Evaluation of 1GHz vs 2GHz RF frequency in the damping rings

1. Evaluation of 1GHz vs 2GHz RF frequency in the damping rings Yannis PAPAPHILIPPOU and Alexej Grudiev April 16th, 2010

2. Background • Baseline: RF frequency of 2GHz, 1 train of 312 bunches spaced by 0.5ns produced and transmitted along injector complex and DRs. But: • Power source and RF design needs R&D (high-peak power, short train, transient beam loading) • Alternative solution: RF frequency of 1GHz with 2 trains of 156 bunches and bunch spacing of 1ns, separated by half the damping ring circumference minus the length of a train • A delay line with an RF deflector is needed downstream of the DRs for recombining the two trains and providing the nominal 2GHz bunch structure.

3. 1 vs. 2GHz in the PDR • Larger bunch spacing (1 vs. 0.5 nm) halves harmonic number (1326 vs. 2581), and increases momentum acceptance by 40% (1.7 vs. 1.2%), thereby making the capture efficiency of the positron beam even better • For keeping the same momentum acceptance, the RF voltage can be reduced (~10 vs. 6.8MV) • All the rest of the parameter changes are the same as for the damping rings

4. New DR parameters * The ring circumference was shortened after relaxing longitudinal parameters in order to reduce space-charge ** Using Bane approximation. Piwinski theory gives (310,4.4,6100)

5. Damping rings (I) • In the DRs, the harmonic number reduction, raises the equilibrium longitudinal emittance (bunch length). • In order to keep it to the same level (IBS effect), the RF voltage should be increased reducing stationary phase (RF bucket becomes more linear). • For shorter ring (space charge reduction), stationary phase gets increased (quite big for 2GHz), i.e. voltage should be increased and momentum compaction factor reduced (relaxing arc cell focusing) • Extraction kicker rise time becomes smaller but it is still long enough (~550ns). This might eliminate the possibility to use IGBT switches. • The 2-train structure may require two separate extraction kicker systems (two pulses of equal size and flat top of 160ns as in the present case) or one kicker with a longer flat top (1μs). • RF frequency of 1GHz is closer to existing high-power CW klystron systems used in storage rings or designed for NLC damping rings (714MHz). An extrapolation of this design should be straightforward. • Larger bunch spacing reduces peak current and power by a factor of 2 (beam loading reduction)

6. Damping rings (II) • The e-cloud production and instability is reduced with the larger bunch spacing. • In the e- rings, the fast ion instability will be less pronounced due to the larger bunch spacing by doubling the critical mass above which particles get trapped (not allowing the trapping of H2O+ and probably CO+). The reduced number of bunches per train will reduce the central ion density, the induced tune-shift and will double the rise time of the instability, thus relaxing the feedback system requirements. • A bunch-by-bunch feedback system is more conventional at 1 than at 2 GHz • The parameters corresponding to CLIC@1TeV are not compatible with the 1GHz train structure and need to be re-worked in order to prevent the luminosity reduction

7. Delay line layout • Two configurations: an α-shape (as in CTF3) or an Ω-shape • In the α-shape the same RF deflector can be used for both injection and extraction (maybe also jitter feedback), whereas the Ω-shapeshould use 2RF deflectors or a kicker and RF deflector α-delay line Ω-delay line RF deflector RF deflector RF Deflector / kicker

8. Delay line layout II • The α-shape has a circumference equal to half the damping ring length (~210m) • It can be inserted in between the damping rings in order to be used for both electrons and positrons with a delay of ~1DR revolution time • The Ω-shapeis larger by the length of the (straight) line between the injection and the extraction point • It can be divided in 3 arcs with opposite bending angle satisfying the relationship • There is a geometrical relationship imposed to the length of the straight line depending on the bending angles and the arcs radii • The optics can be tuned to be isochronous for not perturbing the longitudinal beam characteristics

9. Delay line impact • Delay line does not contribute to emittance growth due to incoherent or coherent synchrotron radiation due to low energy and relatively short length • Any systematic trajectory errors corrected by orbit correctors and proper choice of optics functions and phase advances. • The systematic energy loss will be roughly half of the damping rings (~same energy and bending radius), i.e. 500keV, which is around 0.16% of energy difference. Corrected with RF cavities of a few hundred kV. • Can be used for timing jitter feedback if special optics used • Main issue: stability of RF deflector for keeping (horizontal) emittance growth small (<10% of the beam size). • Experience with the CTF3 RF deflectors instrumental for determining and achieving the requested tolerances

10. RF deflector stability • The angular deflection of the kicker is defined as • Large beta functions and π/2 phase advance necessary for minimizing kicks • Injected beam position at the septum • Typically, injection is dispersion free • Number of injected beam sizes set to Nx=6-10 • The thickness of the septum cannot be smaller than 2-3mm • Kicker jitter produces a beam displacement transmitted up to the IP. • Typically a tolerance of σjit ≤0.1σxis needed • Translated in a relative deflection stability requirement as • As beam size is around 10-5 m,position at the septum dominated by septum thickness • The tolerance remains typically a few 10-3 (more relaxed for larger beam sizes and lower septum thickness) • Maybe a double RF deflector system can further relax the tolerance

11. RF system @ 1 vs 2GHz 1 or 2 GHz, pros and cons

12. Summary

13. Recommendation • The 1GHz frequency in the damping rings is easing beam dynamics in the CLIC DR and drives the RF system to more conventional parameters • The added complication of the train interleaving regarding the RF deflector jitter does not seem a show-stopper as: • A conceptual design of the delay loop has been already worked and will be further refined for the CDR • Typically the RF deflector power source in CTF3 have shown stability of around 10-3 • The jitter in beam position can be further studied in CTF3 and measurements can back up this choice for the CLIC CDR • The parameters for CLIC@1TeV have to be reworked to be compatible with the 1GHz frequency • It is recommended thus to choose as baseline in the CLIC DR an RF frequency of 1GHz • A conceptual design for the 2GHz RF system has to be undertaken in parallel as a back-up solution