• J.J. Welch, H.-D. Nuhn, J.A. Sevilla
  SLAC, Menlo Park, California

Undulator magnets for the LCLS need to be maintained at a very stable and accurate temperature in order to stay within the tolerance required for the FEL. At the LCLS the temperature of the undulator magnets is mainly determined by the temperature of the surrounding air. Furthermore, the climate control system which controls the temperature of the air must never accidentally go out of a safe operating range of ± 2.5 C or the magnets may lose calibration and have to be removed and remeasured. This was one motivation for the sighting the Unduator Hall underground in a tunnel where the thermal inertia of the surrounding earth provides stability. In this paper we describe the technical solution adapted by the LCLS for controlling the air temperature in the Undulator Hall and its initial performance. We also discuss thermal issues of heat balance and steady state and transient temperature behavior of the undulators system and the surrounding earth.