Beam based alignment in storage rings Beam dynamics meet magnets II 02 December 2014 R. Bartolini Diamond Light Source and John Adams Institute, University of Oxford

Outline Motivations Review of tools for beam based alignment BPM-quad centring ORM alignment Girder based alignment Conclusions Beam dynamics meet magnets II 02 December 2014 Thanks to M. Apollonio, M. Boge, R. Dowd, X. Huang, L. Nadolski, J. Safranek, V. Sajaev

Motivations A correct alignment of machine component allows reaching the nominal performance of the ring Misalignments left over after installation or developing with time require corrections because: feed down effect from magnets (et al.) orbit distortion dispersion (emittance) tune shifts + beta beating generate linear coupling excitation of nonlinear resonances dynamic and momentum aperture (injection efficiency and lifetime) misalignment with photon beamlines … Beam dynamics meet magnets II 02 December 2014

Alignment Simulations usually assume (Diamond, NSLS-II, ASLS, …) ~ 100 um rms offset for girders ~ 30 um offset for magnets on girders ~ 200 urad for rolls Beam dynamics meet magnets II 02 December 2014 New trends (NSLS II, ESRF II, Diamond DDBA, …) employ alignment in situ with vibrating wire or a stretched wire measurement. Resolution of finding magnetic centre is 5-10 micron. Magnets are locked onto girder then installed. Overall magnet to magnet positioning claimed better than 30 micron. Many light sources use laser trackers and magnet fiducials. Magnet to magnet positioning ~ 50 um (but rely on the correct fiducialisation of the magnetic field to the mechanical targets).

e.g. in situ alignment Alignment towers in place for central group SPring-8 concept based on NSLS-II vibrating wire method - K. Soutome “In-situ” and beam-based magnet measurement and alignment ESRF type stretched wire for Diamond II DDBA

Tools for beam based alignment If the alignment is still not good or changes occur after installation we have to rely on beam based alignment techniques BPM - Quadrupole centring (BBA) – or sextupole centring e.g. Diamond, SPEAR III, SLS, … BPM rolls e.g. Diamond, SLS, … Individual magnet alignment e.g. APS, ASLS Girder based alignment SLS Diamond Beam dynamics meet magnets II 02 December 2014

Quadrupole centring Procedure for aligning the BPM centre of the quad centre Beam dynamics meet magnets II 02 December 2014 crucial in the commissioning phase (with loco and orbit correction) routinely repeated at light sources (every run ~ every month) BPM dipole corrector quadBPM scan quad gradient if orbit moves, beam is not at centre of quad change upstream corrector(s) to change position of the beam in quad when orbit does not move redefine the zero of the adjacent BPM orbit correction will put the beam at the centre of the quad

Quadcentre – MATLAB middlelayer Example of quad scan for Diamond BPMS (same at all light sources using Middlelayer) Beam dynamics meet magnets II 02 December 2014 BPM reading quad. centre Orbit change Orbit change due to a quadruple gradient change measured at 5 different corrector settings corresponding to 5 positions of the beam in the quad A fit allows determining the centre of the quad

BPM - Quadrupole offsets (Diamond) First runs during Diamond commissioning Quite large, but: - electrical centres of the BPMs were not calibrated - no accurate mechanical survey. Black dots are primary BPMs (different geometry) Reproducibility in adjacent runs less than few 10s  m unless some work has been carried out in CIA or in the tunnel (few100s  m) Watch out for current dependence of BPM response Heat load on chamber matters! Procedure is slow (few h for full machine)

BPM - Quadrupole offsets (SPEAR III) Beam dynamics meet magnets II 02 December 2014 Courtesy J. Safranek Quad to BPM offset < 700 um; reproducibility 23 um after 38 h

BPM - Quadrupole offsets (SLS measurements) Courtesy M. Boge

BPM - Quadrupole offsets (SLS) Courtesy M. Boge

BPM rolls from LOCO (Diamond) LOCO uses the off diagonal clock of the response matrix to fit a number of machine parameters which include the BPMs gain and coupling. This is a measure of the BPM mechanical roll and electrical cross talk between the X and Y electronics channels

BPM rolls from orbit cross talk (SLS) Similar analysis based on in-house software; Measured via corrector pattern analysis: exciting V orbit bumps at each BPM and measuring H correctors Courtesy M. Boge Dispersion function looks more like a betatron wave after cross talk correction Results cross checked with LOCO

BPM- sextupole offset A similar technique can be applied to sextupoles SLS used BPM to sextupole alignment in the sextupoles which are also equipped with skew quadrupoles. Assuming that skew quadrupole magnetic centre coincide with sextupole magnetic centre Beam dynamics meet magnets II 02 December 2014 BPM dipole corrector sextBPM Not routinely used as the BPM-quad centring

Sextupole alignment from orbit response matrix (APS) At APS the orbit is deliberately sent off axis to the sextupoles to comply with users golden orbit. Need to know the offset at sextupoles. LOCO approach can be extended to measure magnet misalignment by introducing “virtual” quadrupoles at the sextupoles location to compute the feed-down effect due to sextupole misalignment quadrupole fits the H offset at the sextupole skew quadrupole fits the V offset at the sextupole Procedure: scan sextupole by family (S1 – S4) measure ORM fit the quadrupole and skew quadrupole component linear fit on the gradient to find the H and V offset at the sextupoles

Sextupoles alignment from orbit response matrix (APS) This technique was further developed at the Australian light source (ASLS) Beam dynamics meet magnets II 02 December 2014 Offset reconstruction is in good agreement with the golden orbit provided by the BPMs (WIP) Courtesy V. Sajaev

