Needle
cathodes are useful when high brightness is more important than high
current. Besides electron microscopes, examples include Smith-Purcell
and other FELs for which the focal volume is so narrow that space-charge
forces limit the useful current.
The basic advantage of needle cathodes is the high electric field at
the tip, as much as 1010 V/m. This avoids space-charge limits on the
current density and introduces new physics into the emission process.
Needle cathodes vary in size over more than six orders of magnitude.
Millimeter-size needles work like ordinary photocathodes with quantum
efficiencies enhanced by the Schottky effect. Micrometer-sized needles
exhibit field emission and photo-enhanced field emission. Nanometer-sized
needles, such as carbon nanotubes recently exhibited the highest cw
brightness ever recorded.
In the future, it appears that arrays of needles produced lithographically
from silicon and diamond will be important when electron beams are required
with a higher current or a specific shape. These and other developments
will be discussed.