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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 5700 P.O. Box 2008 Oak Ridge, TN 37831-6169, USA
Editor: James A. Rome Issue 129 December 2010
E-Mail: jar@ornl.gov Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
An overview of LHD results
Recently, the journal Fusion Science and Technology, published
a special issue (Vol. 58, No.1) on results from the
Large Helical Device (LHD). The issue comprises 13
chapters with 60 papers and covers not only the progress
of experimental studies for past 12 years, but also work on
diagnostics, heating devices, theory including threedimensional
(3D) effects, and engineering related to superconductivity.
Several key experimental results are discussed
here.
LHD experiments started in 1998. Since then, studies to
understand the intrinsic physics of net current-free plasmas
have successfully proceeded. The LHD operational
regime has been extended by increasing the heating capability
to 23 MW of neutral beam injection (NBI), 3 MW of
ion cyclotron resonance frequency (ICRF) heating, and 2.5
MW of electron cyclotron resonance heating (ECH). In
addition, a new 5 MW perpendicular neutral beam injection
system and a new ICRF antenna were added in
FY 2010. Significant physical achievements include high
beta (5.1%), high density (1.2 × 1021 m−3), and steadystate
operation (3200 s with 490 kW), supported by a reliable
cryogenic system that has safely operated for >56,000
hours.
Global energy confinement obtained with a configuration
optimized according to neoclassical theory, has proved
comparable to that of tokamaks running in ELMy Hmode,
exhibiting a gyro-Bohm-like property as seen in the
International Stellarator Scaling (ISS95). Significant collisionality
dependence (predicted by neoclassical theory)
has not been observed (Fig. 1). The optimization according
to neoclassical transport theory successfully
demonstrated that anomalous transport is reduced simultaneously.
Fig. 1. Dependence of the ratio of confinement enhancement
factor to the ISS95 scaling on collisionality. Three
cases with different magnetic axes are plotted [1].
In this issue . . .
An overview of LHD results
Recently, the journal Fusion Science and Technology,
published a special issue on the Large Helical Device
(LHD), summarizing 12 years of progress in heliotron
research related to LHD. Several major LHD experiments
are briefly reviewed here. ............................ 1
Engineering for installation of W7X superconducting
coil leads
An assembly cart to enable attachment of the coil
leads has been designed by a team from the U.S. as
part of the Wendelstein 7-X (W7X) collaboration. .. 4
Public Day at IPP
More than 2600 visitors took the opportunity to view
the construction of W7X and to learn more about
fusion. ..................................................................... 6
Stellarator News -2- December 2010
An improved electron energy confinement mode, found in
several helical devices, has been characterized by a highly
peaked electron temperature profile in the core region, and
appears when the centrally focused ECRH power exceeds
a certain threshold value. The threshold power has been
determined to be related to the transition of the radial electric
field from the ion root to the electron root, based on
the bifurcated nature of the radial electric field due to the
ambipolarity condition of neoclassical transport fluxes.
In ion heating experiments with high-Zeff conditions, a
central ion temperature of 13.5 kV was achieved in an
argon-seeded plasma, strongly suggesting the capability of
the helical configuration to confine high-performance
plasmas. In low-Zeff experiments, ion heat transport
improved in core plasmas heated by high-power NBI. The
ion temperature has a peaked profile with a steep gradient
in the core region (ion internal transport barrier). Transport
analysis indicates that anomalous transport is reduced in
the core region, where a negative radial electric field is
predicted by neoclassical ambipolarity. Improvement of
ion heat transport with positive radial electric field was
also successfully demonstrated utilizing strongly focused
ECRH, suggesting further improvement of ion heat transport
in reactor-relevant plasmas.
Spontaneous toroidal flow driven by the ion temperature
gradient and an extreme hollow profile of carbon impurities
(“impurity hole”) is observed associated with the
increase in ion temperature gradient (Fig. 2).
The positive radial electric field drives spontaneous flow
in the counter-direction at the plasma edge and in the codirection
near the magnetic axis. The component of the
spontaneous toroidal flow driven by the ion temperature
gradient is clearly observed and expected to be one of the
dominant components of toroidal flows in high-ion-temperature
discharges in LHD. Transport analysis of the carbon
impurity in discharges with an impurity hole reveals a
low diffusion coefficient and an outward convection
velocity, whereas inward convection is predicted by neoclassical
theory at half the minor radius.
