Why cryogenic copper structures sustain much higher accelerating gradients
In plain words
Copper structures cooled to about 45 K have held fields roughly twice as strong as room-temperature ones, and the reason is not settled.
Precise statement
X-band Cu cavities cooled to $T \sim 45-77\,\mathrm{K}$ reached accelerating gradients near 2.5 MeV/cm at low breakdown rate (SLAC tests reported 2018). Determine whether the gain comes from higher yield strength and reduced dislocation mobility, from lower surface resistance and reduced pulsed heating, or from another effect, by predicting the temperature dependence of the breakdown threshold between 20 and 300 K for fixed geometry and pulse length.
What would settle it
Breakdown-threshold measurements on one structure geometry at several temperatures between 20 and 300 K compared with competing models that are fixed before the data.