I want to charge a sphere using a van de graff generator , it would sound easy , but using a 1 MV or a 900 KV is really hard, it is enough to break like 1 meter of air so whenever i try to ground the sphere there is spark . So i wanted to know if it will help if i covered the sphere with a layer of plastic or an insulator , or if there was a vacuum between the the generator and the sphere. And another thing, can a graphite rod be charged like a metal one ?
Anything that increases the strength of the dielectric between the sphere and the ground will improve your odds of success. Achieving a nearly perfect vacuum will help, but in general, a better, but imperfect vacuum will actually hurt (the lower air density reduces the collision rate faster than it reduces the number of carriers). You are better off attempting to insulate with known insulators (rubber, glass, etc.)