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Published by Oak Ridge National Laboratory
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Editor: James A. Rome Issue 151 December 2015
E-Mail: [email protected] Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
Recent Milestones for
Wendelstein 7-X: Commissioning
and Operations
After the confirmation of the magnetic flux surfaces in
Wendelstein 7-X (see Stellarator News No. 149, August
2015), the commissioning continued with the preparation
for plasma operations. In August, the plasma vessel and
the ports were baked to 150°C with hot water in the cooling
circuits. The outsides of ports and flanges were heated
electrically to the same temperature. During the 2.5 days
of heating, the plasma vessel expansion amounted to about
15 mm. Also, the mechanical stresses were measured and
compared to the results of finite element calculations. The
mechanical measurements matched the predicted values
very closely and once again confirmed the proper manufacturing
and assembly of the device.
After 4 days at 150°C, the plasma vessel was cooled down
again over 2 days. The water content in the plasma vessel
exhaust dropped considerably and the total pressure
decreased by more than one order of magnitude.
Following cooldown, measurements of the magnetic flux
surfaces continued. Primarily, we attempted to find an
equilibrium sensitive to error fields that could be corrected
using the trim coils. These experiments are ongoing. Most
of the time was used to bring diagnostics into operation
and to develop and test the central safety control system
(cSS). This was a very tedious process, accompanied by
the auditors of the Technical Supervisory Association
(TÜV) who were appointed by government authorities to
evaluate the set-up of operations, including radiation protection
measures, safety systems, and the planned operation
processes. In November, the TÜV accepted all
measures (after some additions), and the authorities of
Mecklenburg-Vorpommern have announced that the operation
permit will be granted in the second week of December.
At this writing, final preparations are under way: the commissioning
of the valve control, glow discharge cleaning.
and central operations management (cOPM), which controls
the plasma discharges, the gas inlet, the heating
power, and the triggers for diagnostics.
Update: W7-X completed its first discharge (Fig. 1) on
December 10:
https://www.euro-fusion.org/2015/12/wendelstein-7-xbegins-
fusion-journey/
Fig. 1. The first plasma produced by the Wendelstein 7-X
stellarator. (Photo: IPP)
In this issue . . .
Recent Milestones for Wendelstein 7-X: Commissioning
and Operations
Wendelstein 7-X passed rigorous safety tests and has
been approved for operation in the middle of December.
.......................................................................... 1
20th International Stellarator-Heliotron Workshop
The workshop, hosted by the Max Planck Institute for
Plasma Physics, was held 5–9 October 2015, at Greifswald,
Germany. There were 163 presentations. 2
Stellarator News -2- December 2015
Hans-Stephan Bosch
for the W7-X Group
20th International Stellarator-
Heliotron Workshop
The 20th International Stellarator-Heliotron Workshop
(ISHW) was held 5–9 October 2015, at Greifswald, Germany.
The Max Planck Institute for Plasma Physics hosted
this workshop, which was planned to take place against
the background of initial experiments in Wendelstein 7-X
(W7-X). The workshop was held at the Alfried Krupp
Wissenschaftskolleg, which is located in the medieval city
center of Greifswald, next to the 14th-century cathedral.
Head of the International Programme Committee (IPC):
Prof. Katsumi Ida, National Institute for Fusion Science,
Toki, Japan. Head of the Local Organizing Committee
(LOC): Prof. Per Helander, IPP, Greifswald, Germany.
This workshop consists of 1 overview session and 6 topical
sessions. The topics of this workshop are listed below
and the distribution of presentations is shown graphically
in Fig. 1.
1. Overview (4 invited)
2. Impacts of magnetic topology/three-dimensional (3D)
effects (6 invited, 8 orals, 56 posters).
3. Edge-core coupling of turbulence and transport (6
invited, 8 orals, 56 posters)).
4. Interactions among energetic particles, MHD, and
transport (6 invited, 8 orals, 56 posters).
5. Impurities, neutral sources and sinks, and transport (3
invited, 4 orals, 16 posters).
6. Reactor perspectives (3 invited, 4 orals, 16 posters).
7. Coupling of core optimization to the plasma boundary
and plasma-materials interaction (PMI) (3 invited, 4
orals, 16 posters).
Overview session
The following four overview talks were given in the Overview
session.
1) Recent Progress of Japanese heliotrons, LHD and
Heliotron J by T. Morisaki (National Institute for Fusion
Science),
2) Overview of TJ-II, TJ-K, and H-1NF experiments by C.
Hidalgo (Laboratorio Nacional de Fusion, CIEMAT),
3) Recent results and program on the HSX device by S.
