All opinions expressed herein are those of the authors and should not be reproduced, quoted in publications, or
used as a reference without the author’s consent.
An international journal of news from the stellarator community
Editor: James A. Rome Issue 171 December 2020
E-Mail: James.Rome@stelnews.info Phone: +1 (865) 482-5643
On the Web at https://stelnews.info
Fast ion physics is heating up
on Wendelstein 7-X
The goal of nuclear fusion is to create a self-sustaining
fusion reaction whereby the kinetic energy of the fusion
by-products is converted into heat both for energy generation
and for sustainment of the temperatures necessary for
the fusion reaction. The most likely candidate reaction is
between isotopes of hydrogen (deuterium and tritium),
producing 14.1 MeV neutrons and 4.3 MeV helium. The
conditions under which this reaction occurs results in a
plasma state, implying that the helium by-product is in a
charged state. The goal of magnetic confinement fusion is
to use magnetic fields to confine both the thermal hydrogen
isotope plasma and the resulting energetic helium
nuclei (alpha particles). Thereby the confined helium can
collide with the deuterium and tritium, heating it and sustaining
the reaction. Demonstration of the confinement of
such energetic particles is a key goal of the Wendelstein 7-
X (W7-X) project, as this is a necessary condition for
bringing forward the Helias line of stellarators as a power
plant.
The W7-X stellarator seeks to demonstrate reactor-relevant
plasmas, including demonstration of good fast ion
confinement. As the device will not be fueled with tritium,
and is smaller and lower field than an equivalent power
plant, proxy particles must be generated for demonstration
of good alpha confinement. The neutral beam injection
(NBI) and ion-cyclotron resonance heating (ICRH) systems
serve this purpose, with 55 keV deuterium ions serving
as proxy particles for fusion alphas in the larger
reactor. In addition to simply demonstrating acceptable
levels of fast ion confinement, the maximum-J (the orbit
bounce-averaged adiabatic invariant) nature of the W7-X
magnetic field is predicted to improve fast ion confinement
as the plasma beta is increased [1, 2].
In the previous experimental campaign on W7-X, the first
neutral beam experiments were conducted. In these experiments,
two of four ion sources were commissioned in
neutral beam box NI21. A second neutral beam box
(NI20) is being prepared for the future. The commissioning
was smooth, achieving 5 s of continuous NBI, and no
observed duct beam blocking. Discharges into the empty
plasma torus provided a means to calibrate simulation
model geometry [3], while spectroscopic measurements of
the plasma–neutral beam interaction provided validation
of neutral beam deposition simulations [4]. Such work
highlights the neutral beam systems dual role as both heating
system and plasma diagnostic [5].
In this issue . . .
Fast ion physics is heating up on Wendelstein
7-X
The Wendelstein 7-X (W7-X) stellarator commissioned
the first of two neutral beam boxes during the
previous experimental campaign. Data from these
preliminary experiments is being used to validate simulations
of neutral beam injection in stellarators. Predictive
simulations are helping to inform future
upgrades to and experiments on W7-X.. ................. 1
Observation and prediction of the fast-ion distribution
in the Large Helical Device using
FIDASIM
The National Institute for Fusion Science and University
of California, Irvine have started an international
research program for the observation and prediction
of the fast-ion distribution in the Large Helical Device
using fast-ion D alpha with FIDASIM since October
2018. The FIDA diagnostic is able to be a useful tool
for understanding the fast-ion distribution in a threedimensional
magnetic field device. We describe the
current status of the FIDA diagnostic and FIDASIM
computer code on the LHD and show representative
results. .................................................................... 3
Online resources for learning about fusion
research
We provide an overview of freely available educational
online resources relevant to plasma physics and
fusion research. We hope that this list might constitute
an entry point for young students, people in other
fields, and teachers. ................................................ 5
Stellarator News -2- December 2020
Experiments scanning magnetic configuration, plasma
density, and electron cyclotron resonant heating (ECRH)
power were conducted. In general, plasmas in which the
ECRH power was greater than the 3.6 MW of NBI power
showed a consistent asymptotic density rise when the neutral
beam was turned on with little response in plasma
temperature. In contrast, discharges heated solely by NBI
showed nearly constant plasma temperatures, a continual
density rise, and strong density peaking in the core. These
discharges reached the highest densities achieved in W7-X
to date. Reintroduction of 1 MW of ECRH into these discharges
arrested the density rise, flattened a region of density
in the plasma core, and resulted in ion temperatures
reaching 2 keV before dropping to 1.5 keV. These experiments
are qualitatively in agreement with experiments
conducted on W7-AS, and complementary experiments
have been proposed for the Large Helical Device.
