In this letter, we report on experimental measurements of an instability in
the large-area electron-beam diodes used to pump krypton-fluoride (KrF) la
sers. The instability is identified as the transit-time instability and it
is shown that it modulates the electron beam (spatially and temporally), pr
oducing a wide spread in the energy and momentum distributions of electrons
emerging from the diode. These effects can enhance the energy deposited in
the foils and adversely affect the energy-transfer efficiency to the KrF g
as. Analysis and simulations of the instability suggest that resistively lo
aded slots in the cathode should eliminate the instability. [S0003-6951(00)
03232-0].