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.
Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy.
Published by Fusion Energy Division, Oak Ridge National Laboratory
Building 5700 P.O. Box 2008 Oak Ridge, TN 37831-6169, USA
Editor: James A. Rome Issue 118 December 2008
E-Mail: jar@ornl.gov Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
UST_1, a small, low-cost
stellarator
UST_1, Ultra Small Torus (shown in Fig. 1), is a very
small (R = 119 mm) modular stellarator built in a personal
laboratory. Two main objectives were pursued: developing
innovative low cost construction techniques and allowing
the author to learn all aspects of stellarator design, construction,
and operation. UST_1 is located 65 km north of
Valencia, Spain, and was designed and built during 2005/
06 and operated during 2006/07. Successful experiments
to validate the quality of the design and construction have
been carried out, particularly field mapping experiments
and basic plasma pulses. UST_1 has proved that low-cost
techniques to build accurate stellarators exist. Very probably
it is the third modular stellarator in the world, the most
economical with acceptable quality, and the first designed
and built by only one person.
Similar small devices with reactor-like geometries probably
would be useful to improve the conceptual designs and
maintenance procedures for future fusion reactors.
Summary of features and parameters
UST_1 is a 2-field period modular stellarator with an
aspect ratio 6 formed by 12 resistive partially optimized
modular coils. Each coil is formed by 6 turns of flexible
copper conductor wound in a groove machined in a circular
torus. The grooves were accurately machined into a
single plaster frame by a specially designed toroidal milling
machine. Electron cyclotron radio-frequency heating
(ECRH) at the second harmonic (B0 = 46 mT and eventually
B0 = 90 mT) heats the plasma using a 0.8-kW, 2.45-
GHz commercial magnetron. Typical length of the plasma
pulse is 2 s at 46 mT. Toroidal field (TF) current per coil is
2.3 kA-turn.
Additionally, a vacuum system, control and diagnostics
systems, and power supplies complement the stellarator.
Fig. 1. The UST_1 stellarator and some of its auxiliary systems.
In this issue . . .
UST_1, a small, low-cost stellarator
The Ultra Small Torus is the world’s smallest and lowest-
cost modular stellarator with acceptable quality. It
was designed, built, and operated by one person in
near Valencia, Spain to learn about fusion. UST_1 is a
two field period modular stellarator with an aspect
ratio 6 formed by 12 resistive partially optimized
modular coils. only 2700 € were spent on materials for
the entire Facility...................................................... 1
Retirement ceremony and Stern-Gerlach medal
for Friedrich Wagner
On 27 November, 180 invited guests celebrated the
retirement of Friedrich (“Fritz”) Wagner from the Max-
Planck Institut für Plasmaphysik (IPP). In addition to a
scientific colloquium, an exhibition of Wagner’s paintings
opened in the main IPP hall. It has been
announced recently that Prof. Wagner will receive the
Stern-Gerlach medal, which is the highest award of
the Deutsche Physikalische Gesellschaft, given for
extraordinary contributions in experimental physics. 7
Stellarator News -2- December 2008
The cost of the installation was minimized and only 2700
€ were spent on materials for the whole facility. The size
of the device was adapted to the available economical and
technical resources and to the indeterminacies of building
a first device.
The plasma volume is 1.1 L, major radius R = 119.2 mm,
average minor radius a 21mm. I selected a low shear
configuration with 0 = 0.32 and a = 0.28, optimized to
occupy a narrow range just below 1/3 in order to avoid
high-order rationals and large magnetic islands. Additionally,
UST_1 is optimized for other important plasma
parameters, such as large plasma size, deep magnetic well,
low ripple, and low variance of the minima of |B|. Optimization
is modest because the coils are constrained to lie on
a circular torus.
Plasma parameters for this small device deduced from
ISS04v1 [1] with B0 = 0.1 T, enhancement factor = 0.1,
and PECRH = 400 W are very modest, on the order of
Te ~ 2 eV, n ~ 2 1017 m3, and E ~ 0.2 s, with ~ 0.
