Write a note on electrostatic focusing.
Electrostatic deflection is the method of aligning the path of charged particles by applying the electric field between the deflecting plates.
Diagram above represents the electrostatic focusing.A and B are two co-axial cylinders with potentials V1 and V2 such that V2>V1.R is the equipotential ring placed between A and B.
(1) Consider electron beam 1:
It will remain normal to all the equipotential surfaces and hence it is simply accelerated without any deviation of the path.
(2) Consider electron beam 2:
It will have following 2 effects:
(a) On the L.H.S off R:The parallel component of FP will move the electron towards right while the normal component FN will move the electron downwards by applying Fleming’s left hand rule at point C.
(b) On the R.H.S off R: FP and FN will move the electron towards right and towards up respectively by applying Fleming’s left hand rule at point D.
(3) Consider electron beam 3:
It’s path will be as shown with same case as case(2).
The focal length can be changed by varying V1 and V2
(4) Bethe’s laws is also followed in electrostatic focusing :
`(v_2)/(v_1) = (sin θ_i) /(sinθ_r)`
(5)Electrostatic focusing is used for accelerating and focusing electron beams.