Notes • Dipole calibration = scan dipole current and measure the change in BPM measurement • Change in beam position can be predicted from change in dipole current • Not an absolute calibration because cannot determine absolute beam position in BPM • In my plots I just arbitrarily chose the zeros of the true vs measured data to coincide, i.e. line forced to go through origin. • 2 × OTR calibration can provide an absolute calibration but can only be done for INJ-BPM-03 and possibly ST2-BPM-01
Current Calibration • ‘Electrostatic’ calibration (ask R Smith for details) • Simulate stripline voltages for different beam (vertical) offsets (horizontal offset = 0?) • Beam position is estimated to be proportional to yest = (VN-VS/VN+VS) • Thus, calibration factor is calculated by real_beam_offset / yest • Then you have real_beam_offset = cal_factor * yest • For ALICE striplines in main loop (50 mm diameter) cal_factor is 13.69 • Ian Kirkman uses CST 3D and has ~13.0 for EMMAcylindrical bpms(48mm radius) (uses simulation of a thin wire ~μm thick) (has also done bench tests with wire and got good agreement) • Linearity in original ALICE calibration is only good over about +/- 5 mm • Ian’s calibration looks good only over about +/- 6 mm
Current Calibration • Linearity in measurement looks much better than in simulation Measurement #2302 ST2-BPM-04 Indicates linear response over at least 14 mm (but what if x true centre is really at -7 mm?) Simulation indicates linear response only over +/-6 mm
#2286\\dlfiles03\Astec\Projects\ALICE\Work\2011\02\19\Shift 2 • 60 pC throughout. 81.25 MHz/5. Train length 30 µS throughout • ST2-BPM-04 • Dipole calibration. ST2-DIP-04 • Data no good: left ST2-Q5 ON • ST1-BPM-02 • Dipole calibration. ST1-DIP-03 #2286. ST1-BPM-02 OTR calibration of ST1-BPM-02 Period 9-10. does not compare with
#2302\\dlfiles03\Astec\Projects\ALICE\Work\2011\02\25\Shift 2 • 80 pC (probably). 30-50 μS. 81.25 MHz/5 • ST2-BPM-04 • Results depend on train length, but slope of measured/true position (~0.4-0.5) agrees with other stripline BPMS from Period 9. 30 μS 50 μS
# 2321\\dlfiles03\Astec\Projects\ALICE\Work\2011\03\04\Shift 2 • 60 pC. 81.25 MHz/5. 30-50 μS • INJ-BPM-03. • OTR calibration INJ-YAG-02 and INJ-YAG-03 30 μS 50 μS All data Data around x measured = 0 mm
#2321. INJ-BPM-03 • Shape of beam changes significantly (booster focussing)when scanning • Beam size is large compared to how much it moves • These things may contribute to the non-linear beam based calibration curve • But wasn’t this also the case with Period 9 data?
#2321 • ST2-BPM-01. Quads ON. Effect of separately varying train length and charge 30 μS 60 pC 30 μS 20 pC 50 μS 60 pC
#2328\\dlfiles03\Astec\Projects\ALICE\Work\2011\03\07\Shift 2 • INJ-BPM-03 • 40 pC, 81.25 MHz/5. Vary the train length • Final two points changed the burst mode to /4 rather than /5 and then back again. Position changed by ~ 0.5 mm. Changed divisor to /4 then back to /5
Conclusions So Far • BPM position measurement seems to depend on beam current • Depends on train length? (several observations, stabilises > 50 μS? #2328) • Depends on bunch charge? (evidence #2321) • Possible causes of the dependence? • BPM electronics response? • Beam related factors? (beam dynamics changes (i.e. space charge), beam loading, beam background) • Dependence on charge is seen in #2321 ST2-BPM-01 (slope of x-pos vs dipole current) • Unlikely to be beam dynamics due to space charge since post-linac • Unlikely to be beam loading since running in /5 burst mode and maintained good ER throughout. • Beam background? Narrow valve in ST2, different amount of background scattering off it depending on charge? Could feasible affect slope? But probably not since good ER maintained throughout (beam loss would make ER poor) • Dependency on train length • 30 vs 50 μS dependency has been observed on several different BPMs • One experiment on INJ-BPM-03 where varies bunch length in steps from 5 to 90 μS • Both indicate that BPM position measurement is not stable to train length below 50 μS. • Data from period 8. Linearity of voltages vs. charge. • Does BPM position measurement depend on bunch rep frequency? I thought I changed this on #2328 from divisor 5 to divisor 4 and there was no change. • Noticed changes compared to Period 9-10 beam based calibration INJ-BPM-03. (Card changes?, Quads not turned off in either data set?) • Significant offset on INJ-BPM-03 absolute OTR calibration • To get good quality beam based calibration on INJ-BPM-03 may need to spend time producing a set-up for that purpose.
