Energy Spectrum Measurement of the Multipacting Electrons in the SPS. Analysis of the Possible Utilisation of the BGIP Monitor.

An electron cloud cascade driven by the proton beam space charge field is expected to occur in the LHC. This charge cloud composed of secondary, ionisation and photo - electrons is accelerated under the influence of the proton beam electric field as a function of both bunch intensity and period. This process results in the production of a large heat load on the surface (especially on the dipole section beam screen at cryogenic temperature), multiplication of space charge in the chamber, coupling between electrons and the beam and increased vacuum pressure that could ultimately cause the loss of the proton beam itself.In-depth theoretical and experimental work has recently been carried out at CERN [1-1b]. In this framework, a numerical simulation code has been developed [2-2b] following the analytical approach to the problem [3] and a deep research program has been executed. To test the validity of the simulation code, experimental results were obtained in a TW test chamber simulating the proton beam by means of RF pulse trains. Good agreement was observed between experiments and simulations when measuring the energy spectrum of the emitted electrons and the current as a function of the RF pulse parameters. Furthermore, in the SPS-MD with an LHC-type beam, above a beam intensity threshold a pressure rise by more than a factor of 50 has been observed. Multipacting has been measured and unambiguously confirmed, because preliminary test ruled ion-induced desorption out. An extensive program is underway for the SPS in order to test possible remedies and to avoid the detrimental effect of the electron-cloud in the LHC.To validate the simulation code by testing it on a real accelerator, the missing but essential measurement is the determination of the multipacting electrons energy spectrum produced in the SPS by the LHC - type proton beam.