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WARNING!!!

WARNING!!!. Data taken on these shifts had attenuation factors set incorrectly and problems with faraday cup bunch charge measurements. #3091. Effect of attenuation and timing on the BPM readings Final “AP” conclusions might depend on attenuation/timing!

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WARNING!!!

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  1. WARNING!!! • Data taken on these shifts had attenuation factors set incorrectly and problems with faraday cup bunch charge measurements

  2. #3091 • Effect of attenuation and timing on the BPM readings • Final “AP” conclusions might depend on attenuation/timing! • Or, more provocatively, can you prove any “AP” result you want by changing the attenuation and timing? • How do we know what the ‘correct’ attenuation/timing is • Ignore for now? will require time consuming investigation • Transient in first part of train in ER mode measured on AR1-BPM-1 • Linac set to give bunch minimum energy spread. • BPM response to varying bunch charge, consistency with AR1-1 screen observations

  3. #3091 Horizontal Transient AR1-BPM-02 Standard 60 Pc, with ok-ER, after tuning attenuation and setting timings to the ‘correct’ values. There is initial steep transient then a shallower one. Overall about 2 mm. 2 mm After improving ER. Transient has reduced to ~ 1.2 mm overall. Side note: note the average x position has changed too. 1.2 mm

  4. #3091 Beam Dump In Good ER, see previous slide Beam dump in

  5. #3091 Effect of Bunch Charge Good ER, see previous slide, LA = 1.0 LA = 0.5. Note the average x position has changed by 1 mm

  6. Conclusions • Observed transient in x position, y position and ‘charge’, x transient much larger than y • Observed an effect of bunch charge on BPM reading

  7. X-transient • What is the cause of the x-transient? • 1. Intrinsic bunch energy variation over train • 2. Intrinsic bunch x position variation over train • 3. Intrinsic bunch charge variation over train  variation in BPM response to charge • Various combinations of 1,2,3

  8. X-transient Discussion • On #3091 before we improved the ER we made the following observation. • Dispersion on AR1-BPM-02 is 0.33 m (checked in MAD and ELEGANT by James and me) • So a ~ 2mm transient = 0.6 % momentum transient (using beam momentum = 26.5 MeV) • The dispersion on AR1-1 is 0.88 m (MAD value) • So the x-variation over train (if due only to 0.6 % energy variation) at AR1-1 should be ~ 5.5 mm • But the observed size on AR1-1 at 10 uS was ~ 2 mm full width (no picture unfortunately)  inconsistent! • One reason for the inconsistency could be that the x-variation is not really 2 mm at AR1-BPM-02. The apparent x-variation could be due to the varying bunch charge over the train and BPM non-constant response to different bunch charges • AW theory, and supported by the observation that the average beam position on AR1-BPM-02 does change with LA • But could changing the LA really change the beam position, from dynamical effects of charge variation? • Is the AR1-1/BPM-02 comparison too simplistic/invalid? Would 0.6 % bunch energy variation really give 2 mm full width on AR1-1? Several problems • We only use 10 us on AR1-1 whereas the 2 mm variation on the BPM is over the WHOLE TRAIN. • e.g. if energy variation isn’t linear over train? • e.g. if the first ~ microseconds of the train have lower bunch charge? • A. Kalinin made point that you would expect to see a similar size transient in x AND y if the BPM charge-response is to blame. This is not observed.

  9. Shifts #3121,3122,3123, Fri-Sat 14-15th September • AR1-BPM-01 used for first time. • Other AR1 BPMs used while varying dispersion using AR1-Q1. • Also ST2-BPM-3 used. • Various bunch charges used from 30 pC to 150 pC, in the confusion of scope settings. • Pop-in Dump in and out • 16 MHz bunch rep, 100 uS

  10. AR1BPM3-4 ST2BPM3 #3121 Fri 14th Sep Shift 3 PW Q1/4 = 2.05 • All data at 30 pC bunch charge. • Different AR1 quad strengths used • Largest dispersion is at ST2-BPM-3 mid chicane ~ 0.5 m • On this shift had AR1-BPM-01, AR1-BPM-03, AR1-BPM-04, AR1-BPM-05 available • Difficult to get large dispersion (in ER conditions) on any AR1 BPMs, can get -0.3 m on BPM3-4 with Q1/4 = 2.38 A • Collected much data with pop-in dump in and out Q2/3 = 1.0 A Q1/4 = 2.2 Q1/4 = 2.05 A Q1/4 = 2.2 A Q1/4 = 2.38 Q1/4 = 2.38 A See slide of extra notes for dispersion calculations

  11. #3121 energy variation/transient over the train • On a previous shift (#3091) had seen transient of ~ 1mm in first 10 uS on AR1-BPM-02 where we think the dispersion is fixed at + 0.33 m • Any transient seen on this shift? Of ALL the data taken, most obvious transient seen on ST2-BPM-03 with AR1Q1/4 = 2.2 and dispersion at this location predicted as + 0.5 m Compare with AR1-BPM-02 observation on #3091 (dispersion = + 0.33) BUT DIDN’T SEE MUCH EVIDENCE OF THIS TRANSIENT, ON THE OTHER BPMS

  12. #3121 energy variation/transient over the train • Do see a small transient on AR1-BPM-01 at which we expect dispersion to be zero, and upstream of the quads we’re using • 4 separate observations • This transient was not seen on any other BPM except ST2-BPM-03 shown on previous slide

  13. #3121 energy variation/transient over the train • Generate ‘large’ dispersion of -0.3 m on AR1-BPM 3,4 using Q1/4 = 2.38 A • What is this? Linear energy variation over train? Why? Are we sure ER is maintained here? • If the previous observations show an energy transient at start of train, it is not seen here. BPM3 BPM4

  14. #3121 Effect of Pop-In dump • AR1-BPM-04 with large dispersion = - 0.3 m • Bunch position change over the train actually seems smaller with the dump IN • Overall shift of train position to more negative values. • If dispersion is negative, this implies an overall DECREASE in energy of the train when the dump is in. NEED MORE EXPLANATION HERE. • Effect of dump doesn’t seem to introduce a ‘droop’ • Somewhat inconclusive dump out dump in

  15. #3121 Extra notes • AR1 set to Q1/4 = 2.2 A, • Q2/3 = 1.0 A • These are equivalent to • K(Q1/4),(Q2/3) = 9.20,-4.22 • For K.E. = 26.0 MeV. • I have my own spreadsheet to convert current -> K, and cross-checked it with the magnet table.

  16. #3122 • On this shift we had ~ 150 pC • Again see transient on AR1-BPM-01. It’s “up and down” here. • Massive transient on ST2-BPM-03 dump out dump in

  17. #3123 • Charge problem solved and back to normal 60 pC. • Dump out vs dump in investigations • Some varying of AR1-QUADS but don’t know how useful this is with these two BPMs dump out dump in ST2-BPM-03

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