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Published by Fusion Energy Division, Oak Ridge National Laboratory
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Editor: James A. Rome Issue 132 June 2011
E-Mail: jamesrome@gmail.com Phone (865) 482-5643
On the Web at http://www.ornl.gov/sci/fed/stelnews
Wendelstein 7-X News,
April 2011
Current leads enter serial production
Wendelstein 7-X (W7-X) will use superconducting modular
coils to generate the confining field. This will enable a
discharge length of 30 min.
Connecting the superconducting coils to their power supplies
represents a considerable technical effort. The current
leads provide the electrical connection, as well as a
transition from cryogenic temperature to room temperature.
In operation, the W7-X coils will have to conduct
18,200 A at 269°C. In the case of a sudden shutdown, the
components have to cope with voltages of several thousand
volts; they have been tested up to 13,000 volts.
Staff at Karlsruhe Institute of Technology (KIT), in collaboration
with the Swiss CRPP Institute, have been able to
meet these extremely difficult specifications. In 2003 they
produced a demonstration current lead for ITER. Figure 1
shows a photograph of a prototype current lead, based on
the ITER design but developed specifically for W7-X, that
passed all tests at the end of last year. The core is a hightemperature-
superconductor (HTS)—a material that
becomes superconducting at comparatively “high” temperatures
around 163°C. However, due to the high currents
and magnetic fields in W7-X it will be kept below 
213°C. The advantage of using ceramic HTS is to provide
a junction that combines practically zero electrical resistance
with good heat insulation.
As strange as it sounds, the cooling of the HTS module
uses the heat conduction in the materials. The “cold” side
is connected to components that are cooled to 269°C by
liquid helium. The “warmer” side is kept at 223°C by
helium gas fed into a heat exchanger. The external connection
can be kept at room temperature during operation.
One special feature of the newly developed current leads is
that they allow the cold end of the connection to be at the
top. In contrast to all previously constructed current leads,
in Wendelstein 7-X the current enters the machine from
below. This increases the effect of convection, which
transports heat upwards and could compromise the function
of the heat exchanger. The design has been modified
to suppress convection, and the recently completed tests
showed that the new W7-X design can be implemented
successfully.
A total of 14 serially produced current leads will be manufactured
and tested at KIT by the end of 2012. The first
serially produced pair successfully passed the acceptance
test in late April 2011. Assembly of the current leads in
W7-X is planned by IPP as a cooperation with Oak Ridge
National Laboratory (see Fig. 2). Assembly is facilitated
by the short distance from the coils to the heavy-duty
power supplies, which are located in the basement below
W7-X.
Fig. 1. W7-X current lead prototype developed at KIT and
IPP. During operation, the left side is at room temperature,
while the right side is connected to the superconducting
coils. The heat exchanger is based on a development from
CERN. Photo: KIT.
In this issue . . .
Wendelstein 7-X News, April 2011
Four of five modules are now in their final position on
the machine foundation. The current leads, which
must make the transition from superconducting temperature
to room temperature, have been successfully
designed, tested, and are now being manufactured. 1
Stellarator News -2- June 2011
The results of the tests for the prototype current leads
developed at KIT are so impressive that current leads
using the same design are planned for other fusion experiments.
Starting in 2013 these current leads will be built
into the superconducting tokamak JT-60SA, a joint project
between the EU and Japan.
Wendelstein 7-X status
For the most part, the assembly of W7-X is progressing on
schedule. Four of five modules are now in their final position
on the machine foundation. One of the biggest
remaining tasks is mounting the 254 ports. The challenge
here is to keep the deviations from the intended placement
within the allowed limits. This requires somewhat more
effort than originally assumed; however, the use of time
buffers means that deadlines remain unaffected. An overall
view of the W7-X torus hall is shown in Fig. 3.
M. Hirsch
Max Planck Institut für Plasmaphysik (IPP)
Greifswald Germany
E-mail: Matthias.Hirsch@ipp.mpg.de
Fig. 2. CAD diagram of the assembly procedure for the current
leads. The assembly ramp, marked in yellow, guides
the current leads into the W7-X cryostat from below.
Fig. 3. The W7-X torus hall, with 4 modules in their final position on the machine foundation.
Photo:Tino Schultz