Length monitor for 1 mm SLC bunches

A non-intercepting RF bunch length monitor for σ_z=0.5 to 2.0 mm long electron and positron bunches in the Stanford Linear Collider (SLC) has been built with a design similar to a previous device for longer bunches. For this device, fields from the beam pass through a ceramic gap, enter receiving cavities, are then measured with power detectors, and finally are recorded by the SLC control computer. The designs of the receiving cavities (25 and 36 GHz) are described as well as the choice of the RF power distribution and measuring systems. Beam measurements have been taken as a function of bunch compressor RF voltage, bunch intensity, and beam position. Long term bunch length measurements were recorded during SLC colliding beam operation indicating that the bunch length is constant to about 3%. Thus, 1 mm length monitors operating at 25 and 36 GHz have successfully monitored long term bunch length changes at the few percent level in the SLC