Design of a telescopic zoom system for electron acceleration with lasers

A telescopic zoom system made of three spherical mirrors has been designed for the purpose of electron acceleration with lasers at LULI's Apollon facility. This system is based on a telescope with 3 or 4 mirrors, the distances of which can be varied continuously. We are constrained by laser damage considerations which prevents us from reducing the dimension of the incident laser beam and we will show that the 3-mirror solution can be made of a first convex mirror, a second concave mirror and a third convex mirror. It is possible to get a continuous range of focal lengths when translating the second mirror such that the final focal length will vary from 1 to 4 (zoom ratio 4x) and that the final focal spot will not move. When dealing with on-axis mirrors, we will get a central obscuration and the next step will be to go off-axis such that no obscuration will occult the beam propagation. Moreover our laser beams are fairly well collimated with a residual divergence much less than 100 μrad which means that we are not considering any field of view like it is for astronomical systems. The purpose of this paper is to describe the step-by-step method leading to the final compact zoom system that allows the focal length to be varied continuously. A mock-up of the system at a reduced scale is being built, first as a proofof-principle and second to work on the alignment of the 3-mirror zoom.