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Published by Oak Ridge National Laboratory
Building 5600 P.O. Box 2008 Oak Ridge, TN 37831-6169, USA
Editor: James A. Rome Issue 153 June 2016
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
Summary of WEGA Operation
at IPP Greifswald
After more than 12 years at the Max Planck Institute for
Plasma Physics (IPP), Greifswald, Germany, the operation
of the WEGA stellarator was ended in November 2013 in
order to free up resources in the institute for the upcoming
flagship experiment, the stellarator Wendelstein 7-X (W7-
X) that has just finished its first operation phase, OP 1.1.
WEGA, originally the Wendelstein Experiment in Grenoble
for the Application of RF heating, was built for the
study of RF heating methods as a joint undertaking
between the CEA in Grenoble (France), ERM in Brussels
(Belgium), and IPP Garching (Germany) [1].
The machine, which was initially operated from 1974 to
1981 in Grenoble, is an early member of IPP’s Wendelstein
stellarator family. After a stay at the IPF Stuttgart
from 1982 to 1999 the experiment was relocated in 2000
to IPP Greifswald, with the aim of bridging the time gap
until the beginning of W7-X operation. After about one
year of reassembly and preparation of the infrastructure
plasma operation started in July 2001.
At IPP Greifswald the acronym WEGA was reinterpreted
as Wendelstein Experiment in Greifswald für die Ausbildung
(for education) because the major goal was the training
and education of young scientists. The machine was
also used for the application of new diagnostics for testing
of the control and data acquisition system developed for
W7-X, and for basic research in plasma physics.
WEGA is based on a unique concept: the machine, with a
major radius R = 0.72 m, can either be operated as a tokamak
or, by equipping the plasma vessel with additional
helical coils, as a classical stellarator. The machine was
used nearly exclusively as a tokamak in Grenoble but as a
stellarator only in Greifswald.
The magnetic configuration space of the stellarator configuration
is very flexible. The magnetic field system consists
of a set of 40 toroidal field coils required for both the tokamak
and the stellarator version. The poloidal field for the
stellarator configuration is generated by 4 coils consisting
of 14 filaments each, winding helically around the torus
with l = 2 and five-fold toroidal symmetry. The rotational
transform  (for WEGA as a stellarator) is determined by
In this issue . . .
Summary of WEGA Operation at IPP Greifswald
WEGA operated as a stellarator in Greifswald to train
new plasma scientists and to develop diagnostics
and operating scenarios for Wendelstein 7-X (W7-X).
................................................................................. 1
W7-X program and plasma evaluation workshop
A workshop was held in Greifswald at the end of May
to discuss initial W7-X results and plans for the next
operational phase. .................................................. 4
Report on the 15th Coordinated Working Group
Meeting
Activities at the 15th CWGM, held in Greifswald from
21–23 March, 2016, are discussed. ....................... 4
Fig.1. WEGA stellarator at IPP Greifswald.
Stellarator News -2- June 2016
the current ratio in the helical and toroidal field coils,  
(Ih/Itf)2, and can be varied over 0 < 1. Depending on
the rotational transform, a maximum average plasma
radius of about a = 11 cm is achievable, corresponding to
an aspect ratio R/a ~ 7 and a plasma volume V = 0.16 m3.
For radial positioning control of the plasma, two pairs of
vertical field coils in a Helmholtz configuration were
available. In addition, a 5-arm iron transformer with a flux
change of 0.4 Vs could be used for current drive experiments.
At low magnetic fields of about B0 = 70 mT,
steady-state plasma operation for about one hour with the
water-cooled magnetic field coil system was demonstrated,
and quasi-stationary operation at B0 = 0.5 T with a
pulse length of about 20 s and a subsequent cooling phase
of about 5 min could be achieved.
A set of standard diagnostics such as a magnetic flux surface
diagnostic, an 80-GHz single channel interferometer,
different Langmuir probes and probe arrays, bolometers,
video cameras, spectrometers in the visible and UV range,
mass spectrometer, calorimeter, high-frequency (HF)
probes, soft X-ray detector, electron cyclotron emission
(ECE), Rogowski coils, and a diamagnetic loop was routinely
used for the determination of the basic plasma
parameters such as plasma density, electron temperature,
stray microwave radiation levels, and for surveillance of
the plasma vessel [2, 3, 4, 5, 6].