Chronological description of the development
A description of UST_1 in chronological order is presented
to better show not only what the stellarator is, but
also what it was not, and why.
The conception of UST_1 started in June 2005. All the initial
decisions were critical because frequently revising the
base design of the project could have easily led to a dead
end. Some of the key decisions taken are described next.
What to build?
Only tokamaks and stellarators were considered due to
pragmatic reasons.
A tokamak?
Initially the idea of a small tokamak around 6 times
smaller than the Brazilian tokamak ETE seemed attractive.
The power supplies, coils, heating power, length of pulse,
etc., were estimated by rough calculations. These estimates
were sufficient to rule out the construction of a tokamak
due to three insolvable difficulties: (i) the high
voltage and power (~ 1 MW) needed for the central solenoid
(CS) coil presented insurmountable cost and safety
issues. (ii) Te was too low for adequate plasma resistivity
and plasma current. (iii) Pulses would be of the order of
milliseconds or less because of CS flux and heat limitation
(very expensive diagnostics). The idea of such a small
tokamak was abandoned.
A stellarator?
Estimates for UST_1 and study of W7-X [2], CTH [3],
and also LHD, TJ-II, and NSCX gave the first ideas for the
design. A 1/10-scale CTH torsatron was the starting point.
The estimates provided insight that at least a stellarator
working at some few eV of plasma temperature could be
obtained without major difficulties. A stellarator was chosen.
Superconducting or resistive coils?
The need for vigorous power supplies was already a concern.
Therefore some research was done to check the feasibility
of high temperature superconducting (HTS) coils.
Bi2223 HTS was chosen and conceptual design and calculations
were performed. However, the cost of the HTS
wire alone would have been 6300 € — too much. I
selected resistive coils.
Coils outside the vacuum vessel or inside?
More global drawbacks than global advantages were discovered
for coils inside (e.g., QPS, CNT). I picked coils
outside the vacuum vessel.
Classical stellarator, torsatron or modular stellarator?
A modular stellarator seemed favorable due to construction
and power supply simplicity but no means to calculate
and build the modular coils were available at that moment.
Thus the starting point was the definition of the coils
kindly supplied by the CTH team. A similar classical stellarator
was also considered.
A JAVA code named SimPIMF was developed to calculate
three-dimensional (3D) magnetic fields. Later, and partially
because the use of NESCOIL was impossible, the
code evolved, and it is now able to calculate/simulate by
field line tracing: Poincaré plots (Fig. 2), rotational transform
and magnetic well profile, plasma size, orbit simulation
with drifts, particle losses, other ‘plasma’ parameters,
minimum distance between coils, and optimization of such
parameters by iterative generation of parametric 3D coils.
Simultaneously, more accurate estimates of the voltage,
current, power, and heating rate of the coils, magnetic field
B0, and other key parameters for several sizes of stellarators
and combinations of winding packs were carried out.
Forces on the support frame were negligible for B0 = 0.1 T.
Pulses about 1 s long were achievable without excessive
Fig. 2. Magnetic surfaces, magnetic islands and stochastic
region in UST_1 for = 0.
Stellarator News -3- December 2008
coil temperature rise (<35ºC). A test of a power supply
consisting of a commercial 12V battery was carried out,
obtaining pulsed current of 1200 A. Also E and Te were
calculated (ISS04v1 [1]) for diverse combinations of
inputs (ECRH power, enhancement factor and plasma
size).
A basic vacuum system, composed of a mechanical pump,
a diffusion pump, and a thermocouple gauge, was installed
and tested. This basic system has been upgraded several
times. Also, after some fruitless tries, a toroidal vacuum
vessel was conceived and built using five commercial copper
elbows.
As a result of the parallel development of all the different
components of the stellarator, including the SimPIMF
code, partially optimized classical coils were obtained in
January 2006.
However, just then, and inspired by the code developed for
classical stellarators and by the work of Dolan [4], a new
idea for the optimization of modular coils was conceived.