Anderson (Wisconsin Univ.), and
4) 15 years of construction of Wendelstein 7-X by T.
Klinger (Max-Planck Institute for Plasma Physics).
Recent research progress on each concept was reviewed
by the overview speaker, and key issues for each concept
were discussed. This session delineated current understanding
and remaining issues for scientists working on
other concepts.
Topical sessions
Impacts of magnetic topology/3D effects
Edge-localized mode (ELM) mitigation by 3D fields is an
important issue in both tokamak and helical plasmas. An
ELM suppression experiment using a resonant magnetic
perturbation (RMP) field in a tokamak was reported by W.
A recent picture of Wendelstein 7-X taken by Glen Wurden
during the ISHW 2015 tour.
Fig. 1. Distribution of talk topics.
Impact of magnetic
topology and 3D
effects
Interactions among
energetic particles,
MHD and transport
Impurity neutral
sources sinks
and transport
Edge-core coupling
of turbulence and
transport
Reactor
perspectives
Coupling of core optimization
to the plasma boundary and PMI
Stellarator News -3- December 2015
Suttrop. Small magnetic perturbations (0.1% of toroidal
field) to the axisymmetric tokamak are used to form a
ergodic divertor, compensate error fields, and most importantly,
control ELMs and other MHD instabilities. Differences
in shielding effects between tokamak and helical
plasmas were discussed by K. Y. Watanabe. The collisionality
dependence on penetration threshold current is a negative
dependence in the Large Helical Device (LHD). The
penetration threshold current of RMP decreases as the
density and collisionality increase in LHD. This density
dependence is opposite to that observed in tokamak.
Another approach to suppress ELMs was presented by Y.
Liang. The lower hybrid current drive (LHCD) experiment
in EAST has demonstrated that LHCD can be a powerful
tool to suppress giant ELMs. The field-aligned helical current
filament of the LHCD antenna produces an optimized
perturbation spectrum. M. Rack presented calculation
results showing that the magnetic perturbation due to
LHCD near the plasma edge has an ideal mode number for
the ELM suppression.
Many presentations addressed the effects of RMP on particle
transport in both experiment and simulation. O.
Schmitz presented the experimental results of helium control
by RMP in TEXTOR and LHD. RMP fields are a finetuning
actuator for improvement of divertor functionality.
M. W. Jakubowski also presented that the effect of pumpout
by RMP is also observed in L-mode and H-mode plasmas
in tokamaks. A comparison of helium profiles near
the plasma edge obtained from the charge-exchange spectroscopy
measurements and an EMC3-EINERE simulation
code was presented by K. Ida, A. Bader, and M.
Kobayashi. The experimentally observed effect of RMP is
reproduced by simulation; however, the slope is opposite.
Close correlation between magnetic topology and turbulence,
transport, and plasma potential is observed in the
RFX-mod reverse field pinch, H-1NF heliac, and Helically
Symmetric Experiment (HSX). M. Spolaore presented an
experimental observation in RFX-mod. This observation
highlights a tight and dynamical correlation between
small-scale electromagnetic filamentary structures and the
actively modulated magnetic topology. C. Michael presented
experimental results from H-1NF. The change in
density/pressure is most closely localized to a zero shear
region, and a standing wave structure which is frozen to
the magnetic configuration is observed. A. R. Akerson discussed
the 3D scrape-off layer (SOL) transport in HSX.
Density and space potential are measured with probes and
the observations indicate the presence of both SOL and
diffusive transport channels at the edge of HSX.
Edge-core coupling of turbulence and transport
An internal electron transport barrier is commonly
observed in stellarator, heliotron, and heliac plasmas. Various
interesting experimental results were presented for
LHD, Heliotron-J, and HSX. S. Anderson presented the
internal electron transport barrier observed in HSX. A
peaked electron temperature profile is observed when the
deposition of the electron cyclotron resonant heating
(ECRH) is focused at the magnetic axis. This observation
is explained by a lack of stiffness of the electron transport.
H. Takahashi presented the internal electron transport barrier
in LHD. N. Pablant showed clear evidence of the
radial electric field using an imaging X-ray spectrometer.
The reduction of turbulence transport associated with the
formation of a radial electric field on the electron root was
discussed. Similar experimental results in Heliotron-J
were presented by T. Minami.
Key aspects of the internal electron transport barrier were
discussed. The important issues raised are (1) the relationship
between the radial electric field, turbulence suppression,
and temperature gradient, (2) the contribution of
neoclassical anomalous transport, and (3) the role of rational
surfaces. T. Estrada showed the relationship between
turbulence and radial electric field shear in the limit cycle
oscillation (LCO) observed in TJ-II. Turbulence leads the
process and produces an increase in the EB flow shear.
