An open-cell cavity having six rf cells was tested first to explore gradient
limitations we will face in a cooling channel. In the initial tests, without
a magnetic field, significant dark current was observed at high gradients,
with an intensity that follows a Fowler-Nordheim relationship (scaling as
about the tenth power of gradient). However, the parameters obtained from
fitting give unreasonably large values for the field enhancement factor, $\beta_{FN}$, which is not presently understood \cite{DarkCurrentnote}. Some
evidence was seen for ``conditioning,'' that is, we observed a decrease over
time in dark current at fixed gradient, though this has not yet been studied
systematically. Measurements of the momenta of accelerated electrons are
consistent with a significant fraction of the particles traversing the
entire length of the 6-cell cavity. When operated in the solenoid field, the
cavity required reconditioning, suggesting that the field emission sources
moved to different locations on the cavity wall. There was also evidence for
copper ions being sputtered from the cavity iris and accelerated to the
titanium vacuum window on the cavity. This eventually led to a vacuum leak
in the window. Inspection of the cavity inner surfaces showed pitting in the
iris region