The competition between the bunching force of the ponderomotive potential and the repulsive force of the self electrostatic potential in a tapered free electron laser limits the beam density, deceleration rate due to tapering, and the local electric field spatial exponentiation rate. The limit on the spatial exponentiation rate is below that predicted by linear theory at the transition from the Compton to Raman regimes. Furthermore, this limit is independent of the form of the particle energy distribution function.

• Received 23 September 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.211