Free electron laser beam quality — from Castelgandolfo to Rome

Since the Castelgandolfo FEL conference photo injectors and advances in beam transport has lead to many orders of magnitude improvement in beam brightness as well as our understanding of what constitutes a high-quality beam. We find that beam compression may increase brightness, but not beam quality; and that, when the relation λ = πε is satisfied, the FEL is well into the kinetic regime of operation. A good measure of beam quality is the scaled thermal velocity. In some cases space charge may improve beam quality.