As far as I can understand superconductivity, the devices are similar but opposite. Superconductors have a both a critical current (in the absence of a field),

As far as I can understand superconductivity, the devices are similar but opposite. Superconductors have a both a critical current (in the absence of a field), and a critical magnetic field (in the absence of a current). So, cranking the control line raises the field, which squelches current capacity to zero (and/or since there's an excess of either/both, superconductivity is lost).