As a result, a modular stellarator was chosen and modular
coils were defined and optimized during the next months.
Tens of thousands of coil structures were evaluated by
simulation until the UST_1 coils were selected.
How to build the modular coils?
Modular coils have the advantage of requiring, in the simplest
way, only one layer of coils and only one set of coils
connected in series. Therefore the system is much less sensitive
to inaccuracies, specially of the power supply. Nevertheless,
accurate (~0.1% optimum, ~0.3% maximum
errors) fabrication and installation of modular coils is not
an easy task. Some methods that had been used to build
modular coils (HSX, W7-X, NCSX, CTH frame [3]) were
studied.
Device to mechanize modular coils.
A device to mechanize modular coil construction for stellarators
was conceived, and such a machine (Fig. 3) was
constructed after some first tests and some improvements.
It is a milling machine working on toroidal coordinates
able to machine grooves on a toroidal surface that is kept
fixed on the milling machine for the whole series of 12
coils. The result was successful and it is one of the main
results of the project: the demonstration that low-cost techniques
to build accurate stellarators exist.
The main advantages of this machine are:
1. Positioning and adjustment of the coils is not necessary
because all the grooves are machined on a single toroidal
surface. Only machining of the grooves is performed.
2. Fabrication errors are similar to those in CNC milling
machines—very small. Errors <~0.3 mm with respect to
the designed position of the coils have been achieved using
a medium quality toroidal milling machine. This is critical
in such a small machine where a given error in millimeters
is a larger fraction of the size.
3. Construction time is reduced and the process simplified.
A special conductor slightly over 3.5 mm in diameter was
finally selected and manufactured. The conductor is
formed by copper threads compacted by a commercial
shrinkable sleeve. The winding pack is a double pancake
of 3 layers and 2 rows. The width of the grooves was
thought to laterally compress the two pancakes on the
sides of the groove in order to facilitate the winding of the
coil (otherwise many stops and fasteners are necessary
during the winding process), avoid unwinding and reduce
positioning errors. A groove of 7 mm width and 12 mm
depth was finally defined.
The 12 grooves in the plaster frame were satisfactorily finished
in May 2006, Fig. 4. The machining of each groove
took ~2 h. Adequate crossovers for the conductors were
chosen and the conductor was compressed against the bottom
of the groove for an accurate positioning. The low
number of turns implies increased field errors due to positioning
errors of only one turn. The final assembly is
shown in Fig. 5.
Fig. 3. Milling machine working on toroidal coordinates.
Stellarator News -4- December 2008
Validation and operation of UST_1
Field line mapping
The traditional fluorescent movable rod method was considered
[5, 6] and implemented. A very simple mechanism
was used, composed of fluorescent ZnO deposited on a
copper wire of 1.5 mm diameter and ~120mm length that
is balanced as shown in Fig. 6. A short external magnetic
pulse is applied to a tiny ferromagnetic piece fixed on one
end of the wire so it oscillates at a frequency compatible
with the frame rate of the digital camera (30 fps) and the
length of the pulse (~2 s at B0 = 46 mT). Two e-guns were
built following the method used in CNT [7] and a commercial
FireWire camera was installed.
Figure 7 shows the experimental setup and Fig. 8 a sample
experimental image, obtained from pulses 198 and 202.
The image is a superposition of frames at 30 fps.
Fig. 4. Grooves for the coils in a plaster toroidal surface.
Fig. 5. Top view of UST_1. 12 modular coils located in
machined grooves.
Fig. 6. The oscillating wire used for the flux surface measurements,
showing the pivot and the ferromagnetic piece
(right). The wire is not yet coated in this photo.
Fig. 7. Field mapping experimental setup.
Stellarator News -5- December 2008
The experimental conditions during pulses 198 and 202
were B0 = 34 mT, e-beam 84 and 95 eV respectively, vacuum
3 103 Pa, plasma flat top = 2.5 s, date 24 August
2006. During pulse 204 the e-gun filament failed and field
mapping was concluded.
The recording of such low-quality images was really difficult.
