Discussion and preliminary conclusions LIU-SPS ZS Electrostatic Septum Upgrade Review M.J. Barnes

Present Performance The ZS has sparked since introducing LHC type beam. During 2009 ZS outgassing was an important limiting factor. Outgassing depends strongly on beam parameters, in particular the bunch length. In 2011 sparking problems with bright beams – 100kV was set to avoid sparking (due to long-time constants full voltage modulations, cycle to cycle, are not possible). Maximum ZS intensity, during operation, to date is with 25ns beam 1.2x10 11 (i.e. OK for nominal LHC but is about half the intensity required for the HL-LHC). Worst pressure rise is with ZS5 – the reason for this is not understood: maybe ecloud occurs in nearby equipment? Sparking occurs when beam is extracted from SPS (but not in ZSTF) – the reason for sparking is not understood. ZS is not presently an important factor in the overall SPS impedance – but could become so in the future.

Studies/Simulations Required Measurement of beam coupling impedance of a ZS, with and without electrical circuit to understand effect of circuit. Compare measurements with suitable predictions to validate model. Damping, e.g. lossy material such as cable with ferrite loaded rubber, to be studied for electrical circuit in tank. Measurements to understand if wire versus foil make a difference to beam coupling impedance (measurements on a short model?). Determine contribution of pumping ports, in ZS interconnects, to SPS impedance. NEG coating of anode – does NEG still have a low SEY (e.g. 1.4 or less) after saturation; if so, ZS does not need to be bake able to benefit from NEG. Carry out a failure analysis for moving of pumps from interconnects to ZS tanks. Ecloud simulations with electrical field map (considering effect of A-K field leakage between the wires) – with and without ion trap voltages. Cost benefit analysis of having an individual power supply for each ZS.

Ideas To determine the impact of the pumping modules, to verify the resonance frequencies of the ion traps as well as to determine the influence of the RF screen, impedance measurements of a connected ZS with pumping modules and with/without RF screen are needed. Explore the possibilities to reduce e-cloud in ZS anode on ion traps (longitudinal grooves, coating?). Ecloud could also be occurring in the interconnects. Consider NEG coating or solenoids of interconnects. Explore suggestions to reduce e-cloud production in the pumping modules (solenoid, additional ion traps, NEG coating, even suppressing pumping modules). Ion traps could be extended into interconnects….. Note: alternating ion trap voltage, per ZS, is no longer required. Cycling ZS generators helps sparking problems, but may not help multi-cycling – time-constant is probably the same with individual power supplies for each ZS, hence probably no benefit. Pumps on each ZS tank, sectorisation of vacuum (valves on both ends of each ZS tank). Increase pumping speed with NEG cartridges. Study of ion trap efficiency – beam induced effects are modulating voltage and therefore probably reducing efficiency. Study the possibility to make improvements to the ion trap box, in particular reduce resistance. Assess the need for the Anode current measurement. If not needed, put the anodes directly to ground (RF screen?).

Others The ZS test facility (ZSTF), although not totally representative of the installation, is invaluable for testing ideas (e.g. connection box modifications) and thus MUST be retained. All potential “solutions” must be tested in ZSTF before being deployed. PRIORITIES FOR IMPROVEMENTS TO BE DECIDED.