Luminosity Tuning
Franz-Josef Decker 26-Oct-2006
1. Hints for tuning areas
2. What tools are available besides tunes
3. Examples for coupling and dispersion
4. What is changing
5. How to change a beta beat
6. By 4 pattern tuning reveals coupling effect of parasitic crossing
7. Lifetime bumps [found due to trouble during wiggler experiment]
Fit with kick here
We get hints from our “regular” characterization, where improvements might be helpful
Control system gives you 6*8*2 = 96 bumps
Effects and uses of sextupole bumps[picture from SLC Final Focus paper]
Anti-symmetric: (2 phases: ,’)
In y: y-dispersion, fixes eta_y due to orbit variationsIn x: x-dispersion, used for LER x-emittance control
Symmetric: (4 phases: xy, yx)
In y: Coupling, SD: xy, SF: yx
In x: Beta beat and (tune shift) SD for y, SF for x (only ONE phase per arc!) [two “canceling” sym x in SDs increases lifetime!]
Symmetric y-bumps at SD2s (coupling): 50 m causes 2.5% less luminosity
Howard Smith
Anti-symmetric y-bumps (dispersion): 210 m causes 2.5% less luminosity
Howard Smith
ARC 11 ARC 05
Symmetric bumps 165 um 50 um 90 um 70 um (coupling)
Anti-symmetric bumps 560 um 210 um 410 um 285 um(y-dispersion)
Measured Luminosity Sensitivity
Bumps necessary to reduce luminosity by 2.5 %
Dispersion is about 4 times less sensitive than coupling, so it is better to use SYM and ASY knobs than individual sextupole bumps. Operators fine tune these to about half these offsets or 0.5% luminosity change
Q4R raft motions
Q2 chamber seen as Q4 raft motion causes luminosity dipsup to half an hour after filling from scratch
25% Luminosity dips
LER: Low current minus high current orbit difference
Big kicks near septum (?) and at Q2 chamber. After new Q2 chamber HER injection was impossible without lowering septum bump.
20% of nominal
Correctors Dispersion for 1mm bump1. Chicane: + – – + zero 2. 60 deg lattice: + – + 2 mm HER3. 90 deg lattice: + + 4 mm LER4. Double bump: + – 8 mm
5. ASYM SF1: 74 mm6. ASYM SF2: 140 mm7. ASYM SD1: 152 mm8. ASYM SD2: 294 mm
9. 4-corrector: 7 mm10.Septum-bump: 80 mm (for 12 mm bump)
(not compensated in design)
Dispersion Bumps in Numbers
How to change a beta beat
1. Find location withclear max or min at sextupole pair(SF for x, SD for y)
2. Sym. +x bump willwill make quad stronger
3. Like IP with dynamic betamore focusing reduces beta
4. Therefore moveto –x in this case
5. Depending on tuneamount is about 1-2 mm for a beatof 4
6. Fix tune change
Design
Luminosity Tuning for By 4 Pattern (no parasitic crossing)Parasitic crossing has HUGE coupling effect,: 550 um sym. y-bump necessary
By 2 By 4
1 hrtuning
[(550/50)^2 * -2.5% = -300% ?]
Heating due to wigglerHere we have 2000 mA in LER and we need about a limit of 175 deg F to be able to go all the way to 3000 mA. Synchrotron light is linear.
Wiggler bump (anti-symmetric in x) to reduce eta_x at wiggler to have mainly a damping wiggler (15% more damping 5% more luminosity)
nux nuy beam lifetime 15% bump: -781E-6 0.0110 103 minutes 30% bump: -468E-5 0.0253 103 minutes 40% bump: -664E-5 0.0398 very low
Summary of Wiggler Experiment
The Bad:1. Need to raise 150F limit to 175F for 3000 mA2. LER x-emittance too big, low luminosity3. Nu_y change not understood4. Eta_x reducing wiggler bumps create VERY low lifetime
The Good:1. Found quadrupole solution which directly reduces curly H2. VERY low lifetime triggered search for cause and finally
found lifetime knobs.
Effects of x-bumps at FOUR sextupole pairs
SD1 SD2 SD2 SD1 typical arc
x x X X X X x x 180 90 180 90 180 90 180
Bump signs: + + y-beta beat and tune change – – + + y-beta beat only– – – – + + y-beta beat just in between big effect on lifetime!
After bumps of -400 um and +200 um in three LER arcs:
Less loss at septum: 120,000 20,000 [counts]More lifetime: 45 150 [min]Less DCH background: 1000 800 [counts]
Lifetime Bumps
LER beam loss monitor at septum reduced from 120,000 to 40,000 after first arc bumps.
Wrong way
Reduced Drift Chamber background -12% and -8%
Yuri Nosochkov showed, that the beta beat has a small effect while the created phase shift can cancelthe third order chromaticity.
Before (design) after bumps
Summary
1. Tuning is still very necessary
2. We have knobs which control:a) dispersion x and yb) coupling xy, yxc) beta beat in x and yd) new: lifetime e) Wx, Wy [G. Yocky’s talk]
3. Often machine changes, like By4, MIA solutions, wiggler, etcrequire big tuning efforts
4. Many thanks especially to the newest operators, which push the machine to higher luminosity peaks!
Outlook
More cross fertilization between tuning to modeling
1. Tuning Modeling [tune shift, dynamic beta, coupling] not in model yet: a) beam-beam crossing at IP (partly done) b) beam-beam effect at parasitic crossing c) distributed tune shift due to elliptic chamber vs local compensation d) higher order effects
2. Modeling Tuning a) more tries to dial in dispersion or coupling solutions into machine b) localizing strong coupling changes by looking at “coupled phase”
69: PR06 1072 SD2 -2.1 mm+1.8kG126: PR04 1022 SCX3 ? 133: PR02 4112 SCY3/SK6 ?147: PR02 3149 SK4 +1.4 kG more210: PR12 3092 SKG2 -1.6 kG