Data from these experiments is currently being simulated
using the BEAMS3D [6], ASCOT [7] and VENUSLEVIS
[8] stellarator energetic particle codes. Analysis of
these discharges is serving as a benchmarking activity for
these codes, helping to validate their physics models and
analysis workflows. Simulations such as these are an
important part of the data analysis, as they provide the
source terms in the radial transport equations and equilibrium
calculations (current density). In Fig. 1, such terms
for the heating and current drive are shown for a a 5-s pure
NBI discharge in W7-X. Work is underway to validate
these simulation results against experimental data. And
such simulations are already helping to guide future experiments
in W7-X.
For the next experimental campaign, a second neutral
beam box will be brought into operation and the ICRH
antenna will begin operation. These upgrades will more
than double the ion heating capabilities of the experiment.
The ICRH system will bring direct ion heating to W7-X
and possibly produce higher energy fast ions than the NBI
system. The second NBI beam line will not only double
the heating power, but also allow for balanced beam injection.
This allows for easy assessment of co- versus counter
passing particle physics. Finally, efforts to improve the
NBI pulse length, achieve higher neutralization efficiency
and access high-density confinement modes will be
explored.
This work has been carried out within the framework of
the EUROfusion Consortium and has received funding
from the Euratom research and training programme 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.
Samuel Aaron Lazerson, Ph.D.
Research Scientist - E3 Stellarator-Heizung und -Optimierung
Max-Planck-Institut für Plasmaphysik
Wendelsteinstr. 1, D-17491 Greifswald Germany
+49 03834 88-2796
E-mail: samuel.lazerson@ipp.mpg.de
References
[1] Drevlak, M., Geiger, J., Helander, P., Turkin, Y.
(2014). “Fast particle confinement with optimized coil
currents in the W7-X stellarator,” Nucl. Fusion, 54 (7),
073002. https://doi.org/10.1088/0029-5515/54/7/
073002
[2] Strumberger, E. (2000). “Deposition patterns of fast
ions on plasma facing components in W7-X,” Nucl. Fusion,
40 (1), 1697–1713. https://doi.org/10.1088/0029-
5515/40/10/302
[3] Äkäslompolo, S., Drewelow, P., Gao, Y., Ali, A., Asunta,
O., Bozhenkov, S. A., et al. (2019). “Validating fast-
Fig. 1. Profiles of NBI in W7-X plasma heating (left) and current drive (right) as calculated from the BEAMS3D code for a
pure NBI discharge in W7-X. Bounds indicate changes over 5 s.
Stellarator News -3- December 2020
ion wall-load IR analysis-methods against W7-X NBI
empty-torus experiment,” J. Instrum. 14 (07), P07018–
P07018. https://doi.org/10.1088/1748-0221/14/07/
P07018
[4] Lazerson, S. A., Ford, O. P., Nuehrenberg, C.,
Äkäslompolo, S. J., Poloskei, P. Z., Machielsen, M., et
al. (2020). “Validation of the BEAMS3D neutral beam
deposition model on Wendelstein 7-X” Nucl. Fusion 60
(7). 076020. https://doi.org/10.1088/1741-4326/ab8e61
[5] Ford, O. P., Vano, L., Alonso, J. A., Baldzuhn, J.,
Beurskens, M. N. A., Biedermann, C., et al. (2020).