Sextupoles alignment from orbit response matrix (ASLS) Beam dynamics meet magnets II 02 December 2014 A prerequisite is a clean linear machine (with LOCO). Main target is the V offset in sextupoles individually to reduce the V emittance. Instrumental in achieving the quantum limit 0.35 pm Used the V offset to shim each individual magnet on the girder (the sextupoles were systematically lower vertically). Shim in units of 25 um, total shim reached 150 um in many cases Courtesy R. Dowd

Sextupoles alignment from orbit response matrix (ASLS) Beam dynamics meet magnets II 02 December 2014 Shimming sextupoles for V alignment at ASLS Courtesy R. Dowd

Survey data for girders: SLS Aiba et al NIMA 694, 133,. (2012) SLS found (2010) that the main source of quadruple misalignment is actually due to the girder misalignment (correlation in offset of individual quads on the girder) Beam dynamics meet magnets II 02 December 2014

Survey data for girders: Diamond Beam dynamics meet magnets II 02 December 2014 Survey August 2013 H-plane Survey January - August 2013 V-plane best fit plane

Survey data for girders: Diamond Beam dynamics meet magnets II 02 December 2014 H-plane Survey March 2014

Survey data for girders: SPEAR III Beam dynamics meet magnets II 02 December 2014 Courtesy J. Safranek

Girder shift vs corrector: SLS Courtesy M. Boge Such girders shift produce a characteristic corrector pattern and induce V dispersion and optics perturbation

Girder remote control movers: SLS Courtesy M. Boge

Beam assisted girder alignment Courtesy M. Boge Girder realignment based on quadrupole survey data (2011). Girder pitch and heave were fitted to the quadrupole data. Girders were moved according to these fits on survey data, with stored beam and running FOFB. Resulting corrector changes used to calculate the girder movement to confirm the proper mechanical re-alignment online Quadrupole-BPM BBA done successively

Beam assisted girder alignment Courtesy M. Boge Girder realignment instrumental in reducing the vertical coupling to 1.2 pm and 0.9 pm. New campaign of BAGA foreseen based on survey data taken in 2013

Girder movers experience at Diamond LVDT motion sensors G3 end Camsha ft Axes of rotation Bearin g Range of motion Uncontrolled cam motion cracked a bellow in a mock up cell. LVDTs were installed

Modelling effect of girder misalignments Beam dynamics meet magnets II 02 December 2014 AT model MATLAB functions change magnet positions according to survey girder  (sway, yaw, heave, pitch) quadrupoles/sextupoles magnetic centres moved w.r.t. to girder according to survey data (residuals <50um) BPM in the model primary fixed to floor, secondary anchored to girder DX YAW primary BPM Bending secondary BPM QUAD SEXT

Realignment strategy Used both survey data and correctors pattern to devise the required re-alignment Survey data have to be used with care absolute survey data do not reproduce the corrector pattern in the model – this is basically still unclear why! however small changes in girder positions (~ um) can be reproduced well and generate a distinctive corrector pattern after orbit correction. Machine and model agree well. Additional (and severe) complications arise by the constrain of not moving the photon beam down the beamline (IDs and bendings) We re-aling the girder in collaboration with the beamlines: usually we introduce a golden offset that restores the beam position to exactly the same point in the beamline (transparent realignment)

Girder shift vs correctors: machine vs model original position new position C03 G1 G2 G3 HC3G2 sway=+324um - orbit variation correctly reproduced - CM variation correctly reproduced - local reduction in total CM kick angle reasonably reproduced

Girder based alignment (cell 3) (1) (2) 22/10/2013 VC3G1 heave = -245 um pitch = urad GO =191 um model data (A)BPM offset after BBA cell-3 VC3G1 - 22/10/2013 heave = -245 um pitch = urad MOPRO101, IPAC2014 (SP,  ) = (-95um,-28urad) (A)Orbit after move (B)Effect of BBA (C)Effect of GO(3,1) = 181um XBPM-measured tilt: -28.7urad / AT prediction: -28. urad

Conclusions Beam dynamics meet magnets II 02 December 2014 A collection of tools for beam based alignment is available and has been well tested at several different light sources ORM technique at ASLS shows that sextupole horizontal and vertical offsets can be measured with the beam to a accuracy of microns depending on the magnet position. Girder alignment with remote control proven at SLS and encouraging results are obtained at Diamond Beam based alignment techniques can potentially achieve measurements of the magnetic centre of magnets to precision comparable to the best bench-top techniques

Conclusions We should include the BBA and tuning technique in the definition of the tolerances for alignment and for the magnet design (e.g. supports) This is not commonly done in the design stage error study generally assume orbit corrections, tune corrections chromaticity correction but not BBA It is likely that the requirement will ease to “just allow the beam to be stored” without maxing out the correctors. Beam dynamics meet magnets II 02 December 2014

Conclusions Adapted from SLAC DLSR Workshop, Accelerator Session Close-Out December 11, 2013 Tolerances on alignment and field quality Over-specifying tolerances can be very expensive There was a general consensus that the tolerances assigned to magnet alignment and the field quality are computed with a pessimistic approach. The possibility of beam based correction should be considered in specifying the tolerances. This is true for alignment errors both for orbit and for optics corrections. Beam based correction tools like BBA, LOCO, coupling free steering can significantly relax the tolerances. It as suggested to explore setting tolerances based on first-turn just to store the beam.