Beam Optics Set-Up at SLAC End Station A Frank Jackson ASTeC, Daresbury Laboratory

Introduction to End Station A End Station A is an ILC Test Facility at SLAC. 28 GeV beam with ILC-like bunch length and charge (single bunch) Running since 2006: Jan, April, July Collimator wakefield measurements Energy spectrometer BPMS FONT BPM performance Experiments share slot of beam time and run parasitically with PEP-II.

Not designed for very small beam sizes (like FFTB) ESA Overview Each ILC test experiment requires different beam profile in the ~100 m scale Challenges of A-line; high dispersion and synchrotron radiation in bend Not designed for very small beam sizes (like FFTB) Experimental Alcove The ‘A-line’

ESA Optics Description Dispersion (, ’) minimisation by quadrupole pair at bend midpoint Beam profile tuning achieved by 4 quadrupoles downstream of bend Diagnostics, 2 wire scanners in alcove + synch light monitor (energy spread)

ESA Optics Simulation Strong bend (synchrotron radiation) make optics modelling difficult Linear optics (MAD) not valid for entire line Can use ELEGANT to simulate bunch transport with SR, for varying input conditions Beam optics (Twiss) at end of line sensitive to input conditions (emittance, energy spread) Expect factor of 4-5 growth in horizontal emittance from linac exit

ESA Optics Method Initial beam set-up for good quality beam through A-line to focussing quads Measure optics at entrance to focussing quads, rather than rely on beam transport simulation Use measured optics as input to MAD linear optics fitting for beam profiles in Alcove

Preliminary Optics Set Up Preliminary set up includes steering, dispersion minimisation and energy spread tuning. Steer to BPM adjacent to focussing quads. x-dispersion minimisation by tuning knob of Q19, Q20. Measure correlation of bend/alcove BPMs while dithering beam energy. Energy spread sensitive to linac phasing and beam charge. x(alcove BPM) < 5mm p = 0.25%

x = 5.5e-09 m 5% y = 3.4e-10 m Optics Measurement Use quadrupole scan method with Q27 (first focussing quad after bend) Emittances Beam Twiss params measured to < 10% accuracy x = 5.5e-09 m y = 3.4e-10 m 5%

Optics Tuning Requirements Collimator wakefield tests Vertical collimators ~1.4 mm wide Small vertical beam 100um to scan up and down gap Horizontal size less important 1 mm Energy spectrometer BPM Require horizontal focus of ~200 um through ‘chicane’ Vertical size less important  1mm

Quadrupole focussing determined using MAD Optics Tuning Results vertical beam size 83m for collimator wakefield tests Quadrupole focussing determined using MAD Measurements taken using wirescanners WS1 just downstream of collimator WS2 at mid-chicane Measured and predicted beamsizes agree to within ~20% WS1 horizontal beam size 240m for BPM studies WS2

These optics solutions provide long focus waists Beneficial for experiments Beam size (mm) s (m)

Optics Stability Beam from linac is not perfectly stable (particularly in vertical plane) Day-night temperature variations affect RF Vertical instability had been seen previously in E158 expt at ESA Solved by emittance coupling (skew quad) Not desirable for wakefield experiment (beam tilt) Linac tuning (phasing, position/angle feedback setpoints) all necessary on shift-by-shift timescale to ensure good vertical emittance

ESA Optics Summary and Future Able to use ‘simple’ optics measurements and tuning to achieve desired beam profiles in ~100 m range Beam stability is big issue and must be continually monitored. Practical steps - realignment of focussing quadrupoles Other optics plans will depend on future of End Station Beam damage tests considered but would require very small beam sizes ~ 10 m