SPS Orbit studies Contents: - Motivation - Stabilization of orbit at extraction points - Search for orbit drift sources presented by Eliana GIANFELICE (Fermilab and CERN) APC Seminar, Fermilab, November 21, 2013 1/61 < > � � ⊖ ? i P ≪ ≫ �
Motivation This work has been triggered by the difficulties by transferring the beams to LHC ob- served after the SPS optics was changed to the Q20 one in September 2012 and transfer lines changed consequently. Being the measured beam emittance in LHC apparently not affected, an optics mis- match SPS-line-LHC is unlikely. The hypothesis that the SPS orbit at extraction is not well reproducible has been inves- tigated. Issues to be addressed: • Are the observed orbit changes relevant? • Is it possible to correct them? • Is it possible to find a source? 2/61 < > � � ⊖ ? i P ≪ ≫ �
Preamble The CERN SPS was brought in operation in 1976 and accelerated protons for fixed p collider, e ± injector for LEP and currently target experiments, thus converted into a p ¯ used as LHC injector, for fixed target experiments and for producing neutrinos beams for the Gran Sasso experiments (CNGS). The orbit at extraction is not corrected by active elements. Few interlocked bumpers are used for steering the beam at the extraction points. At the beginning of operation orbits at top energy are recorded and quadrupoles moved for minimizing the orbit ex- cursion wrt reference. The SPS optics for neutrino production and for LHC injection being different, the ma- chine alignment is done for the more intense CNGS beam (going to be changed!). 3/61 < > � � ⊖ ? i P ≪ ≫ �
Orbit reproducibility at extraction (Q20, Fall 2012 measurements) • The variations are larger in the horizontal plane. • Is the spread relevant for the transmission stability in the lines? 4/61 < > � � ⊖ ? i P ≪ ≫ �
Those orbits at flat top have been analyzed and used for simulating the resulting trajec- tory in TI8/TI2. The starting conditions are found by a Fourier analysis of the difference closed orbit wrt. average. Resulting TI8 trajectories (computed) 5/61 < > � � ⊖ ? i P ≪ ≫ �
3 σ envelopes ( ǫ N =1.7 × 10 − 6 ) with collimators at 5 nominal σ , including dispersion contribution with dp/p =0.0295% and excluding worst case (after run stop): 6/61 < > � � ⊖ ? i P ≪ ≫ �
• Results are consistent with the trajectory actually measured (L. Norderhaug-Drøsdal et al., IPAC13) • The SPS orbit variations may explain the losses in the transfer lines. 7/61 < > � � ⊖ ? i P ≪ ≫ �
Trajectory mis-match, which would lead to a large emittance dilution, is compensated by dampers in LHC, but time consuming manual steering by the machine crew was needed to keep transfer clean. Ongoing studies/improvements may solve/mitigate the problem. If not: • Use of the already interlocked extraction bumpers for automatically correcting beam position and slope at the extraction points wrt golden values. Advantages: – Compensating directly the source for the changing trajectory in extraction lines. – Potential for time saving. Best strategy (if any) to be found by simulations. Use of the large aperture BPMs located at the extraction points (BPCEs) may be problematic. 8/61 < > � � ⊖ ? i P ≪ ≫ �
Strategies for correcting the orbit at the extraction points Investigation goal: can we infer beam position at extraction points from orbit measured at BPMs along the ring (no BPCEs)? Simulation steps: • Introduce random radial misalignments and assign orbit to reference orbit . • Add new radial misalignments. – Use all available “correctors” for steering to the reference orbit, thus compute orbit due to correctors only. a ( “fake correction” ). or – Use Fourier analysis of difference orbit at BPMS and use main components to compute orbit all over the ring and in particular at the extraction points. or – Trajectory amplitude and phase fit. • Use bumpers for restoring radial position and angle at extraction septum keeping orbit unchanged outside the bump. a actually as we are not correcting the orbit, we can possibly use a larger number of elements if convenient. 9/61 < > � � ⊖ ? i P ≪ ≫ �
