Update on electron-cloud power deposition for the Large Hadron Collider arc dipoles

Abstract

We revisit the estimation of the power deposited by the electron cloud (EC) in the arc dipoles of the Large Haydron Collider, by means of simulations. We adopt, as simulation input, a set of electron-related parameters closely resembling those used in recent simulations at CERN [F. Zimmermann, in LTC Meeting No. 40, CERN, 2005]. We explore values for the bunch population N_{b} in the range 0.4×10^{11}≤N_{b}≤1.6×10^{11}, peak secondary electron yield δ_{max} in the range 1.0≤δ_{max}≤2.0, and bunch spacing t_{b} either 25 or 75 ns. For t_{b}=25 ns we find that the EC average power deposition per unit length of beam pipe, dP[over ¯]/dz, will exceed the available cooling capacity, which we take to be 1.7 W/m at nominal N_{b} [F. Zimmermann, in LHC MAC Meeting No. 17, 2005], if δ_{max} exceeds ∼1.3, but dP[over ¯]/dz will be comfortably within the cooling capacity if δ_{max}≤1.2. For t_{b}=75 ns dP[over ¯]/dz exceeds the cooling capacity only when δ_{max}>2 and N_{b}>1.5×10^{11} taken in combination. The rediffused component of the secondary electron emission spectrum plays a significant role: if we artificially suppress this component while keeping δ_{max} fixed, dP[over ¯]/dz is roughly cut in half for most values of N_{b} explored here, and in this case we find good agreement with earlier results [F. Zimmermann, in LTC Meeting No. 40, CERN, 2005], as expected. We provide a fairly detailed explanation of the mechanism responsible for such a relatively large effect. We assess the sensitivity of our results to numerical simulation parameters, and to physical parameters such as the photoelectric yield, bunch train length, etc. Owing to the lack of detailed knowledge of the electron emission spectrum, the sensitivity of dP[over ¯]/dz to the rediffused component appears to be the most significant source of uncertainty in our results. Nevertheless, taking our results as a whole, the condition δ_{max}≤1.2 seems to be a conservative requirement for the cooling capacity not to be exceeded.