LCLS Timing Requirements - and how they were arrived at Patrick Krejcik September 16-17, 2003

1. LCLS Timing Requirements- and how they were arrived atPatrick KrejcikSeptember 16-17, 2003 • Accelerator physics requirements • Scientific user requirements • Compatibility and operational requirements Patrick Krejcik, LCLS

2. Accelerator physics requirements • Stating the obvious • Timing and RF phase control are synonymous in LCLS • From an accelerator physics point of view: • Ensuring the correct electromagnetic field in the device at the time the electron bunch passes by • LCLS bunches have sub-picosecond duration • Bunch timing tolerance is also sub-picosecond • Controls perspective can distinguish between • fine resolution (e.g. phase locking) • and coarse resolution (choosing an RF bucket) • And synchronization with AC power (timeslot) • Operational perspective distinguishes between stability and tuning (e.g. feedback) Patrick Krejcik, LCLS

3. Resolution versus stability • Resolution can be coarse or fine • Coarse resolution means putting the bunch (or observing) the correct S-band RF bucket • Coarse timing needs to be reproducible after power dips and other resets • Fine timing, or phase control • Pulse-to-pulse stability, or jitter, is the performance criteria • Longer term drift can be corrected on time scales ~10 sec. Patrick Krejcik, LCLS

4. LCLS Principal Components – excluding DC devices • Gun laser • Laser diagnostics • Gun RF • Linac L0, L1, X1, L2, L3 RF • Electron bunch diagnostics • Gated readback of devices • Active devices using • pulsed laser synchronized to e-beam • RF deflecting cavity • Pump-probe laser for user experiments • X-ray diagnostics Patrick Krejcik, LCLS

5. Allocation of major components of the LCLS rf system Patrick Krejcik, LCLS

6. Why not just ask for infinite precision? • Timing jitter is a performance parameter for the user • Shouldn’t we at least aim to demand the best available? • Ultimately limited by inherent stability of SLAC klystrons • Measurements on SLAC klystrons show they have jitter of ~0.1 deg. S-band • Simulations used to determine the expected performance given this level of jitter Patrick Krejcik, LCLS

7. Pulse-to-pulsejitter estimates based on machine stability Simulate bunch length variations… …and bunch arrival time variations… 0  0.26 psec rms 82  20 fsec rms P. Emma • linac phase 0.1 deg-S rms • linac voltage 0.1% rms • DR phase 0.5 deg-S rms • Charge jitter of 2% rms Patrick Krejcik, LCLS

8. Tolerance budget (ptol) for <12% rms peak-current jitter (column 3) or <0.1% rms final e− energy jitter (column 4). The tighter tolerance is in BOLD text and both criteria, |ΔI/I0| < 12% and |ΔE/E0| < 0.1%, are satisfied if the tighter tolerance is applied. The voltage and phase tolerances per klystron for L2 and L3 are Nk larger. Patrick Krejcik, LCLS

9. Distributions of core-slice-averaged values for the beam and FEL for 227 seeds Patrick Krejcik, LCLS

10. Scientific user requirements • Maintaining saturation in the FEL • Provide femtosecond timing for pump-probe experiments • Time stamp arrival of FEL pulse w.r.t. an optical laser pulse • i.e. synchronize user laser with linac RF reference • Again, coarse timing of RF bucket (see OTR diagnostic) • And jitter at subpicosecond level Patrick Krejcik, LCLS

11. Compatibility and operational requirements • Timing system also has a supervisory role • Software inputs distinguish between different beam pulses (beam codes) and controls repetition rate • LCLS upgrades are foreseen for bunch trains to multiple users, with differing beam parameters • Timing logic also includes MPS and PPS functions • Single bunch dumper, or gun veto • Timing must coexist with controls of PEP II beams in adjacent beam lines, through the same micros. • Linac configuration for LCLS must be revertable to fixed target operation (interleaving not yet foreseen) Patrick Krejcik, LCLS

