The energy transfer between the laser and a bunch of electrons in LACARA may be understood as follows.If one fixes oneself in the rest frame of a relativistic electron moving along a magnetic field, circularly-polarized laser photons (which are also travelling parallel to the electrons) are Doppler-downshifted to match the gyrofrequency of the electrons. Thus the electrons can acquire energy via a cyclotron-resonance interaction with the transverse optical fields of the laser.  The laser beam is configured as a lowest-order HEM₀₀ (“Gaussian”) mode, and so its effective index of refraction in vacuum  is slightly less than unity as one passes through the focus.  The variables for exact autoresonance are related through , where the magnetic field strength is B(z), the index is n(z), the electron energy factor is g (z) and normalized axial velocity is _z(z) During acceleration, the normalized transverse velocity of the electrons increases as the electrons execute about one gyration for each Rayleigh length in the optical field./span>Electrons radiate very little synchrotron radiation, since << 1. The magnetic field required scales favorably as 1/: Symbol'>g, and electrons entering with all optical phases are accelerated equally with no requirement of pre-bunching.  In order to obtain a good quality accelerated bunch of electrons, the tolerances on initial beam emittance and misalignment are quite tight, albeit similar to other laser experiments that recently have been successful at ATF (e.g. VISA and STELLA). The LACARA can have a high efficiency for power transfer from laser to beam.