Furthermore, in cooperation with the W7-X team and
external colleagues, additional diagnostics such as a
coherent imaging system, a heavy ion beam probe, a
supersonic helium beam, X-ray silicon drift detectors, and
neutral pressure gauges have also been implemented and
tested.
Plasma Heating Scenarios
Plasma heating was realized by two high-frequency heating
systems operating at 2.45 GHz (26 kW, cw) and
28 GHz (10 kW, cw). In addition ,the transformer could be
deployed simultaneously for reaching otherwise nonaccessible
plasma regimes due to synergetic effects. Different
advanced heating scenarios were experimentally
established and theoretically analyzed.
The first harmonic O-mode heating (O1) with 2.45 GHz
waves at a resonant magnetic field of 87 mT limits the
electron density to a cutoff value of ncut,2.45 < 7.45 
1016 m3. However, electron densities of
ne > 20·ncut,2.45 could be achieved for ω/ωce < 0.7 with a
heating power of several kW. The coupling mechanism
was explained by finite Budden tunneling through the evanescent
layer, which is amplified by multiple reflections.
The dependence of the absorbed power on the magnetic
flux density was treated with a resonator model, being in
good agreement with experiments emphasizing the excitation
of whistler waves as the most probable candidate.
An entirely new heating concept was investigated during
the simultaneous use of both HF heating systems. At a
magnetic field of 0.5 T, the 28 GHz extraordinary polarized
(X2) waves first generated a target plasma with densities
up to the cutoff value of ncut,28 < 0.4 1019 m3 and a
bulk temperature of the order 10 eV. Seed electrons with
an energy above 50 keV started to interact effectively with
the near field of the 2.45 GHz heating antenna similar to
the process of Landau damping [7]. For this reason, the
heating scenario was called stochastic Landau acceleration
(SLA). Electrons up to energies of MeV verifiable by their
X-ray, gamma ray and synchrotron emission could be
detected [7]. Furthermore, the confinement of electrons
with energies higher than 200 keV depends strongly on
their momentum in relation to the magnetic field vector
leading to a toroidal net current of up to a few hundreds of
Amperes, verified by extensive particle tracing simulations.
Both microwave systems, 2.45 GHz and 28 GHz, were
also used in a single operation to excite electron Bernstein
waves via a two-step conversion process (OXB) within the
plasma edge from an ordinary wave (O-wave) into an
extraordinary wave (X-wave) and finally into a Bernstein
wave. Such electrostatic Bernstein waves (EBW) have no
upper density limit and are able to carry the heating power
into the highly overdense plasma center. The OX conversion
process, which takes place around the O-mode cutoff
layer within the density gradient area of the plasma, was
measured for the 2.45 GHz waves with the aid of HF
probes [8]. Furthermore, the antennas of both heating systems
were optimized to achieve a maximum OX conversion
efficiency, which was measured for the 28 GHz case
as being in good agreement with 2D finite-difference timedomain
full wave calculations. On the other hand, the
small wavelength of the electron Bernstein waves is of the
order of the electron Larmor radius, allowing the modeling
of their propagation by means of a 3D ray tracing code [9].
For both frequencies the code predicts a dependence on
the magnetic field strength and the formation of a high
parallel wave number N>> 1 allowing current drive
mainly carried by a suprathermal electron component. The
resultant Doppler downshifted absorption and the accompanying
current drive efficiency could be proven in the
experiment by a decrease of the magnetic flux density,
whose absolute values were in very good agreement with
the code predictions [10].
A further aspect also related to the operation of W7-X was
the investigation of the X2 startup utilizing the 28 GHz
microwave source. The experiments were continued at
Heliotron J and LHD and led to a better understanding of
the start-up behavior’s dependence on the gas species,
neutral gas pressure, rotational transform, and input power
[11].
