In particle accelerators, a common mechanism for accelerating a charged particle beam is via copper resonant cavities in which electric and magnetic fields form a standing wave, the mode of which is designed so that the E field points along the axis of the accelerator, producing forward acceleration of the particles when in the correct phase.
The maximum electric field
achievable is limited by a process known as RF breakdown.
The reliable limits for various RF frequencies
[1] An approximate relation by least-square optimization of the data yields[2]
This relation is known as the Kilpatrick Limit.