C. Dobrowolski
Fabrico Inc.
Atlanta, Ga.
A challenge faced by Princeton Plasma Physics
Laboratory (PPPL) researchers was to devise both
electrical isolation and structural support for conductive coils that will see 2 kV when the National
Compact Stellarator Experiment (NCSX) goes live in
2009. The NCSX produces hot-ionized gases or plasmas confined by a magnetic field. The field is generated by 18 coils made by winding copper cables
onto a stainless-steel form.
When the gases reach the right temperature and
density, two nuclei fuse together forming a heavier
nucleus. The process releases vast amounts of energy. Harnessing this energy provides an inexhaustible electric power source that is safe and environmentally friendly.
COILING UP
The flexible copper cables are made from over
3,000 strands of 0.0063-in.-diameter wire compacted
into a rectangular shape with a cross section of
less than 0.15 in.2 The cables wind onto the forms
in rectangular packs four layers wide and 10 or 11
layers high. The coils must be electrically isolated
from the stainless-steel winding form to prevent
shorting.
The coils also need a stable support. Unrestrained
coiled copper cables would "come alive" when energized and move relative to each other. Even the
smallest movement is bad because the magnetic
field in the Stellarator device depends, in part, on
the shape and position of the coils. Movement could
also damage the coils and shorten their life.
Two critical elements provide the required electrical isolation and stability. One is an electricalinsulation material (ground wrap) wrapped around
the copper cables. The other is a hard epoxy that
encases the flexible cable-and-insulation assembly
in a solid structure and gives some additional electrical insulation.
For the insulation material, designers had a choice between Mylar and Kapton tape. Both provide
needed electrical isolation. But Kapton, a polyimide
film, better withstands cryogenic temperatures during NCSX operation without turning brittle or losing
critical properties. Temperature was also a factor in
selecting epoxy. It must hold up under liquid-nitrogen operating temperatures without cracking.
Although Kapton has enough insulation for the copper cables, it posed a design challenge. The polyimide
film won't absorb epoxy and would remain flexible,
It was also not thick enough (0.005 in.) to insulate
adequately. Fabrico Inc., helped solve these issues.
Fabrico is a contract design and manufacturing firm
specializing in the conversion of flexible materials. Its
engineers helped develop a conservative ground wrap
that not only meets insulation requirements for normal operation
at 2 kV but also withstands errant
voltage spikes as might come from
a fault in the electrical system.
The ground wrap consists of
a fiberglass layer attached to the
Kapton tape. Fiberglass easily
absorbs epoxy and helps it flow
into the innermost parts of the
coil. Once hardened, the epoxyreinforced fiberglass on the Kapton tape provides the required
structural support for the coils.
Fabrico is one of the few fabricators able to join Kapton tape and
fiberglass. S-glass was chosen for
the NCSX coils. It is stronger and
less expensive than E-glass, and
is treated in a way that improves
its ability to absorb epoxy. S-glass
also has a slight edge over E-glass
in withstanding exposure to radiation. This is important because
fusion devices such as NCSX are
radioactive during operation.
Conservative insulation design
called for extra-thick 5-mil (0.005-in.)
Kapton tape. But 5-mil Kapton tape
is exotic and particularly difficult to
obtain. As an alternative, Fabrico
suggested laminating together two
layers of more readily available
2-mil (0.002-in.) Kapton tape. Any
current spikes would generate magnetic forces on the coils. Wraps had
to allow for such effects by adding
structural reinforcement.
The insulation design required
ground wrap to go on the coil
winding form in multiple halflapped layers — each strip covering roughly half of the strip laid
down before it. With the ground
wrap so positioned, the fiberglass
layer had to be wider than the
Kapton tape to expose the glass.
If fiberglass didn't extend past the
ends of the tape, the overlapping
of the insulation layers might
have made it difficult for epoxy to
reach the glass. This would have
reduced structural stability.
The ground wrap went on the
coil winding forms with the Kapton side facing the copper cables.
One end of each insulation strip
attached to the winding form, while the other end was
temporarily free. The copper cables were wound
onto the steel forms over
the top half of the insulation strips. With the cables
in place, the free ends of
the insulation strips were
folded up and brought together with the attached ends so
the ground wrap completely covered the cables.
In the final step, a tight-fitting
rubber mold enclosed the copper
cable and ground wrap. This assembly then went in an autoclave.
Under vacuum, technicians filled
the mold with epoxy. Over 8 hr the
epoxy worked its way between the
strands of the copper cables, completely wetting the insulation materials, and filling the voids around
the coiled cable.
The epoxied assembly cured
in the autoclave or in a vacuum
oven at 110°C for 5 hr followed by
15 hr at 125°C.
MAKE CONTACT
Fabrico Inc., (800) 351-8273,
fabrico.com