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An international journal of news from the stellarator community
Editor: James A. Rome Issue 165 June 2019
E-Mail: Phone: +1 (865) 482-5643
On the Web at
Pellet particle-fueling experiments
in the Wendelstein
7-X stellarator
During the last two experimental campaigns (OP1.2a and
OP1.2b) at the advanced stellarator Wendelstein 7-X (W7-
X), pellet fueling experiments were performed [1]. W7-X
is situated at the Max Planck Institute for Plasma Physics
in Greifswald, Germany. For the sake of deep and efficient
particle fueling, series of pellets were injected into W7-X
discharges. A blower-gun pellet injector [2] that was originally
used at the Max Planck Institute in Garching, Germany,
was loaned to W7-X, in order to investigate the
fueling needs of a large stellarator.
For future campaigns, a steady-state pellet injector is presently
under construction at Oak Ridge National Laboratory
(ORNL), USA. The design of this new injector is
derived from a former prototype that was developed for
the tokamak ITER [3]. The new injector will be the result
of a collaboration between ORNL, the Princeton Plasma
Physics Laboratory (PPPL), the National Institute for
Fusion Science (NIFS), Japan, and the Laboratorio Nacional
de Fusión (CIEMAT), Spain. All activities are organized
under the aegis of the European Atomic Energy
Community (EURATOM) on the European side, and the
US Department of Energy (USDOE) on the US side.
The Garching blower-gun injects hydrogen or deuterium
ice pellets, with a maximum velocity of about 300 m/s.
Each pellet is of cylindrical shape with a length and diameter
of 2 mm; hence each hydrogen pellet contains nominally
3.3 1020 atoms. Helium is used as the propellant
gas. Up to ≈ 40 pellets are frozen for one plasma discharge,
which can then be injected with a variable repetition
frequency between 2.5 Hz and 30 Hz. Previously this
injector was used on ASDEX-Upgrade [4] with a single
barrel; this design was changed for W7-X to a two-barrel
injector. Consequently, pellets can be injected into the
inboard port (AEL41) on the magnetic high-field side
(HFS), and the outboard port (AEK41) on the magnetic
low-field side (LFS). Hence, both HFS and LFS pellet
injection are possible into one discharge.
Figure 1 shows a sketch of the arrangement. Two pellet
guide tubes connect the blower-gun with W7-X, with
lengths of 14.5 m for the LFS pellets and 29 m for the HFS
pellets. Microwave mass detectors [5] allow for the measurement
of pellet mass and velocity in flight. A fast photodiode
records the ablation light intensity, and a fast
camera system (with a frame rate up to 350 kHz) observes
the ablation clouds simultaneously from two sides [6]. A
“burst-mode” Thomson scattering system [7] allows for
the measurement of electron density and temperature profiles,
with a time resolution ≈ 100 μs. These fast data are
rapidly evaluated, in order to investigate the final deposition
of particles after radial drift of the ablated pellet.
In this issue . . .
Pellet particle-fueling experiments in the
Wendelstein 7-X stellarator
Deep and efficient particle fueling experiments were
performed in the stellarator Wendelstein 7-X (W7-X).
Series of hydrogen ice pellets were injected by means
of a blower-gun. The question was tackled, whether
there is a difference in the fueling efficiency between
the inboard and the outboard machine side. In addition,
the injection of series of relatively small and slow
pellets was tested, in preparation for a future steadystate
pellet injector with faster and larger pellets. .... 1
PLADyS: Advanced Core-to-Core Network for
High-Temperature PLAsma Dynamics and
Structure Formation Based on Magnetic Field
This 5-year program is designed to elucidate the origin
and the dynamical evolutions of the structural formations
that are ubiquitously observed in nonlinear and/
or nonequilibrium systems. The focus is on two key
aspects to understand their mechanism: the dynamics
of turbulence and energetic particles in high-temperature
plasmas, and the resultant emergence of global
spontaneous flow (structure, such as geodesic acoustic
modes and zonal flows). ..................................... 5
Stellarator News -2- June 2019
Fig. 2 shows the shape and size of the two guide tubes.
