Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

ed multiple times with no degradation. This would enable superconducting research facilities in
which major components could be readily tested and replaced, and enhance maintainability, availability
and inspectability of a demo. This is a nontrivial task, since not only should there be excellent
electrical connection, but also structural, cooling, and insulating connection. significant
resources are thus warranted to achieve this challenging goal.
Quench Detection and instrumentation
Quench detection is the achilles’ heel of a superconducting magnet in an erratic pulsed field environment.
The specific weakness of tokamak magnets is the plasma disruption, which is unscheduled
and varying, making it impossible for its signal to be completely zeroed out predictively. arbitrary
reliability can be built into the power supply interrupters through series connections and
redundancy. however, this is much harder to do for quench detectors, which are built into the
coils with signal and noise ratios that are intrinsic properties of the sensors. a simplified way of
stating the problem is that the magnet voltages are on the order of 10 kv, while the quench signals
desirable to detect are on the order of 100 mv, implying a need for five orders of magnitude in
noise rejection. various methods of quench detection needing further development include balanced
voltage taps on coil segments, co-wound voltage taps for intrinsic inductive signal cancellation,
and co-wound optical fibers that can sensitively measure temperature and strain over a wide
range of operating conditions.
The following goals should be achieved:
• demonstration of standoff voltages in voltage sensors of > 500 v, including helium
infiltration over a range of partial pressures and over a range of quench/dump temperatures.
• demonstration of leak-tightness of sensor extraction ports over experiment-relevant number
of cool-down cycles. Goal should be a leak-tightness of < 10 -6 torr after 50,000 cycles.
• The intrinsic strain rejection of a fiber-optic thermometer, before filtering and advanced
signal processing, should be reduced by a factor of 10 3 , over the range of 4 k-150 k. in other
words, a change in the conductor strain of 0.1 % should change the effective differential
optical path by no more than 1 ppm. The higher the critical temperature of a superconductor,
the more attractive fiber optic thermometry becomes for quench detection, because of the
rapidly improving temperature sensitivity of the index of refraction.
• demonstrate feasibility of commercial fiber use with an hts magnet.
Prototype Magnet Development
although lab scale tests and component development can lead to viable solutions for the issues
discussed above, integration of these components into a magnet is nontrivial, and may lead to
complications and synergistic effects, resulting in a magnet that is unable to achieve all its design
goals. Therefore, once the oFes strategic planning process identifies a next-step Us device,
the design goals should be then focused on those required by the device concept. Then all development
steps can be proven on a relevant-scale prototype coil or coils, and tested under fullscale
operating requirements. depending on the scale of the magnet, existing facilities for testing
should be used or modified to carry out the test program.
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