VIPER: an industrially scalable high-current high-temperature superconductor cable is the 2020 Superconductor Science and Technology paper in which 📝Zachary Hartwig and 28 co-authors qualified the conductor that 📝SPARC's magnets are wound from.
The problem it solves is not that 📝REBCO tape carries current — that was known — but that a fusion magnet demands a great deal more of a conductor at once. It must not degrade under enormous Lorentz forces, must survive repeated cooling to 20 kelvin and back, must join to its neighbors through low-resistance connections that can be taken apart, and must announce a 📝quench before the stored energy destroys the coil. Earlier cables satisfied some of these. The 📝VIPER cable — vacuum pressure impregnated, insulated, partially transposed, extruded and roll-formed, which is what the acronym unpacks to — satisfied all of them together for the first time.
The qualification is the substance of the paper, and it is experimental rather than predictive: 📝critical currents held stable across thousands of mechanical cycles at extreme electromechanical loading, across multiple cryogenic thermal cycles, and through dozens of quench-like transients. Two independent integrated fiber-optic quench detectors outperformed conventional voltage-based detection. That last result matters beyond this cable, because a 📝no-insulation coil cannot be protected the way a conventional one can — and at 20 📝tesla there is no second chance at protection.
The 2024 successor is 📝PIT VIPER: A Low-AC-Loss Cable for Pulsed Magnets, which takes the same cable architecture into service where the field changes fast and 📝AC loss, rather than critical current, becomes the limiting term.
VIPER is the conductor that made the 📝Toroidal Field Model Coil (TFMC) possible a year later. Everything we have built at 20 tesla is wound from this cable or its descendants.