“Charge exchange recombination spectroscopy at Wendelstein
7-X”. Rev. Sci. Instrum. 91 (2),1–12. https://
doi.org/10.1063/1.5132936
[6] McMillan, M., and Lazerson, S. A. (2014). BEAMS3D
Neutral Beam Injection Model. Plasma Phys. Controlled
Fusion 56 (9), 095019. https://doi.org/10.1088/
0741-3335/56/9/095019
[7] Varje, J., Särkimäki, K., Kontula, J., Ollus, P., Kurki-
Suonio, T., Snicker, et al. (2020). “High-performance
orbit-following code ASCOT5 for monte carlo simulations
in fusion plasmas,” arXiv: 1908.02482.
[8] Pfefferlé, David “Energetic ion dynamics and confinement
in 3D saturated MHD configurations.” PhD thesis,
EPFL, Lausanne, 2015. doi: 10.5075/epfl.thesis-
6561.
Observation and prediction of
the fast-ion distribution in the
Large Helical Device using
FIDASIM
A magnetic confinement fusion reactor requires the sustainment
of a high-temperature and high-density plasma
by energetic alpha particles from fusion reactions. Therefore,
it is important to understand the behavior of energetic
particles in magnetic confinement devices. The study of
energetic particle confinement has advanced in tokamak
type devices [1] and is one of the main topics in ITER [2].
On the other hand, the study of energetic particle confinement
in helical devices is rapidly developing. At the Large
Helical Device (LHD), magnetohydrodynamics (MHD)
mode-induced transport and loss of fast ions have been
studied with high-power neutral beam injection (NBI) [3].
In the LHD, we have conducted only hydrogen experimental
campaigns for several years. To understand the
physics of fast-ions, deuterium experimental campaigns
have been performed in the LHD since March 2017. The
National Institute for Fusion Science (NIFS) and University
of California, Irvine have started an international
research program (Fig. 1) for the observation and prediction
of the fast-ion distribution in the LHD, located in
Toki, Japan, using fast-ion deuterium-produced alphas
(FIDAs) with FIDASIM since October 2018.
In the FIDA diagnostic, the Doppler-shifted D alpha light
from fast-neutrals is utilized as a signal from the energetic
particles. These fast neutrals are produced by the chargeexchange
process between fast ions in plasmas and
actively introduced neutrals from the NBI [4]. The advantages
of the FIDA diagnostic are the velocity and spatially
resolved measurement of fast ions at the crossing point
between its line of sight and the incident line of the neutral
beams.
Fig. 1. Photographs of a research meeting and preparation for experiments by researchers at the National Institute for
Fusion Science and the University of California, Irvine.
Stellarator News -4- December 2020
Originally, the FIDASIM computer code predicted signals
produced in two-dimensional (2D) axisymmetric devices.
However, the effect of three-dimensional(3D) fields on
fast-ion confinement is important. Therefore, FIDASIM
has been enhanced to simulate signals produced in 3D
geometry and is used here to predict FIDA signals at LHD.
The code requires a distribution function, plasma profiles,
magnetic equilibrium, and diagnostic geometry as inputs.
GNET [5] solves a drift kinetic equation in five-dimensional
phase space using a Monte Carlo technique, taking
into account the guiding-center motion in the 3D magnetic
configuration. It is used to calculate the distribution function.
In order to validate FIDASIM, the radial profile of
fast ions was measured using the FIDA diagnostic in
MHD-quiescent plasmas [6].
Figure 2 shows measured spectra from the FIDA diagnostic
and various sources of light in the D alpha spectral
band calculated by FIDASIM in the case of ne_avg= 0.65
×1019 m3 at R = 3.597 m. The black solid line is measured
spectra of the FIDA diagnostics with a calibration
factor (fcalib). The colored solid lines are calculated by
FIDASIM. As a result of the comparison of the FIDA
diagnostic data with FIDASIM results, they demonstrate
good agreement.
