Results on RF-Measurements on 3-Convolution HL-LHC fingers Branko Popovic, Christine Vollinger IWG meeting #32, 14-May-19

Overview HL-LHC RF fingers with three convolutions are planned to be used in the LHC triplets installation as means to reduce the contribution of the bellow modules to beam impedance. In the past, measurements have been carried out on modules with 2-convolution RF fingers (C. Vollinger, impedance meeting 15-Dec-2014) as well as measurements and simulations on 3-convolution RF-fingers (K. Sjobak, e.g.: https://indico.cern.ch/event/556760/contributions/2243231/attachments/1323591/1986039/2016-08-15_Impedance-ACE3P.pdf). Request to the current RF-measurements is to validate their shielding effect on the bellow modules. Measurements of HL-RF fingers, IWG meeting #32, 14-May-19 B. Popovic, C. Vollinger (BE/RF-BR)

HL-LHC triplet bellows and 3-convolution RF fingers In total, 20 modules will be assembled in the triplets (5 per triplet). The solution is proposed for other locations as well, if approved. RF-finger design from TE-VSC, J. Perez Espinos, C. Garion Measurements of HL-RF fingers, IWG meeting #32, 14-May-19 B. Popovic, C. Vollinger (BE/RF-BR)

Measurement description Single wire measurements Details see: HL-LHC prototype, J. Perez Espinos, LHCVSMPI0001 Measurements with probes and single wire have been carried out on the bench in different longitudinal and transverse positions. Expected longitudinal movement is max. 12 mm, transverse movement is max. 10 mm. Max coupling between inner part and outer bellows is expected for compressed longitudinal (nominal -12 mm), max. transverse movement. Probe measurements: all modes below 2 GHz could be identified as probe modes → abandoned. Measurements of HL-RF fingers, IWG meeting #32, 14-May-19 B. Popovic, C. Vollinger (BE/RF-BR)

Single wire measurements Measurements in different longitudinal and transverse positions did virtually not change the modes inside the structure, except for some variation of the resonance peaks. The measured geometry is artificially closed on both ends, so resonances coming from this effect should be suppressed. We added absorber foam padding on the end plates of the measurement set-up to mimic the extended beam pipe of the true structure. Measurements of HL-RF fingers, IWG meeting #32, 14-May-19 B. Popovic, C. Vollinger (BE/RF-BR)

Single wire measurements The remaining modes are slightly shifting downwards in the resonance (also a feature of the strong coupling which we introduced now), but are most gone. The largest mode at 2.08 GHz has a remaining Q-value of 60 and is considered uncritical. Measurements of HL-RF fingers, IWG meeting #32, 14-May-19 B. Popovic, C. Vollinger (BE/RF-BR)

Conclusion Probe measurements (not shown) and single wire measurements have been carried out on the 3-convolution RF-fingers for the HL-LHC bellow modules. Measurements in different longitudinal and transverse positions within the expected range of mechanical movement did virtually not change the modes inside the structure, except for some variation of the resonance peaks. Measurements with absorber foam indicate no critical resonances in the structure. Based on these measurements, we suggest that an OK from IWG is given for the use of these 3-convoluted RF-fingers to shield the required 20 bellow modules in the LHC triplets. Remark: for the suggested geometry, we see a good shielding effect. Changes in the geometry of the RF-fingers (size scale, diameter, etc.) can result in different shielding properties, e.g. if different frequencies are to be expected. The results cannot be extrapolated to other shield sizes. We recommend to measure each bellow module and its shielding individually. Measurements of HL-RF fingers, IWG meeting #32, 14-May-19 B. Popovic, C. Vollinger (BE/RF-BR)