Update on new triplet beam screen impedance B. Salvant, N. Wang, C. Zannini 7 th December 2015 Acknowledgments: N. Biancacci, R. de Maria, E. Métral, N. Mounet, N. Kos Agenda:  Impact of coating on the new triplet beam screen  Impact of coating on the current beam screen  Impact of various weld scenarios of weld of new triplets

Resistive wall impedance of the new beam screens Materialσ el [S/m]εrεr Thickness [µm] aC coating Titanium coating4* Copper Stainless steel Vacuum01Infinity With Coating 5 layers structure 1 st layer (aC) 2 nd layer (Ti) 3 rd layer (Cu) 4 th layer (StSt) 5 th layer (Vacuum) Without Coating 3 layers structure 1 st layer (Cu) 2 nd layer (StSt) 3 rd layer (Vacuum) Updated Thanks to Nicolo Biancacci’s measurements Info from N. Kos

Geometries of the Hi-Lumi IR1 and IR5 beam screens (triplet) MagnetCold bore ID (mm) Beam screen ID between flats (mm) Beam screen length (m) Q / Q2a / Q2b / Q / CP / D / DFXJ / D29587/ Q / Info from N. Kos

Longitudinal impedance of beam screen in triplets (IR1&IR5) (2D calculation with ImpedanceWake 2D – N. Mounet)  Significant impact of coating on imaginary part  Longitudinal effective impedance of the beam screen multiplied by 3 because of the coating.  Still expected to be in the background of the total LHC impedance (~90 mOhm)

Transverse impedance of beam screen in triplets (IR1&IR5) (2D calculation with ImpedanceWake 2D – N. Mounet)  Significant impact of coating on imaginary part  Vertical effective impedance of the beam screen increased by 70% because of the coating.  Still expected to be in the background of the total LHC impedance (~20 MOhm/m) For lattice version V1.2 beta*=15cm/15cm

ElementMax betax/betay [m/m] Z T,eff1 without aC coating [kOhm/m] Z T, eff2 with aC coating [kOhm/m] Z T,eff2 -Z T,eff1 [kOhm/m] Collision Round21758/ Collision Flat43154/ Presqueeze optics6776/ VDM optics 30m618/ Compare between different Optics (IR1, IR5 new beam screens)

ElementLength [m]Beam screen ID between flats (mm) betay_ave [m]Z T,eff1 without aC coating [kOhm/m] Z T, eff2 with aC coating [kOhm/m] Z T,eff2 -Z T,eff1 [kOhm/m] Triplets Q / Q / Total  Longitudinal/transverse impedance of beam screen increased by 40%/14%  Still expected to be in the background of the total LHC impedance. For lattice version V1.2 beta*=15cm/15cm Effect from coating all LLS (IR1, IR5) ElementLength [m]Z l,eff1 without aC coating [Ohm] Z l, eff2 with aC coating [Ohm] Z l,eff2 -Z l,eff1 [Ohm] Triplets--3.6E-59.6E-56.0E-5 Q E-61.8E-51.1E-5 Q E-62.0E-51.3E-5 Total--5.1E-513.5E-58.4E-5

Impact of coating on the current beam screen (IR2 and IR8) Magnet Beam screen ID (mm) Beam screen length (m) Average betax (m) Average betay (m) Q140.4/ Q2+Q350.4/ CP+D1 61.0/ For lattice version V1.2 beta*=15cm/15cm  Significant impact of coating on imaginary part  Longitudinal effective impedance comparable with the impedance of the new beam screen.  Transverse effective impedance is about 1/10 of the new beam screen impedance.

Conclusion for coating Small impact expected on impedance of coating the new beam screens or the whole LLS for IP1 and IP5. Same level of longitudinal impedance contribution by coating the current beam screen in IR2 and IR8, while the transverse impedance contribution is about 1/10 of the new beam screen impedance.

Effect of the weld(s) -“Transverse” weld -“Longitudinal” weld  note: not easy to get convergence with CST simulations for this case  Requires refined mesh near the weld.

Effect of transverse weld (IR1, IR5 new beam screens) ElementLength [mm]Power loss [W] Q112*40.30 Q212*80.54 Q312*40.27 CP8*40.18 D18*40.18 DFXJ4*40.09 D216*40.51 Q412*40.47 Total Power loss per meter for 1 beam (for D2) in weld: 7.9 [W/m] in beam screen: 0.3 [W/m] Assuming a weld falls on a max of beta function (30 km):  Effects on the imaginary impedance  Power loss for 2748 bunches at 2.2e11 p/b  Small effect expected

Effect of longitudinal weld  Effect of weld is visible, but still far from a solid stainless steel beam screen  Effect of different types of weld (1 weld, 2 welds, and smaller) appears small

Still numerical issues with rotated weld  Still working on … Frequency (GHz) Impdance in Ohm

Longitudinal Impedance ratio with respect to copper beam screen Frequency (GHz) Ratio vs copper  1 weld (2mm) is better than 1 weld (4 mm) and is better than 2 welds (2*4 mm)  increase of an order of 50% to a factor 2 of both real and imaginary longitudinal impedance

Transverse dipolar impedance Frequency (GHz) H dip impdance in –Ohm/10mm

Frequency (GHz) Ratio vs copper  1 weld (2mm) is better than 1 weld (4 mm) and is better than 2 welds (2*4 mm)  increase of an order of 50% to a factor 3 of both real and imaginary longitudinal impedance

Impedance of partially penetrated welds in LLS beam screens The LHC/HL-LHC LSS beam screens have so-called contact rings welded to both extremities of the beam screen. The weld penetrate 40-50% in the beam screen and a gap between the beam screen and the contact ring is <0.05 mm. electroplated with 5 microns of gold The impedance of the gap is calculated by ABCI:  The impedance contribution to the total LHC impedance is expected to be small. Assuming a weld falls on a max of beta function (30 km):

Summary We should complete the study (rotated welds) No showstopper in the various options unveiled so far. It could be interesting to rotate the weld locally to the plane where the beta functions are smaller. To be checked if the optics do not change too much. BETYBETX