Review on electron cloud simulations for the SPS electrostatic septum (ZS) G. Rumolo and G. Iadarola (20/02/2013)

CERN, LIU-SPS ZS Review, 20/02/20131Brief review on electron cloud simulations for the SPS electrostatic septum (ZS) G. Rumolo and G. Iadarola in LIU-SPS ZS Review, 20/02/2013ZS with LHC-type beamsStudy of electron cloud build up thresholds in a ZS-like geometry with LHC25 beams:Without external fieldsWith the voltage from the ion trapsSome conclusionsNew simulations run with PyECLOUD with the correct model/geometrySome practical information on the ZS20mmV=-220kV-6kV-3kVAnodeCathodeIon-traps2From J. Wenninger, Introduction to Slow Extraction to the North Targets2000 W.75Re.25 septum wires.Wire diameter 50 mm (first ZS) to 100 mm.Wire spacing 1.5 mm.Instructions when high intensity LHC beams are in the SPS: Retracted girder (not anymore after 2010 to allow || MDs & conditioning) HV 0 kV (not anymore after 2010, 20-100 kV applied, reduces outgassing) Ion traps onObservations at the ZS with 25 ns high intensity beam (see also talks from Bruno and Karel):Above certain intensities (nominal beam, more than 1 batch injected) vacuum spike in the ZS observedThe increased vacuum levels can provoke a vacuum interlock, which stops the ion traps and hence the beam in the machine. In this sense, ZS limits the LHC beam By increasing the bunch length the vacuum does not degrade. Sparking occasionally occurs (ramp, ejection)Ion trap voltage drop and current measured off the plates Does not appear to be the principal problem, rate decreasing Some hints that e-cloud could build up in the ZS, even if the presence of a voltage should clear electrons. ZS during SPS operation with high intensity LHC beams3

In absence of voltage from the ion traps significant electron cloud builds up for dmax > 1.5 the electron cloud between bunches is uniformly distributed over the chamber cross section Electron cloud simulationsthe ZS geometry without external fields4lec = 1010 e-/m46 mm140 mm

Assuming a voltage of 3 kV between the bottom and top plates the electron cloud is suppressed at least up to dmax =2.0 (different build up curves are all below the one for dmax =2.0)Electron cloud simulations including the voltage from the ion traps5Elec = 103 e-/m46 mm140 mmAssuming a voltage of 3 kV between the bottom and top plates the electron cloud is suppressed at least up to dmax =2.0 (different build up curves are all below the one for dmax =2.0) the electrons are fully cleared between subsequent bunches and there is no visible dependence on the SEY of the plate.Electron cloud simulations including the voltage from the ion traps6E46 mm140 mm

Zoom of previous plot in the first 0.3 ms

For V=100 V the SEY threshold lies also around 1.5Assuming dmax =1.7 and scanning the voltage between the bottom and top plate the electron cloud is found to be fully suppressed for 500 V V < 4 kV V=100 V is not sufficient and a strong electron cloud is formed (with a faster rise time than V=0 but a faster decay, too, due to the clearing voltage)Electron cloud simulationsscanning the voltage values7

V = 100 VChanging the voltage, we actually change the clearing efficiency electrons are cleared more efficiently (i.e. more quickly) with higher voltages However, if the voltage becomes too low (