However, the radial propagation of the EB shear location
and the foot-point of the electron internal transport
barrier does not match in LHD experiments, which is
inconsistent with turbulence suppression by EB shear.
Edge-core coupling of turbulence and transport was discussed
by H. Takahashi. Simultaneous increase and
decrease of temperature gradients are observed during
ramp up/down of ECRH power. With an increase in
ECRH power, the spatiotemporal coherence is decreased
due to the change of the correlation length and the turbulence
lifetime. F. Fernández-Marina presented radial correlation
length studies performed in the TJ-II stellarator
using both Doppler reflectometry experiments and simulations.
Isotopic effects on turbulence are important in toroidal
(tokamak and stellarator) plasmas. C. Hidalgo presented
an isotope effect on experimental turbulence in TJ-II,
while S. Ohshima presented experimental results in Heliotron-
J. The long-range correlation increases as the hydrogen
ratio increases in TJ-II, however it decreases as the
ratio increases in Heliotron-J. The discrepancy of the
experiments in TJ-II and Heliotron-J is due to the complex
excitation and damping mechanism of zonal flow. The isotopic
effect on the long-range coherence of turbulence
may depend on the radial electric field and damping rate
which are sensitive to magnetic field configuration.
Stellarator News -4- December 2015
Isotopic effects on turbulence have also been studied in
simulations. M. Nakata showed the impact of isotope species
and collisionality on the ion temperature gradient
(ITG) mode and the trapped electron mode (TEM) instabilities
in helical plasmas. The zonal flow energy in deuterium
plasmas is larger than that in hydrogen plasmas.
Similar results are also obtained by the transport model
presented by S. Murakami.
Interactions among energetic particles, MHD, and
transport
Interactions among energetic particles, MHD, and transport
were also discussed. T. Estrada discussed the experimental
study of Alfvén eigenmodes (AE) in focusing on
nonlinear coupling in TJ-II. An MHD mode with frequency
20 kHz due to nonlinear coupling with the highfrequency
AE modes was reported. These observations
suggest the existence of an interplay between the oscillating
EB flow and the MHD mode as a precursor for the
LCO onset. F. Castejon presented the well-depth dependence
of turbulence and the frequency of Alfvén modes.
Increasing the magnetic well depth decreases the turbulence
level, and the frequency of the Alfvén mode
increases. It is open to question whether this correlation is
coincident or the result of causal relation.
The simulation of AE instabilities driven by fast ions was
discussed. Improved understanding of these modes is
motivated by observations that they can lead to enhanced
fast ion transport and lowered heating efficiencies. D. A.
Spong described recent development and testing of a
global gyrokinetic PIC (particle-in-cell) simulation model,
based on the GTC code, for analyzing Alfvén instabilities
in 3D configurations. Results were presented for both linear
and nonlinear stability analyses of these instabilities in
the LHD and W7-X stellarators. The AE amplitudes
showed couplings in toroidal mode number over field
period intervals, which is an expected characteristic of
macroscopic modes in stellarators. G. W. Bowden reported
simulation results for AE instabilities in H1-NF and C. R.
Cook and C. C. Hegna discussed the island-induced AE
predicted with the SIESTA code. The eigenfunction of AE
instability was found to be localized close to island core.
Impurities, neutral sources and sinks, and transport
This session centered on the poloidal asymmetry in the
impurity density.
A. Alonso reported the observation of poloidal asymmetry
of impurity density measured with a bolometer in TJ-II
and compared it to simulation results. M. Landreman presented
calculations of poloidal and toroidal variations of
potential as candidate mechanisms for this poloidal asymmetry.
Impurity transport studies using tracer encapsulated solid
pellets (TESPEL) was reported by N. Tamura. In LHD
experiments, ECH suppresses the emission intensities
from a highly ionized V tracer impurity, providing clear
evidence of impurity exhaust due to ECH.
Neutral density also has a strong impact on transport. M.
Goto reported the measurement of neutral density in the
core of LHD. The reduction of neutral density makes it
easier to achieve high ion temperature plasmas. U. Wenzel
discussed neutrals in the divertor region of W7-X. The
neutral density is expected to have a maximum value at
the transition from attached to detached plasma.
Verification, validation, and prediction of simulation
codes for transport and ray tracing were discussed. S.
