Transient mirror heating theory and experiment in the Jefferson Lab IR Demo FEL

During commissioning of the IR Demo FEL at Jefferson Lab, we noticed that the FEL exhibited a rapid power drop with time when the first set of 3 μm mirrors was used. Though the rate of power drop was unexpected, it was thought that it could be due to a distortion of the mirrors during a time short compared to the thermal diffusion time. This transient distortion might affect the laser more than the steady state distortion. This paper presents some analysis of the transient mirror heating problem and some recent experimental results using different mirror substrates and coatings. It is found that the behavior of the first mirror set cannot be reconciled with the observed power falloff if a linear absorption is assumed. The power drop in more recent experiments is consistent with linear thermal analysis. No anomalous transient effects are seen.