Studies of emittance preservation methods and the status of their experimental validation Erik Adli 1, W. Farabolini 2, Reidar Lillestol 1, Jürgen Pfingstner 1 1 University of Oslo, 2 CERN ECFA Linear Collider Workshop 2016 May 30

Outline 1.Introduction 2.Beam-based alignment 3.Experimental WFM verification 4.On-line DFS 5.Conclusions

1. Introduction

Static misalignments and mitigation methods ILCCLICCured problem Mechanical alignment 1. Element placement in tunnel to ref. points. 2.a Elements aligned w.r.t. each other. 2.b Stretched wire system (around IR). 1. Element placement in tunnel to ref. points. 2. Stretched wire system. 2. Most tolerances satisfied, e.g. element roll, cavity offset (not CLIC).

Static misalignments and mitigation methods ILCCLICCured problem Mechanical alignment 1. Element placement in tunnel to ref. points. 2.a Elements aligned w.r.t. each other. 2.b Stretched wire system (around IR). 1. Element placement in tunnel to ref. points. 2. Stretched wire system. 2. Most tolerances satisfied, e.g. element roll, cavity offset (not CLIC). Beam-based alignment 3. One-to-one steering. 4. Dispersion-free steering or kick minimisation. 3. One-to-one steering. 4. Dispersion-free steering. 5. RF alignment. 3. QP offset, structure tilt. 4. BPM offset. 5. Structure offset CLIC.

Static misalignments and mitigation methods ILCCLICCured problem Mechanical alignment 1. Element placement in tunnel to ref. points. 2.a Elements aligned w.r.t. each other. 2.b Stretched wire system (around IR). 1. Element placement in tunnel to ref. points. 2. Stretched wire system. 2. Most tolerances satisfied, e.g. element roll, cavity offset (not CLIC). Beam-based alignment 3. One-to-one steering. 4. Dispersion-free steering or kick minimisation. 3. One-to-one steering. 4. Dispersion-free steering. 5. RF alignment. 3. QP offset, structure tilt. 4. BPM offset. 5. Structure offset CLIC. Tuning 5. Different emittance tuning bumps. 6. IP luminosity tuning. (5. Bumps are a reserve.) 6. IP luminosity tuning. 5. General improved performance. 6. For special tolerances of final focus.

Static misalignments and mitigation methods ILCCLICCured problem Mechanical alignment 1. Element placement in tunnel to ref. points. 2.a Elements aligned w.r.t. each other. 2.b Stretched wire system (around IR). 1. Element placement in tunnel to ref. points. 2. Stretched wire system. 2. Most tolerances satisfied, e.g. element roll, cavity offset (not CLIC). Beam-based alignment 3. One-to-one steering. 4. Dispersion-free steering or kick minimisation. 3. One-to-one steering. 4. Dispersion-free steering. 5. RF alignment. 3. QP offset, structure tilt. 4. BPM offset. 5. Structure offset CLIC. Tuning 5. Different emittance tuning bumps. 6. IP luminosity tuning. (5. Bumps are a reserve.) 6. IP luminosity tuning. 5. General improved performance. 6. For special tolerances of final focus. … Contributions from the University of Oslo

2. Beam-based alignment

One-to-one steering Initial misalignments after mechanical alignment cause strong emittance increase. The beam oscillations are reduced by steering beam in centre of BPMs. Sequential strategy: 1.Use first quadrupole to steer beam through first BPM 2.Use second quadrupole to steer beam through second BPM 3.… Emittance can be drastically reduced, but is not yet sufficient, due to remaining dispersion.

Dispersion-free steering (DFS) Method: Step 1: The dispersion η at the BPMs is measured by varying the beam energy. Step 2: Corrector actuations Δy 1 (quadrupole movements) are calculated to minimise dispersion η and the beam orbit b. Considering many BPMs and quadrupoles leads to linear system of equations [4]: DFS is applied to overlapping sections of the accelerator (36 for ML of CLIC). η 0 especially important for ILC: dispersion by design

Before correctionAfter 3 iterationsAfter 1 iteration Beam profile measurement DFS verification at FACET/SLAC Emittance artificially increased (March 2013) Emittance fully recovered x: 43.2x10 -5 m -> 3.71 x m y: 27.82x10 -5 m -> 0.87 x m Emittance growth corrected with wakefield free steering (March 2014) 11 A. Latina, J. Pfingstner, D. Schulte, E. Adli, F. J. Decker and N. Lipkowitz, Phys. Rev. ST Accel. Beams 17, (2014).

RF alignment (only CLIC) Problem: After DFS, still too large Δε due to transverse wakefields (WF). Transverse WFs prop. to beam structure offset. Kick applied to tail of bunch. Results in beam-breakup instability (BBU). Especially severe for CLIC since Courtesy of D. Schulte Solution: Alignment of structures to beam after 1-to-1 steering and DFS. Beam offset is measured in each super- structure with wakefield monitors (WFM). Alignment per girder not per structure Correction performance depends on accuracy of WFM.

WFM specifications From simulations of full RF alignment (D. Schulte). WFM accuracy chosen to be 3.5μm to only cause Δε y of 5%. Larger values cause strong Δε y. RF alignment is zeroing technique but spec has to achieved despite: 1.Mechanical alignment of cells to each other (accuracy). 2.Noise in signal and electronics (resolution). Averaging can help.

