HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study - PDF document

CERN-ACC-SLIDES-2014-0061 HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study Presentation “Impedance Considerations for the Design of the Triplet / D1 Beam Screen” Mounet, N (CERN) et al 05 March 2014 The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404. This work is part of HiLumi LHC Work Package 2: Accelerator Physics & Performance . The electronic version of this HiLumi LHC Publication is available via the HiLumi LHC web site <http://hilumilhc.web.cern.ch> or on the CERN Document Server at the following URL: <http://cds.cern.ch/search?p=CERN-ACC-SLIDES-2014-0061> CERN-ACC-SLIDES-2014-0061

Impedance considerations for the design of the triplet/D1 beamscreen N. Mounet, B. Salvant, C. Zannini and E. Métral Acknowledgements: G. Arduini, C. Boccard, G. Bregliozzi, D. Draskovic, L. Esposito, S. Fartoukh, C. Garion, G. Iadarola, R. Jones, R. Kersevan, T. Lefevre, N. Kos, A. Mostacci, A. Nosich, G. Rumolo, M. Taborelli and E. Todesco 1 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

HL LHC: impedance considerations for the new beam screens in IR1 & 5  Reminder on current and HL-LHC final triplet layout and beam screen geometry  Power loss estimates  Update on beam screen contribution on total impedance budget 2 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

HL LHC triplet layout (IR1 & 5) Taper (30mm) Taper (77mm) Taper (35mm) BS 53V BS 63V BS 74 Warm pipe 7.9 m 23.7 m 27.25 m 2.7 m Beam screen 2w 2b Old Model (elliptic cylinder, 50 µ m Cu + inf. stainless steel) 3 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

HL LHC triplet layout (IR1 & 5) Taper (30mm) Taper (77mm) Taper (35mm) BS 53V BS 63V BS 74 Warm pipe 7.9 m 23.7 m 27.25 m 2.7 m Beam screen 2w 2b Old Model (elliptic cylinder, 50 µ m Cu + inf. stainless steel) r New Model: s. steel cylinder - with 0.5 µ m amorphous carbon New BS, 9.5m, New BS, r =60.5mm ”aC” ( ρ =10 -2 Ω. m – M. Taborelli) r =50.5mm and 50 µ m Cu. Taper 4 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Resistive-wall impedance of new beam screens: impact of the weld 3 different positions tested, with either 1mm or 2mm height (CST):  Pos. 1 Pos. 2 Pos. 3 C. Zannini 5 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Resistive-wall impedance of new beam screens: impact of the weld 3 different positions tested, with either 1mm or 2mm height (CST):  Pos. 1 Pos. 2 Pos. 3 Finally, baseline close to pos. 3 (2mm) (but in another corner): ⇒ Relatively small impact of the weld. C. Zannini 6 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Resistive-wall power loss in the triplets ‑ HL-LHC triplets beam screens resistive wall power loss (gaussian bunches)  of each kind of device in the triplets, including BS weld factor: IR2 & 8 (HL-LHC parameters - worst case): 7 TeV, N b = 3.5 10 11 , M=2*1404 bunches, σ z =7.5cm BS type BS 53 BS 63 BS 74 b [mm] 20.2 25.2 30.5 P loss /L [W/m] 1.2 1.0 0.8 IR1 & 5 (HL-LHC parameters - worst case), with 2mm weld in a corner: large, aC + small, aC + Cu BS type Cu coating coating b [mm] 60.5 50.5 P loss /L [W/m] 0.37 0.31 7 HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013

Resistive-wall power loss in the triplets ‑ HL-LHC triplets beam screens resistive wall power loss (gaussian bunches)  of each kind of device in the triplets, including BS weld factor: IR2 & 8 (HL-LHC parameters - worst case): 7 TeV, N b = 3.5 10 11 , M=2*1404 bunches, σ z =7.5cm BS type BS 53 BS 63 BS 74 b [mm] 20.2 25.2 30.5 P loss /L [W/m] 1.2 1.0 0.8 IR1 & 5 (HL-LHC parameters - worst case), with 2mm weld in a corner: large, aC + small, aC + Cu BS type Cu coating coating b [mm] 60.5 50.5 P loss /L [W/m] 0.37 0.31 Note: these assume no interferences between the 2 beams EM fields. Effect of these can be evaluated (see studies by G. Rumolo, G. Iadarola, C. Zannini); this was done for the current beam screens, where it tends to decrease the final power loss. 8 HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013