Stellarator News -3- June 2016
Plasma-Wall Interactions
One issue in OP1.1 of W7-X was the conditioning of first
wall components saturated by hydrogen after e.g., plasma
collapse. In contrast to predecessor stellarator experiments,
the superconducting coils will be deenergized only
overnight, so that the confining magnetic field does not
allow the use of glow discharges for intermediate wall
conditioning. For this reason, 28 GHz electron cyclotron
resonant heating (ECRH) was tested in a conditioning
campaign and directly compared with a 9 MHz, 3 kW ion
cyclotron resonance heating (ICRH) wall conditioning
system at WEGA [12]. Optimized sequences of short
ECRH discharges were developed and additionally combined
with off-axis heating and magnetic field sweeps. In
comparison to ICRH, a cleaning efficiency of up to 50%
could be obtained, leading to the decision to go without an
exclusive ICRH cleaning system in OP1.1. The basic
cleaning of the W7-X vacuum vessel by cleaning agents
was followed by a 3-day baking of the plasma vessel and
the ports at 150°C. The later application of glow discharges
was limited because of unprotected copper surfaces
in this first commissioning phase. However, ECRH
was successfully used for the basic conditioning in the first
3 days of plasma operation as well as for reconditioning of
saturated walls after extensive hydrogen operation.
Prototype Implementation of the W7-X Control
and Data Acquisition System
The W7-X prototype CoDaC implementation at WEGA
included an integrated test of control, data acquisition, and
processing, as well as the implementation of diagnostics in
an environment similar to the later W7-X operation [13].
Key aspects were the control of the experiment operation
via segment programs, real-time control, and a continuously
operating data acquisition system with many channels
at high speed. Further topics were the validation of
the W7-X safety concepts and the experimental test of the
application software for planning, control, and observation
of the experimental program.
The project was divided into two sequences. During the
first, the existing control system was replaced by the
implementation of the W7-X control and data acquisition
concepts for the central control and also for the technical
components and first diagnostics. In the second phase, the
safety system and the segment control system were added
to support a technically and physically oriented experimental
program. Since 2011 the new CoDaC system has
been nearly exclusively used for plasma operations,
including sophisticated experiments such as event detection
in plasma parameters and the subsequent change in
microwave heating power. In addition, further W7-X prototype
diagnostics such as a neutral pressure gauge, a sensitive
integrator for the magnetic diagnostics, and a
multichannel Langmuir probe array have been implemented
and tested.
Summary
After more than 12 years and 47,613 discharges the operation
of the WEGA stellarator at the IPP Greifswald was
ended in November 2013.WEGA has been intensively
used for the education of young scientists. In total. 6 PhD
theses and more than 15 diploma, bachelor, and master
theses were finished. A few tens of students gained a first
insight into the field of magnetically confined plasma
physics during internships. But WEGA was also intensively
used in preparation for W7-X operation in the fields
of the development and testing of new diagnostics, the
application of new plasma heating scenarios, and plasma
wall cleaning discharges utilizing the HF heating system
and as a test bed for CoDaC’s control and data acquisition
system.
Outlook
Despite its age of 40 years, the shutdown at IPP Greifswald
was not the end of the machine. WEGA has found a
new home! In 2014 the experiment was disassembled, and
the whole machine including the power supplies, a few
diagnostics, and further equipment were transferred to the
University of Illinois at Urbana-Champaign (USA). Here,
at the Center for Plasma-Material Interaction (CPMI),
WEGA is being rebuilt now as the Hybrid Illinois Device
for Research and Application (HIDRA) and will be used
for studying the interaction of hot plasma with plasma-facing
components (PFCs).
CPMI is one of the leading laboratories in studying
plasma-material interactions and the effects of liquid lithium
on fusion plasmas, as well as developing the technology
required to use liquid metals in a fusion device.
HIDRA now provides CPMI with a quantum leap in its
capabilities. With its own toroidal device to study liquid
metals and plasma-material interactions, CPMI can now
more efficiently study different PFC concepts and technologies
before their deployment on larger machines around
the world.
References
[1] R. Fritsch et al., 9th Symposium on Fusion Technology,
Garmisch-Partenkirchen (1976).
[2] M. Otte et al., AIP Conf. Proc. 993, 3 (2008).
[3] M. Otte et al., Contrib. Plasma Phys. 50, 780 (2010).
[4] M. Otte et al., Nukleonika 57,171 (2012).
