Ion induced pressure instability: the threat for

Transcript:Ion induced pressure instability: the threat for:
Ion induced pressure instability: the threat for positively charged beams O.B. Malyshev ASTeC Vacuum Science Group, STFC Daresbury Laboratory, UK ECLOUD’22, Isola d'Elba, Italy, 26-30 Sep. 2022 What is the ion induced pressure instability? Ion induced instability with the negatively charged beams O.B. Malyshev ECLOUD’22, Isola d'Elba, Italy, 26-30 Sep. 2022 3 CO H2 CH4 CO2  beam Residual gas molecules are ionised by the beam The positively charged ions build up an ion cloud along the negatively charged beam path The ions cause the ion induced beam instability Mitigation: Better pumping system Ion collectors + + + + + + + + + + + + + + Positively charged beams Electrons Electrons appear in vacuum chamber due to photoemission and electron from a beam induced gas ionisation Electrons are accelerated by the beam charge and drift between bunches These electrons may strike the vacuum chamber wall causing Secondary electrons Electron stimulated gas desorption (ESD) These electrons build up an electron cloud that cause a beam emittance ‘blow-up’ Gas density increases causing a generation of more electrons from beam induced gas ionisation Ions Ions appear in vacuum chamber due to a beam induced gas ionisation Ions are accelerated (repelled) from the beam by the beam charge and drift between bunches These ions strike the vacuum chamber wall causing Ion stimulated gas desorption (ISD) Secondary electrons Gas density increases causing a generation of more ions from beam induced gas ionisation The electrons will contribute to an electron cloud built up O.B. Malyshev ECLOUD’22, Isola d'Elba, Italy, 26-30 Sep. 2022 4 e- CO H2 CH4 CO2 e- e- e+ or A+ beam            H2+ CO H2 CH4 CO2 e-  Pressure instability and critical current O.B. Malyshev ECLOUD’22, Isola d'Elba, Italy, 26-30 Sep. 2022 5 Critical current, Ic, is a current when pressure (or gas density) increases dramatically. Mathematically: First observed at ISR storage ring at CERN in 1971 Studied for SSC (USA) LHC design (CERN) includes results of modelling ion induced pressure instability Then studied for ILC-DR In present is under investigation for FCC Theoretical model Equation of gas balance The equation for the volumetric gas density inside a room temperature vacuum chamber  – ion stimulated desorption yield, I – beam current, – a cross section of the gas ionisation, qt – electron charge; h – photon stimulated desorption