12. Operational tuning requirements • LCLS is a single pass machine, not a storage ring • Pulse-to-pulse jitter is a performance parameter • It must be diagnosed • And minimized through tuning • Diagnostic devices*, especially BPMS, need single pulse readback • All BPMS, from gun to laser, should readback on the same pulse (single pulse orbit display) • All BPMS should have 120 Hz buffered data acquisition capability • * other gatable devices include FEL powermeters etc. Patrick Krejcik, LCLS

13. Operational tuning requirements: beam based feedback • Pulse-to-pulse stability is set by the hardware • Long-term drifts are to be corrected by feedback • Feedbacks control (experience with SPPS) • Transverse position and angle launch in the undulator • Energy at the injector, chicanes, undulator • Beam phase • Bunch length • Feedbacks should operate as fast as possible • 120 Hz Patrick Krejcik, LCLS

14. Some concepts • The timing system has 3 levels of inputs • An RF clock harmonic of 2856 MHz RF • A 360 Hz clock, phase locked to the AC power • A software beam code from a master patern generator (MPG) Patrick Krejcik, LCLS

15. RF frequency choices • The PEP II timing system is derived from the 8.5 MHz damping ring revolution frequency • The 56th harmonic, 476 MHz is transmitted on the main drive line (MDL) • In the crate a PDU counts cycles of the 4th subharmonic at 119 MHz to allow step changes of 8.4 ns Patrick Krejcik, LCLS

16. RF frequency choices – LCLS gun laser • A local oscillator isolates the laser from noise on the MDL and from phase jumps during PEP II injection • It relocks to the MDL between PEP II cycles • LCLS beams are on a different time slot to PEP II • The choice of local oscillator frequency is driven by compatibility with the (unused) 8.5 MHz and by the length of available commercial laser cavities Patrick Krejcik, LCLS

17. RF frequency choices – LCLS gun laser • CDR quotes laser frequency of 79.33 MHz • This is the 36th sub-harmonic of 2856 MHz. • For comparison, the damping ring 8.5-MHz revolution frequency is the 336th sub-harmonic of 2856 MHz. • The sector-0 master oscillator VCO and the LCLS VCO frequencies are therefore in the ratio 6:56. • Design would be simplified if laser could be made to operate at 119 MHz • Technology is improving • SPPS Ti:Sapphire pump-probe laser is now operating at 102 MHz, te 12th harmonic of 8.5 MHz Patrick Krejcik, LCLS

18. Timing and rf distribution in sector-0 and sector-20 of the linac Patrick Krejcik, LCLS

19. Functional Requirements • Maximum Link Length 2 Kilometers • Timing Stability (Long Term) < 5 picoseconds • Timing Jitter RMS < 0.5 picoseconds • RF Phase Stability (1 second) < 0.07 deg. S-band • RF Phase Stability (Long Term) < 5 picoseconds • RF Phase Jitter RMS < 0.07 degree S-band • Phase Transmission Frequency 2856 MHz • Timing Resolution (Normal) 350 ps (S-band bucket) • Timing Resolution (with vernier) 1 ps • Required Frequency Stability 3x10e-9 Patrick Krejcik, LCLS

20. Control system components at each klystron station Patrick Krejcik, LCLS

21. Schematic of two adjacent nominal sectors showing distribution of rf power to the klystrons Patrick Krejcik, LCLS

22. Klystron phase stable to <0.1 deg. S-band over ~10 sec. Pulse-to-pulse phase variations, and histogram, measured at PAD of a single klystron shows 0.07-degree S-band rms variation over 17 seconds. Pulse-to-pulse relative amplitude variations measured at the PAD of a single klystron shows 0.06% rms variation over 2 sec (horizontal axis is in 1/30-sec ticks). Patrick Krejcik, LCLS

23. 0.5 deg. S-band klystron phase variation over several minutes Phase variations measured at the PAD of a single klystron over a period of minutes. Each point is an average over 32 beam pulses. Patrick Krejcik, LCLS

24. Long term stability dominated by RF phase drifts Measurement of the phase variations between two adjacent linac sectors over a period of several days Measurement of phase variations seen along the linac main drive line over a period of several days. Patrick Krejcik, LCLS