[5] D. Zhang et al., Nucl. Fusion 52, 043002 (2012).
[6] P. Drewelow et al., Rev. Sci. Instrum. 80, 123501
(2009).
[7] H. Laqua et al., Plasma Phys. Control. Fusion 56,
075022 (2014).
[8] Y. Podoba et al., Phys. Rev. Lett 98, 255003 (2007)
Stellarator News -4- June 2016
[9] J. Preinhaelter et al., Plasma Phys. Control. Fusion 51,
125008 (2009).
[10] Y. Podoba et al., AIP Conf. Proc. 993, 235 (2008).
[11] M. Preynas et al., AIP Conf. Proc. 1580, 498 (2014).
[12] T. Wauters et al., AIP Conf. Proc. 1580, 187 (2014).
[13] J. Schacht et al., Fusion Eng. Des. 83, 2–3 (2008) 228–
235.
Matthias Otte for the WEGA team
IPP Greifswald, Germany
Daniel Andruczyk for the HIDRA team,
Center for Plasma Material Interactions
University of Illinois at Urbana-Champaign, USA
W7-X program and plasma
evaluation workshop
The first W7-X program and evaluation workshop took
place at the end of May 2016. Many colleagues from near
and far came to Greifswald, Germany, spending an interesting
four days at the Krupp-Kolleg in a stimulating
atmosphere. From the approximately 130 participants,
about 40 were from collaboration partners around the
world, including scientists from various European labs,
from the United States, and from Japan.
During the four days spanning 23–26 May, many interesting
presentations were given and lively discussions paved
the way for starting the scientific and organizational preparations
for the next operational campaign.
The first day of the workshop was devoted to the results of
the first experimental campaign (OP 1.1),giving a comprehensive
overview of the status of the data analysis, already
indicating many interesting plasma properties.
The second day focused on the scientific objectives of the
next campaign (OP 1.2) for which the installation of an
inertially cooled divertor just started. The partners from
the United States, from Japan, and from the EUROfusion
Consortium presented their views and ideas to further
extend the collaboration topics.
On the third day, presentations and discussions concentrated
on all the issues associated with the device: control,
data acquisition, data access, and data analysis. Of course,
these are topics which have many aspects and concern the
complete chain from taking first data to providing a full
scientific analysis. Also remote participation was discussed,
which is of particular importance for all those not
always working on site.
Finally, the last day dealt with all questions around the
organization of experiments, ranging from the developing
experimental proposal to conducting the experiment and
starting the data evaluation.
All in all, many lively and very constructive discussions
provided a large collection of interesting and important
ideas and proposals aiming at smooth conduct of future
experiments. The many comments and ideas now definitely
form a good basis for the preparation of OP 1.2.
The organizers would like to thank all those who helped
preparing the workshop and who participated and devoted
four days to the future success of Wendelstein 7-X.
Prof. Dr. Robert Wolf
Max-Planck-Institut für Plasmaphysik
Wendelsteinstraße 1
17491 Greifswald
Germany
Report on the 15th Coordinated
Working Group Meeting
The 15th Coordinated Working Group Meeting was held
in Greifswald 21–23 March 2016. The organizers are
grateful to the host institution, the Max-Planck-Insitut für
Plasmaphysik (Greifswald, Germany), EUROfusion, and
NINS/NIFS for support. The meeting was conducted
under auspices of the IEA Energy Technology Network
(IEA Technology Collaboration Programme for Cooperation
in Development of the Stellarator-Heliotron Concept).
For the first time, a group of coordinators (Ascasibar,
Gates, Yokoyama, and Dinklage) prepared the meeting,
identified topics for international cooperations, and took
over the responsibility to track the agreed actions. Figure 1
shows the on-site workshop participants.
Given that the first experimental campaigns of W7-X
ended only 2 weeks prior to the meeting, a session about
first findings on W7-X attracted large attention from the
roughly 20 external participants on site. Session attendees
from the host institute and many remote participants
(Japan, US, Ukraine, EC) added up to about 50 colleagues
attending the meeting in total.
W7-X and comparative plasma start-up studies
First findings from W7-X confirmed low error fields, in
agreement with magnetic surface measurement experiments.