An interesting high-density operational regime with an
internal diffusion barrier (IDB) has been observed. Typical
profiles are shown in Fig. 3. The IDB is characterized by a
steep density gradient in the core plasma, and the attainable
central density approaches 1.2 × 1021 m−3 while a relatively
low-density mantle plasma continues to surround
the core. The maximum central pressure reaches 150 kPa
for an optimized magnetic configuration. Such a high central
pressure causes a very large Shafranov shift, more
than half the radius. Core fueling is absolutely essential for
the IDB formation, and the IDB is reproducibly obtained
using intensive multiple-pellet injections. The attainable
density is restricted by limited heat deposition in the core
plasma due to the strong attenuation of the neutral beam in
the high-density plasma.
Fig. 2. An “impurity hole” carbon density profile [2].
Fig. 3. Electron density and temperature profiles in an IDB
plasma. [1]
Stellarator News -3- December 2010
Easy access to the high-density regime without fatal disruptive
phenomena is one of the attractive characteristics
of helical devices (Fig. 4). The operational density is considerably
higher than the Greenwald density limit that
applies to tokamak plasmas. In LHD, the density limit is
reached when the edge density at the last closed flux surface
exceeds a value approximately equivalent to the Sudo
density limit, which increases with the square root of the
heating power. IDB plasmas with a peaked density profile
can be maintained as long as the edge density does not
exceed the Sudo limit.
High-beta (>5%) plasmas have been successfully obtained
without major disruption by suppressing the Shafranov
shift that would reduce the heating efficiency of NBI and
increase the amount of helical ripple.
The dominant MHD instabilities have been observed in
the peripheral region with a magnetic hill. The amplitudes
of the modes clearly depend on the magnetic Reynolds
number, and the dependence is almost the same as that of
the linear growth rate of the resistive interchange mode.
Near the ideal stability boundary, a strong MHD mode
appears and deforms the pressure profile near the resonant
surface.
Spontaneous change in MHD equilibrium is a key issue
for high-beta plasma production. Theory predicts that the
magnetic field structure in the periphery becomes disordered
due to finite-beta effects, whereas experiments show
that significant pressure is maintained in the edge region in
the high-beta regime. The relationship between the predicted
magnetic field structure and collisionality, mean
free path of particles, and other factors has been investigated.
Also, an understanding of the spontaneous dynamics
of magnetic islands, self-healing, and growth is
important for maintaining high-beta equilibria. Experiments
using a resonant magnetic perturbation indicate that
self-healing occurs in the higher-beta and lower-collisionality
plasmas, while a magnetic island grows in the lowerbeta
and higher-collisionality region. Magnetic configuration
effects on island dynamics also have been investigated.
Energetic ion-driven MHD instabilities such as Alfvén
eigenmodes and energetic particle modes (EPMs) and
their impacts on energetic ion confinement are being studied.
Two types of toroidicity-induced Alfvén eigenmodes
(TAEs) are typically observed in LHD plasmas that are
heated by tangential NBI. One is localized in the plasma
core region near a central TAE gap, and the other is a
global TAE having a radially extended eigenfunction.
Core-localized TAEs with even and odd radial mode parities
are often observed. The global TAE is usually
observed in medium- to high-beta plasmas with broad
regions of low magnetic shear. Helicity-induced Alfvén
eigenmodes (HAEs), which exist in gaps, are unique to
three-dimensional plasmas that have both toroidal and
poloidal mode couplings and were detected for the first
time. Recently, reversed magnetic shear Alfvén eigenmodes
(RSAEs) having characteristic frequency sweeping
were discovered in reversed magnetic shear plasmas produced
by intense counter-neutral beam current drive. In
the reversed-shear plasma, the geodesic acoustic mode
(GAM) excited by energetic ions was also detected for the
first time in a helical plasma.
Many other important results are included in this special
issue of Fusion Science and Technology. Please visit the
American Nuclear Society site
http://epubs.ans.org/?p=fst
to obtain the whole issue.
References
[1] H. Yamada et al., Fusion Sci. Technol. 58, 1 (2010) 12–
28.
[2] M. Yoshinuma et al., Fusion Sci. Technol. 58, 1 (2010)
103–112.
Satoru Sakakibara
National Institute for Fusion Science
Toki, Japan
E-mail: sakakis@lhd.nifs.ac.jp
Fig. 4. Comparison of experimental electron density with
the Greenwald density limit (dashed line) [1].
