Physical Description

The physical diagram shows how the electron passage produces infrared beams that are split, filtered, and converted to a voltage by means of a pyroelectric detector. The existence of 2 detectors is for redundancy, in case one fails. The infrared mirror can be moved out from the electron way, what causes the impossibility to measure the bunch length.

 "The light passes through a filter that transmits low frequencies (long wavelengths) and rejects high frequencies. As the bunch gets shorter, more of the light is in high-frequencies, and so the signal drops; the opposite happens if the bunch gets longer. We can’t measure the bunch length without a filter. There are actually two filters with different cut-offs to span different ranges of bunch length; only one is used at a time, and that’s why the filters are movable. Normally our bunches are short enough that only one filter is used routinely." (text by Alan S. Fisher).

Mirror and Filters Control

The mirror and 4 filters can be moved out from the beam or in front of the beam with pneumatic actuators. Each of these 5 elements has a sensor for the IN or OUT position. The diagram shows how the control elements are connected to industrial pack modules responsible for digital input and output signals.

The EPICS database that gives access to the control system of the mirror uses a CALC record to show if the element is in the IN or OUT position, or if it is moving.

The EPICS database to control the filters is simpler. Each filter has an actuator and two position sensors.

Pyrodetector Digitalization Process