Trim coils were used to confirm good magnetic
fields and to assess the coils as a tool for edge studies.
Even in the first weeks of operation, a substantial number
of sophisticated instruments were employed to investigate
plasmas with both helium and hydrogen fill gases. Edge
cooling was successfully demonstrated with nitrogen
Stellarator News -5- June 2016
puffs. Unexpected phenomena such as rotating filaments
in the plasma edge are under investigation. Heating with
X2 mode was proven to work reliably both on- and offaxis.
Electron cyclotron (EC) current drive was demonstrated
as a proof-of-principle. Even first O2 mode heating
could be demonstrated. The global particle balance in
comparison to HSX and W-7X employing filterscope measurements
was discussed and compared to modeling. The
role of magnetic islands and rotational transform on particle
confinement was discussed.
First experiences for wall conditioning in W7-X, both with
glow discharge and electron cyclotron resonance heating
were reported. The major impurities in the device as determined
from residual gas mass spectrometry, are water and
carbon mono- and dioxides. Shot-to-shot behavior of the
ratio of outgassing level indicates improvements over the
campaign. The wall conditioning scenarios for the next
campaign of W7-X were proposed and discussed.
Features of RF plasma production were studied in detail at
the Uragan-3M device. Low-frequency oscillations are
detected and analyzed at this machine in a low plasma
density regime. A simple method to control the edge
island zone was suggested for the Uragan-2M machine.
Self-consistent RF wall conditioning discharge modeling
for W7-X is in progress. A new antenna for plasma production
and heating in the Alfvén resonance regime is
being modeled for the H-1 heliac.
A 0-D model simulation could reproduce the experimental
observation of neutral beam injection (NBI) plasma startup
in Heliotron J. The important process is a positive feedback
among production of fast hydrogen ions, electron
heating, and ionization/dissociation of the main gas puff.
One prediction of the model was that plasma startup using
perpendicular NBI could be possible in W7-X if a sufficiently
dense seed plasma is used as a target.
Impurity transport
Recent measurements of different impurity lines in the
VUV range showed the ability to obtain estimates of
impurity confinement time in the first plasmas of W7-X.
The relationship of impurity density asymmetries (i.e.,
angular variations of the moments of the impurity distribution
function) with radial transport was discussed. A
method was proposed to assess the presence of impurity
density asymmetries from 0-D signals by identifying radiation
oscillations following sudden changes in plasma
parameters. This has been consistently observed after pellet
injection in TJ-II and successfully modeled with a fluid
code. Recent extensions of neoclassical theory for high-Z
species are being tested in several codes (EUTERPE,
SFINCS) and could allow a critical assessment of the
nature of impurity transport in stellarators by direct comparison
with flux-capable diagnostics.
A TESPEL system was installed on the pellet injection
line of the TJ-II stellarator in 2015, with several successful
injections showing a clearly separated evaporation of the
polyethylene shell followed by the release of the tracer
material.
The following coordinated actions were agreed to:
 Prepare a comparison of discharges with Fe flakes for
an LHD ICH plasma and a W7-X ECH plasma. This
might give a hint about plasma sustainment.
Fig. 1. On-site participants at the workshop.
Stellarator News -6- June 2016
 Assess the presence of radiation oscillations in the
range of the EB rotation frequency after sudden
events such as pellet injections, central Te crashes,
ELMs, etc., as a signature of density asymmetries.
 Initiate a validation activity for the computation of
electrostatic potential variations on flux surfaces with
EUTERPE/SFINCS/FORTEC-3D.
3-D equilibria
The first 3-D equilibrium reconstruction of W7-X has
been performed using the V3FIT/PARVMEC code. From
diamagnetic loop signals, V3FIT can reconstruct a finite
beta equilibrium. This result was made possible through
the development of new tools for the W7-X experiment.
These tools, access magnetic diagnostic data from the
W7-X CoDaC data archive system and generate input files
for VMEC and V3FIT. Using this, researchers have a turnkey
solution to provide the equilibrium state, allowing better
understanding of W7-X results.