Several motivations exist for the injection of pellets in
W7-X. The most important is the mitigation of hollow
density (or even pressure) profiles, for the case of dominating
neoclassical particle transport in the core of W7-X.
The off-diagonal term (thermo-diffusion) driven by the
electron temperature gradient can induce a significant outward
particle flux. Neoclassical transport calculations predict
the requirement of a central particle-fueling rate in the
range of 10 21 s1 with a heating power of 10 MW. Only
pellets can provide such a high central fueling rate.
Another motivation is the test of HFS pellet injection, in
order to take advantage of the grad-B drift. This technique
provided a considerably improved fueling efficiency in
tokamaks [8]. Although the grad-B term in W7-X is
smaller than in a tokamak, pellet code calculations predict
a favorable fueling efficiency for HFS pellets compared to
LFS pellets.
In addition, the technique of injection of series of pellets
was investigated, with comparably small and slow pellets.
The use of small pellets is of particular interest in W7-X,
because here electron cyclotron resonance heating
(ECRH) is used, which exhibits a cut-off density of 1.2 
1020 m3 in the X2-mode polarization at 2.5 T main magnetic
field strength. Only very large pellets offer the possibility
of penetrating to the magnetic axis (as required for
central fueling, see above). However, pellets of that size
might violate the upper density boundary. A solution for
this dilemma is, to some extent, the O2-mode polarization.
However, large pellets will then reduce the single-path
power absorption, which is a function of the electron temperature
Te. The lower the central Te, the poorer the singlepath
absorption. Hence, unacceptably high stray radiation
levels might be produced in the W7-X vessel.
Finally, pellets are known to induce modes of improved
energy confinement, for instance by the formation of
peaked density profiles, resulting from central particle
fueling [9]. Such enhanced confinement modes could be
demonstrated in W7-X with series of pellets.
The formation of hollow density profiles was not an issue
during the first two experimental campaigns. Even without
central particle fueling by pellets, the density profiles were
always flat or slightly hollow. Therefore, enhanced central
particle fueling was never required to avoid destabilization
of deleterious MHD modes. Fast and efficient particle
fueling could be demonstrated successfully nevertheless,
as shown for instance in Fig. 3. A series of five pellets
enhanced the central electron density faster than expected
from a diffusive time scale. Within <30 ms about 80% of
all pellet particles were deposited inside an effective minor
plasma radius of 0.3 m. Such behavior was typical for
most of the discharges.
In total, 147 discharges were fueled with series of pellets.
Figure 4 is a photograph of the first HFS pellet ever
injected into W7-X. In many cases, a distinct peaking of
the density profile could be observed, as shown in Fig. 5.
During a series of 15 pellets, the line density increases
from 3  1019 m2 to 9  1019 m2 (corresponding to an
averaged density of very roughly about 2  1019 m3 to 7
1019 m3). Each pellet can be seen as a sharp jump
upward in the density signal. After the series of pellets, the
line density decays on a time scale of about 1.3 s. The density
profile peaking is defined as the ratio of the central to
the average density. It increases considerably during the
series of pellets, because of the enhanced central particle
Fig. 1. Arrangement of the blower-gun in the diagnostic
hall, which is separated from the torus hall by a radiation
shield (the wall of the torus hall). Two steel tubes guide the
pellets from the blower-gun to W7-X through a hole in the
wall, the hole having a diameter of 80 mm. The guide tubes
have an outer diameter of 10 mm. As the guide tubes are
very long, they are built in shorter sections, which are connected
by CF40 vacuum flanges. Halfway between the
blower gun and W7-X, two pumping stations pump away
the eroded hydrogen of the pellets. They also contain the
microwave measurement systems. Shutters in front of the
ports AEK41 (LFS pellets) and AEL41 (HFS pellets) separate
W7-X from the guide tubes. Not to scale.
