RF Stability Working Group: Key Findings & Recommendations

1. RF Stability Working GroupJorn Jacob (ESRF), John Byrd (LBNL) General Issues • RF phase and amplitude noise • filtered by cavity and translate into timing and energy jitter of beam • slow phase shifts cause timing jitter • fast phase and amplitude shifts (f~fs) cause energy jitter. IR Beamline typically most sensitive to fast jitter. • stretched bunches complicate issues • Coupled bunch instabilities • driven by cavity higher-order-modes • vertical instabilities driven by resistive wall and ion effects (not covered in this workshop) • Single bunch instabilities (not covered) • driven by broadband vacuum chamber impedance (i.e. tapers, etc.) • longitudinal microwave instabilities • vertical mode coupling instabilities

2. Findings • Most of the users request about 1 deg phase stability, typical for 3GLS. No reference specified. • 0.1 deg is a more challenging objective. This request comes from time resolved experiments. It looks like the RF needs 0.1 deg stability above about the 100 Hz range, whereas the experiment can lock on larger residual phase fluctuation below 100 Hz. • Energy oscillation has only been specified as a fixed, frequency independent quantity. Again time resolved experiments give the most stringent upper limit of DE/E < 5 10-5 . • Choice of RF transmitter technology: the current design foresees the use of Klystrons. Solid state amplifiers are discussed, having intrinsically less phase noise and no saturation. • Concern with klystron approach: phase noise from power supply ripple ~1.2 deg., saturation of klystron makes rf loop difficult: suggestion to use “scalar” phase loop • Tied with the above, proper design of RF fast and slow control loops, including or not beam phase • Looked at necessity of harmonic cavities for lifetime: impacts number of injections/minute. Together with ion gap in filling keeping phase transients reasonable pushes for superconducting main and harmonic cavities • With harmonic cavities, the phase and amplitude transfer functions from klystrons to beam and cavities become complicated, bandwidth of feedback probably limited to less than 1 kHz. • Simulations indicate that LCBI from high frequency resonances are mitigated by the landau cavity.

3. Recommendations (1) • The exact specification for phase and energy jitter should be clearly defined as a function of frequency. • The transfer from phase to energy oscillation is substantial at low frequency, peaks at the synchrotron frequency (3 .. 4 kHz) and decreases for higher frequencies. The situation becomes even more complicate with harmonic cavities and needs to be further studied. • The upper limit of DE/E < 5 10-5 should be checked against the required position stability at the locations with dispersion (at the 2nd dipole location with h=5 cm this would give 2.5 mm, a factor 10 above the 0.3 mm specification, e.g. for IR beam lines). • Effects of beam instabilities are source of transverse and/or longitudinal oscillations and need to be investigated. • Further analysis of cavity and beam, phase and amplitude transfer functions are required for the RF system including harmonic cavities. Simulations should include RF loops to check maximum achievable gain / bandwidth. Simulations should lead to a specification of the LLRF system. • Probably, a fast RF feedback will not be compatible with stretched bunch operation. Then a scalar phase loop around the transmitter will have to be optimized to minimize RF phase noise.

4. Recommendations (2) • The choice of SC technology for main and harmonic cavities is endorsed as to be necessary for sufficent lifetime even with a gap in the fill pattern (> 3h lifetime required for less than one injection shot / minute in top up mode). • The use of the harmonic cavities for Landau damping of possible remaining HOMs at higher frequencies up to 7 GHz (due to high vacuum chamber cut off) seems adequate. • The choice of SC cavity technology (CESR B or KEK B) needs to be further discussed. Reliability being also important for an overall beam stability (e.g. thermal transients after trips), the power capability of the RF coupler is an issue for the large beam power at NSLS II. The CESR B cavity would already be operated at its upper power limit. • If noise from klystron transmitter appears to be too high even with an adequate phase loop, check possibility of alternative technologies: • IOT transmitters, as used at Diamond, which don’t convert HV ripple into Phase noise as much as klystrons. • Solid state amplifiers, powered by 100 kHz switched power supplies (far above the synchrotron frequency) , and which provide very low phase noise -> SOLEIL (352 MHz) and SLS (500 MHz). • Moreover, solid state amplifiers are extremely modular and are therefore intrinsicly redundant and provide high reliability (indeed a very low failure rate has been experienced by SOLEIL so far). • A survey of existing LLRF systems should be conducted (development of custom sytems at many labs). FPGA based systems matching NSLS II requirements should be identified. • Opt for as simple as possible robust design. Use high quality components, solid power supplies for all parts of the RF system to minimize noise and drifts. • The timing system needs to be specified in detail.

5. Summary • NSLS-II RF group should be congratulated for an excellent initial survey of noise effects. • Noise requirements need frequency specification. Constant dialogue with users critical to understand noise requirements. • Initial investigation of instabilities promising. Further work needs to be done. Sensitivity to small effects. whining, complaints, bickering, insults, etc.