping at 1 section and
baffle/dome installation at 8 of 10 sections) in LHD; the
pellet injector and second heavy-ion beam probe (HIBP)
(in collaboration with ORNL and Kharkov/Kurchatov
Institute, respectively) in TJ-II; and plasma profile measurement
by means of several diagnostics in H-J. Increases
in device capability should extend the range of joint
research. One specific request to TJ-II and H-J was to perform
deuterium (D) experiments to increase the database
(in addition to previous D experiments in W7-AS) to
resolve isotope effects, which should be a critical issue in
burning S-H plasmas.
The topics discussed at the meeting were the following:
resonant magnetic perturbation (RMP), wall conditioning,
3D equilibrium, flow and viscosity, transport validation
(energy transport: ongoing, and particle transport: kickoff),
Alfvén eigenmode and energetic particles, database
issues, and reactor design and system code. Among these
sessions, joint papers have been accepted on Alfvén eigenmode
(oral, EX/5-2) and transport validation (poster, EX/
P3-14) issues for the 24th IAEA Fusion Energy Conference
(IAEA-FEC) at San Diego in October 2012, and on
database issues (singularization of data subgroups) for the
next EPS conference (Stockholm, July 2012). One more
joint paper that originated from CWGM activities will be
presented at the EPS meeting on magnetic island dynamics,
although the session was not set up at CWGM10.
Summary of each session
RMP
A study on transport modification due to RMP in LHD has
been jointly conducted with the tokamak community. It
has been formulated as task TC-24 (along with 3D effects
on macro- and microstructures) in the transport and confinement
topical group of the ITPA. A further joint experiment
is being planned for the coming 16th experimental
campaign of LHD. It will investigate how the amplitude of
perturbations affects the level of turbulent transport.
Wall conditioning
TJ-II reported on recycling and isotope exchange of H, D,
and He plasmas resulting from Li-wall conditioning
research. Development of a wall conditioning strategy for
W7-X has been relying upon the experiences of other
devices, such as WEGA, LHD, and Tore Supra. Since
ECRH will be the main heating source for the first operational
phase of W7-X, an ECRH wall conditioning strategy
needs to be developed. For this purpose, a joint
experiment is proposed in LHD by utilizing its ECRH
capability.
3D equilibrium
Recent progress on 3D equilibrium studies in LHD highbeta
plasmas was reviewed, focusing on how to identify
the 3D magnetic field structure. One approach uses the
positions of the radial electric field null (Er = 0) or the
maximum gradient of Er, originated by the positive Er generation
due to opening magnetic field lines. This has also
been observed in DIII-D. Identification of stochasticization
in the plasma edge has also been tried using the heat
pulse propagation technique. Necessity of rigorous numerical
treatment outside the last closed flux surface (LCFS)
was pointed out in order to relate measurements at different
scrapeoff layer (SOL) positions and for performing
simulations on edge physics such as EMC3. An open
question raised is how to validate the numerical modeling.
As a follow-up discussion, divertor heat flux measurements
in LHD were introduced, with a statement that the
positions of measured heat flux peaks fit to those predicted
from HINT2 numerical results. Such a validation study,
Stellarator News -4- August 2012
including the identification of the LCFS, was discussed as
a topic for joint experiments.
Flow and Viscosity
A wide range of research on plasma flow and viscosity
issues has been conducted in S-H devices. Based on individual
discussions made at the 18th International Stellarator-
Heliotron Workshop (Australia, 2012), it was agreed to
launch a flow and viscosity session in CWGM. Possible
joint actions discussed so far between NIFS and CIEMAT
were introduced, such as the numerical code verification/
validation and joint experiments on plasma biasing. The
HSX team also joined discussions with its Reynolds stress
and Er measurement, and comparisons to PENTA code
calculations. Joint action plans have been discussed
between NIFS and CIEMAT. Any such proposals will be
sent to HSX group so that they can decide to join/update
these proposals. The topics-oriented joint actions in such
areas as the trigger and dynamics of the L-H transition, 3D
effects on zonal flow, and the impact of self-regulation
mechanisms in transport and stability were also proposed
in connection with the 3D physics session in ITPA.
Transport validation (energy transport)
Transport in S-H plasmas consists of neoclassical and turbulent
contributions. To perform studies of transport
model validation, the impact of neoclassical transport has
been investigated in ion-root plasmas (medium to high
density with comparable electron and ion temperature
under sufficient ion heating power) of W7-AS (previous
documentation), LHD, and TJ-II (new joint experiment).
This is a natural extension of joint efforts on CERC (core
electron-root confinement) plasmas and successfully documented
international benchmarking activities of neoclassical
transport codes. Towards a joint paper at the coming
IAEA-FEC, progress in energy balance analysis was
shared and the formats of materials were discussed along
with “homework” assignments. It was also followed up by
showing examples from the non-local neoclassical transport
code, FORTEC-3D, to provide (in some cases) a
quantitatively different prediction of the neoclassical
ambipolar Er. Certainly, it would be valuable to examine
discharges of joint experiments using several other codes
for validation from different points of view. The XICS (Xray
Imaging Crystal Spectrometer, under collaboration
between PPPL and NIFS) measurement of ion and electron
temperatures was also reported, to provide profile
information for the energy transport analysis. The request
to perform particle transport study also was raised, and
this became a natural introduction to the following session.