Power loss due to pumping holes Thanks to A. Mostacci mathematica notebook, PhD thesis and help:  ➢ power loss due the TEM (coaxial) wave propagating between beam screen & cold bore, ➢ pessimistic approach adopted: assume holes are uniformly distributed (actually they are random in size and position), and all area outside BS is filled with TEM wave, ⇒ result: power loss (per unit length) is negligible compared to the resistive- wall power loss: less than 0.1 mW/m both for LHC and HL-LHC, taking a small beam screen thickness for HL-LHC (0.5mm) ⇒ even smaller in new BS with larger stainless steel thickness (1 mm). 9 HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013

Contribution from the beam screens (and tapers) to the total HL-LHC impedance  Horizontal dipolar impedance (case of round 15cm optics, 50 µ m Cu, 15° angle tapers, holes surface coverage between 2.5% and 3%) Real part Imag. part → very small contribution from triplets at high frequency, up to 3% at first unstable betatron line (8 kHz) (same in vertical and longitudinal – see backup slides) , → pumping holes & tapers give a negligible contribution. 10 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Can we make it better / worse ?  Contribution in percent to total model (horizontal dipolar imp.) Resistive-wall with more copper Pumping holes with more surface (real part of imp.) coverage (4%) (imag. part of imp.) → Low freq. RW impedance can be → Even if pumping holes coverage is decreased to ~1% of total model at 8 kHz. increased to 4%, contribution is marginal. 11 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

A word about BPMs in triplets From R. Jones, HL-LHC PLC meeting (18/01/2013):  Many more stripline BPMs than in LHC triplets & much higher β functions → transverse impedance has to be looked at carefully (ongoing collaboration on BPM design with T. Lefevre – D. Draskovic) . 12 Transverse impedance in the HL-LHC era - WP2 - HiLumi annual meeting 13/11/2013

Conclusions Beam screens from the triplets contribute little to the total HL LHC ‑  impedance budget (less than 3% at worst).  Main (by far) contribution is resistive-wall. At constant aperture & β * , it can be decreased to ~1% by adding more Cu.  Pumping holes contribution is very small (0.2% at most), even in worst case scenario of 4% surface coverage. 13 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Backup slides 14 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Geometric impedance of the tapers Broad-band (BB) impedance of taper evaluated with Yokoya's formula [CERN SL/90-88 ] for  cylindrical geometry, valid under the contitions b θ / σ z <<1 and either a / σ z >>1 or ( b-a)/a<<1 : π ( T = j Z 0 θ b ) , n = j μ 0 θ f 0 L a − 1 1 Z with a the smallest radius, b the largest radius, Z ( b − a ) θ the taper slope, σ z the RMS bunch length and 2 f 0 (~11.2 kHz) the revolution frequency. Taper impedance is directly proportional to its angle. New triplet BS: taper between Q1 and Q2, with standard angle (4 tapers on the whole) :  Taper parameters BSQ1 → BSQ2 a [mm] 50.5 b [mm] 60.5 Taper angle 15° Im( Z T ) [ Ω /m] 53 Im( Z L /n ) [ µΩ ] 19 15 HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013

Contribution from the beam screens (and tapers) to the total HL-LHC impedance  Vertical dipolar impedance (case of round 15cm optics, 50 µ m Cu, 15° angle tapers, holes surface coverage between 2.5% and 3%) Real part Imag. part This is 5 % of full model 16 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014

Contribution from the beam screens (and tapers) to the total HL-LHC impedance  Longitudinal impedance Real part Imag. part 17 Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014