Results of the investigation of error fields at W7-X using
the flux surface measurement technique available at W7-X
and the trim coils were reported. The focus was on the
investigation of the (n = 1, m = 2)-error field in a configuration
especially designed to provide the  = 1/2 resonance
in a region accessible with the flux surface measurement
technique. The special design of the configuration having
an increased (still small) shear allowed a successful comparison
of measurements and calculations to derive first
estimates of the error field component and its phase at low
field strength.
HINT calculations were made for configurations of W7-X
that had been especially designed via an iterative process
of equilibrium calculations and neoclassical transport simulations
(DKES and NTSS) with small bootstrap current (a
few kilo-amperes) for high-performance, high-density
experimental scenarios. The HINT study explored the
divertor compatibility of the low- and the high- configuration,
taking into account the pressure and the current
density profiles of the self-consistent core-plasma equilibrium
and transport analysis. The main results is that for the
high- configuration, increasing beta increases the stochasticity
of the field around the boundary island.
Fueling and particle transport
An overview of energy and particle transport in tokamaks
and helical devices bridged the different magnetic confinement
concepts. Emphasis was placed on the possibility
that some of the features experimentally observed in tokamaks
(pedestal in the H-mode, stiffness of temperature
profiles, and inward particle pinch in the core) could
become more relevant in large optimized helical devices.
The following coordinated actions were agreed to:
 Improve the documentation of the benefit of H-mode
to confinement. Restart a topical group that studies
the role of magnetic topology and radial electric fields
in access to H-mode.
A joint paper with contributions from LHD and TJ-II on
the effect of transient density profile shaping on transport
is being prepared to document fueling effects through
inward transport after injection of a pellet. Similar observations
at LHD and TJ-II were reported. A new observation
on LHD is a change in fluctuations when the density
gradient becomes positive, and thus the ratio of the logarithmic
temperature and density gradients,  becomes
negative. A discussion of analytical and quasilinear
numerical TEM calculations about the impact of  on turbulence
in W7-X showed that the relative orientation of
density and temperature gradients (e.g., positive density
gradient combined with negative temperature gradient)
may have a relevant impact on the level of turbulence.
These findings were complemented by investigations of
fueling in reactor-grade plasmas, in particular by means of
pellet injection. Pellet penetration should be shallow in a
burning plasma, according to ablation models, which predict
that it is improved by high-field-side injection.
Strategic cooperation
and diagnostics cooperation
Plasma-wall interaction (PWI) studies for 3-D devices
were discussed again to bring in experience from tokamaks.
This topic is newly proposed. Examples of PWI
study in tokamaks, which could impact the experiment
plan for OP1.2. in W7-X, were presented. PWI studies in
LHD focused on material migration.
 It was agreed that PWI studies need to develop simulation
codes for 3-D systems that include the wall and
that can predict what happens.
Recently, 3-D edge modeling, especially with EMC3-
EIRENE, became a topic of ITPA SOL/Div, and we can
cooperate with the activity. On the other hand, it was noted
that edge plasma simulation codes are still under development.
For example, detachment plasma and drift are difficult
to treat at this stage. A follow-up meeting will be
organized as a satellite meeting to the forthcoming IAEA
FEC.
Activities at the ITPA Transport & Confinement topical
group meeting, which was held 16–18 March 2016, were
reported. The progress of studies in tokamaks (ITER Hmode
database, low-Z impurities transport, I-mode, L-H
transition, and 3-D) and stellarators (3-D) were briefly
introduced. In the 3-D session, the L-H transition mechanism
in stellarators was discussed.
Studies of the iota window to L-H transition in low-shear
stellarators were introduced. It was emphasized that
Stellarator News -7- June 2016
understanding the iota window is a critical issue for
exploring the L-H transition in low-shear stellarators,
especially for W7-X.
A summary of the first US stellarator workshop, First US
Stellcon, which was held 16–17 February 2016, was introduced.
Areas of consensus in the workshop were:
 A tool to optimize divertors is needed.
 Turbulence optimization is new and exciting.
 Coil simplification is new and exciting.
 Comparisons between experiments and extended
MHD are important.
Progress in International Stellarator/Heliotron Database
(ISHDB) activities was reported. ISHDB was acknowledged
by EUROfusion and supported to be brought to a
common EUROfusion infrastructure. Expansion of
ISHDB with new data sets and physics topics is possible
and desirable.
