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 Oak Ridge National Laboratory
Building CR 5600 P.O. Box 2008 Oak Ridge, TN 37831-6169, USA
Editor: James A. Rome Issue 146 December 2014
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
Installation of magnetic diagnostics
for W7-X completed
This summer, the integration of the magnetic diagnostics
for Wendelstein 7-X (W7-X) operation phase 1 (OP1) into
the core device was completed. The entire set is shown in
Fig. 1. The magnetic equilibrium diagnostics are
 an almost symmetric set of 40 saddle coils on the outside
of the plasma vessel (PV) (black in Fig. 1),
 three Rogowski coils in triangular planes (two on the
outside of the PV, one inside the PV, green in Fig. 1),
 five sets of poloidal magnetic field probes (also called
segmented Rogowski coils), two of which are in a triangular
plane and three in near-bean-shaped planes
(most sets consist of a subset on the outside of the PV
and a subset inside the PV in matching positions; blue
in Fig. 1), and
 three diamagnetic loops inside the PV, two in nearbean-
shaped planes and one (with compensation
coils) in a triangular plane (violet in Fig. 1).
In addition, there are 125 Mirnov coils, most of them
arranged to form one partial and three complete poloidal
arrays in triangular planes (red in Fig. 1).
In this issue . . .
Installation of magnetic diagnostics for W7-X
completed
Design, manufacture, and installation of the numerous
diagnostic coils for Wendelstein 7-X has been completed.
In addition to proper signal detection, these
coils must survive thermal load and not host large
eddy currents that would mask the signals they are
supposed to detect. ................................................. 1
Fig. 1. Complete set of W7-X magnetic diagnostics. For color code see text. The numbering of the 10 half modules (HM) of
W7-X is indicated. The pick-up coils shown in lighter shades of color are additional coils which may be installed at a later
stage.
HM 10
HM 11
HM 20
HM 21
HM 30
HM 31
HM 40
HM 41
HM 50
HM 51
Stellarator News -2- December 2014
Challenges in the design of the W7-X magnetic
diagnostics
Because classical pick-up coils were chosen for the magnetic
equilibrium diagnostics, their signals must be integrated
in time to know the magnetic flux at each moment
of the discharge. Although offset voltages in the coil circuits
can be determined before the start of a plasma discharge,
even a small change or drift in these voltages will
affect the integrated signal strongly in a long-pulse discharge
(W7-X is designed for discharges of up to half an
hour). It was demonstrated that the required long-pulse
accuracy can be achieved in the electronics part of the coil
circuits [1], by using a chopper stage at the front end,
which periodically reverses the polarity of the input signal.
Drifts of offset voltages in the rest of the coil circuit (e. g.,
thermovoltages) can not, as a matter of principle, be distinguished
from “true” signals and must still be kept to a
minimum. The integration is performed numerically after
digitization of the signals.
The pick-up coils located inside the PV can be exposed to
a significant level of stray 140 GHz electron cyclotron resonance
(ECR) radiation, since ECR heating (ECRH) is the
main heating system of W7-X, and some of the heating
scenarios work with low single-pass absorption. In order
to achieve a high winding density, classical polyimideinsulated
winding wire was used for the in-vessel equilibrium
diagnostics. The microwave absorption of polyimide
is high, and the windings must be shielded from the stray
ECR radiation.
An additional thermal load on the in-vessel magnetic diagnostics
originates from the back faces of the wall protection
elements, which are located between the plasma and
the PV wall and which will receive plasma radiation and
convective loads from the edge plasma.
To summarize, the harsh environment inside the plasma
vessel requires a design providing good microwave shielding,
low microwave absorption of the shielding, and either
active cooling or good thermal contact with actively
cooled components, or else the capability to operate at
high temperatures (of the order of several 100 C).
In the design of the ECR stray radiation shielding and the
heat conductors, these structures must not strongly influence
the signals recorded by the respective pick-up coils.
Since Cu provides both ECR shielding at low absorption
and good thermal conduction, it is the preferred material,
but eddy current coupling to the windings would damp and
phase-shift the recorded signals too strongly above some
boundary frequency. In order to achieve a time resolution
of the order of 1 ms for the diamagnetic loop and its compensation
coils in the triangular plane and for the in-vessel
Rogowski coils, the heat conductors are arranged perpendicular
to the windings. The PV is used as the heat sink,
and the pick-up coils are attached to it by adapter blocks
with custom shaped surfaces designed using information
obtained from local laser scans of the PV surface.
