Physics with the CMS forward CASTOR calorimeter

The CASTOR calorimeter is designed for the very forward region (pseudorapidity range from -6.6 to -5.2) in the CMS experiment at LHC. It will be operated in the low luminosity period of pp-collisions up to 2x$10^{33}$cm$^{-2}$s$^{-1}$ and in heavy-ion-collisions. The design of the calorimeter is determined by space constraints inside a shield for radiation and for magnetic field and restricted to materials which tolerate a high radiation level. The calorimeter surrounds the beam pipe as a very compact sampling structure of tungsten and quartz plates with a depth of 10 hadronic interaction lengths. The granularity of 16 transversal segments and 14 longitudinal sections allows to reconstruct shower profiles, to separate electron sand photons from hadrons and to search for phenomena with anomalous hadronic energy depositions as expected from exotica. Performance as measured in test beams, first operation experience and first measurements in the very forward region of CMS will be presented.