Refraction in a plasma and laser resonators for vacuum ultraviolet

Characteristics of propagation of beams of rays are considered theoretically under conditions when their paths become bent as a result of smooth gradients of the electron density in a cylindrically expanding plasma. An estimate is obtained of the combined effects of refraction and diffraction and the case of strong predominance of refraction is analyzed. The construction of resonators for vacuum ultraviolet lasers is considered specifically. Types of resonators ensuring an effective positive feedback in the case of a stronger or weaker bending of rays in a plasma are suggested.