Low Bunch Charge Effects in BPMs

1. Low Bunch Charge Effects in BPMs A. Kalinin EMMA tasks, experiments, and procedures 23 August 2012

2. DJK Program • Run 13 to 15 August. • Injected bunch charge was in range 15pC to 20pC as indicated by WCM-based Charge Monitor. • DJK’s program “Number of shots @ 20 turns vs BPMs” showed: 1) About six of BPMs return zero shots. 2) For a orbit changing, a set of zero return BPMs is also changing. 3) For other BPMs, number of returned shots has periodic variation along the ring.

3. BPM investigation • To remind, each BPM is triggered individually by a built-in Beam Trigger. In circular BPMs, BT uses vertical plane signals as the variation of vertical orbit is minimal. In rectangle BPMs, no preference is there. • Beam Trigger outputs (MR, /WEN, RDY) and card logic signals (NO_BEAM, FULL, EMPTY, ERR) of one of the BPMs (rectangle BPM1-2, card P2) were investigated using scope and/or front panel LEDs. • The investigation environment was suitable: bunch charge was in range 10 to 20pC, turns were varying (that time, extraction was tried). The BPM choice was also appropriate. • The BPM behaviour that was observed, can be classified as follows: 1) ‘good’ beam, the BPM works normally; 2) ‘no beam’, when both signals in a doublet are lower than the BT threshold. The BPM as it should, returns Nturn=0, all LEDs are normal; 3) ‘bad’ beam, when one pulse in doublet is lower than the BT threshold. LEDs are chaotic, sometimes the ERR LED=1.

4. What is a ‘bad’ beam, or why one pulse in doublet does become lower than the BT threshold? For circular BPMs: a) low bunch charge @ some imbalance in doublet due to ‘normal’ vertical bunch offset (typical); b) ‘normal’ bunch charge @ big imbalance due to outrageous vertical bunch offset (rare). For rectangle BPMs: c) ’normal’ bunch charge @ big imbalance in doublet due to big horizontal offset (typical). • This conditions might take place either on the first or other, and only turn, or after a number of ‘normal’ turns/doublets. The first case produces /WEN=H-to-L, RDY=L, the second one produces glitches H-to-L-to-H in /WEN=H and RDY=H. • Note removing the bunch after some number of turns by Extraction Kicker is a good practice when one just avoids a) that may happen with turns due to bunch charge degradation. • Note again the number variation in DJK plots is happening due to a).

5. Low Bunch Charge. What to do? • The ‘single’ pulse cases above can be managed internally by BPM logic (just this is done in new versions of ADC Clock board and VME card). They can be managed externally (this was proposed by GC as a beam-independent RDY that is generated after the event). • But! No use is there in these measures, if the bunch charge is same low. A BPM will show same zeroes as it does now! Please, take this statement in your mind and heart. • ALICE is set to 40pC @ single bunch. A half of this charge is typically lost, the ring gets about 20pC. Now, injection tuning takes best quarter bad whole shift. Probably, no sense is there to fight for better injection, it’s a hard task. Keeping it as it is would be just OK. • Why can’t we set ALICE to 60pC @ single bunch?! With it, we’ll havein the ring about 30pC. Nothing extraordinary is there in this value, 30pC is merely an EMMA Specification value. The BPMs were designed just for this charge. • As for environment radiation load, it is only 1.5 times higher than for 40pC. For 60pC, the extra load is 20pCx(8x60x60x5)bunches~3x10e3nC. Same load occurs on ALICE for ~30 of loss cases per shift, when 1600 bunches @ 60pC each hit the wall beyond the dump. How many of such cases per ALICE shift do typically happen? Nobody never bothered...