Quenching Marital Bliss
Niobium, mined in Brazil, needs to be exquisitely purified. It is one of
26 metals in the periodic table with natural superconducting properties.
Below a critical temperature (9.2 Kelvin for niobium) electrons in a
superconducting metal begin to pair up in happy couples called Cooper
pairs. Cooper was one of three physicists who received the Nobel Prize
for explaining this effect. The colder you make the niobium below 9.2
Kelvin, the more pairs form.
“The electrons get ‘married’ and they behave in a different way,” said
Greg Loew (DO). “They flow freely through the cold niobium, meeting with
absolutely no resistance to their motion. Only the remaining unpaired
electrons, the bachelors, still feel a little residual resistance. If
the critical temperature is exceeded, all couples undergo instant
Couples face one more problem: Even at low temperatures, a strong
magnetic field will quench the superconducting properties. Niobium has a
high quenching field—it takes a magnetic field of 1750 Oersteds to
extinguish its superconductivity. That is crucial in an accelerator
because the longitudinal electric fields which propel the particles are
always surrounded by circular rings of magnetic fields.
“Thus it’s inevitable that magnetic fields will eventually limit the
cold approach to a certain accelerating gradient,” Loew said. “The
secret is how to design the cavity shapes so that the magnetic field
which accompanies the electric field is as low as possible.”