Fig. 2. Geometry of the HFS and LFS guide tubes in real
space. The scale indicates the size of the arrangement.
The blower-gun and W7-X are not shown. The labels A
show the location of the microwave systems, the labels B
the vacuum shutters.
Stellarator News -3- June 2019
HFS pellets show a systematically higher fueling efficiency
than LFS pellets. However, the difference is small
and is, taking into account the measurement uncertainties,
statistically not relevant. The measured pellet fueling efficiencies
are 70% ± 31% for LFS pellets and 88% ± 34%
for the HFS pellets. The uncertainties show the limited
shot-to-shot and pellet-to-pellet reproducibility of the
experiments. It is assumed that the rather small difference
between HFS and LFS pellets is a result of the small grad-
B term of about 0.8 T/m in the pellet injection plane.
These experimental results were compared to the outcome
of HPI2 code calculations. HPI2 is a pellet deposition code
[10] that calculates the deposition of pellet particles, taking
into account the grad-B drift of the pellet plasmoid
(the dense and cold high-beta cloud produced by the pellet
particles). A deceleration of the plasmoid drift, which is
caused by parallel currents inside and outside the plasmoid,
is also considered by the calculation. The experimental
findings are consistent with the calculations.
Figure 6 shows the result of an HPI2 code calculation, performed
for a single pellet. The measured density values
from Thomson scattering (blue circles) are smoothed by a
fit to a Gaussian process regression function (solid red
curve), indicating the uncertainty of the measurement
(dotted red curves). The dot-dashed black curve shows the
result of the calculated particle deposition after the drift.
The blue dashed curve shows the pellet ablation profile.
As expected, the ablated material drifts away from the
plasma center, as can be seen from the outward displacement
of the deposited curve compared to the ablated one.
Fig. 3. Time development of the density profile, as measured
by Thomson scattering. The two arrows indicate the
time interval for the injection of the series of pellets. The
Thomson data were recalibrated with respect to the interferometer
line density measurement. The density value is
color encoded (z-axis).
Fig. 4. Photograph of the first HFS pellet ever injected into
W7-X. The HFS and LFS injection geometry are indicated
by arrows. Some of the W7-X graphite tiles can also be
seen, illuminated by the pellet ablation light. The second,
weaker light trace above the ablation cloud is a reflection
on the wall.
Fig. 5. Time traces of the plasma line density (left y-axis,
solid line) and density peaking factor (right y-axis, dotdashed
line). The two dashed vertical lines indicate the
time interval when the series of pellets was injected.
Stellarator News -4- June 2019
The technique of injection of series of pellets works well,
even with small and slow pellets. The experiments demonstrate
that plasma cooling by the first pellets in a series
supports the deeper penetration of the last pellets in the
same series. Thus, the fueling efficiency is enhanced, and
fast and efficient central particle fueling is facilitated. This
was demonstrated for the first time in LHD [11].
A striking effect in W7-X was the strong transient
enhancement of the energy confinement time that
appeared a few 100 ms after the last pellet of a series. The
experimental energy confinement time exceeded the
expectation from the International Stellarator Scaling law
ISS04 [12] by several 10%. It was, however, not possible
to stabilize the phase of enhanced confinement to stationarity.
Common for these discharges were a peaked electron
density profile, reduced plasma radiation losses, strongly
increased central ion temperatures, and a strong negative
radial electric field. The highest fusion triple product,
observed in a stellarator up to now, 6.5 1019 keV m3 s,
could be achieved, with central ion temperatures well
above 3.5 keV, and energy confinement times above 200
ms. These findings could not be reproduced with gas puffing
as a particle source. Figure 7 shows time traces of the
energy confinement time E for the experiment and the
ISS04 expectation. After the series of pellets, the experimental
E transiently exceeds the ISS04 value.
