Experience from

1
Experience from

• ATF-2 Workshop
• January 5, 2005

2
Overview of FFTB

• Final Focus Test Beam (FFTB) Experiment at SLAC
• Prototype FF for Linear Collider
• Focused 46.6 GeV beam from SLAC linac
• straight-ahead or C-line beam
• ß 10 x 0.1 mm (design)
• ?e 30 x 3 µm (design)
• RMS energy spread 0.3 (design)
• RMS bunch length 0.6 mm
• s 1.8 µm x 60 nm (design, inc. aberrations)
• 30 pulses per second, 1.0 1.6 nC / pulse
• Collaboration BINP, DESY, FNAL, KEK, MPI,
  Orsay, SLAC

3
Location and Orientation
W?E
Looking West from the Alignment Tower
Looking East from inside the BSY
4
Optics
5
Magnet Movers

• One per quad (except final doublet), one per CCS
  sextupole
• x/y/roll motions
• 3 mm range each DOF
• lt 1 µm step each DOF
• Used for BBA

6
BPMs

• Standard 36 cm stripline
• 1 per quad
• 1 µm resolution _at_ 1.6 nC
• Advanced 5.7 GHz cavity
• 4 total
• 25 nm resolution _at_ 1 nC

7
FP Beam Size Measurement
Ion Time of Flight BSM, 92 cm downstream of QC1
quad
Laser-interferometer BSM, 40 cm downstream of QC1
quad
8
FFTB Run Pattern

• FFTB operation was not compatible with SLC
• FFTB had short (1-3 week) runs during SLC
  downtimes
• Establish good beam to dump
• BBA, lattice diagnostics, energy measurement
• Beta match and background tuning
• Acquire signal on BSM
• Global tuning (waist, coupling, etc)
• Other beam size experiments (bandwidth, etc)

9
Beam Size Measurement
Beam size measurement with (left) and without
(right) background subtraction (laser pulses _at_ 10
Hz laser-free pulses used for backsub). Since
beam size is determined by depth of the wave
trough, this was very important! Each point
averages 6 laser-on and 12 laser-off pulses, each
scan takes 30-60 seconds.
10
Beam Size Measurement (2)
Azimuthal distribution of He ions indicates
extremely flat beam. The asymmetry between the
two peaks is due to banana shape of beam
(curvature in xz or yz plane).
11
Minimum Spot Size (Laser-Compton)

• Expect 59 8 nm, including
• nominal beam size 35 nm (emittance and energy
  spread below nominal)
• Beta mismatch 17 nm (ß 125 um, not 100 um)
• Beam and Final Doublet jitter 40 nm
• Limited accuracy of aberration tuning
• Waist 11 nm
• coupling 11 nm
• dispersion 5 nm
• chromaticity 10 nm
• sextupole 10 nm

12
Minimum Spot Size (2)

• Expect 59 8 nm
• Measure 70 7 nm, including laser-power
  imbalance and hourglass corrections (10 effect
  total)
• 23 measurements made over 48 hours

13
Minimum Spot Size (3)
3 scans of sy2 vs e slope indicates ß 130
um, and offset (size at zero emittance) almost 70
nm! Was there some BSM / optical systematic we
were not properly controlling? Admittedly, fits
are not especially convincing!
14
Stuff We Did Wrong

• BSM Systematics
• never convinced ourselves wed found all effects
• Extraction line
• Looks at FD and FP spot
• poor BPMs
• poor optics
• Tight aperture for Compton photons, etc
• Coupling
• Didnt have full control
• Was there a rotation _at_ FP?

• Collimation
• Extremely hard to get OK conditions for BSM
• Took linac collimators 2 sets of jaws in FFTB
• Optics probably halo limited anyway!
• Intermediate small-spot diagnostics
• Wire scanners dont work well at 1001 aspect
  ratio

15
FFTB Graduate Students

• Ghislain Roy
• Thesis on optics design of FFTB
• Currently at CERN AB/OP
• Patrick Puzo
• Thesis on ion TOF BSM
• Currently at Universite Paris-Sud
• Peter Tenenbaum
• Thesis on FFTB commissioning and results
• Currently at SLAC, ILC Department

16
A Few Notes

• From Oides PAC paper on FFTB design to first 70
  nm spots took 5 years (1989 1994).
• In retrospect, the risk of complete failure was
  immense
• Unprecedented BSMs, emittance preservation in the
  linac, backgrounds
• We did it anyway and didnt fail
• Risk was substantially increased by the short and
  intermittent run schedule
• would have benefited from more continual
  operation, like ATF, TTF, NLCTA have had.