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 divorce."

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