Outcome of recent impedance bench measurements on collimators N.Biancacci, D.Amorim, F.Caspers, I.L.Garcia, F.Giordano, T.Kaltenbacher, G.Mazzacano, E.Métral, B.Salvant, L.Teofili, C.Vollinger

Outline Impedance measurements on TCTWH Impedance measurements on TCSPM (preliminary) Conclusions and outlook

Outline Impedance measurements on TCTWH Impedance measurements on TCSPM (preliminary) Conclusions and outlook

TCTWH TCTP with embedded wire for beam-beam compensation [1,2]. Validated with impedance bench measurements on 13-12-2016 Main outcome: same as a TCTP on 31-07-2014, i.e. presence of 87 and 169 MHz transverse HOM -> Detailed analysis confirmed not an issue for beam stability (see also PhysRevAccelBeams.20.011003) Tungsten jaw Ferrite tiles Wire Glidcop [1] D.Pierini, “Advanced collimation design studies (TCT with wire, hollow e lens)” 4th Joint HiLumi LHC-LARP Annual Meeting [2] O.Aberle , “TCTW production, assembly procedure and validation” LHC Collimation Working Group #202

TCTWH TCTWH TCTP 169 MHz 87 MHz

Outline Impedance measurements on TCTWH Impedance measurements on TCSPM (preliminary) Conclusions and outlook

TCSPM Main outcome of the measurements: In operation (MD) can we distinguish the stripes? Yes we can (preliminary result). When not in operation (jaw retracted), any issue? Normally not, but HOMs detected: 455 MHz, 800 MHz and 1.2GHz. To be crosschecked with simulations. HOMs present at all gaps, especially large ones. Which gaps recommended setting for retraction? Would suggest 15mm. So that the long fingers are not sticking out. Simulations ongoing to see where HOMs are located. TiN Mo MoGr

TCSPM TCSPM test bench TiN Mo TCSPM jaw MoGr

TCSPM X In operation (MD) can we distinguish the stripes? Measured longitudinal impedance vs X position Z_Glidcop << Z_stripes: Z_TiN > Z_Mo : Z_Mo > Z_MoGr: X ? Possible explanation: moving the wire we do not only change the material underneath but also: Slightly the geometrical impedance (first taper is shorter). To be estimated with simulations. Characteristic impedance of the wire-DUT TEM line: decreases for |x|>0. To be estimated with simulations GC TiN MoGr Mo GC

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan ~ 1.2 GHz 800 MHz

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan

TCSPM When not in operation (jaw retracted), any issue? Systematic gap scan Observations: Presence of HOMs at all measured gaps (i.e. apparently no dependence on length of the long RF finger) Shunt impedance modulated by jaw position: max Rsh = 35 Ohm Not an issue from heating point of view 10W at max for a mode at 500MHz (1W at 800MHz). To be checked: Further impedance data available (time to process still needed) Origin of HOMs with simulations. Are those transverse HOM?

TCSPM Mode at ~ 1.2 GHz identified with probes in transmission: Q~ 115

TCSPM Mode at ~ 445 MHz identified with probes in transmission: Q~ 120

Conclusions and outlook TCTWH Measurements in line with those ones done on TCTP Not an issue for beam stability No apparent effect of embedded wire TCSPM Still some analysis to do, but main messages can be drawn: Can see different impact of coating on RW impedance. Strange behavior when on MoGr: tbc if geometrical impedance change or change in TEM characteristic impedance could play a role. Identified HOMs at 455 MHz, ~ 800 MHz and ~ 1.2 GHz: work on going with simulations to understand where these are located. Not an issue from heating point of view. Tbc if these are transverse modes -> unlikely to be an issue for transverse stability. Recommended to keep jaws partially closed (closer “parking”). At 15mm the long RF fingers fit in the corresponding hole: can avoid unwanted issues (bent RF fingers towards the beam) Recommended analogous measurements on collimators with similar design.

Many thanks!

Backup

RW theory

RW theory

RW theory Delta ~0.1 Ohm 76 MHz