Breakdown limit of RF cavities inside strong solenoid fields
In plain words
A muon collider needs accelerating cavities placed inside very strong magnets, and such cavities spark at lower fields than without the magnet.
Precise statement
Vacuum normal-conducting cavities at $\sim 200\text{-}800\,\mathrm{MHz}$ in axial magnetic fields $B \sim 3e4\,\mathrm{G}$ show reduced maximum gradient, attributed to field-emitted electrons focused by B onto the walls, causing local heating. Establish the scaling of the safe gradient with B, frequency and wall material up to $B \sim 1e5\,\mathrm{G}$ and above, as required for 6D muon ionization cooling channels.
What would settle it
Gradient-limit measurements versus B up to the cooling-channel field for at least two wall materials and frequencies, compared with an electron-focusing heating model.
Status in the literature
Beryllium-walled and high-pressure gas-filled cavities reached approximately $0.5\,\mathrm{MeV}/\mathrm{cm}$ in fields near $3e4\,\mathrm{G}$ at the Fermilab MuCool Test Area (reported around 2020); higher fields remain untested.
See also
- Prerequisite for Experimental demonstration of 6D cooling with re-acceleration
- Related Reaching muon collider target emittances by ionization cooling
- Related Can final cooling reach collider transverse emittance with acceptable longitudinal growth
- Related Microscopic mechanism that initiates vacuum RF breakdown on copper