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

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 1

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

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 2

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

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 3

HL LHC triplet layout (IR1 & 5)

BS 53V 7.9 m BS 63V 23.7 m BS 74 2.7 m Warm pipe 27.25 m

Taper (30mm) Taper (77mm) Taper (35mm)

Beam screen 2w Model (elliptic cylinder, 50µm

Cu + inf. stainless steel)

2b

Old

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 4

Taper

HL LHC triplet layout (IR1 & 5)

New BS, r =60.5mm

New BS, 9.5m, r =50.5mm

r

Model: s. steel cylinder

• with 0.5µm amorphous carbon

”aC” (ρ=10-2 Ω.m – M. Taborelli) and 50µm Cu.

BS 53V 7.9 m BS 63V 23.7 m BS 74 2.7 m Warm pipe 27.25 m

Taper (30mm) Taper (77mm) Taper (35mm)

Beam screen 2w Model (elliptic cylinder, 50µm

Cu + inf. stainless steel)

2b

Old New

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 5



3 different positions tested, with either 1mm or 2mm height (CST):

Resistive-wall impedance of new beam screens: impact of the weld

• Pos. 1
• Pos. 2
• Pos. 3
• C. Zannini

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 6



3 different positions tested, with either 1mm or 2mm height (CST):

Resistive-wall impedance of new beam screens: impact of the weld

• Pos. 1
• Pos. 2
• Pos. 3
• C. Zannini

Finally, baseline close to pos. 3 (2mm) (but in another corner):

⇒ Relatively small impact of the weld.

HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013 7



HL-LHC triplets beam screens resistive wall power loss ‑ (gaussian bunches)

• f each kind of device in the triplets, including BS weld factor:

Resistive-wall power loss in the triplets

IR2 & 8 (HL-LHC parameters - worst case): 7 TeV, Nb= 3.5 1011, M=2*1404 bunches, σz=7.5cm IR1 & 5 (HL-LHC parameters - worst case), with 2mm weld in a corner:

BS type BS 53 BS 63 BS 74 b [mm] 20.2 25.2 30.5 Ploss/L [W/m] 1.2 1.0 0.8

BS type small, aC + Cu coating large, aC + Cu coating b [mm] 50.5 60.5

Ploss/L [W/m]

0.37 0.31

HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013 8



HL-LHC triplets beam screens resistive wall power loss ‑ (gaussian bunches)

• f each kind of device in the triplets, including BS weld factor:

Resistive-wall power loss in the triplets

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.

IR2 & 8 (HL-LHC parameters - worst case): 7 TeV, Nb= 3.5 1011, M=2*1404 bunches, σz=7.5cm IR1 & 5 (HL-LHC parameters - worst case), with 2mm weld in a corner:

BS type BS 53 BS 63 BS 74 b [mm] 20.2 25.2 30.5 Ploss/L [W/m] 1.2 1.0 0.8

BS type small, aC + Cu coating large, aC + Cu coating b [mm] 50.5 60.5

Ploss/L [W/m]

0.37 0.31

HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013 9



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).

Power loss due to pumping holes

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 10

Contribution from the beam screens (and tapers) to the total HL-LHC impedance

 Horizontal dipolar impedance (case of round 15cm optics, 50µm 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.

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 11

Can we make it better / worse ?

 Contribution in percent to total model (horizontal dipolar imp.)

Resistive-wall with more copper (real part of imp.) Pumping holes with more surface coverage (4%) (imag. part of imp.) → Even if pumping holes coverage is increased to 4%, contribution is marginal. → Low freq. RW impedance can be decreased to ~1% of total model at 8 kHz.

Transverse impedance in the HL-LHC era - WP2 - HiLumi annual meeting 13/11/2013 12



From R. Jones, HL-LHC PLC meeting (18/01/2013):

A word about BPMs in triplets

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).

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 13

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.

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

Backup slides

HL LHC: impedance considerations in IR1 & 5 - N. Mounet et al - 01/07/2013 15



Broad-band (BB) impedance of taper evaluated with Yokoya's formula [CERN SL/90-88] for cylindrical geometry, valid under the contitions bθ /σz<<1 and either a /σz>>1 or (b-a)/a<<1:



New triplet BS: taper between Q1 and Q2, with standard angle (4 tapers on the whole):

Geometric impedance of the tapers

Z

T = j Z 0θ

π ( 1 a− 1 b) ,

with a the smallest radius, b the largest radius, θ the taper slope, σz the RMS bunch length and f0 (~11.2 kHz) the revolution frequency.

Taper parameters BSQ1 → BSQ2 a [mm] 50.5 b [mm] 60.5 Taper angle 15° Im(ZT) [Ω/m] 53 Im(ZL /n) [µΩ] 19

Taper impedance is directly proportional to its angle.

Z

L

n = jμ0θ f 0 2

(b−a)

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 16

Contribution from the beam screens (and tapers) to the total HL-LHC impedance

 Vertical dipolar impedance (case of round 15cm optics, 50µm Cu, 15° angle

tapers, holes surface coverage between 2.5% and 3%)

Real part

• Imag. part

This is 5 % of full model

Technical Meeting on Vacuum for HL-LHC: Impedance considerations - N. Mounet et al - 05/03/2014 17

Contribution from the beam screens (and tapers) to the total HL-LHC impedance

 Longitudinal impedance

Real part

• Imag. part