Question

http://en.wikipedia.org/wiki/Linear_particle_accelerator
Why do we need drift tubes, we could have accelarated particles by just subjecting them to potential difference b/n two plates ?

Answer 1

Are you asking why it's more economical to have several acceleration stages, each at a modest potential, rather than one acceleration stage at an enormous potential? It's sort of why it's more efficient to have a 4-cylinder 1.6L car engine with 5 speed transmission, instead of a 16-cylinder 6L car engine with no need for a transmission at all.

Answer 2

The theory of drift tubes is fairly complicated, but essentially you subject the charged particle to a field which alternates in direction, but when the direction opposes the motion of the particle, the particle is inside the tube, shielded from the force, so it experiences only a net forward force. Otherwise, we'd need extrememly large potential differences to achieve the same acceleration.