Low GAP Beam Position Monitor (LG-BPM)

The Beam Position Monitor (BPM) system provides a measurement of the transverse (X and Y) position of the electrical centre of mass of the electron bunches. The beam is sensed by means of button electrodes housed in the so-called “BPM”. This BPM is a stainless steel block fitted with four 50Ohm SMA vacuum feed-through. The signal induced by the bunched beam (2ns bunches separation) on each electrode is the 500MHz carrier signal, amplitude modulated according to the beam-to-electrode distance. By applying a simple formula (difference-over-sum) to the four output voltages (V1 to V4) of the AM demodulator, it is possible to recover the position information.
A new Beam Position Monitor (BPM) system is presently under design for the Storage Ring of ELETTRA. Its performances rely on the new Low-Gap BPM, specifically designed for the low-gap Insertion Device (ID) Aluminum vacuum chamber, presently under installation at ELETTRA. To develop this new system two main projects have been started the mechanical design to provide the Low Gap BPM, fitted with 14mm diameter buttons, and the relative support system and vacuum chamber interface 
the electronic design to provide a new analogue front-end and a digital amplitude demodulator 
Mechanical design At ELETTRA, the low gap ID vacuum chamber has an elliptical section, with L=81mm and H=14mm. To have the maximum sensitivity in the vertical plane, 14mm diameter buttons have been very closely located, with only 12mm center-to-center distance in the horizontal plane (see fig.1). Two bellows will isolate the BPM from mechanical movement possibly induced by the vacuum chamber.

top view of the Low Gap BPM, including two SMA connectors and the two bellows and flanges. 

The support system of the BPM will provide state-of-the-art both short-term (vibrations) and long-term (thermal) stability to the monitor. Furthermore, a monitoring system, based on capacitive sensors (see fig.2), for BPM sensor motion at sub-micrometer level will be installed, providing the information about any sensor motion.

T:Dhe two armatures of the capacitive sensor (20x20mm each).

Electronic design
The electronics (see fig. 3) of the new BPM system can be divided into the following blocks:
- analog front-end
- digital receiver (AM demodulator)
- DSP processor
    The main function of the front-end is to provide high-stability and resolution for the button signals with sufficient bandwidth for the Local Feedback to operate properly. Two front-end solutions are presently under design: a 4-channel Front-end, developed in collaboration with the Swiss Light Source, and a 4-to-1 Multiplexed Front-end, internally developed. In order to keep matched the gains on the four channels, a pilot frequency signal will be injected at the front-end inputs and the amplitudes of the four outputs will be kept equal by tuning each channel gain. The IF outputs of the front-end will be directly under-sampled at 32*frev being the signal band-limited around the IF frequency of 70 MHz.
The digital receiver block will provide x & y position signals by means of state-of-the-art Digital Demodulator (Digital Down-Converter).
Finally a DSP module will manage the data stream and provide filtered values to the central processor of the Local Feedback

Last Updated on Friday, 16 December 2011 14:47