#2359\\Dlfiles03\alice\Work\2011\03\20\Shift 2 • Previous to this shift, BPM aquisition window (which is supposed to match the noise filter time) was inadverently set to ~32-33 μS, thus 30 μS train length may have been too small (see prev results). Rob/Mike fixed this on ~18 Feb 2001. • Did shift on 20th Feb studying 8 BPMs (one on each card) vs train length and bunch charge • Didn’t do ALL BPMs in machine because custom built software couldn’t read out all BPM data quick enough. • BPMs, in order they are arranged on cards (not machine) • INJ-DIA-EBPM-01, INJ-DIA-EBPM-05, AR1-DIA-EBPM-02, AR1-DIA-EBPM-06, ST2-DIA-EBPM-05, AR2-DIA-EBPM-04, ST2-DIA-EBPM-01, AR2-DIA-EBPM-06 • Used “FEL-02”, 27.5 MeV, 81.25 MHz /5. • Held bunch charge at 40 pC when scanning train length. • Held train length at 100 uS when scanning bunch charge. • Beam was fluctuating during measurements? ER was not always steady.
#2359 BPM vs train length (30 -100 μS) • INJ-DIA-EBPM-01, INJ-DIA-EBPM-05, AR1-DIA-EBPM-02, AR1-DIA-EBPM-06, ST2-DIA-EBPM-05, AR2-DIA-EBPM-04, ST2-DIA-EBPM-01, AR2-DIA-EBPM-06 BPM y position BPM x position E > 4 V S > 4V E > 4 V S > 4V EMMA BPM EMMA BPM E > 4 V N > 4V E > 4 V N > 4V
#2359 BPM vs bunch charge (laser transmission 5-50% 5-100 pC) • INJ-DIA-EBPM-01, INJ-DIA-EBPM-05, AR1-DIA-EBPM-02, AR1-DIA-EBPM-06, ST2-DIA-EBPM-05, AR2-DIA-EBPM-04, ST2-DIA-EBPM-01, AR2-DIA-EBPM-06 BPM y position BPM x position E > 4 V S > 4V E > 4 V S > 4V EMMA BPM EMMA BPM E > 4 V N > 4V E > 4 V N > 4V
#2359 BPM vs bunch charge.Second go after some linac adjustments to make beam steadier • INJ-DIA-EBPM-01, INJ-DIA-EBPM-05, AR1-DIA-EBPM-02, AR1-DIA-EBPM-06, ST2-DIA-EBPM-05, AR2-DIA-EBPM-04, ST2-DIA-EBPM-01, AR2-DIA-EBPM-06 BPM y position BPM x position E > 4 V S > 4V E > 4 V S > 4V EMMA BPM EMMA BPM E > 4 V N > 4V E > 4 V N > 4V
#2359 Conclusions • Some BPMS the buttons are fixed > 4 V at all beam conditions. Broken buttons/electronics, or are we very badly mis-steered. • BPM positions do not seem to change with train length. • Bunch charge makes significant effect on BPM readings in some cases. Is this due to beam changing or BPM response • Check: does beam position on screen change with bunch charge? Could use YAG in mid-chicane to judge effect of single bunch charge. From memory INJ-5 image doesn’t change position very much when charge is varied. Other high regions AR1-2 ? • There was some “jitter” of ER especially during BPM vs charge measurments.
Bunch charge Dependence xbpm ybpm N-S/N+S E-W/E+W N S E W How do the Voltages read from Ben’s program (averaged over 10 shots) relate to the voltages that are used in the electronic position algorithm?