An alternative approach is chosen for the Mirnov coils: A
high winding density is not desired for them, and a metallic
ECR shield would be particularly harmful due to the
high frequencies to be measured. Therefore, a Konstantan
® wire is wound on an aluminum nitride (AlN) coil former,
which is attached to the cooling pipes of the wall
protection elements by a CuCrZr clamp (see Fig. 2). The
coil former can tolerate heating to peak temperatures of
460° C, according to ANSYS® finite element simulations,
in spite of the high heat conductivities of AlN and CuCrZr.
During the engineering design activities, mock-ups and
prototypes of the different coil designs were tested in a
pair of Helmholtz coils to assess their electromagnetic
response, in the MISTRAL test chamber to assess their
microwave resilience [2, 3], and in a vacuum chamber
equipped with a special thermoelectric heater to assess
their heat conductance.
In-vessel continuous and segmented Rogowski
coils
In Fig. 3, an in-vessel Rogowski coil segment is depicted
during assembly. It consists of 10 solenoids, each with 2
winding layers on a stainless steel tube as core. The solenoids
are connected electrically in series. Each solenoid is
inserted in an outer stainless steel tube with 0.25 mm wall
thickness that serves as a stray ECR radiation shield. This
wall thickness is chosen to preserve sub-1 ms time resolution
and to allow bending the solenoid into proper shape.
The shapes were individually designed for a total number
of 452 in-vessel solenoids, due to the strongly threedimensional
(3D) shape of the PV. After solenoids were
Fig. 2. Mirnov coil attached to a cooling pipe of the wall
protection elements by a CuCrZr clamp (right). The Konstantan
® winding wires of the two winding layers continue
through a Cu pipe (toward the top right, parallel to the cooling
pipe). Inside the Cu pipe, the two wires are twisted and
insulated by silicate sleeves (visible in front of the CuCrZr
clamp).
Stellarator News -3- December 2014
bent, each segment was assembled on an individual rig
with the end and intermediate holders used to attach it to
the PV. Between the stainless steel shielding tubes, Cu
profiles are clamped; these will conduct the heat deposited
on the surface of the shielding tubes to the end and intermediate
holders, where it is transmitted to the PV through
the Cu adapter blocks. The heat conductance of the thin
stainless steel shielding tubes is entirely insufficient for
this purpose, but the use of tubes with sufficiently high
heat conductance in the longitudinal direction (e.g., made
from Cu with a larger wall thickness) would also increase
the electric conductance parallel to the winding (i.e.,
around the tubes) to intolerable values.
In one of the end holders, which is later closed by a Cu
cover for microwave screening, the winding wires are
crimped to signal cables, which run in Cu tubes and Cu
cable ducts to the vacuum feedthroughs at the ports, such
that continuous microwave screening for the polyimideinsulated
cables and wires is established.
Inside some of the core tubes, Pt1000 sensors are placed
for temperature surveillance in locations where the most
critical thermal loads are expected. For the continuous
Rogowski coils, segments of this type are lined up poloidally
in a closed circle with the poloidal gaps between
adjacent segments minimized. The individual segments in
this case are electrically connected in series. In the segmented
Rogowski coils, each segment is contacted individually
by its own signal cable. Whereas the continuous
Rogowski coils are optimized for measuring the total
plasma current through their openings, the segmented
Rogowski coils provide additional information on the
poloidal and also the radial distribution of the plasma current
density.
Diamagnetic loop with compensation coils
As a second example, the triangular plane diamagnetic
loop is shown in Fig. 4 in a folded state and after installation
inside the PV. The articulated design was chosen to
make it possible to complete the winding of the main loop
outside the PV and to transport the coil to its positions
inside the PV through a narrow port. The introduction of
multi-pin connectors between different sections had been
discussed as an alternative, but the thermovoltages at these
connectors were found to introduce intolerably large
uncertainties in long-pulse integration of the signals.
In the sections with compensation coils, the structural elements
are side walls laser-cut from a ceramic fiber-reinforced
ceramic (Keramikblech®) with 3 mm thickness
(see Fig. 5). The winding area of the compensation coils is
perpendicular to the toroidal direction. Therefore, eddy
currents in highly conductive side walls would reduce the
time resolution of these coils. The stray ECR radiation
shielding is therefore manufactured from 0.1 mm stainless
Fig. 3. Assembly of an in-vessel Rogowski coil segment on
a purpose-built rig. The surfaces of the supports at the end
and intermediate holders correspond to the surfaces of the
Cu adapter blocks to which the segment will be mounted in
the plasma vessel. The Cu heat conduction rails (lower left)
are ready for insertion between
the individual solenoids.
