Question

what is parameters on which the focal length of an electrostatic lens depend on

Answer 1

An electrostatic lens is a device that assists in the transport of charged particles.[1][2][3] For instance, it can guide electrons emitted from a sample to an electron analyzer, analogous to the way an optical lens assists in the transport of light in an optical instrument. Systems of electrostatic lenses can be designed in the same way as optical lenses, so electrostatic lenses easily magnify or converge the electron trajectories. An electrostatic lens can also be used to focus an ion beam, for example to make a microbeam for irradiating individual cells