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

2. 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.

3. 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’

4. 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)

5. 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

6. 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

7. 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%

8. 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%

9. 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

10. 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

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

12. 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

13. 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