Fig. 4. Diamagnetic loop in fully folded state (top) and after
integration into the W7-X plasma vessel in a triangular
plane (bottom). This loop consists of four sections with
compensation coils and a short flat section with the main
loop only (to the left in the bottom view), where the available
space between plasma vessel and wall protection elements
was insufficient for a further compensation coil. In
the bottom view, the wall protection elements are not yet
installed.
Stellarator News -4- December 2014
steel sheets that are plated on the outside with 3 m Cu to
reduce the microwave absorption. Nevertheless, the heat
conductance of the thin stainless steel sheets is still insufficient.
Therefore, there is a 10 mm 4 mm cross-section
Cu heat conductor on each side, every 50 mm poloidally
along the loop that directly connects to a Cu adapter block
bolted to the PV. The high electric conductance in radial
direction does not affect the time resolution of the compensation
coil, since its winding runs in poloidal direction
for most of its length. Additional 1 mm Cu heat shields are
located toward the back faces of the wall protection elements,
such that thermal radiation from these elements
does not hit the 0.1 mm stainless steel ECR shielding.
Each of the Cu heat shields is directly attached to two of
the heat conductors on the two sides of the loop, and poloidal
gaps between the individual heat shields prevent their
high electrical conductance from jeopardizing the time
resolution of the compensation coils.
Outlook
Presently, the signal cables from the vacuum feedthroughs
to the positions of the electronic and data acquisition cubicles
are laid, and the electronics and data acquisition
equipment are prepared. As soon as these are functional,
the integration of the magnetic signals into the W7-X
machine control system will start.
Two calibration phases are foreseen: First, the signals generated
by changes in the field coil currents will be
recorded in order to be able to distinguish them from signals
generated by plasma currents during discharges. In
addition, it is planned to introduce specially designed
exciter coils into the plasma vessel, in particular to explore
the influence of eddy currents in in-vessel components on
the signals recorded by the magnetic diagnostics.
Acknowledgement
From the first drafts of the W7-X magnetics in 1999 to this
date, some 60–70 colleagues at the IPP Garching and
Greifswald sites contributed significantly to the realization
of these diagnostics, with the support of many staff at
external suppliers. Most of the time, the core team consisted
of 2–3 physicists, 1–3 engineers, 3–5 designers, and
2–5 technicians. Without the accurate work of all these
people it would not have been possible to build these complex
diagnostics and to integrate them into the W7-X
device.
References
[1] A. Werner, Rev. Sci. Instrum. 77, 10E307 (2006),
[Proc. 16th Topical Conf. on High-Temperature Plasma
Diagnostics, Williamsburg, VA, USA, 7–11 May
2006].
[2] M. Hirsch et al., The impact of microwave stray radiation
to in-vessel diagnostic components, in Proc. of the
Int. Conf. on Fusion Reactor Diagnostics (9–13 September,
2013, Varenna), 2013.
[3] D. Hathiramani et al., Fusion Eng. Design 88, 1232
(2013), [Proc. 27th Symp. Fusion Technology (SOFT-
27), Li ege, Belgium, 24–28 September 2012].
Michael Endler for the W7-X Magnetics Team
E-mail: michael.endler@ipp.mpg.de
Max Planck Institute for Plasma Physics,
Wendelsteinstr. 1,
17491 Greifswald, Germany
Fig. 5. Assembly of a compensation coil section of a diamagnetic
loop. The side walls of 3 mm ceramic fiber-reinforced
ceramic (white) are covered on the outside by 0.1
mm stainless steel sheets with a 3 m Cu layer. The windings
are formed by 25-wire ribbon cable with the individual
wires connected in series. The pivot connecting to the next
section is visible at bottom left. The winding of the compensation
coil is already installed; the winding of the main loop
(one further layer of ribbon cable) is installed once the sections
of the loop are connected. Finally, the opening
between the side walls is closed by further 0.1 mm stainless
steel sheets with a Cu layer. At the bottom of the section,
the heat conduction rails/brackets of 4 mm Cu are
visible They provide for attachment of the entire structure
to the PV via Cu adapter blocks. The heat conduction rails/
brackets on the top are not yet installed.