Some of the difficulties were:
Collision of the beam with the rear part of the e-gun
after the third turn of the beam due to ~ 1/3. A very
small diameter e-gun and accurate strategic positioning
of the e-gun were required to solve the rear collision.
The size of the e-gun (only 8 mm external diam),
makes it a tiny fragile device.
Inadequate vacuum level resulting in an electron
mean free path corresponding to 10–100 beam turns.
Low sensitivity of the digital commercial camera,
which meant that maximum energy of electrons is
favorable for higher brightness but large drifts appear
because of the small size of UST_ 1 and low B0, see
Fig. 9.
Main conclusions from field mapping
A notable degree of accuracy has been achieved in the
design and construction of the stellarator and in the simulation
code, because magnetic surfaces agree with the
designed surfaces.
Adequate accuracy was obtained because the whole process
from the conception to the manufacturing and assembling
(type of stellarator, type of conductor, method to
build modular coils, etc) has been successful and suitable
to the technical and budget constrains.
Heating system and plasma pulses
The objective was to measure Te and ne for certain power
absorbed in the plasma to roughly compare with calculations
from scaling laws.
A simple heating system based on a commercial microwave
magnetron at 2.45 GHz was installed. The microwave
power is transmitted into the vacuum vessel (VV) by
a coaxial cable and an antenna because the small size of
the VV port hinders the use of a waveguide for 2.45 GHz.
The system is composed of a waveguide to coaxial adapter
located inside the microwave oven (for extra safety), a
dual directional coupler, two biased Schottky diode detectors
to measure the forward and reflected power, a double
stub microwave tuner to tune impedances, a ¼ lambda stub
antenna, two microwave leak detectors, and RF cables and
connectors.
The data acquisition system receives signals from: one
inverted magnetron for vacuum measurement, current
(voltage) in the modular coils, two signals from the ECRH
Schottky diode detectors, and two signals from the Langmuir
probe.
Fig. 8. Comparison of experimental field mapping results
(cyan points) and simulated points (circles). Note: Some
experimental points faded due to the superposition process,
but they are visible in the original frames.
Fig. 9. Magnetic surfaces for: (a) electrons at ~90 eV (red
points, large drifts) and, (b) electrons at ~0 eV (yellow, no
drifts). The simulated electrons are emitted from the same
point in both cases.
Stellarator News -6- December 2008
Pulses from 248–260 produced acceptable plasmas. In
particular, the chronogram for pulse 251 is shown in Fig.
10. The conditions during this pulse were B0 = 4 6mT,
plasma flat top = 2 s, pressure before the pulse = 2 mPa.
The pressure (red line) increases immediately after the
plasma starts, probably as a result of desorption of gases
from the walls. The difference between the RF forward
and reflected power is small, indicating poor coupling,
although the absorbed power is higher during the pulse.
The plasma is generated only when the TF current is ON
even if the RF power is kept on longer. It suggests that, at
least partially, heating is produced by resonance at the second
harmonic.
Pulses from #261 to #265 additionally recorded the signal
from a Langmuir probe located at ~0.5a from the magnetic
axis. The experiments stopped at the end of June 2007 and
improvement of the Langmuir experiments was not carried
out. Only relatively inaccurate data were obtained.
Summary and comments
The construction of a modular stellarator was decided on
as a good candidate for a low-cost fusion device. Other
alternatives were abandoned due to excessive cost or technical
difficulties. An innovative construction method was
devised, a milling machine working on toroidal coordinates
able to create a groove in only one poloidal turn and
able to create all the grooves in a single plaster frame. It
notably increases accuracy and avoids the need for 3D
adjustment of the coils.
The low cost is achieved by the use of inexpensive materials
and the particular construction of the coils. A few turns
of conductor are compressed inside the accurate grooves,
therefore simplifying the winding process.
The validation of the correctness and precision of the
design has been achieved up to a certain degree. Field
mapping experiments have been carried out. Even if the
quality of the experiments is poor, mainly due to economic
limitations, agreement between the simulations and the
experimental results indicates that a notable degree of
accuracy in the magnetic field has been achieved.
