Linear focal Cherenkov-ring camera for direct longitudinal phase space measurements

A test accelerator for the coherent THz source (t-ACTS) has been constructed at Tohoku University, in which the generation of intense coherent THz radiation from subpicosecond electron bunches will be demonstrated. The distribution in the longitudinal phase space extracted from an electron gun is crucial to achieve extremely short electron bunches. For relatively low electron energy, to avoid a space charge effect during measurement is required. Employing velocity dependence of the opening angle of Cherenkov radiation, combined use of a streak camera and a 'turtle-back' mirror which resolves and confines the Cherenkov light with different opening angle onto a line may allow us to observe the longitudinal phase space distribution directly. Energy resolution can be improved by the '4f' imaging system consisted of two off-axis parabolic cylinder mirrors, and more practical experimental setup has been investigated. The sufficient energy and time resolution was derived by the numerical ray tracing. The current status of this development will be presented. (author)