  With the aim to obtain short-pulse bunches with high peak current for a
terahertz radiation source, an EC-ITC (External-Cathode Independently Tunable
Cells) RF gun was employed. As the external injecting electron source of the
ITC RF gun, a gridded DC gun plays a key role, the performance of which
determines the beam quality in the injector and transport line. In order to
make the beam well compressed in the ITC RF gun, the energy of the electrons
acquired from the gridded DC gun should be 15 KeV at most. A proper structure
of the gridded gun is shown to overcome the strong space- charge force on the
cathode, which is able to generate 6 {\mu}s beam with 4.5A current
successfully.