It is assumed, as a working hypothesis, that the enhanced
density gradients after pellet injection might help stabilize
ion-temperature gradient (ITG) modes. This mechanism is
supported by the advanced concept of W7-X including an
approach to quasi-isodynamicity [13], reducing the W7-X
sensitivity against electrostatic turbulence.
This work has been carried out within the framework of
the EUROfusion Consortium and has received funding
from the Euratom research and training program 2014-
2018 and 2019–2020 under grant agreement No. 633053.
The views and opinions expressed herein do not necessarily
reflect those of the European Commission.
Jürgen Baldzuhn for the Wendelstein 7-X Team
Max Planck Institute for Plasma Physics
Greifswald, Germany
Fig. 6. Result of the HPI2 pellet code calculation. Shown is
the measured Thomson density profile (circles) together
with a smoothing regression to a Gaussian process function
(solid red line) and the interval of reliability of the
regression (dotted red lines). The HPI2 result for the pellet
ablation is the dashed blue curve, and the final deposited
particle density is indicated as the black dot-dashed line,
being superposed on the density profile prior to pellet injection.
The maximum of deposited particles is further outside
the plasma compared to the maximum of the ablation, as a
result of the grad-B drift.
Fig. 7. Time traces of the energy confinement time for the
experimental data (solid line) and the ISS04 expectation
(dashed line). The two dashed vertical lines indicate the
time interval of pellet injection.
Stellarator News -5- June 2019
[1] T. Klinger et al., Plasma Phys. Contr. Fusion 59, (2017)
[2] M. Dibon, Master-Thesis (2014) TU Munich, Max-
Planck Institut für Plasmaphysik.
[3] B. J. Green et al, Plasma Phys. Control. Fusion 45
(2003) 687.
[4] P. T. Lang et al., Nucl. Fusion 58 (2018) 036001.
[5] S. K. Combs et al., Rev. Sci. Instrum. 77 (2006)
[6] G. Kocsis et al., Fusion Eng. and Design 96–97, (2015)
p. 808.
[7] Damm, H. Master-Thesis (2018), Garching, Max
Planck Institut für Plasmaphysik
[8] P. T. Lang et al., Phys. Rev. Lett. 79 (1997) 1487.
[9] F. Wagner, European Phys. J. H (2017) 10.1140/epjh/
[10] B. Pégourié et al., Plasma Phys. Contr. Fusion 47
(2005) 17.
[11] R. Sakamoto et al., Nucl. Fusion 46 (2006) 884.
[12] H. Yamada et al., Nucl. Fusion 45 (2005) 1684.
[13] H. E. Proll et al., Phys. Rev. Lett. 108 (2012) 245002.
Introduction to Stellarators
August 19–23, 2019
This summer school, which is supported jointly by the
Princeton Plasma Physics Laboratory and the Simons
Foundation Collaboration in Mathematical and Physical
Science, will provide an introduction to Stellarators for
graduate students and postdoctoral fellows in applied
mathematics, computer science, fluid dynamics, and
plasma physics. The course will be mostly self-contained,
and no previous background in fusion or plasma physics is
needed. The emphasis of the course will be on mathematical
and physical foundations of Stellarators, and in
addressing optimization methods for designing Stellarators.
The lecturers of the course are drawn from members
of the Simons Foundation Collaboration on “Hidden Symmetries
and Fusion Energy.”
The lectures will be given at the Princeton Center for Theoretical
Science. The lecture schedule includes problemsolving
sessions by students and a computer lab at the
Princeton Plasma Physics Laboratory. Students will have
an opportunity to present and discuss their research in a
poster session (optional).
The application deadline is July 1, 2019. Please apply on
the website (
PLADyS: Advanced Core-to-
Core Network for High-
Temperature PLAsma Dynamics
and Structure Formation
Based on Magnetic Field
The Institute of Advanced Energy (IAE) [1], Kyoto University,
Japan (Prof. Kazunobu Nagasaki being the PI), has
won the Core-to-Core Program implemented by the Japan
Society for the Promotion of Science (JSPS) [2]. This program
is designed to create world-class research centers
that partner over the long term with other core research
institutions around the world in advancing research in
leading-edge fields on issues of high international priority.
