LC-ABD2 Work Package 2: Damping Rings

1. LC-ABD2Work Package 2: Damping Rings Andy Wolski University of Liverpool and the Cockcroft Institute 12 April 2007

2. WP2 contains two key contributions to the damping rings R&D • Task 2.1: Damping Rings Beam Dynamics (Manager: A. Wolski) • Evaluate impedance-driven instability thresholds and growth rates. • Evaluate the effects of beam loading, injection/extraction transients and long-range wake fields in the damping rings under a range of operational conditions.* • Develop techniques for low-emittance tuning. • Task 2.2: Damping Rings Vacuum Systems (Manager: O. Malyshev) • Calculate the average pressure and pressure profile in the damping rings and, in the context of the results of these calculations, evaluate the technology options for the damping rings. • Produce technical designs for components in the vacuum chamber in the arcs and straights, and use these designs for developing an impedance model. • * Supported by CI core grant.

3. Damping Rings must provide very high quality, very stable beams Damping ring parameters are very demanding in terms of beam stability:no operating machine meets all the parameters simultaneously. Beam quality High Current

4. Major issues for beam stability • Electron cloud effects in the positron damping rings • One of the top priorities for damping rings R&D: already receiving major attention from groups around the world. • Ion effects in the electron damping rings • Still some uncertainty in likely impact on damping rings performance. Can probably be mitigated with feedback systems and a well-designed vacuum system. • Impedance-driven beam instabilities • Wide experience from operating facilities; we expect the damping rings to operate in a challenging regime. • Long-range wake fields can drive multibunch instabilities, and couple jitter from freshly-injected bunches to damped bunches awaiting extraction. • Short-range wake fields can drive single-bunch instabilities, which can appear as emittance increase, or a “bursting” type of instability. • These effects require careful study, with beam dynamics models closely connected to the technical design of the vacuum system.

5. Single-bunch instabilities are diverse and complicated. There is a lot of operational experience of these effects, but a good understanding for any given machine generally requires a lot of hard, detailed work. • Single-bunch instabilities were a major problem for the SLC damping rings: eventually, the vacuum chamber had to be rebuilt. Evaluate impedance-driven instability thresholds and growth rates. Single-bunch instability in the SLC damping rings. Left: Experimental observation(B. Podobedov, BNL). Right: Simulation(K. Oide, KEK). Mode amplitude (arb. units) Time (ms) We shall collaborate with LBNL and SLAC in the construction of impedance models (using technical designs of the vacuum chamber, to be performed in Task 2.2) and the evaluation of the resulting instabilities.

6. Instability studies supported by CI core grant • Evaluate the effects of beam loading, injection/extraction transients and long-range wake fields in the damping rings under a range of operational conditions. Using a time-domain simulation code, we studied the coupling of injection jitter to damped bunches in the NLC damping rings. t = 0 ms t = 10 ms Similar (or stronger) effects are expected in ILC. Studies must include a detailed impedance model (resistive wall and HOMs), lattice model, radiation damping and feedback system. Our present code does include these effects

7. Task 2.1 Goal 2 • Lowest achieved vertical emittance (after significant effort) is 4.5 pm in KEK-ATF. The ILC specification is for 2 pm. Develop techniques for low-emittance tuning. 10% agreement Emittance-tuning using ORM analysis in the KEK-ATF. Several techniques (orbit/dispersion/coupling correction; orbit response matrix analysis…) work well in simulation, but practical implementation with the necessary accuracy and precision is still extremely challenging. We need to demonstrate a technique that can be routinely applied to a (6 km) ring to achieve vertical emittance of 2 pm on a regular basis.

8. Task 2.2 Goal 1 • Calculate the average pressure and pressure profile in the damping rings and, in the context of the results of these calculations, evaluate the technology options for the damping rings. Calculation of the pressure in a section of the ILC damping rings in two different scenarios for the vacuum system, as a function of the spacing between the pumps. Left: Stainless steel tube. Right: NEG-coated tube. (O. Malyshev, ASTeC) Initial evaluations have been performed, as part of the EUROTeV programme, and have indicated the benefits of NEG-coated vacuum chamber. Detailed studies are now needed to evaluate the benefits of NEG-coating, and to produce technical specification for the vacuum system (apertures; antechambers; material and coating; pumping locations; pumping speeds etc.) Vacuum studies must be well-integrated into studies of electron cloud and ion effects.

9. Task 2.2 Goal 2 Produce technical designs for components in the vacuum chamber in the arcs and straights, and use these designs for developing an impedance model. • Technical designs of components in the vacuum chamber are essential for constructing an impedance model. • Need to include bellows, flanges, tapers, pumping ports, BPMs, antechambers, kickers and septa… • Close collaboration with other technical groups (e.g. instrumentation) is essential. • Producing a complete, detailed model is a significant amount of work, but is essential for a reliable evaluation of the impact of collective effects. Calculation of trapped modes in PEP II bellows. Higher-order mode heating is a significant problem for PEP II, and a potential problem for the ILC damping rings. (Cho Ng, SLAC). We will collaborate with LBNL on the technical design, and with SLAC on the impedance modelling.

10. Task 2.2 Goal 2 • The goal of producing a detailed impedance model for the EDR, based on technical designs of the important components, is ambitious. • The Damping Rings Workshop at Cornell, 26-28 September 2006, outlined a staged plan, with specified milestones towards the goal of a complete evaluation of the impedance-driven collective effects. • Begin with constructing an impedance model based on scaling components from existing facilities, in parallel with the technical design of the damping rings vacuum. • Proceed iteratively to improve the model, using the results of the scaled impedance model to guide the design work, so as to achieve a specified impedance budget. • Our proposed work on the vacuum system fits extremely well with the timescales and methodologies. • If the hoped-for contributions from other labs (LBNL and SLAC) are not provided, we still make an essential contribution towards a reliable impedance model. Produce technical designs for components in the vacuum chamber in the arcs and straights, and use these designs for developing an impedance model.

11. Final Words • Work Package 2 addresses two critical and related issues for the ILC damping rings: • Dynamical effects that potentially limit beam quality and stability. • Vacuum system specification and design. • We will make an important contribution to the ILC in these areas. • The work we are proposing will produce results needed for the EDR on an appropriate timescale. • We will collaborate with identified international partners to maximise the benefit of the resources that are available. • The tasks are closely connected to other work packages within LC-ABD, for example: • WP6: Alignment and Survey – precision magnet alignment and stability are critical for achieving ultra-low emittance. • WP7: Beam Feedback and Control Systems – fast bunch-by-bunch feedback will be essential for maintaining beam stability. • WP8: Advanced Beam Diagnostics – laser wire will be required for measurement of ~ pm emittance.