Measuring Emittance Using Beam Position Monitors

The Los Alamos Advanced Free-Electron Laser uses a high charge (greater than 1 nC), low-emittance (normalized rms emittance less than 57 ~ mm mrad), photoinjector-driven accelerator. The high brightness achieved is due, in large part, to the rapid acceleration of the electrons to relativistic velocities. As a result, the beam does not have time to thermalize its distribution, and its transverse profile is, in general, non-Gaussian. This, coupled with the very-high brightness, makes it difficult to measure the transverse emittance. Techniques used must be able to withstand the rigors of very-intense lectron beams and not be reliant on Gaussian assumptions. Beam position monitors are ideal for this. They are not susceptible to beam damage, and it has been shown previously that they can be used to measure the transverse mittance of a beam with a Gaussian profile [l]. However, this Gaussian restriction is not necessary, and, in fact, a transverse mittance measurement using beam position monitors is independent of the beam’s distribution. I