Update on LHCb VELO impedance Benoit Salvant Olga

Update on LHCb VELO impedance Benoit Salvant, Olga Zagorodnova (DESY) Many thanks to Massimiliano Ferro-Luzzi, Juan Esteban Mueller, Alexej Grudiev and Elias Metral.

Agenda • Status of computations • Status of 2015 observations

Imaginary longitudinal resistive contribution Computations with impedance. Wake 2 D (N. Mounet) Distance to beam NEG Al. Mg 4 Torlon To be compared to 90 m. Ohm quite small even for low distance

Imaginary transverse resistive contribution Distance to beam NEG Al. Mg 4 Torlon To be compared to 30 MOhm/m (and beta function is very low) quite small even for low gaps

Impedance simulations

Impedance simulations A lot of leaking to the outside pipe

Imaginary longitudinal impedance in Ohm Longitudinal impedance (real) Box displacement in mm Frequency in GHz Non zero real longitudinal impedance at low frequency due to “global step out” Clear presence of longitudinal modes above 400 MHz, but less linked Olgaconnections Zagorodnova HL-LHC 7 tank to the geometry of the box than to the between the boxes and the outside

transverse impedance (imaginary) Imaginary transverse impedance in Ohm/m Box displacement in mm Frequency in GHz ~1 MOhm/m when closed, but impact mitigated by increased stabilization in collision and low beta* Clear presence of low frequency transverse modes from 100 MHz onwards Olga Zagorodnova HL-LHC 8

Longitudinal impedance (imaginary) Imaginary longitudinal impedance in Ohm Box displacement in mm Frequency in GHz Large low frequency imaginary impedance, most likely due to the bellow-like structure of the box Im(Zeff/n)~4. 7 m. Ohm (full LHC is 90 m. Ohm). Olga Zagorodnova HL-LHC Large contribution! Is that an issue? 9

Margin for longitudinal impedance? -/+ 12% impedance J. E. Mueller Good agreement between model and measurements at low intensities. Extrapolation to higher intensities: there seems to be margin there, but are

Updated resistive power loss as a function of distance to the box Nb=1. 15 e 11 p/b M=2748 bunches Bunch length=1 ns

Prediction for HL-LHC parameters Nb=2. 2 e 11 p/b M=2748 bunches Bunch length=1. 08 ns

Observations during 2015 • Insertions of Roman pots in beam during collisions: also 2 to 3 % of total LHC impedance no visible impact so far • Temperature monitoring: very insensitive to beam parameters. • Can we extrapolate for HL-LHC?

Beam induced heating? Energy Intensity Temp TOTEM Roman pots Temp MKI Temp ATLAS-ALFA Roman pot TOTEM “IN” Example of temperature of certain LHC devices during physics fills MKI: injection kicker TOTEM Roman pots ATLAS-ALFA Roman pots Temperature increase due to the interaction of beam induced wake fields with the surrounding Beam induced heating has affected operation since intensity ramp up started in mid-2011 14

LHCb temperatures

LHCb temperatures Temperatures seem more affected by an external source than the beam itself

LHCb temperatures

Energy error No clear evidence of energy lost by the beam when inserting the VELO More precise measurement available with the new obs box by Juan.

Conclusions • No showstopper so far, but several warnings • The geometry of the box is less critical for the higher order modes which can be studied separately together with the wakefield suppressors • Important to perform measurements on a mockup and be ready to add mitigation measures and sensors (e. g. couplers, ferrites) • Try to probe Hilumi intensities with the LHC beam in single bunch next year to see the available margin