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<p>FERMI C-BPM</p>


The FERMI Cavity Beam Position Monitor (FERMI C-BPM) is a vital beam diagnostic instrument for the Free Electron Laser. It allows for the measurements of the electron beam trajectory to be taken in a non-destructive way. With sub-micron resolution, the beam must be aligned with the seed laser, if present, and with the FEL-emitted radiation.


FERMI C-BPM is a complete solution. It covers the analogue signal acquisition (C-BPM Pick-Up), the detection of the signal amplitude (RF Front-End), the digital signal acquisition, processing and transferring the data to the control system (Digital Back-End).


C-BPM Pick-Up consists of two mechanically tunable copper cavities with a 6.5 GHz resonance frequency. One of these cavities generates the monopolar component (reference signal), that is a damped sine wave at 6.5 GHz with a time constant t of approximately 300 ns. The amplitude of the reference signal is directly proportional to the charge. The second cavity (position signal) generates two X and Y dipolar components on the output connectors. The amplitude of the position signal is directly proportional to the charge and to the distance of the beam from the cavity longitudinal axis. The mechanical tuners fitted to each cavity on the C-BPM allow for the alignment of three frequencies (reference, horizontal and vertical positions) within 200 kHz around a nominal value of 6.5 GHz.


Signal Detection RF-Front-End directly detects the envelope of the RF signals pre- processed by an RF hybrid. The position signals, phase shifted and amplified, are combined with the reference signal in a 180° Double Arrow Hybrid Coupler. The hybrid coupler is a passive component performing both the sum and difference of its two input signals. These output signals pass through a digital 64 step attenuator. The amplitude envelope is measured by a precision RF Detector with a baseband band-width of 12 MHz.


Digital  Signal Acquisition:  the signals from the tunnel electronics are acquired by the in-house developed 4 channel 160 Msps digitizer board (ADO), housed in the μ-TCA crate. The ADO board is connected to an OPTO-IO board, which directly controls the RF-Front-End. In addition, the μ-TCA crate houses the timing generator board (MitiCH) and a dual channel 320 Msps signal generator board that also controls all the self-calibration system mechanism and acquisition modes. All the digital boards are equipped with a 10/100/1000 ethernet interface, which can communicate with any control system.