Stellarator News -4- December 2010
Engineering for installation of
W7X superconducting coil
leads
For the past year, a U.S. team led by J. H. Harris (ORNL)
and G. H. Neilson (PPPL) has been working with the Wendelstein
7-X (W7X) stellarator assembly team headed by
L. Wegener [Max Planck Institut für Plasmaphysik (IPP),
Greifswald] to design tooling and techniques for the precision
assembly of the high-current (17 kA) leads connecting
the W7X stellarator coil set to the cables from the
W7X power supply. This activity is part of a larger joint
PPPL-ORNL-LANL project to prepare for a long-term
U.S. partnership on W7X that has received initial funding
of $7.5M for FY 2011–13 from the U.S. Department of
Energy Office of Fusion Energy Sciences.
The first task of this project was to develop a scheme to
lift and support the heavy (~500 kg) lead assemblies and
align and hold them securely in position so that the leads
could be welded to the coil current feeds in five positions
around the torus. The temporary supports must then be
lowered and the loads shifted precisely to the permanent
support structure without disturbing the joint itself. The
working space in and near the lead fixing boxes is barely
large enough for a technician’s arms, with millimeter-scale
tolerances for placement of the leads. The design work
therefore has had to be carried out using 3D computer
techniques supported by real-world checks at ORNL and
IPP using mock-ups of the relevant parts of the W7X
device.
The overall approach was developed by M. J. Cole
(ORNL), P. J. Fogarty (ORNL), and T. J. Brown (PPPL). It
features a custom-built, movable lead installation cart with
precision hydraulic positioners for the current feeds (Fig.
1), which can be used in a variety of configurations to
translate, raise, position, support, and remove the lead
assemblies, as illustrated in Fig. 2.
Detailed design of the cart, its fixtures, and the procedures
to be followed required the preparation of 122 engineering
drawings at ORNL by M. J. Cole, K. Logan, and G.
McGinnis and supporting engineering analysis by A.
Lumsdaine of ORNL, working with the IPP engineering
team.
Nearly all of the work was carried out with the teams
working at their home institutions and communicating in
weekly three-way teleconferences (IPP-PPPL-ORNL) and
via e-mail and computer file exchange. P. J. Fogarty visited
IPP-Greifswald for a week in 2009 to start work on
the concept, and M. J. Cole and A. Lumsdaine visited IPP
for two weeks in mid-2010 to verify key aspects of the
design in discussions and mock-up trials with the IPP
team.
Final versions of the drawings were completed at ORNL
and posted for download by IPP on 10 September 2010.
IPP conducted final checks before sending out a tender for
fabrication of the cart and associated fixtures by industry
during the week of 20 September; delivery of these items
Fig. 1. The lead assembly cart.
Stellarator News -5- December 2010
is expected in early 2011. In this time frame, IPP will also
verify the lead assembly process using its on-site mockup.
IPP is now preparing for coil lead installation on the
W7X machine, which will begin in the Summer of 2011.
Jeffrey H. Harris
Oak Ridge National Laboratory
P. O. Box 2008, Oak Ridge, TN 37831-6169
E-mail: harrisjh@ornl.gov
Fig. 2. The assembly cart in action.
Stellarator News -6- December 2010
Public Day at IPP:
Stellarators are attractive
More than 2600 visitors took the opportunity to see Wendelstein
7-X at its construction site during the Public Day
at IPP Greifswald. It was a unique chance to see the interior
of the five modules of the superconducting stellarator
in different states of assembly, like in a textbook—top
view and poloidal cut—before they are finally hidden in
the vacuum vessel. Guided tours were given in groups of
15 to 20 persons to facilitate explanations despite the
reverberant torus hall acoustics. At the entrance of the
experiment building, queuing was unavoidable and
patiently accepted. For those waiting and those who
wanted to learn more, presentations were given in the also
well-filled lecture hall—on W7-X physics, the design and
engineering activities, the cryo-technics applied, and particular
assembly issues. A real highlight was the 3D movie
contributed by the IPP theory department showing the
device interior, its complexity, but also its symmetry properties,
for example, from the viewpoint of a drifting particle.
Finally, the main corridor hall of IPP Greifswald was
used both for an arts exhibition—the so called magistrale,
and for children’s activities and some do-it-yourself experiments,
providing IPP members with the opportunity to
attract a new generation of fusion scientists.
M. Hirsch
Max Planck Institut für Plasmaphysik (IPP)
Greifswald] Germany
E-mail: Matthias.Hirsch@ipp.mpg.de
Fig. 1. A view of the visitor’s platform in the torus hall. (Photo: IPP, Anja Richter-Ullmann)