A schematic of ITER’s central solenoid assembly. Credit: Oak Ridge National Laboratory |
Scientists and engineers at the Spallation Neutron
Source at Oak Ridge National Laboratory are working with the U.S. ITER Project
Office at ORNL, the Japanese Atomic Energy Agency, and the ITER Organization to
resolve issues with a critical component of the experimental fusion energy
facility ITER. The VULCAN Engineering Diffractometer at SNS is being used to
examine superconducting cables for ITER’s central solenoid magnet, which
induces the electrical current needed to confine and shape the plasma inside the
reactor.
ITER is the international research facility in
southeastern France
whose mission is to demonstrate the feasibility of fusion as a practical
long-term energy source. SNS, located a few miles down the road from the U.S.
ITER Project Office, is the world’s most powerful pulsed neutron source.
Ned Sauthoff, U.S. ITER project manager, said the VULCAN
measurements have provided useful data and that VULCAN will have a role in the
continuing investigation. “Having seen the initial results, I think we
have sufficient evidence to state that initial measurements have demonstrated
the capability of VULCAN to measure important material properties, that
meaningful results were achieved, and that the team is now focused on using the
unique capability to gain important understanding aimed at solving the
problem,” Sauthoff said.
The central solenoid, a joint Japan-U.S. ITER
responsibility, is on a tight schedule. The superconducting cables, supplied by
Japan,
cost more than $3,000 per meter. Improving the cable performance by reducing
the degradation of the superconducting strands is important to staying on
schedule and on budget. “We are working on an important problem that will
have an immediate impact on science and technology on an international scale,”
said Ian Anderson, head of ORNL’s Neutron Sciences directorate, which operates
SNS.
The team of Japanese, U.S. and ITER Organization
engineers discovered in late 2010 in a sample test that the superconducting
cables making up the central solenoid magnet at the core of the ITER design
were losing their current-carrying capacity over time to an extent well beyond
that experienced in an earlier ITER model coil test. The cables can generate a
magnetic field as strong as 13 tesla, and the electromagnetic (Lorentz) force
exerted on the wires by the high magnetic field and powerful current is known
to cause some degradation over a period of constant magnetic cycling. The exact
cause of the degradation in the conductor sample is unknown. In addition to the
Lorentz force, it may also be attributable to the sample manufacture or the
particular sensitivity of the wires to the loads. The magnet team at the U.S.
ITER Project Office in Oak Ridge
consulted with scientists at SNS about using neutron scattering to examine the
states of materials inside the cables. The samples examined at SNS are sections
1.65 inches in diameter and several inches in length cut from the much longer
cables. The cables have a complex structure—copper wires interspersed with
superconducting wires of a niobium-tin alloy—all contained in a stainless steel
tube.
Neutrons are highly penetrating and nondestructive, so
neutron scattering can return detailed data about the structure of the cable
sections without destroying or altering them. SNS is the ideal facility for
studying the thick cables because it has the most intense neutron beams of any
pulsed neutron source in the world, said VULCAN instrument scientist Xun-Li
Wang. And VULCAN is the ideal instrument because it is designed to handle large
industrial-sized specimens rather than small lab samples.
The multi-phase material making up the cable is perfect
for characterization by a time-of-flight diffractometer such as VULCAN, Wang
noted.
“Neutron diffraction is a well-known technique for
mapping strain or stress in engineering materials,” Wang said. “With
VULCAN we will be able to determine the deformation induced by the Lorentz
force. “On a fundamental level, we can also study in detail how the
critical current in a superconducting wire responds to applied stress and
develop a predictive model for the wires.”
A plan for future study is being developed with the
Japan Atomic Energy Agency, which is responsible for central solenoid conductor
fabrication and the ITER Organization.