Figure 3 shows a comparison of the FIDA diagnostic
radial profiles and the FIDASIM radial profiles for three
different electron density discharges after integration over
wavelength. The x-axis is the normalized minor radius of
the LHD, and the y-axis is the radiance after spectral integration.
The result indicates that, on average, the measurement
and the simulation are consistent for these discharges
at the center of the plasma (reff/a99= 0.28~0.05) when the
line-averaged electron density is
ne_avg ≤ 0.805 ×1019 m3. Overall, the FIDA diagnostic
results and the FIDASIM results are in good agreement in
MHD-quiescent plasmas at the center of the plasma for coinjection.
This study indicates that the FIDA diagnostic can be a
strong tool for understanding the fast-ion distribution in a
3D magnetic field device. For future work, we will
endeavor to understand fast-ion behavior with the MHD
instabilities using the FIDA diagnostic and the latest
FIDASIM in the LHD.
This research was supported by the NINS program of Promoting
Research by Networking among Institutions
(Grant Number 01411702), the NIFS International Collaboration
Research programs (NIFS18/KLPR047 and
NIFS07/KLPH004), the LHD project budget (ULRR006,
ULRR035, ULRR036, and ULRR702) and (in part) by
U.S. DOE DE-SC0018255. A part of this work was performed
on “Plasma Simulator” (Fujitsu FX100) of NIFS
with the support and under the auspices of the NIFS Collaboration
Research program NIFS18KNST135.
Y. Fujiwara,1 S. Kamio,1 H. Yamaguchi,1 A. V. Garcia,3 L. Stagner,
4 H. Nuga,1 R. Seki,1,2 K. Ogawa,1,2 D. Lin,3 M. Isobe,1,2 M.
Yokoyama,1,2 W. W. Heidbrink,3 M. Osakabe,1,2 and LHD Experiment
Group1
1National Institute for Fusion Science, National Institutes of Natural
Sciences, 322-6 Oroshi-cho, Toki 509-5292, Japan.
2The Graduate University for Advanced Studies (SOKENDAI),
322-6 Oroshi-cho, Toki 509-5292, Japan.
3University of California, Irvine, Irvine, CA 92697, USA.
4General Atomics, San Diego, CA, USA.
Fig. 2. Measured spectra of FIDA diagnostic (black line)
and various sources of light in the D alpha spectral band
calculated by FIDASIM in the case of ne_avg= 0.65 × 1019
m3 at R = 3.597 m.
Fig. 3. Comparison of the FIDA diagnostic and FIDASIM
radial profiles for three different discharges of spectral integration.
The radiance and spectral integration profile are
for Doppler shifts (660.05 nm ~ 665.30 nm).
Stellarator News -5- December 2020
References
[1] Heidbrink ,W. W . and Sadler, G. J. Nucl. Fusion 34,
535 (1994).
[2] Fasoli, A. et al. Nucl. Fusion 47 S264 (2007).
[3] Toi, K. et al, Fusion Sci. Technol. 58, 186 (2010).
[4] Heidbrink ,W. W. et a.l Plasma Phys. Control. Fusion
46, 1855 (2004).
[5] Murakami, S. et al Nucl. Fusion 40, 693 (2000).
[6] Fujiwara, Y. et al. Nucl. Fusion 60, 112014 (2020).
Online resources for learning
about fusion research
At the beginning of this year, lectures being taught online
were still a minority in a typical student’s schedule. While
the concept of Massive Open Online Courses (MOOC)
was a growing trend in higher education over the last
years, these courses were often single courses given by
educators and/or universities specializing in remote teaching.
Now, almost a year (or two semesters) into a worldwide
pandemic situation, basically all universities around
the world have been forced to transit to online teaching.
