ATF 2 Report ALCW 2015 04 Kiyoshi KUBO

ATF 2 Report ALCW 2015. 04. Kiyoshi KUBO, for ATF 2 Collaboration

ATF, Accelerator Test Facility (KEK) ATF 2: Final Focus Test 1. 3 Ge. V

Goals of ATF 2 +a • 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 (P. Raimondi and A. Seryi, Phys. Rev. Lett. 86, 3779 (2001)) 6 -pole magnets Geometric Correction 6 -pole magnets Chromatic Correction Final Quadrupole magnets collision ~np Horizontal Dispersion

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

Presented in IPAC 14 History of measured minimum beam size

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 b*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 (? )

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

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

Data of June 2014 Presented in IPAC 14 IPBSM Modulation (174 degree Crossing angle) Beam Size Evaluated from Modulation (no systematic error assumed) Mean: 0. 58 Standard dev. : 0. 05 Bunch charge ~ 0. 16 n. C Mean: 44 nm Standard dev. : 3 nm

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 (QF 1) – New magnet with large aperture is heavy for the present support (mover) and had large vibration.

Upstream magnets (QF 1 X, QD 2 X ) vibration reduced Jitter predicted from motions of magnets • 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) Orbit jitter before and after J. Pfingstner, et. al. , Phys. Rev. ST Accel. Beams 17, 122801 Measured jitter by BPM

QF 1 (Final H-focus Q) vibration QF 1 QD 0 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 (cav. BPM, 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 ATF 2 Line than in ILC Final Focus. • Low beam energy, long bunch

Presented in IPAC 14 Beam Size Depends on Bunch Intensity IPBSM modulation as function of bunch population. Measured with crossing angle 174 degrees (left) and 30 degrees (right). ATF 2 design bunch charge ~1. 7 n. C

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

OTR monitor View Port Shield No shield by A. Lyapin Photo by D. Mc. Cormick Remove vertical asymmetry With shield Reduce position dependent wake (factor 0. 6) by A. Lyapin

Wakefield measurement and compensation setup J. Snuverink, et. al. , ATF 2 Project Meeting 201502 LAPP

Recently improved calculation J. Snuverink, et. al. , ATF 2 Project Meeting 201502 LAPP

Good agreement with experiment J. Snuverink, et. al. , ATF 2 Project Meeting 201502 LAPP

Remaining studies on Goal 1 On going • Confirm emittance of incoming beam and optics matching (b*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 BPMs – Bolted aluminum plates, no brazing because of Invacuum. – BPM A&B bolted together. – BPM C is independent. BPM C IP Piezo mover Piezo Movers (Cedrat) Piezo Movers (PI) Vertical and horizontal offsets, pitch, rotation of each block can be adjusted. (no yaw) – BPM units are mounted on the base with three piezo movers. – Dynamic range of each mover is +/- 150 um. Initial alignment need to be better than this. Slide from Terunuma

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 N~5 E 9 Residual (Measure – Predicted) Fitting From P. Burrows, in ATF 2 Project Meeting 2015. 02 LAPP 42 nm IPA 66 nm IPB 47 nm From Siwon Jang, in ATF 2 Project Meeting, 2015. 02. LAPP 42 nm Geom 47 nm 55 nm IPC 48 nm

Jitter vs. QD 0 FF setting (waist scan) N~5 E 9 66 nm (single) 49 nm (avg. ) Set waist at BPM Slide from P. Burrows, in ATF 2 Project Meeting 2015. 02 LAPP 28

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

Slide from P. Burrows, in ATF 2 Project Meeting 2015. 02 LAPP FONT 5 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 ATF 2 Project Meeting 2015. 02 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

Best IPFB results (2014 December) Bunch 1: not corrected, jitter ~ 400 nm Bunch 2: corrected, jitter ~ 67 nm 2 bunch operation. Measure 1 st bunch position and feedback to 2 nd bunch. Only 1 BPM was used. Corrected jitter 67 nm resolution 47 nm (If resolution is dominant) 32 Slide from P. Burrows, in ATF 2 Project Meeting 2015. 02 LAPP

Best IPFB results Note the different scales 33 from P. Burrows, in ATF 2 Project Meeting 2015. 02 LAPP Slide

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