Particle transport
Density control is one of the key issues that must be comprehensively
understood for optimal operation in large SH
plasmas and in reactors. The summary of observations
on particle transport in LHD is as follows: the density profile
becomes more hollow in the outwardly shifted configuration
with decreasing collisionality. The density profile
is determined not by particle fueling (NBI fueling and wall
source) but by transport—the neoclassical contribution is
larger (smaller) for convection (diffusion). Gyrokinetic
quasilinear analysis shows qualitative agreement of the
zero flux condition, indicating that an anomalous feature
plays a role in transport. It was pointed out that locating
the separation of core and peripheral regions by investigating
the penetration depth of neutral particles would be one
approach to be considered for particle transport. A joint
experiment in LHD was proposed to further investigate
particle transport by decoupling heating and particle
sources such as ECRH and/or pellet injection.
A summary of methods and results for particle transport
studies in TJ-II contributed to this new topic. A poor particle
confinement regime identified in low-density plasmas
appears to be considerably affected by kinetic effects from
ECRH. This identification must be discerned in order to
plan a joint contribution. NBI plasma studies are ongoing,
but the analysis done so far points to a linear dependence
of particle confinement time with density. There seems to
be, however, a strong degradation with heating power. Particle
confinement improves when the L-H transition
occurs, but no steady-state H-mode is available to make a
quantitative description.
Alfvén eigenmodes, energetic particles
This is the third session on this topic, after the launch at
the 8th CWGM (NIFS, March 2011). Joint experiments
have successfully evolved between H-J (low rotational
transform ) and TJ-II (high ) to understand Alfvén eigenmodes
in low-shear S-H plasmas in both devices; this will
be reported in a joint paper at the next IAEA-FEC. The
comparative study will be expanded to LHD. So far, an
independent database in each device has been utilized for
numerical code validation, along with ongoing code verification
among several codes. It was pointed out that the
accuracy of equilibrium, especially the -profile, should be
carefully considered such as for -scan experiments and
those interpretations by numerical codes. The Bernsteinwave
heated plasmas in the WEGA stellarator were also
reported to show the existence of suprathermal electrons in
the keV range, and also to show direct momentum transfer
with the addition of 2.45 GHz ECRH.
Database issues
ISS04 has been established based on the International Stellarator-
Heliotron Confinement Database (ISH-CDB) using
data from existing devices. As the next step, the assessment
of energy confinement for future devices has been
tried by utilizing dimensionless variables and following
the principles of similarity and scale invariance. Clustering
of data using several sets of dimensionless variables
Stellarator News -5- August 2012
has been going on, and a lot of effort will be still be
required. Extension of CDB with new data was requested
to increase the capability for discriminating datasets.
Progress of the Profile Database (ISH-PDB) was also
reported. Equilibrium information corresponding to registered
discharges, such as wout and input files for
VMEC2000, has been registered (requiring authentication
as joint analysis). Reading routines will be provided to
cover possible different versions of VMEC2000 (currently,
equilibria from version 6.90 and 8.00 coexist in the
database). This “de facto standard” platform should
enhance the validation activity of numerical codes/modeling
by easing the possible difficulties of their applications
to experimental data.
Reactors, system codes
One of long-term goals of the CWGM activity is to define
the data basis for S-H reactor studies. Thus, it is meaningful
for the CWGM activity to share the current status and
future prospects of S-H reactor scenario and system code
development to guide the research direction. At CWGM4
(CIEMAT, October 2008), a reactor session was held,
focusing on engineering issues. At this CWGM, reactor
system code application to reactor design was also emphasized.
The European fusion road map was introduced and
discussed, identifying required steps (the “step ladder”)
towards DEMO and a commercial fusion power plant.
The design status of HELIAS 5-B (a 5-period HELIAS
reactor) was reported with emphasis on engineering
issues. Predictive simulation using 1D transport models
(including the neoclassical database and anomalous modeling)
has been performed for a “upscaled” W7-X, to particularly
find the renormalization factor (in ISS04 scaling),
which tends to decrease with size of the plasma. This finding
provides impact by taking the confinement scaling law
into the system code. The application of the reactor system
code HELIOSCOPE and the current status/future prospects
of FFHR-d1 (the heliotron reactor in the design
study) were also reported. HELIOSCOPE has been utilized
to specify a design window with several engineering
and physics (basically the scaling law, right now) conditions.
Fast and accurate insertion of 3D equilibrium and
physics modules into the system code will be pursued to
obtain a more robust design. It was concluded that the
interaction between system code development/application
and CWGM activity in this regard should be enhanced.
Optimization of the TJ-II configuration has been conducted
utilizing DAB (Distributed and Asynchronous
Bees) algorithm on grid computers (http://fusion.bifi.unizar.
es/).
Figure 1 shows a slide prepared for the final concluding
session, summarizing work plans for 2012/2013 (and
beyond). Programmatic joint experiments/activities were
formulated based on the meeting discussions. These are
open to everybody, of course. Thus, if you/your colleagues
are interested in joining, please let us know.
Finally, it was agreed that the next CWGM11will be
hosted by CIEMAT. The details will be announced once
they become available.
The presented materials in previous CWGMs (except the
first one, unfortunately) are reachable through either the
IPP or NIFS CWGM website (http://www.ipp.mpg.de/
~dinklage/CWGM10/ or http://ishcdb.nifs.ac.jp/). Looking
back at the contents of previous CWGMs should be
instructive when considering the future directions for this
activity.
Acknowledgements
Prof. R.Wolf, Dr. A. Dinklage and Ms. M. Radau (IPPGreifswald)
are greatly appreciated for their extensive
efforts in hosting CWGM10. Mr. R. Klatt (IPP-Greifswald)
and Dr. T. Akiyama (NIFS) are also appreciated for
their kind support in making the video conference run
smoothly. CWGM10 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 of the participants of
CWGM10
E-mail: yokoyama@LHD.nifs.ac.jp
Fig. 1. Slide prepared for the concluding session of
CWGM10.