Timing and RF Distribution NLC -> ILC

1. Timing and RF DistributionNLC -> ILC Josef Frisch

2. History • RF phase and timing distribution concept developed for NLC • Prototype of phase stabilized long fiber links tested • Redundant high reliability design concept developed • Many design concepts transfer to ILC • Note: Presentation is a slightly updated version of the NLC system (without much reference of the current ILC timing / phase distribution designs). • Discussion to focus on Availability / Reliability.

3. Requirements (for purposes of discussion – NOT a specification) • RF phase distribution with stability to ~ 1 picosecond peak to peak. • The compressor has tighter requirements which may require a special system. • Trigger timing distribution with stability to a fraction of a cycle of L-band: 100ps peak – peak (~30ps RMS). • This is to allow resynchronization circuits to reliably select a single cycle of L-band. • Single point failure resistant.

5. Components: Fiducial Generator • The machine is assumed synchronized to a 5 (or possibly 10) Hz fiducial. The fiducial generator would be enabled under software control, synchronized to the 60Hz power line, and to the 1.3GHz RF. • This is a single point failure, but there is only one unit in the accelerator, so the failure rate is expected to be low.

6. Components: Master Oscillator • This must be low noise 1.3Ghz oscillator. • A variety of technologies are available, for example Sapphire disciplined by Rubidium or GPS. • PSI specifies a sapphire based oscillator at • -120dBc/Hz at 100Hz • -152dBc/Hz at 1KHz • -160dBc/Hz at 10KHz and above. • This should meet ILC phase noise requirements. • The Master Oscillator is a single point failure, but there is only one unit, so failure rate is expected to be low.

7. Fiber Links • Point to Point links using standard telecom fiber. • 1550nm laser diode source, modulated at RF • 357MHz for NLC test, 1300MHz OK for ILC. • Fiber spool in oven for fiber length compensation.

8. Phase shift for 10 degree C fiber change, 1 month (note 1 degree X-band = 250 fsec).

9. Components: Fiber Transmitter (1) • Use conventional Telecom laser diode at 1550nm, directly modulated with RF. • NLC tests done at 357MHz • Modern diodes OK for direct 1.3GHz modulation, and have (20dB) lower noise • Best to pulse diode so that reflected power measured with transmitter off. • Note, must limit transmitter power to ~ 1mW, or get nonlinear effects in fiber which degrade performance. • Fiber length compensation using ~5Km spool of fiber in oven • Requires few X 100 Watts: • Continuously cool, heat with fan and wire grid. • Get ~10 second time delay from fiber. (with integration term). • Easy to close feedback loop • Reflected phase measurement same as for receiver. Use downmix and digitizer system. • Fiber transmitter is broad band, so fiducial can be applied as a bipolar phase shift to the RF. • System cost low – all conventional components (except oven!). • Requires ~6 rack units per transmitter.

11. Components – Fiber transmitter (2) • Ovens are “unpopular”: large and consume power • Alternate scheme: Use wavelength tunable fiber working against the fiber dispersion. • Need approximately 4nm/C tuning range, with 0.5pm wavelength resolution. • Available commercially 100nm range, without mode hops, .02pm resolution. • Cost ~$25K. (From New Focus). • Expanded use of DWDM telecom systems may substantially reduce the price of tunable laser systems. • Scheme was briefly tested for NLC and worked, but at that time wide band, hop free tuning was not available. • Probably this is the technology of choice as the laser costs decrease.

13. Fiber Transmitter Reliability • Transmitters are redundant. Auto fail over is performed by the phase comparison unit. • Transmitters can detect broken fibers from reflected signal • In principal can automatically TDR fiber to help quickly find break or reflection.

14. Components: Fiber • Long haul fibers can use standard SMF-28 telecom fiber. • Need low reflections – want fusion splices, not connectors except at transmit and receive chassis. • Note that standard SMF-28 fiber is about as radiation sensitive as a human: a few hundred Rads can degrade its performance. • This varies dramatically with the exact fiber composition. • Need to test transmission system with real installed fiber.

15. Components: Fiber Receiver • Simple • Converts optical to electrical signal • Re-generates fiducial • Error checking on optical signals • Redundant, fail over in phase comparison unit.

17. Phase Comparison Unit • Located at the “crate” level • System design should allow accelerator operation with a failed “crate”. • Local narrow band Phase Locked Loop • Lock to either fiber system • Standby system has phase shifted to match active system • Prevents sudden global phase shifts (MPS issue) • Diagnostics to determine which fiber system is bad • Must be relatively low cost – high multiplicity item.

19. Phase Compare Unit – detecting failed channel • If channel signal level drops, or if fiducials are not detected. • Phase noise relative to narrow band VCO • If slow drift is detected, use head / tail monitors, or beam phase measurement to decide • Phase shifter on standby channel allows smooth changeover

20. Phase Control Unit: Low Noise VCO • Need good narrow band noise to allow phase memory between pulses • Need low cost since this is a high multiplicity unit. • Commercial multiplied VCXOs • Integrated phase noise few ps at 1Hz • Slew rate limit PLL feedback for MPS to prevent sudden beam phase shifts • Can detect noise in fibers at frequencies above ~30Hz

21. Head / Tail Monitor • Phase detection to compare neighboring sectors. • Used in conjunction with Phase Comparison units to detect failed fiber transmission systems.

22. Beam Phase Monitor • Can use Monopole HOM modes. • (have a hammer, everything looks like a nail!) • Data taken at TTF as part of HOM alignment / BPM experiments • Experiment was primarily looking at Dipole modes – Monopole modes were only used for testing system • Directly digitize cavity HOM signals with fast (5Gs/s) scope • Look at phase of HOM Monopole modes relative to 1.3GHz phase reference • HOM modes are a good detector – high Q and mechanical stability (in helium) give accurate measurement.

24. HOM phase results / comments • 1.4 ps RMS for each mode. • Mode difference 1ps RMS -> mode noise of 700fs RMS • Can probably do much better with an optimized system. • HOM couplers also see 1.3GHz signal in cavity. • Provides a direct comparison of 1.3GHz vs beam time. • Electronics specific to monopole modes would be low cost (standard down-mix / digitize system) • Effect of Lorentz detuning not known – could be a major problem for this type of measurement. • If so, can always use conventional phase cavities.

25. Triggers • Assume triggers derived from 1.3GHz countdowns, reset by Fiducial. • 1.3GHz too fast for present day programmable logic – limit few hundred MHz • Can work at a divided down frequency • Expect faster programmable devices by time ILC is constructed • Countdowns similar to SLAC PDUs (now running at 476Mhz). • Due to need to reset frequency dividers running at 1.3GHz, need trigger stability <100ps!

26. Issues / Conclusions • Base technologies for a redundant phase and timing distribution system for the ILC have been demonstrated • Compressor phase is the exception! Needs R+D. • Much engineering required to build a complete system • Various alternate technologies available • Example is fiber laser based phase / timing distribution system developed at MIT and DESY. • Need to do detailed engineering to evaluate trade-offs.-