ATF2 Report ALCW Kiyoshi KUBO, for ATF2 Collaboration

1.3 GeV ATF2: Final Focus Test ATF, Accelerator Test Facility (KEK)

Goals of ATF2 +  Achievement of small (37 nm) beam size (Goal 1) – Demonstration of final focus system based on local chromaticity correction Control of beam position (Goal 2) – Demonstration of beam orbit stabilization with nano-meter precision at the IP, using intra-pulse feedback Understand (and solve, if possible) beam size intensity dependence (“Goal 3”) Other studies: Lower betay* (mainly for CLIC) Ground motion – orbit feedforward Development of instrumentations (beam monitors) etc.

Local chromatic correction Final Quadrupole magnets collision Chromatic Correction 6-pole magnets Geometric Correction 6-pole magnets ~n  Horizontal Dispersion (P.Raimondi and A.Seryi, Phys. Rev. Lett. 86, 3779 (2001))

Status of Goal 1 44 nm (or smaller) beam size confirmed (design: 37 nm) at low intensity (June 2014). Small beam can be achieved repeatedly and quickly, even after machine shutdown. Local Chromaticity correction was demonstrated. (Without chromatic correction, beam size is ~450 nm.) Strong intensity dependence was observed. (It had not been expected.)  studies continued

History of measured minimum beam size Presented in IPAC14

What Contributed to the Improvement? Cures for Higher Order Magnetic Field Errors. – Multi-pole field components of Quadrupole magnets Adopt optics with 10 times larger  * x than nominal, smaller beam size at magnets  reduce x-y coupling effects Replaced final QF magnet (max. horizontal beam size) (Small aperture, strong multi-pole fields  Large aperture, weak multi-pole fields) Found one coil of strongest 6-pole magnet was shorted Exchange with weakest one (January 2013) Turned off, by changing 2 nd order optics (April 2014) Suppress Orbit Drifts and jitter in Final Focus Beam Line – Improvement of orbit feedback – Reduce magnet vibration Improvement of Beam Size Monitor Wakefield reduction (?)

Beam Size Tuning after 3 weeks shutdown Small beam (~60 nm) observed ~32 hours from operation start ~10 hours of IP beam size tuning  y (nm) Time (hours) from Operation Start after 3 Weeks Shutdown Week 2014 April 7 Beam Size Tuning after 3 days shutdown Small beam (~60 nm) observed ~16 hours from operation start ~8 hours of IP beam size tuning Week 2014 April 14 Presented in IPAC14

Figure from: Y. Yamaguchi, Master thesis at Graduate School of Science, The University of Tokyo, 2010 Beam Size Monitor at Focal Point (IPBSM) using interference of laser beam Scan interference fringe phase. Fit modulation M : Fringe phase Gamma-ray signal G Example Small M Large beam Large M Small beam

Data of June 2014 Mean: 44 nm Standard dev.: 3 nm Beam Size Evaluated from Modulation (no systematic error assumed) Mean: 0.58 Standard dev.: 0.05 IPBSM Modulation (174 degree Crossing angle) Bunch charge ~ 0.16 nC Presented in IPAC14

Smallest observed beam size was 44 nm. Why not 37 nm? Beam was jittering? May increase apparent beam size by 2-3 nm. Effect of wakefeild even at low intensity? May increase beam size by 2-3 nm. Beam size monitor was not accurate enough? – Stability of interference fringe phase, intensity, etc. etc. ?

Recent improvements in beam jitters caused by magnet vibrations Upstream Quad magnets – Induced by cooling water. Final H-focusing magnet (QF1) – New magnet with large aperture is heavy for the present support (mover) and had large vibration.

Upstream magnets (QF1X, QD2X ) vibration reduced We found large vibrations of the first two quad magnets in Extraction Line had been induced by a cooling water pipe. The pipe was moved. Vibrations reduced and beam orbit jitter significantly reduced. (amplitude factor ~0.7) J. Pfingstner, et.al., Phys. Rev. ST Accel. Beams 17, Orbit jitter before and after Measured jitter by BPM Jitter predicted from motions of magnets

QF1 (Final H-focus Q) vibration QF1 QD0 Original small magnet: Vertical displacement 4 nm, main resonance freq. 66 Hz (2008) Replaced by a large magnet in November 2012 (better field quality) Vertical displacement  21 nm, resonance  31 Hz (2013 ) 2014 Sims inserted for tighter support Vertical displacement  15 nm Plan in 2015: New stronger support Measured by A.Jeremie, et.al.

Intensity Dependence – Wakefield? Beam size at IP strongly depend on bunch intensity Various efforts to reduce wakefield (bellows shield, removal of unused structures, moves of structures from high beta to low beta region) Theoretical and Experimental estimation of wakefield strength of structures (cavBPM, Bellows) Found wake field of OTR monitor chamber affected beam size and orbit. Further experimental studies performed: Test of Wakefield Free Steering, Intensity dependent orbit measurement, etc.. Wakefield much stronger in ATF2 Line than in ILC Final Focus. Low beam energy, long bunch

IPBSM modulation as function of bunch population. Measured with crossing angle 174 degrees (left) and 30 degrees (right). Beam Size Depends on Bunch Intensity Presented in IPAC14 ATF2 design bunch charge ~1.7 nC

Effect of OTR monitor chamber (beam size monitor in EXT line) to IP vertical beam size was found (June 2014) Tested removal of all monitors: Reduce intensity dependence ~1/2 Optimized positions: Reduce intensity dependence ~1/2