Satake and S. Matsuoka reported the comparison of the
radial heat flux in LHD experiments to that predicted by
neoclassical transport codes. T. Tsujimura presented the
the ray tracing calculation for ECH (LHD-Gauss) in the
LHD experiment. ECH injection in LHD is optimized
using these ray tracing results. W. Kernbichler reported the
prediction of electron cyclotron current drive (ECCD) efficiency
in W-7X using the NEO-2 code. J. H. E. Proll presented
calculations of heat flux driven by trapped electron
modes (TEMs) for W7-X, HSX, and DIII-D. The radial
heat flux increases with the increase of the electron density
gradient as normalized by (n/a), where a is the minor
radius.
Reactor perspectives
Research toward a helical reactor was reported in this session.
R. C. Wolf reported on the prospects of a HELIAS fusion
power plant, and F. Warmer reported on the European
roadmap for stellarator research. For the heliotron concept,
T. Goto used an integrated 1D code to examine plasma
operation and control scenarios in the LHD-like FFHR-d1
helical reactor. A. H. Boozer reported on the concept of
nonresonant divertors in which the sharp edges act as Xpoints
and divert field lines to the walls; its advantage is
that it is insensitive to plasma currents.
Coupling of core optimization to the plasma boundary
and PMI
Two important issues in the coupling of core optimization
to the plasma boundary and PMI ares wall retention and
the effects of magnetic islands. H. Kasahara reported the
change in recycling after a long pulse of 2000 s in LHD.
This was because the wall retention was affected by a
mixed-material layer during the long discharge.
Numerical study of W7-X magnetic island divertor performance
was discussed. Y. Suzuki presented the effects of
Stellarator News -5- December 2015
beta and current density, which significantly change the
edge magnetic topology in the W7-X magnetic island
divertor. Y. Feng studied the effect of magnetic islands on
the carbon concentration using the EMC3-EIRENE code,
which predicts the minimum carbon concentration at a
medium island size.
Summary
A summary was given by K. Ida (NIFS) at the end of the
workshop. He highlighted the following achievements of
the last two years:
Completion of W7-X construction.
Simultaneous achievement of electron internal transport
barrier (e-ITB) and ion-ITB in LHD.
High beta (~4%) with low collisionality.
d) Steady-state operation up to 48 min with 1.2 MW of
heating in LHD.
Then he summarized the physics issue on each topic:
1. Impact of magnetic topology and 3D effects
• ELM suppression
LHCD is found to be as good as (or better than?)
RMP as a technique to suppress ELMs.
The role of 3D fields on ELM suppression should
be investigated by various approaches.
Interplay between turbulence and magnetic topology.
Turbulence localization due to magnetic topology was
observed. (An interesting research area?)
2. Edge-core coupling of turbulence and transport
• Mechanism of e-ITB
The mechanism of the formation of e-ITB is open
to question.
The reduction of transport by Er due to the transition
to electron root is insufficient to explain this.
• Isotope effect
The isotope effect on long-range correlation has
been studied in experiment, and may vary depending
on the magnetic topology (the relation of
damping and excitation to Er). Experiments on
isotope effects in LHD and W-7X are quite important.
3. Interactions among energetic particles, MHD, and
transport.
• Nonlinear coupling of AE modes and dependence
on well depth are observed. Development of a
simulation code for AE modes (AE in magnetic
island) is needed.
4. Impurities, neutral sources and sinks, and transport.
• Impurity transport
ECH contributes to the exhaust of impurities
(Mechanisms? Er and potential asymmetry).
Coexistence of asymmetry of potential and impu-
20th ISHW attendees.
Stellarator News -6- December 2015
rity density is observed.
Neoclassical parallel transport with friction, inertia,
parallel electric field are important.
• Helium transport
Magnetic islands can reduce the helium density in
the core.
Some inconsistencies between measurements and
simulation (EMC3-EIRENE) of the screening
effect in the stochastic region are observed when
the helium fraction is high.
International Program Committee:
• Katsumi Ida. chair (NIFS, Japan)
• Boyd Blackwell (ANU, Australia)
• Robert Wolf (IPP, Germany)
• Kieran McCarthy (CIEMAT, Spain)
• Vladimir Voitsenya (Kharkov, Ukraine)
• Oliver Schmitz (University of Wisconsin, USA)
Local Organizing Committee (IPP Greifswald, Germany):
• Per Helander, chair
• Jutta Gauger, conference secretary
• Beate Kemnitz, public relations
• Matthias Borchardt, scientific programme, technical
support
• Carolin Nührenberg, scientific programme
• Karin Schier, finances
• Andrea Kleiber, website, registration
Katsumi Ida
National Institute for Fusion Science
Tel +81-572-58-2182
Fax+81-572-58-2624
E-mail [email protected]