Beam-based alignment techniques for linacs Masamitsu Aiba, PSI BeMa 2014 workshop Bad Zurzach, Switzerland Thanks to Michael Böge and Hans Braun

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Contents Motivation/Necessity of BBA in linacs Terminology: Beam-Based Alignment BBA techniques FEL BBA Linear collider BBA Summary

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Motivation/Necessity (1) “Linear accelerator” is supposed to be “straight” but actually not… – Because of static and dynamic* imperfections: misalignments, geomagnetism, stray field, ground/mechanical vibration, etc. – Adverse effects, spurious dispersion and wakefield, need to be suppressed by beam-based alignment * This talk covers BBAs for static imperfections

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Motivation/Necessity (2) Major sources of (projected) emittance growth – Dispersion in accelerating structure – Transverse wakefield Other sources: CSR, micro-bunching etc.

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Beam-Based Alignment? BBA: Alignment of accelerator components based on beam measurements – However, the components, namely quads and BPMs, can be virtually displaced: BPM centre recalibration with soft off-set Quad shift with corrector integrated into (close to) quad By x Q Q+C C Shift Soft offset Beam X=X read -X off-set The term “BBA” is used even when no physical component alignment is performed

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI BBA techniques (0) One-to-one – Procedure Just steer the beam to the centre of BPMs – Feature Used for the initial “tunneling” Most likely, it is not sufficient but gives a good starting point Initial alignment dependent Model independent (if the orbit response matrix for steering taken from the machine)

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI BBA techniques (1) Ballistic alignment – Procedure Divide linac into bins Align the first bin and then next ones Turn off the magnets in the bin under alignment Determine reference straight in a bin (the first and the last BPM) Align the BPMs within the bin w.r.t. the reference straight – Features Simple procedure Affected by error (environment) field Initial alignment dependent Model independent Simulation Figure taken from T.O. Raubenheimer and D. Schulte, Proc. of PAC’99, pp

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI BBA techniques (2) Dispersion free steering * – Procedure Measure orbit difference (dispersion) by varying either the beam energy or scaling the magnets Compute possible steering (corrector setting) to minimize the dispersion – Features Error field is taken into account Initial alignment independent Model dependent Extended to dispersion+wakefield free steering See T.O. Raubenheimer, NIM-A, 306, pp (1991) Measurement at SLC Figure taken from R. Assmann, et al., SLAC-PUB * T.O. Raubenheimer and R.D. Ruth, NIM-A, 302, pp (1991)

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI BBA techniques (3) Emittance tuning bump – Procedure Introduce orbit bump over quad => Dispersion correction Introduce orbit bump over cavity => Wakefield correction Find optimum bumps based on emittance measurement – Features Handy and applicable to almost any location Based on emittance measurement Model independent Measurement at SLC Figure taken from, C. Adolphsen, F.-J. Decker and J. T. Seeman, Proc. of PAC’93, pp

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI BBA techniques (4) “Empirical tunings” – Procedure Establish target function (ex. luminosity, FEL power) through BBA Optimise target function(s) using available/effective knob(s) Algorithms: Dither, Simplex, Random optimisation, etc. – Features Handy and easy to implement Overcome limitations in BBA Model independent Dither Luminosity optimisation at SLC Figure taken from, L. Hendrickson et al., SLAC-PUB-8027 (1999) Simple feedback does not work for quadratic response X IP L

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI FEL BBA (1) General remarks on FEL BBA – Linac BBA is rather easy Linac is relatively short => Negligible nonlinear dilution Trajectory error tolerance on the order of 100  m DFS and/or Ballistic alignment would be enough – Undulator section requires precise BBA Electron and photon beams overlap for lasing Trajectory error tolerance on the order of 1  m!

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI FEL BBA (2) LCLS undulator BBA – Procedure Record absolute BPM reading for various beam momenta Compute BPM and quad misaligments and align their positions Repeat above until the orbit difference (dispersion) is minimised – Features Similar to DFS but find absolute BPM and quad misalignments Requires high-resolution BPM to meet the tolerance Requires large momentum change Simulation for LCLS undulator section Figure taken from P. Emma, R. Carr and H.-D. Nurh, NIM-A, 429, pp (1999) This method successfully achieved X-FEL lasing at LCLS !

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI FEL BBA (3) Photon beam based undulator alignment – Procedure Open all the undulator gap except for the first one Align electron beam orbit through the undulator such that spontaneous radiation points to the centre of a downstream photon BPM Open the gap of the first undulator and continue with the down stream undulators – Feature Electron orbit angle is well corrected while offset may be left This method is employed and successful at SACLA, etc.

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI FEL BBA (4) “Corrector-based undulator alignment”* – Procedure Steer the beam to bring the beam to the centre of BPMs and record the corresponding corrector strengths Strong correction indicates a large misalignment! Find BPM positions to minimise the deviation of corrector strengths w.r.t. their average values (Steer the beam to BPM centre when BPM are shifted) Corrector strengths will be all zero if no misalignments and no error field A running orbit feedback eases the above procedure (safe for undulator) – Feature Random error field dependent (Undulator error fields must be small!) No momentum change is required and thus quick * M. Aiba and M. Böge, Proc. of FEL’12, pp Corrector strengths must be the same value for periodic line

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI FEL BBA (4’) Use the magnet excitation current as the observable: BPM reading Corrector strength (or Feedback) Simulation for SwissFEL undulator section * See also application to ring: M. Böge et al., Proc. of IPAC’13, pp

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Linear collider BBA (1) General remarks on LC trajectory control – Long linac Nonlinear emittance dilution can be accumulated Emittance tuning bumps are essential in addition to DFS and/or Ballistic – High energy Limited corrector strength Corrector field => Radiation – Small beam at collision Vibrations even in nano-meter scale have impact Intra bunch-train feedback is essential (also for European X-FEL) A lot of challenges!

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Linear collider BBA(2) Simulation for CLIC, DFS+bumps Figure taken from P. Eliasson and D. Schulte, PR ST-AB, 11, (2008) Intra-train feedback experiment Figure taken from, R. J. Apsimon et al., Phys. Procedia, 37, pp (2011) IP BPM R&D achieving 22 nm resolution Figure taken from, S. Jang et al., Proc. of IPAC’14, pp (2014)

BeMa 2014, Bad Zurzach, Switzerland, Masamitsu Aiba, PSI Summary BBA techniques for linacs – One-to-one, Ballistic, DFS, Empirical tunings – Model independent methods except for DFS (but an iteration covers deficiencies of the model) Quadrupole calibration tolerance is loose and may be determined by the requirements from optics measurement (typically on the order of 0.1%), not from BBA DFS result is independent from initial alignment errors FEL BBA – Relatively easy for short linac – Electron and photon beam based undulator section alignments have been established (Undulators are aligned w.r.t. the aligned BPMs) – Corrector based alignment proposed Linear collider BBA – A lot of challenges and many R&Ds are underway