Photo by D. McCormick OTR monitor View Port Shield No shield With shield by A. Lyapin Remove vertical asymmetry Reduce position dependent wake (factor 0.6)

Wakefield measurement and compensation setup J. Snuverink, et.al., ATF2 Project Meeting LAPP

Recently improved calculation

J. Snuverink, et.al., ATF2 Project Meeting LAPP Good agreement with experiment

Remaining studies on Goal 1 On going Confirm emittance of incoming beam and optics matching (  * y ) – Including lower betay* optics study Detect/correct beam position drift/jitters – High resolution BPMs at IP region will solve the question. – Orbit feedback with 2 bunch mode, and measurement of 2 nd bunch beam size Study of intensity dependence (Wakefield) Near Future Nominal optics (nominal horizontal beta*) Experimental simulation of ILC beam tuning

Goal 2 (stable beam by Feed Back) Main Program since Oct 2014 New IPBPM installed and commissioned – low Q, short decay time (~15 nsec) for ~200 nsec bunch separation – Major issues with short decay time Separation “transient” signals (undesired) and dipole signal (used for beam position evaluation). – Another present issue is “static” signal components (does not depend on beam position), overlapped with dipole signal. Affect dynamic range (about ± 5 um for nanometer resolution) Preliminary resolution estimation Intra-pulse feedback test at IP

In vacuum IP-BPMs and piezo movers BPM A&B BPM C Piezo Movers (PI) Piezo Movers (Cedrat) BPMs – Bolted aluminum plates, no brazing because of In- vacuum. – BPM A&B bolted together. – BPM C is independent. Piezo mover – BPM units are mounted on the base with three piezo movers. – Dynamic range of each mover is +/- 150 um. IP Slide from Terunuma Initial alignment need to be better than this. Vertical and horizontal offsets, pitch, rotation of each block can be adjusted. (no yaw)

IPBPM Resolution estimation (preliminary) High beta-function optics (small angle jitter) – 3 BPMs: Compare position of one BPM with predicted from the others. Nominal (low beta) optics – Set beam waist (focal point) at one BPM and look at the position jitter. Resolution < Jitter of position reading

Large beta (small angular jitter): Position jitter at all 3 BPMs within dynamic range Small beta (large angular jitter) Position jitter at only 1 BPM can be within dynamic range

High beta optics, 3 BPMs FittingGeom 42nm 47nm55nm From P.Burrows, in ATF2 Project Meeting LAPP IPA 66nmIPB 47nm IPC 48nm From Siwon Jang, in ATF2 Project Meeting, LAPP Residual (Measure – Predicted) N~5E9

28 Jitter vs. QD0FF setting (waist scan) 66nm (single)  49nm (avg.) Slide from P.Burrows, in ATF2 Project Meeting LAPP Set waist at BPM N~5E9

Feedback Test (FONT) 2 bunches in one pulse with ILC-like bunch spacing (up to ~300 ns) Measure position of the first bunch and correct following bunch

FONT5 operation modes Aim to stabilise beam in IP region using 2-bunch spill: 1. Upstream FB: monitor beam at IP 2. Feed-forward from upstream BPMs  IP kicker (IPK) 3. Local IP FB using IPBPM signal and IP kicker Slide from P.Burrows, in ATF2 Project Meeting LAPP

2013 beam stabilisation results 1.Upstream FB: beam stabilised at IPB to ~ 300 nm 2. Feed-forward: beam stabilised at IPB to ~ 106 nm 3. IP FB: beam stabilised at IPB to ~ 93 nm Slide from P.Burrows, in ATF2 Project Meeting LAPP

32 Best IPFB results Bunch 1: not corrected, jitter ~ 400nm Bunch 2: corrected, jitter ~ 67nm Corrected jitter 67nm  resolution 47nm Slide from P.Burrows, in ATF2 Project Meeting LAPP (2014 December) 2 bunch operation. Measure 1 st bunch position and feedback to 2 nd bunch. Only 1 BPM was used. (If resolution is dominant)

33 Best IPFB results Slide from P.Burrows, in ATF2 Project Meeting LAPP Note the different scales

Other important studies performed Beam tuning with low betay* optics (together with study for Goal 1) – Betay* 0.05 mm (design x 1/2) (betax* 40 mm, design x 10) – Beam size tuning etc. performed. – Study continued for more accurate optics setting (accurate measurement of emittance and beta function, etc.) Ground motion – orbit feedforward study – Correlation between motion of magnets and orbit change observed.  possibility of feedforward Beam halo measurement – Diamond sensor installed and data taking – Compare with wire scanner data, etc.

SUMMARY Small beam size at IP (Goal 1) – 44 nm, confirmed at low intensity. Close to designed 37 nm. – Compare with size calculated without chromatic correction, 450 nm.  Local chromatic correction scheme (used at ILC) has been demonstrated. – Small size routinely observed with short time (~8 h) tuning. – Improvements in beam orbit jitters, etc. Intensity dependence – Reduced by reducing wakefield. But not yet fully understood. Study continued. Near future: Operation with nominal (horizontal) optics and simulation of ILC FF tuning Position stabilization at IP (Goal 2) – New IPBPM (Low Q for multi bunch) installed and operation started and some preliminary results obtained. – Preliminary resolution ~ 50 nm (Should be improved.) – Successful feedback (residual jitter ~ BPM resolution ) Other studies continued