The comparison of the experimental data with the theoretical
energy confinement time has only been started, but the
preliminary results are promising. Nevertheless the scaling
laws might be inapplicable to such a small stellarator, and
besides the absorbed ECRH power is unknown.
Despite the innumerable difficulties that appeared during
the project a very low cost stellarator fusion device of
acceptable quality has still been possible.
The experiments stopped in June 2007 due to a change in
the labor conditions of the author. My interest now has
somewhat shifted to the development of innovative
designs to lower the construction and downtime cost in
fusion reactors. Small-scale devices of industrial reactorlike
structure would probably be useful to improve the
conceptual designs and maintenance procedures for future
reactors.
Presently a small stellarator is being developed in the
Instituto Tecnológico de Costa Rica in order to learn and
experiment with magnetically confined plasmas and, particularly
in fusion technologies, simulation and plasma
physics. The general idea of the stellarator is similar to the
UST_1 but it is aimed an improvement in the plasma volume,
heating, and number of diagnostics [8].
Further details about the conception, design, constructive
methods, manufacturing, JAVA code, field mapping,
plasma pulses, photos, and videos are available at http://
www.fusionvic.org
Acknowledgement
The author thanks the companies, friends, and researchers
who altruistically contributed to the UST_1 development,
and the friends and relatives who patiently endured my
concentration on UST_1.
Vicente M. Queral
Laboratorio Nacional de Fusión
CIEMAT, Spain
E-mail: vicentemanuel.queral@ciemat.es
(During UST_1 project: Personal laboratory, Spain)
References
[1] Stellarator News issue 92, May 2004.
[2] Craig Beidler et al., “Physics and engineering design
for W7-X,” Fusion Technol. 17, 148 (1990).
[3] G. J. Hartwell, S. F. Knowlton, et al., “Design and construction
progress of the Compact Toroidal Hybrid,”
13th International Stellarator Workshop, 2002.
[4] T. J. Dolan, Fusion Research, Pergamon Press, 1980, p.
398.
Fig. 10. Chronogram for pulse 251.
Stellarator News -7- December 2008
[5] X. Sarasola, T. Sunn Pedersen, et al., “Field line mapping
results in the CNT stellarator,” 32nd EPS Conference
on Plasma Physics, Tarragona, 27 June–1 July
2005, ECA 29C, P-1.058, 2005.
[6] M. Otte and J. Lingertat, “Initial Results of Magnetic
Surface Mapping in the WEGA Stellarator,” 29th EPS
Conference on Plasma Physics and Controlled Fusion,
Montreux, 17–21 June 2002, ECA 26B, P-5.036, 2002.
[7] Personal communication from X. Sarasola, 2006.
[8] Information from Dr. Iván Vargas, Instituto Tecnológico
de Costa Rica, 2008.
Retirement ceremony and
Stern-Gerlach medal for
Friedrich Wagner
More than 180 invited guests celebrated the retirement of
Prof. Friedrich (“Fritz”) Wagner (Fig. 1) at the Greifswald
branch of the Max-Planck Institut für Plasmaphysik on
Thursday, Nov. 27. After early works at the Tokamak Pulsator
in Garching, Friedrich Wagner discovered the Hmode
on the ASDEX tokamak; his corresponding paper
“Regime of Improved Confinement and High Beta in Neutral-
Beam-Heated Divertor Discharges of the ASDEX
Tokamak” [1], remains the most cited plasma physics
paper ever. In 1986 he became the head of the ASDEX
project and was appointed Scientific Member and Director
of the Max-Planck Institute für Plasmaphysik. In 1993,
soon after he moved to the stellarator field and became
responsible for Wendelstein 7-AS, he showed that the Hmode
regime could also be achieved for the first time in a
non-tokamak device, thereby demonstrating the universality
of this toroidal confinement regime. After moving to
the new Institute branch in Greifswald, Wagner was the
head of the Wendelstein 7-X (W7 -X) project from 2003 to
2005. In his role as the speaker of this second IPP branch
(1999–2007) he managed the embedding of the new fusion
research site into the frame of the old Hanseatic and university
town, Greifswald. Since 1999, Wagner has been
lecturing as a Professor of the Ernst Moritz Arndt University.
