  Conditioning of a metal structure in a high-voltage system is the progressive
development of resistance to vacuum arcing over the operational life of the
system. This is, for instance, seen during the initial operation of radio
frequency (rf) cavities in particle accelerators. It is a relevant topic for
any technology where breakdown limits performance, and where conditioning
continues for a significant duration of system runtime. Projected future linear
accelerators require structures with accelerating gradients of up to 100 MV/m.
Currently, this performance level is only achievable after a multi-month
conditioning period. In this work, a pulsed DC system applying voltage pulses
over parallel disk electrodes was used to study the conditioning process, with
the objective of obtaining insight into its underlying mechanics, and
ultimately, to find ways to shorten the conditioning process. Two kinds of
copper electrodes were tested: As-prepared machine-turned electrodes ("hard"
copper), and electrodes that additionally had been subjected to high
temperature treatments ("soft" copper). The conditioning behaviour of the soft
electrodes was found to be similar to that of comparably treated accelerating
structures, indicating a similar conditioning process. The hard electrodes
reached the same ultimate performance as the soft electrodes much faster, with
a difference of more than an order of magnitude in the number of applied
voltage pulses. Two distinctly different distributions of breakdown locations
were observed on the two types of electrodes. Considered together, our results
support the crystal structure dislocation theory of breakdown, and suggest that
the conditioning of copper in high field systems such as rf accelerating
structures is dominated by material hardening.
