1. Longitudinal Diagnostics for start-up
Christopher Gerth (DESY)
On behalf of XFEL WP18 and many others …
Workshop on Linac Operation with Long Bunch Trains
23 February 2010 8

2. Introduction
Set-up of the machine = find operating points a) old settings: save&restore b) predicted operating points from simulations Diagnostics: magnets => orbit,matching => transverse diagnostics (BPM, OTR, BLM, …) modules => energy, bunch compression => longitudinal diagnostics => coarse diagnostics needed with large dynamic range (this talk)
Stable operation of the machine = lock operating points a) single-bunch resolution b) feedback capability => high-resolution diagnostics needed (talk before lunch)

3. Injector laser & Gun set-up
See talk by Siggi …

4. Measurement of absolute Beam Energy in BC2
OTR
EBPM
SR camera
Alignment
laser
BPM
317.6 mm
(survey data)
BPM
Absolute position of alignment laser: ~ ±0.5 mm
Absolute BPM measurement: ~ ±0.5 mm
Error of beam energy ~ 1mm/300mm ~ ±0.3%
Absolute B field of dipoles: ~ ± 0.2%
Error of absolute gun energy ~ ± 0.3 MeV
=> Error of measured energy gain in ACC1 ~ ±0.7% => ± 130MeV

5. Minimum bend angle for SR Camera
Beam centre: 292 mm ~ 16.5 deg
However, best to stay at 18 deg for beam energy measurement if no particular angle is needed.
10 mm + 7 mm
26 mm
317.6 mm
Angle under which SR is imaged:
6 mrad

6. 1) Find on-crest phase (OTR,SR Camera in BC2)
Setup of ACC1 phase:
1) Find on-crest phase (OTR,SR Camera in BC2)
dE <= minimize energy spread (not exactly on-crest) => accuracy ± 0.5 deg
Igor’s 1D simulation tool
~ 1 deg towards compression

7. 1) Find on-crest phase (OTR,SR Camera) 2) Pyro phase scan
Setup of ACC1 phase:
1) Find on-crest phase (OTR,SR Camera)
2) Pyro phase scan
9DBC2.3
Weaker signal in case
9DBC2.1 is saturated!
~ on-crest
~ SASE
9DBC2.1

8. 1) Find on-crest phase (OTR,SR Camera) 2) Pyro phase scan
Setup of ACC1 phase:
1) Find on-crest phase (OTR,SR Camera)
2) Pyro phase scan
Saturated?
Not centered in DR ? …

9. 1) Find on-crest phase (OTR,SR Camera) 2) Pyro phase scan
Setup of ACC1 phase:
1) Find on-crest phase (OTR,SR Camera)
2) Pyro phase scan
Save&Restore required for Pyro Parameters!
Especially if someone changes the setup!
Accuracy ???
Saturated but scan is still working.

10. 1) Find on-crest phase (OTR,SR Camera) 2) Pyro scan
Setup of ACC1 phase:
1) Find on-crest phase (OTR,SR Camera)
2) Pyro scan
3) Start slow feedback
=> Ideally you have an old target value …
Noisy signal (averaging required)
See talk by S. Wesch
Set target
~ 0.6deg in 2 hours
Main beam-based slow feedback for compensation of phase drifts!

11. Save&Restore list for Pyros

12. 1) Find on-crest phase (OTR,SR Camera) 2) Pyro scan
Setup of ACC1 phase:
1) Find on-crest phase (OTR,SR Camera)
2) Pyro scan
3) Start slow feedback
Proposed operating points for SASE.
Igor’s talk
Now: 2 phases to be set and controlled!
Strong compression required if pyro detectors to be used as phase monitors.
Other detectors: e.g. antennas in GHz range, EO, SR camera, …?
=> Operating points for 9mA run with 3rd harmonic need to be studied!

13. Measurement of Beam Energy in BC3
Only OTR screen at centre of vacuum chamber available
Camera not aligned at the moment
EBPM will become available for rel. beam energy measurements (has low prioritity) …?

14. Energy in dogleg/bypass
See talk by Raymund Kammering …

15. Conclusion
Absolute beam energy monitoring important for all accelerating sections! Needs proper design - you cannot simply take a dipole …
Wrong beam energy (ca. 320MeV instead of 280MeV assumed) was one of the main reasons that prevented us from first lasing at TTF1!