Overlapping interests emerged from presentations on diagnostics
(neutron diagnostics and pulse-height analysis). It
was agreed to pursue this field of common activities to
leverage expertise in the stellarator community.
Alfvén Eigenmodes
In order to externally control and/or stabilize energetic
particle (EP)-driven MHD instabilities which could induce
a redistribution and/or loss of energetic ions (including
alpha particles in a fusion reactor), we attempted to apply
ECH/electron cyclotron current drive (ECCD) to three
stellarators: LHD, Heliotron J and TJ-II, in which several
kinds of Alfvén eigenmodes (AEs) have been observed.
Global AEs (GAEs) were successfully mitigated by
increasing ECH power or EC-driven plasma current in
Heliotron J. Both stabilization and destabilization of toroidal
AEs (TAEs) were observed during additional ECH in
LHD. In the case of GAE stabilization by ECCD in Heliotron
J, a continuum damping rate, which is a main damping
mechanism of this case, will increase because of the
formation of magnetic shear induced by ECCD. On the
other hand, effects of ECH on AEs are under investigation
in both Heliotron J and LHD. In TJ-II, the mode behavior
of the observed AEs changes from continuous to burst
when ECH/ECCD is applied. A candidate driver for the
changing mode behavior is the changing trapped electron
fraction, which may have influence on fast ion drag. When
a refractive index N|| is changed to induce EC-driven
plasma current, changes in mode behavior (e.g., chirping
frequency and amplitude) are observed.
Modes with a frequency strongly dependent on the plasma
current are observed in TJ-II and Uragan-3M. An explanation
was put forward about the nature of some low- to
moderate-frequency MHD modes observed in TJ-II plasmas
that cannot be identified if closed and nested flux surfaces
are assumed. The interaction of shear Alfvén waves
with static magnetic island chains opens new gaps in the
continuum spectrum, which enables the existence of
weakly damped Alfvénic modes, magnetic island-induced
AEs (MIAEs). In addition, the coupling of island rotation
modes with Alfvén waves has been identified for the first
time in TJ-II. Fluctuations of 1–500 kHz were investigated
in the Uragan-3M torsatron. High-frequency fluctuations
in the range of 20–500 kHz are observed in low-density
plasmas only. The observed frequency depends on plasma
current rather than square root of electron density.
The next step for intermachine comparisons is to investigate
modes in a normalized parameter space (e.g., stability
condition vs resonance condition).
Turbulence Optimization
Past and recent advances on turbulence optimization [ion
temperature gradient (ITG), trapped electron mode
(TEM)] in stellarator configurations were addressed. A
simple proxy function, including “bad curvature” and the
surface compression due to 3-D shaping, is able to control
ITG turbulence. Starting from the W7-X-high-mirror configuration,
we found a new quasiomnigeneous configuration,
termed MPX, with lower turbulent transport. At the
same time, the effective helical ripple (acting as a proxy
for neoclassical transport in the core) was also reduced. A
realizable experimental setting was proposed, in order to
test the theoretical finding that elongation is a critical geometrical
factor for the control of ITG (and perhaps other
types of) turbulence. In particular, it was suggested that a
high-iota W7-X configuration (with large elongation)
should be identified with stronger turbulence than a lowiota
configuration. For TEM, a proxy has also been found
that separates the region of trapped particles from those
with strong bad curvature. The University of Wisconsin
was provided with the configuration information.
 A report on the electron temperature gradient type of
turbulence for W7-X, compared directly to ITG, will
be prepared in order to assess the relative merit of
electron and ion scales.
 Attempts will be made to combine the aforementioned
approaches; that is, we will try to create new
turbulence-optimized configurations for TEM, while
also providing coil-current information for configurations
with low ITG transport.
Beyond a brief follow-up meeting within the IAEA FEC,
it was agreed to continue the Coordinated Working Group
Meetings in Spring 2017 at PPPL in Princeton.
A. Alonso, E. Ascasibar, M. Beurskens, A. Dinklage, D. Gates,
J. Geiger, S. Masuzaki, V. Moiseenko, J.-L. Velasco,
P. Xanthopoulos, and S. Yamamoto