It also concentrates on fostering the next generation of
trailblazing young researchers. (These two sentences are
cited from the JSPS Website [2].) The program is
approved for 5 years (April 2019–March 2024), with a rigorous
interim evaluation. The approval rate for this fiscal
year was 8 out of 52 applications (~15%) for all research
fields, which demonstrates the competitiveness of this
The title of the approved program is Advanced Core-to-
Core Network for High-Temperature Plasma Dynamics
and Structure Formation Based on Magnetic Field Diversity.
The objectives of this program are summarized as follows
(translated mostly from the application form).
One of the largest scientific goals is to elucidate the trigger
(or the origin) and the dynamical evolutions of the structural
formations that are ubiquitously observed in nonlinear
and/or nonequilibrium systems in physical,
astronomical, biological, and social phenomena. In laboratory
experiments on magnetically-confined high-temperature
plasmas, the structural formations, which cause the
transition to a new state with improved confinement, are
not only observable, but also actively controllable through
various means such as heating and magnetic configurations
This program will focus on two key aspects to understand
the mechanism of these structural formations: the dynamics
of turbulence and energetic particles in high-temperature
plasmas, and the resultant emergence of global
spontaneous flow (structure, such as geodesic acoustic
modes and zonal flows). Recent research has demonstrated
that such structural formations can be controlled by
means of the three-dimensionality of the magnetic field
configuration (magnetic field diversity).
Stellarator News -6- June 2019
This program aims to establish an international core-tocore
network for emerging academic research on structural
formation phenomena in nature by utilizing high-temperature
plasmas in a variety of magnetic configurations. We
have negotiated with partner institutions (abroad) since we
started the application procedure, and have formed the
core-to-core network shown in Fig. 1.
This program will facilitate international collaborations
mainly through three activities: (1) research collaborations,
(2) seminars, and (3) visiting research exchanges
(younger generations).
In support of activity 1, we have set up three task forces
(leading institutions designated), as summarized in Table
In support of activity 2, annual seminars will be held in
turn hosted by core institutions. The kick-off seminar is
scheduled to be held in Kyoto (23–25 March 2020) jointly
with the 20th Coordinated Working Group Meeting
Several young Japanese researchers have been already
selected to be dispatched to collaborating institutions
[long-term visit research exchanges (activity 3) for the
2019 Japanese fiscal year], and also we are ready to
receive similar visitors from abroad with “matching
funds” that they have received as part of their international
collaboration budgets.
As mentioned, structural formation is a topic of broad
common interest in many research fields. Thus, we will
also foster interaction with other research fields and
should be playing a leading role for emerging new academic
research on structural formation through this program.
We have set up a website for this program [3] (mostly in
Japanese at this moment) and created a logo (Fig. 2)
reflecting images of flows and structure formation. We
welcome expressions of interest in this program to promote
programmatic collaborations.
Kazunobu Nagasaki for PLADyS Local Coordinators [4]
[4] K. Nagasaki and S. Kobayashi (Kyoto University);
S. Inagaki (Kyushu University); M. Nakata, Y. Suzuki,
and M. Yokoyama (National Institute for Fusion Science).
Fig. 1. Core-to-core network of this program.
Fig.2 PLADyS logo.
Table 1. Task Forces formed in this program.
Task Force Name Focus area Leading institution
1. Turbulence and spontaneous
Driving mechanisms of zonal flow and structural formation
by high-resolution measurements and large-scale
Kyoto University
2. Excitation of spontaneous flow
by energetic particles
Driving mechanism of spontaneous flow and structural
formation by nonlinear wave-particle interactions
Max Plank Institut für
3. Control of structural formation
by means of three-dimensionality
(3D) of magnetic field configurations
Controlling structural formation by diversity of 3D fields. University of Wisconsin-

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