“Fake correction method” (simulation for testing purposes) Horizontal misalignment of all quadrupoles δ rms =0.25 mm + 0.10 mm, 10 seeds. Ex- pected A 2 (20 seeds) in TI2, no BPCE’s, no BPMs reading errors: ǫ err ǫ corr x x ( µ m) ( µ m) closed orbit 0.250 ± 0.205 0.002 ± 0.001 trajectory 0.250 ± 0.205 0.003 ± 0.002 Corrector strengths: θ nom θ (mrad) (mrad) kmpsh61402 0.000 0.007 ± 0.038 kmplh61655 0.512 0.502 ± 0.039 kmplh61996 0.094 0.101 ± 0.042 kmpsh62199 0.398 0.388 ± 0.035 10/61 < > � � ⊖ ? i P ≪ ≫ �
Add random BPM calibration errors. BPMs gain errors: EALIGN, MSCALX:=0.25*TGAUSS(1),MSCALY:=0.; 20 15 10 5 0 0.6 0.8 1 1.2 1.4 11/61 < > � � ⊖ ? i P ≪ ≫ �
No BPCE’s, with BPMs calibration errors: EALIGN, MSCALX:=0.25*TGAUSS(1),MSCALY:=0.; Expected A 2 ǫ err ǫ corr x x ( µ m) ( µ m) closed orbit 0.250 ± 0.205 0.009 ± 0.015 trajectory 0.250 ± 0.205 0.010 ± 0.016 Correctors strength θ nom θ (mrad) (mrad) kmpsh61402 0.000 0.005 ± 0.036 kmplh61655 0.512 0.501 ± 0.040 kmplh61996 0.094 0.098 ± 0.040 kmpsh62199 0.398 0.388 ± 0.035 12/61 < > � � ⊖ ? i P ≪ ≫ �
Now test with measured difference orbits. By a SVD fake misalignments for all quads are found which reproduce the measured orbit difference in the MADX model. Example: Ref.: ORBIT SPSRING 18-10-12 18-06-21 LHC2 CY64400 T18500.data (18/10/12) Orbit: ORBIT SPSRING 21-10-12 22-11-58 LHC2 CY4940 T18500.data (21/10/12) Q20 Difference Orbit 1500 Measured Difference 1000 MADX (offsets from fit) 500 x ( µ m) 0 -500 -1000 -1500 0 20 40 60 80 100 120 BPM index 13/61 < > � � ⊖ ? i P ≪ ≫ �
Use 589 measured difference orbits (from October 13 to 29 2012) a . Expected A 2 : ǫ err ǫ corr x x ( µ m) ( µ m) closed orbit 0.016 ± 0.024 0.020 ± 0.014 trajectory 0.016 ± 0.024 0.020 ± 0.014 Correction fails ! What is happening? a L. Norderhaug Drøsdal courtesy 14/61 < > � � ⊖ ? i P ≪ ≫ �
A closer look to one “offending” case: ORBIT SPSRING 19-10-12 10-06-54 LHC2 CY161 T18500.data- ORBIT SPSRING 13-10-12 15-42-21 LHC2 CY52822 T18500.data 4000 measured MADX-reconstructed 3000 Coordinates at septum MADX-corrected 2000 x ′ x ǫ x 1000 (mm) (mrad) ( µ m) 0 0.54602 -0.01384 4.7e-3 -1000 -2000 -0.58676 0.00549 7.6e-3 -3000 0 1000 2000 3000 4000 5000 6000 7000 nb: In the picture there is by mistake a longitudinal mis-match of 706 m between “MADX” and “measured” because of a different starting point! 15/61 < > � � ⊖ ? i P ≪ ≫ �
Including one MKE, MST and MSE for fake correction. 4000 measured MADX-reconstructed 3000 Coordinates at septum MADX-corrected 2000 x ′ x ǫ x 1000 (mm) (mrad) ( µ m) 0 0.54602 -0.01384 4.7e-3 -1000 -2000 -0.54028 0.00417 4.4e-3 -3000 0 1000 2000 3000 4000 5000 6000 7000 nb: In the picture there is a longitudinal mis-match by 706 m between “MADX” and “measured” ! 16/61 < > � � ⊖ ? i P ≪ ≫ �
A closer look to a second “offending” case: ORBIT SPSRING 24-10-12 02-33-37 LHC2 CY9160 T18500.data- ORBIT SPSRING 13-10-12 15-42-21 LHC2 CY52822 T18500.data 5000 measured 4000 MADX-reconstructed Coordinates at septum MADX-corrected 3000 x ′ x ǫ x 2000 1000 (mm) (mrad) ( µ m) 0 -1000 0.7094 -0.0212 10.4e-3 -2000 -1.113 0.012 24.6e-3 -3000 -4000 0 1000 2000 3000 4000 5000 6000 7000 nb: In the picture there is a longitudinal mis-match by 706 m between “MADX” and “measured” ! 17/61 < > � � ⊖ ? i P ≪ ≫ �
Including one MKE, MST and MSE for fake correction. 5000 measured 4000 MADX-reconstructed MADX-corrected 3000 x ′ x S ǫ x 2000 S 1000 (mm) (mrad) ( µ m) 0 0.7094 -0.0212 10.4e-3 -1000 -2000 -0.489 0.0 10.e-3 -3000 -4000 0 1000 2000 3000 4000 5000 6000 7000 In both examples the corrected orbit shows similar spikes and a wiggling around 1300 m. nb: In the picture there is a longitudinal mis-match by 706 m between “MADX” and “measured” due to a different starting position. 18/61 < > � � ⊖ ? i P ≪ ≫ �
Analysis of 589 difference orbits repeated including those “correctors”. ǫ err ǫ corr x x ( µ m) ( µ m) closed orbit 0.016 ± 0.024 0.012 ± 0.012 trajectory 0.016 ± 0.024 0.012 ± 0.012 Correctors strength θ nom θ (mrad) (mrad) kmpsh61402 0.000 -0.003 ± 0.003 kmplh61655 0.512 0.503 ± 0.006 kmplh61996 0.094 0.090 ± 0.004 kmpsh62199 0.398 0.392 ± 0.005 It is better, but this is not the explanation... 19/61 < > � � ⊖ ? i P ≪ ≫ �
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