Update on SPS Wire Scanner Prototype Measurements A

Update on SPS Wire Scanner Prototype Measurements A. Valimaa With support from F. Caspers, E. Piselli, L. Timeo, C. Vollinger

SPS Wire Scanner Structure • The wire scanner has two flanges on the side to connect it to the beam pipe. • Three other flanges are for vacuum pumping or other purposes (top, bottom, back). • Tank is the placeholder for the drum in which the scanning mechanism is build on. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

SPS Wire Scanner Structure • In the centre acts the scanner mechanism, the fork 17/09/2020 Alpo Välimaa, BE-ABP-HSC

SPS Wire Scanner Structure • The scanner consists of a fork and a wire. • They couple to electromagnetic cavity resonances. • For the measurements, we used a copper wire instead of a carbon wire for mechanical reasons; It is less fragile. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

SPS Wire Scanner Structure • The drum is for dismounting the scanner system when the tank is placed in the beam pipe. W. Andreazzi 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Motivation • The carbon wire of the fork of SPS wire scanner burns out due to excessive power dissipation on it. • Structure possibly creates an impedance problem. • The structure of the wire scanner allows resonances that couple to the fork. This is to be prevented. • It is critical to find out which of the cavity resonances are harmful to the fork and find a solution to damp them. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Measurement Setup • Measure S-parameters from two probes set inside cavity along the beam line. • Transmission tells us the cavity modes excited and seen by the beam. • Reflection tells us the amount of coupling between the cavity and the probe • Perturb the system as little as possible (small coupling). 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Measurement Setup • Rotate the fork to see which resonances couple with the fork and how much 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Fork Angle Dependency • Transmission measurement on plain structure in function of fork position • 1 st resonance unaffected by the fork position • The fork seems to strongly couple near 300 MHz and 500 MHz • At high frequencies the probes are too strongly coupled on resonances -> modify couplers 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Mode Damping – Top Flange • We insert an all mode coupler from the top flange in order to damp oscillations inside the cavity. • Coupling strength limited by the depth we can penetrate into the cavity not touching the wire. • Through delicate measurements we can conclude that the coupler is coupling to both electric and magnetic fields. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Mode Damping • Increasing the surface area of the coupler with a copper foil had positive effects via increased capacitive coupling • Some modes still unaltered -> insufficient alone 17/09/2020 Alpo Välimaa, BE-ABP-HSC

What If… • … we took the big flange cover off and let the cavity radiate? • We should see how much it is possible to damp the fields if we were able to damp all the resonances at the flange plane 17/09/2020 Alpo Välimaa, BE-ABP-HSC

…But Nothing Changed! • Then we understood that there is a reentrant cavity structure - between the drum and the tank – that couples to the main cavity inside the drum. • Test: Insert lossy material near the small gap where the fields possibly are most concentrated -> it works! 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Lossy contact over the gap • Test of a possible solution by inserting a number of resistors of 10, 50 and 100 Ohm over the gap between the drum and the tank • The results were promising independent of the resistance within 10 -100 Ohm. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Resistor Number Dependency • Good damping is achieved with already two resistors. • Three resistors really perform well up to 700 MHz 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Resistor Position Dependency • Resistor positioning has some visible effect. • Better damping is achieved with symmetric positioning. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Carbon-Like Wire • To mimic the electrical properties of carbon wire, we decided to replace the copper wire with a series of resistors summing up to 4 k. Ohm • Picture: New wire during assembly. View from the top flange. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Fork Angle Dependency – Carbon-Like Wire • Note: Less fork position dependent behaviour (fork coupling) at low frequencies than in the case of copper wire! • This is to be analysed more delicately. 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Fork Angle Dependency – Carbon-Like Wire and Lossy Contact • Modes up to 700 MHz are clearly damped independent of fork position • At 225° fork very close to the probe at port 1 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Proposed Solution • Add feedthroughs to have the desired resistance over the gap but to place the resistors outside vacuum • Place them with 120 degree symmetry – if mechanical restrictions allow – to avoid azimuthal modes of lowest order • Details are to be optimized in simulations 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Thank you! 17/09/2020 Alpo Välimaa, BE-ABP-HSC

Following steps • Re-measure without the wire • Modify the probe to have good data on high frequencies • Resonant matching technique with the modified all mode coupler to damp selected higher order modes 17/09/2020 Alpo Välimaa, BE-ABP-HSC