Vacuum Laser Acceleration

The purpose of this communication is to comment on and discuss laser acceleration of electrons in vacuum. In particular, we will: (i) critique the recent paper by C.M. Haaland [ 11, titled “Laser electron acceleration in vacuum”, (ii) discuss some general features and characteristics of laser acceleration in vacuum and (iii) propose a vacuum laser acceleration concept called the “vacuum beat wave accelerator”. In Ref. [ 1 ] a vacuum laser acceleration configuration was proposed and analyzed. In this scheme a pair of linearly-polarized laser beams with Gaussian profiles, having the same frequency, are focused and intersected in vacuum, see Fig. 1. Here, the first laser propagates along the z1 axis and the second laser propagates along the z2 axis, where the zI axis and the z2 axis are rotated by the angles of 8 and 19, respectively, with respect to the z axis. The phases of the laser are such that the transverse electric fields cancel along the axis while the axial fields add. Properly phased electrons injected along the z axis can be trapped and accelerated by the net axial component of the laser field. The analysis of this configuration contained in Ref. [ 1 ] is flawed for a number of reasons: e.g., the vacuum fields are physically unrealizable. Ref. [l] arrives at the incorrect conclusion that “the electron will acquire a net energy gain in this scheme when the laser-electron interaction is integrated along the z axis from minus to plus infinity”. We find, on the other hand, that the net energy gain vanishes over an infinite interactionregion, which is in agreement with the Lawson-Woodward [ 2-51 acceleration theorem. The axial electric field associated with the intersecting laser beams can be calculated by writing the laser