3. Experimental WFM verification

Wakefield monitors Similar to cavity BPM with low Q and low impedance. Two WF modes are tested based on simulation results: – TM mode: 16.9 GHz – TE mode: 27.3 GHz Exact frequency depend on cell due to tapering CLIC structure. To suppress structure tilt effect, a cell close to super-structure centre is chosen (2 nd cell of 2 nd structure).

Wakefield monitor setup at CALIFES WFM test are performed at CALIFES at CTF3. Two TD26 super-structures are equipped with WFM. TM/TE & vertically/horizontally: 16 channels.

Typically measured signals Plotted are the envelope functions of the WFM signals. No drive beam present. Signal buildup for several bunches. Steady-state and signal decay indicate low Q. Different signal shapes are due to different phase relations and Q factors for TE and TM mode.

Signal noise reduction Challenge: Measurement with drive beam as a strong noise source. Indeed, significant noise level in WFM signals observed. Identified causes: 1.Drive beam. 2.But also CALIFES klystron. Improvements: 1.Better cables and connectors at WFM and in gallery. 2.Improved grounding. Situation much better, but still some noise left (ongoing effort).

Signal analysis via down-mixing Motivation: Get spectra of signal and noise. Verify location of signal peaks from simulations. Adapt waveguide filters to optimise S/N ratio. Since a direct measurement is not possible (27 GHz) signals are down-mixed to baseband. Signal analysis for single bunch is difficult due to short bunch length. Spectrum is combination of beam and train spectrum. Results: Spectra are similar to simulated ones (ongoing work).

WFM resolution Without drive beam – Correlation analysis: To remove correlated beam motion (dispersion, drifts). WFM signals of two structures are correlate. TE mode: 15 – 25 μm (h / v). TM mode: 6.6 μm / 17 μm (h / v). – SVD analysis: Removal of dispersive modes. TE mode: 2.3 – 4.6 μm (v/h). With drive beam – SVD analysis TE mode: 20 – 30 μm TM mode: 20 – 30 μm – Comment: tests without drive beam in the same week also showed um (ongoing work). R.L. Lillestøl, E. Adli, J. Pfingstner, N. Aftab, S. Javeed, R. Corsini, S. Döbert, W. Farabolini, A. Grudiev, W. Wuensch, contributed IPAC16 talk WEOBB02.

4. On-line dispersion-free steering

Motivation for on-line DFS Initial beam-based alignment: Long-term ground motion (> 1 minutes): Effects on the main linac of CLIC: -Ground motion model: ATL law [1] with constant A of μm/m/s. -Emittance increase Δε y ≈ 7.5% / hour (scaling law from simulation). -E.g. Δε y of 100% in 13 hours. Orbit feedback steers beam onto golden orbit. Orbit feedback steers beam onto dispersive orbit.

On-line DFS algorithm Goal: Perform DFS parasitically during physics data taking. Problem: -Only very small beam energy variation δ acceptable (< 1 per mil). -Measurement are strongly influenced by BPM noise and usual energy jitter. Solution: -Many measurements are averaged. -Use of a Least Squares estimate (pseudo-inverse), which can be significantly simplified by the choice of the excitation: Assumed problems: -BPM resolution and main beam energy jitter. -Not a big issue if N < 100 is large enough. Overall correction time 7 minutes.

On-line DFS performance ATL motion (13h): – correction for different ω. – Δε y = 0.2% after 3 rd iteration. BPM noise: – Effective BPM noise of 10nm. – Average time of 144s x 3 iter. (7 min). – Δε y = 0.2% after 3 rd iteration. Robustness: – Short-term ground motion – Energy fluctuations – Several other tested imperfections. – Only two surprises.

Unexpected problem 1: wakefields Problem: Also wakefields create dispersion due to applied dipole kick. Effect is larger for smaller δ (non-linear dispersion). WF dispersion cannot be cancelled by quadrupole dispersion, since kick is different along bunch. Solution: WF dispersion is lowered if δ is created shortly before correction bin (local energy change). Realised for CLIC by changing charge in one decelerator only.

Unexpected problem 2: structure tilt Problem: Structure tilts create an artifact in dispersion measurement. Changed gradient to create δ changes also kick due to tilt. This orbit change is interpreted as dispersion and deteriorates correction. Effect is much stronger for local energy change. Solution: Dispersion artifact can be fitted and removed in Software due to different shape.

Correction results On-line DFS including: -Local energy via decelerator charge change. -Structure tilt artifact removal. Parameter scan for ω: -Parameter window rather small. -Improvement possible by variable ω along linac. -Final Δε ≈ 20% (budget 50%). Parameter β not used: -Unconstrained. -20 μm movements. J. Pfingstner, E. Adli and D. Schulte, Phys. Rev. ST Accel. Beams, in review.

5. Conclusion Emittance preservation is besides acceleration a central design issue of ILC and CLIC. An especially challenging imperfection is component misalignments. The mitigation strategy is similar for ILC and CLIC, but tolerances are more relaxed for ILC. University of Oslo is contributing to the studies via two contributions: 1.Experimental verification of wakefield monitors -System tests at -Signals noise has been strongly reduced. -Resolution is closed to nominal value of 3.5 μm, but repeatability improvement necessary. 2.On-line dispersion-free steering -Parasitic DFS with small energy changes to mitigate ground motion effects. -Unexpected sensitivity to two static imperfections. -Solutions have been found and simulations show satisfying performance.

Thank you for your attention!