Damping Rings James Jones ASTeC, Daresbury Laboratory

Personnel ASTeC Oleg Malyshev – Vacuum Studies James Jones – Low emittance tuning studies + Engineering Support Liverpool University Andy Wolski – Low emittance tuning, Vacuum Design 1 RA currently employed looking partially at impedance effects RA – Vacuum Design (LC-ABD 2) RA – Vacuum Design (LC-ABD 2) Larisa Malysheva – Polarisation issues (As part of a wider collaboration)

Damping Rings must provide very high quality, very stable beams ILC DRPEP II LERHERA eSPring-8 Energy5 GeV3.5 GeV27.5 GeV8 GeV Circumference6694 m2199 m6360 m1435 m Average current400 mA2450 mA58 mA100 mA Number of bunches2767 – Particles per bunch2×10 10 – 1× × × ×10 10 Extracted bunch length (rms)6 mm12 mm9 mm6 mm Horizontal normalised emittance8 μm200 μm1000 μm90 μm Vertical normalised emittance0.02 μm10 μm180 μm0.09 – 0.27 μm DR Damping ring parameters are very demanding in terms of beam stability: No operating machine meets all the parameters simultaneously.

Previous Work ASTeC was, and is, committed to damping ring work as part of the EUROTeV framework: Both O. Malyshev and J. Jones have small but significant work packages within this framework EUROTeV WP 3 Task 2 deals with electron cloud issues within the positron damping ring, and mitigation schemes. O. Malyshev was a major contributor to vacuum simulations for this task. EUROTeV WP 3 Task 3 deals with low emittance tuning simulations for the damping rings. The work is coordinated by J. Jones.

Vacuum Simulations Complete simulations of damping ring vacuum systems, including both the dipole induced photo desorption, along with analysis of pump location and speeds Conventional vacuum technology does not allow to reach required vacuum after 100 Ah conditioning NEG coated chamber provides cheapest and simplest vacuum solution for dumping ring: less number of pumps less pumping speed required lower bakeout/activation temperature (180C in stead of C) low SEY to suppress e-cloud effect

Low Emittance Tuning Have a large scale emittance tuning simulation environment for the RDR damping rings. Includes full orbit, dispersion and coupling correction. Models the effects of ATL-like ground motion on the time evolution of the output emittances. None CO Full Initial Correction Only Full Correction every 6 Days

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.

Using a time-domain simulation code, we studied the coupling of injection jitter to damped bunches in the NLC damping rings. Task 2.1 Goal 1 Evaluate the effects of beam loading, injection/extraction transients and long- range wake fields in the damping rings under a range of operational conditions. t = 0 mst = 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: alternatives may be available (e.g. MULTI-TRISIM).

Task 2.1 Goal 2 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). 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.

Task 2.1 Goal 3 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. 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. Emittance-tuning using ORM analysis in the KEK-ATF.

Task 2.1 Goal 3 The main facilities used so far for experimental studies of low-emittance tuning have been the KEK-ATF and the LBNL-ALS. The ATF will continue to be available. The main limitation so far has been the availability of personnel. Producing a high-quality beam from the storage ring will be essential for ATF2. Beam time at the ALS is generally available at monthly intervals. The main limitation tends to be the availability of staff to run the experimental studies. There are presently two serious proposals for future damping rings test facilities: CESR-tf could start operations for damping rings studies as early as June HERA-DR could start operations in late In both proposals, low-emittance tuning would be an important part of the programme. Further opportunities are provided by other machines. Light sources, e.g. DIAMOND. KEK-B (proposed damping rings study programme starting in 2009, to include low- emittance tuning). Develop techniques for low-emittance tuning.

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. 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. 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)

Task 2.2 Goal 2 Determine conditioning rates for NEG coatings under various conditions. We know that: the initial pressure in a NEG coated vacuum chamber activated at 180 C is better by two orders of magnitude than that in a stainless steel vacuum chamber baked in-situ to 300 C; NEG outgassing rates reduce with accumulated photon dose. Data from the ESRF show that this reduction could be up to 2 orders of magnitude; NEG coating simplifies and reduces the cost of the pumping system, and works to mitigate multipacting. In other words, we know that NEG coating is worth using. For the design of the vacuum system, we need to know the photon and electron-stimulated desorption yields: as functions of photon or electron dose, up to very large doses; as functions of photon or electron energy; as functions of NEG activation temperature (from room temperature up to 250 C); after air vent to different pressures (from mbar to atmosphere), to determine whether recovery after an accident requires reactivation. As a result of the experimental studies: we will be able to produce (for the TDR) a vacuum system design optimised for performance and cost, including spacing and required pumping speeds of the lumped pumps; we will gain invaluable experience in the use of NEG coatings under a wide range of conditions.

Task 2.2 Goal 3 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.

Task 2.2 Goal 3 The goal of producing a detailed impedance model for the TDR, based on technical designs of the important components, is ambitious. The Damping Rings Workshop at Cornell, September, 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.

Final Words The damping ring work proposed 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 a leading contribution to the ILC in these areas. The work we are proposing will produce results needed for the TDR on an appropriate timescale. We will collaborate with identified international partners to maximise the benefit of the resources that are available. Vacuum studies have the potential for industrial involvement, and a major contribution (> 13 km of vacuum system) during construction. The tasks are closely connected to other work packages within Cockcroft, for example: Effects of linac wakefields depend on beam stability from damping rings Instrumentation and feedback essential for maintaining stability