Vertical Beam Position Interlock system (BPI)
Functionality :
The continuous measurement and monitoring of the vertical (Z) e-beam position in the Storage Ring at 8 locations around the 844m SR circumference and the fast triggering of a beam-dump-interlock in case this Z displacement would exceed a threshold value (typically at 0.7mm) from a pre-defined reference position. The response time is ~1ms.
The eight BPI stations are fully independent and the measurement and interlock functions are fully hardwired to assure ultra-high reliability which is its primary requirement. The system needs no input parameters from the operator to function and it can not be stopped or strapped remotely.
Each BPI station works autonomously, i.e. it detects itself the presence of beam (it activates itself above a pre-set intensity threshold, typically 10mA), is not perturbed by beam injection and is not dependant on different beam filling patterns.
Each BPI station is equipped with a so-called data-logger. It stores 1024 measurements and is stopped (on all 8 BPI stations) if a beam interlock is triggered. This facility permits the operator to verify in case of an interlock that the measurements of the BPIs are correct and coherent with each other.
Purpose :
The BPI system protects the vacuum chambers against damage (thermal, melting) by the powerful synchrotron radiation beams in case of a strong missteering in the vertical plane of the electron beam.
Ultra-high reliability is a prerequisite while the number of 8 independent stations provide a degree of redundancy so that in case of non-functioning of one or two stations the security purpose is still maintained.
The BPI blocks and their location :
The BPI blocks have the same geometry and pick-up electrodes as those of the Closed Orbit BPM system. In fact, out of the total of 224 BPMs blocks for the C.O.BPM system, 8 blocks have been dedicated for the BPI system.
They are located in cells 1, 8, 10, 14, 16, 20, 24 and 27. The selected stations in these cells (7 stations per cell) is number 3 or number 5, at both locations the vertical Beta value is large (~35m).
The choice of the 8 BPI locations is such that independent of the origin and cause of a missteered vertical closed orbit (and independent of the location of this cause) a maximum of the 8 BPI stations will detect it.
With the vertical C.O. integral tune number at 14 and the above location of the BPI stations it can be shown that this criteria is respected.
Concept / Signal Treatment :
The signal treatment concept is in its principle identical to that of the Closed Orbit BPM system (time-multiplexing of the 4 RF signals from the electrode) but operates at a higher frequency, i.e. 8 KHz.
The de-multiplexing of the 4 signals is done after down-conversion of the 352MHz signal to 10.7Mhz, amplification, filtering and level detection. The Z position is calculated (by analog components) every 130us (although it takes 4 times this value to completely recalculate a position after a rapid beam movement).
Precision, resolution, reproducibility :
The absolute precision (with respect to the magnetic center of the Quadrupoles) is estimated at 100um rms and obtained by simply adjusting the Z measurement to zero.
This is done only twice a year (if the Z measurement has drifted more than 100um) after a reference value for the Closed Orbit (based on the 214 active BPM stations) has been determined (which is generally within 100um rms).
The resolution of the yielded BPMmeasurement (1KHz) is about 50um.
The reproducibility at medium and longer time scales, and against varying beam conditions is as with the C.O.BPM system.