Why no electric field in the dees? Why is there no electric field in the hollow space of the dees of a cyclotron though it is open at one face?
Is electrostatic shielding applicable for space(hollow space) inside open conductors(like dees) also? 
 A: Who says there is no electric field in the dees? The early designs by Lawrence included various grid or slit designs to close off the dees and keep the electric field out of the dees. But, in April 1931 Livingston (Lawrence's grad student, see APS.org) tried the cyclotron without those devices in an attempt to increase the beam current. To quote from "From Nuclear Transmutation to Nuclear Fission, 1932-1939" by Per F Dahl (Institute of Physics, 2002), p. 75-76:

In the absence of a grid or similar metallic shield across the dees of a cyclotron ... there is a slight penetration of the accelerating electric field near the entrance to the dee. Fortuitously, the penetrating field lines, or 'lines of force,' will act like an electric lens ...

So, yes, the field penetrates slightly into the dees, and helps keep the first few turns of the spiraling ions focused, increasing the current on the output. At higher energies, magnetic focusing is more important. Playing while the boss is away can be a good thing for a grad student.