This has created a massive amount of educational
resources that could in principle be available to everybody
but are in reality often hidden behind a university-based
teaching portal. Still, a few courses are free to access, but
those are difficult to find. This fact motivated the idea to
assemble an overview of freely available educational
online resources relevant to plasma physics and fusion
research. We hope that this list might constitute an entry
point for young students to get interested in the field of
nuclear fusion. It might also serve as an information
source for people from other different fields to get an idea
about what we are researching, or even serve as an inspiration
for teachers for their own lectures. In no particular
order, we have compiled the following list.
Plasma physics: Introduction
This is an MOOC developed by EPFL and freely available
at edX [1], EPFL courseware [2], or YouTube [3]. The
course covers the basics about plasma physics and is an
excellent starting point. It requires basic knowledge about
electromagnetism, thermodynamics, and statistical physics
(all topics covered by a typical physics bachelor’s degree).
[1] https://www.edx.org/course/plasma-physics-introduction
[2] https://courseware.epfl.ch/courses/coursev1:
EPFL+PlasmaIntroduction+1T_2018/about
[3] https://www.youtube.com/channel/UCM91JfbMXvtwKYNQAVl0YuQ/
playlists
Plasma physics: Applications
This course is the continuation of the previous course and
covers plasma applications in industry, astrophysics, and
nuclear fusion. It is available at edX [4], EPFL courseware
[5], and YouTube [6].
[4] https://www.edx.org/course/plasma-physics-applications
[5] https://courseware.epfl.ch/courses/coursev1:
EPFL+PlasmaApplication+1T_2018/about
Stellarator News -6- December 2020
[6] https://www.youtube.com/channel/UCM91JfbMXvtwKYNQAVl0YuQ/
playlists
Fusion research lecture
This lecture is usually given by the author of this article at
the University of Stuttgart (Fig. 1). It requires basic
plasma physics knowledge and covers various aspects of
nuclear fusion research with a focus on magnetic confinement.
It is available at YouTube [7].
[7] https://www.youtube.com/playlist?list=PL9F2aQG5CnOdw9MXqS309tojBUvChQ9jm
Summer school: Fusion Energy and Plasma
Physics Course
The Summer Science Undergraduate Laboratory Internship
(SULI) course is a summer school regularly given at
Princeton Plasma Physics Laboratory (PPPL). It consists
of a series of lectures ranging from an introductory level
into plasma physics and nuclear fusion to specialized talks
about turbulence in plasmas, plasma propulsion, and complex
plasmas, to give just a few examples. The lectures are
available at the PPPL website [8].
[8] https://suli.pppl.gov/2020/course/index.html
Plasma fluid theory (magnetohydroynamics)
This course requires basic undergraduate physics knowledge
and starts with simple fluid mechanics, going then
into a wide range of topics including 2-fluid treatment,
magnetohydroynamics (MHD), and more. It was originally
taught at the National Research Foundation of Korea
and is available on YouTube [9].
[9] https://www.youtube.com/playlist?list=PLJnw7q_gjMgmuGctIqL6crS7cmmALTZLe
Introduction to Plasma Physics
Although the videos for this course were recorded 30 years
ago, they are still relevant as they walk the audience
through Francis Chen’s book Introduction to Plasma
Physics and Controlled Fusion which is considered a standard
text book. This course was given at the University of
Wisconsin–Madison by James D. Callen and is available
on YouTube [10].
[10] https://www.youtube.com/playlist?list=PL1FcfKGHsyMhfKegznP0i7eO804Z5YBdm
Dr. Alf Köhn-Seemann
IGVP, University of Stuttgart
Pfaffenwaldring 31, 70569 Stuttgart
Tel +49 711 685 69686
E-mail: koehn@igvp.uni-stuttgart.de
Fig. 1. Screenshot of the University of Stuttgart fusion
research lecture [7].