In 2007 he became the President of the European
Physical Society (EPS). As recently announced, Prof.
Friedrich Wagner will receive the Stern-Gerlach medal,
which is the highest award of the Deutsche Physikalische
Gesellschaft (DPG) given for extraordinary contributions
in experimental physics.
The celebration colloquium was chaired by the scientific
director of IPP, Prof. G. Hasinger. The first scientific talk
was delivered by Prof. K. Itoh, “Quarter Century of Hmode—
A view of theorists,” and complemented Wagner’s
Hannes Alfvén lecture at the EPS conference 2007 “25
years H-mode research—an experimentalist’s view” [2].
Referring to the fact that the total cost of a fusion reactor
scales with the H-factor as ~H1.3, which for H ~ 2 yields
a cost reduction of about 60%, Itoh finally concluded that
“he brought the sun closer to earth.” In a more personal
review, Prof. Th. Klinger, now scientific head of the W7-X
project, spoke on building science by indicating parallels
with the history of German enlightenment and the creativity
in building devices as well as theories, publications and
paintings—such as Friedrich Wagner did. Prof. Pinkau, for
a long period the scientific director of IPP, reminded the
audience that a long-term commitment and measure of
anticipation have been, and still are, the basis for successful
fusion research. Finally, regards were also given by the
Faculty of Physics connected with thanks for the mutual
support of the plasma physics institutes in Greifswald.
Such an event is also a good occasion for presents: As
Fritz is known to relax by playing cards with others, the
Wendelstein team gave him a collection of self-made
games—such as a memory game with pictures of all his
co-workers, said to be for keeping his brain sharp. Furthermore,
a mock-up of the plasma experiment WEGA and an
artist’s impression of W7-X were presented to Friedrich
Wagner. Finally Friedrich Wagner himself gave an overview
of the structures, teams, and people that over the
years have developed all around him (many of them
attending the ceremony) and gave thanks to all those who
supported and contributed to his efforts.
Fig. 1. Friedrich Wagner giving a sketch of the main development
lines in 33 years of fusion research. (Photo: F.
Noke)
Stellarator News -8- December 2008
After a reception the celebration continued with the opening
of the exhibition “meer und mehr” (“The sea and
beyond it”) giving an overview of Fritz Wagner’s paintings
from the last few years (Figs. 2–4). On his initiative the
bright central axis of the Greifswald IPP building has been
regularly used for exhibitions that bring together inspiring
arts and everyday research. For most of the guests and coworkers
the ceremony continued next day when Fritz
Wager spoke in the IPP colloquium on the main lines that
developed—and still guide—”33 years in fusion research.”
Matthias Hirsch and Beate Kemnitz
Max-Planck Institut für Plasmaphysik
Greifswald, Germany
E-mail: matthias.hirsch@ipp.mpg.de
References
[1] F. Wagner et al. “Regime of Improved Confinement
and High Beta in Neutral-Beam-Heated Divertor Discharges
of the ASDEX Tokamak,” Phys. Rev. Lett 49,
1408 (1982).
[2] F. Wagner et al., “A Quarter Century of H-mode Studies,”
Plasma Phys. Control. Fusion 49, B1 (2007).
Fig. 2. A good view of Wagner’s paintings, many of them
showing structures in nature. The two celebrants are Maurizio
Gsparotto and Lars Sonnerup. (Photo: B. Kemnitz)
Fig. 3. The vernissage of “meer and mehr” at the main gallery
of IPP, which is regularly used for exhibitions. (Photo:
F. Noke)
Fig. 4. “Kurzer Moment der Reflexion / Short moment of
reflection” oil painting by F. Wagner.This picture was used
on the poster for the exhibition.