fp7 high luminosity large hadron collider design study ... · at 3.5 tev, the measured rise-time...

CERN-ACC-2014-0005

HiLumi LHCFP7 High Luminosity Large Hadron Collider Design Study

Milestone Report

INITIAL ESTIMATE OF MACHINEIMPEDANCE

Metral, E (CERN)

23 January 2014

The HiLumi LHC Design Study is included in the High Luminosity LHC project and ispartly 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-2014-0005>

CERN-ACC-2014-0005

http://hilumilhc.web.cern.ch

http://cds.cern.ch/search?p=CERN-ACC-2014-0005

Grant Agreement No: 284404

HILUMI LHC FP7 High Luminosity Large Hadron Collider Design Study

Seventh Framework Programme, Capac i t ies Spec i f ic Programme, Research In f ras t ructu res, Col laborat i ve Pro ject , Des ign Study

MILESTONE REPORT

INITIAL ESTIMATE OF MACHINE IMPEDANCE

MILESTONE: MS29

Document identifier: HILUMILHC-MS29

Due date of deliverable: End of Month 24 (November 2013)

Report release date: 19/11/2013

Work package: WP2: Accelerator Physics and Performance

Lead beneficiary: CERN

Document status: Final

Abstract:

This document summarizes the first estimates of the longitudinal and transverse impedances of the HL-LHC machine, discussing all the equipments which have been taken into account until now and comparing the results to the current LHC machine, whose model has been recently updated.

Copyright © HiLumi LHC Consortium, 2014

Grant Agreement 284404 PUBLIC 1 / 15


Doc. Identifier: HILUMILHC-MS29

Date: 19/11/2013

Copyright notice: Copyright © HiLumi LHC Consortium, 2014 For more information on HiLumi LHC, its partners and contributors please see www.cern.ch/HiLumiLHC 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. HiLumi LHC began in November 2011 and will run for 4 years. The information herein only reflects the views of its authors and not those of the European Commission and no warranty expressed or implied is made with regard to such information or its use.

Delivery Slip

Name Partner Date

Authored by E. Métral CERN 19/11/2013

Edited by E. Métral CERN 19/11/2013

Reviewed by E. Métral [Task 2.4 coordinator] G. Arduini [WP2 coordinator] L. Rossi [Project coordinator]

CERN CERN CERN

21/11/2013

Approved by Steering Committee 23/01/2014


http://www.cern.ch/HiLumiLHC



Date: 19/11/2013

TABLE OF CONTENTS

1. INTRODUCTION ......................................................................................................................................... 4

2. STATUS OF THE LHC IMPEDANCE MODEL IN 2012 ........................................................................ 4 2.1. TRANSVERSE COUPLED-BUNCH INSTABILITIES: SIMULATIONS VS. MEASUREMENTS ................................... 4 2.2. TRANSVERSE COHERENT TUNE SHIFTS: SIMULATIONS VS. MEASUREMENTS ................................................ 5

3. REFINING THE LHC IMPEDANCE MODEL ........................................................................................ 6

4. THE HL-LHC IMPEDANCE ...................................................................................................................... 8

5. FUTURE PLANS / CONCLUSION / RELATION TO HL-LHC WORK ............................................. 12

REFERENCES .................................................................................................................................................... 14




Date: 19/11/2013

Executive summary This document summarizes the first estimates of the longitudinal and transverse impedances of the HL-LHC machine, discussing all the equipments which have been taken into account until now and comparing the results to the current LHC machine, whose model has been recently updated. It must be noted that: 1) an estimate of the errors associated with the simulations and approximations used for the geometries or models has not been done yet; 2) only the (thought to be) most critical components have been considered and simulated so far and therefore the model is not finalized yet; 3) at present the existing LHC impedance model accounts only for a fraction (one third to one half) of the measured transverse instability growth rates and transverse coherent tune shifts, a feature that is also observed in impedance models of several other machines. These aspects will be further addressed in the future by simulations and experiments, in order to assess the expected accuracy and the necessary margins to be taken in the estimate of the HL-LHC intensity limitations.

1. INTRODUCTION The construction of the impedance model is the first very important step before starting to make beam stability analyses and to study the (very significant) interplay with other mechanisms such as beam-beam or e-cloud, to try and optimize parameters such as transverse tunes and tune splits between the two beams, coupling between the transverse planes, chromaticities (value and sign), (Landau) octupoles current (value and sign) to increase Landau damping, transverse damper gain and bandwidth, bunch length and / or longitudinal profile. This first step is presented in this report, discussing first the current LHC impedance model and its recent upgrade. Three talks linked to this subject were recently given [1-3] and the present document is a summary of Ref. [1].

2. STATUS OF THE LHC IMPEDANCE MODEL IN 2012

Up to now, the LHC impedance model included the contributions (all weighted by the local beta functions) of: (i) the resistive-wall impedance of collimators; (ii) the resistive-wall impedance of beam screens and warm vacuum pipe (with several different cross-sections); and (iii) a broad-band model from the design report [4], including pumping holes, BPMs, bellows, vacuum valves, the geometric impedance of collimators (assuming round tapers) and other BI instruments. This first model was initially developed to account well for the transverse coupled-bunch instabilities, whose most critical frequency range is quite low, from ~ 8 kHz to ~ 40 MHz.

2.1. TRANSVERSE COUPLED-BUNCH INSTABILITIES: SIMULATIONS VS. MEASUREMENTS




Date: 19/11/2013

A comparison between HEADTAIL [5] simulations and measurements of the transverse coupled-bunch instability rise-time vs. chromaticity is shown in Fig. 1, where it can be seen that a good agreement was achieved at 450 GeV. At 3.5 TeV, the measured rise-time was larger by a factor ~ 2-3 but a higher uncertainty on the values of the chromaticity was expected due to the octupole feed-down errors which were not correctly taken into account at that time.

Fig. 1 Transverse coherent instability rise-time vs chromaticity (Q’) from both HEADTAIL simulations and

measurements with 12 + 36 bunches (on beam 2) at 450 GeV in May 2011.

2.2. TRANSVERSE COHERENT TUNE SHIFTS: SIMULATIONS VS. MEASUREMENTS A comparison between HEADTAIL simulations and measurements of the transverse coherent tune shifts (when moving collimator families at 4 TeV with Q' ~ 1-5) is shown in Fig. 2, revealing a factor ~ 2 (and a factor ~ 3 at injection). This is a good result, considering the complexity of all the parameters playing a role in the effective impedance and such a factor 2 was also observed in many other machines in the past. However, the impedance model has been refined recently to try and understand this discrepancy factor.




Date: 19/11/2013

Fig. 2 Factor between the measured and simulated transverse coherent tune shifts when moving collimator

families at 4 TeV with Q’ ~ 1-5 (left) and at 4 TeV and 450 GeV (right).

3. REFINING THE LHC IMPEDANCE MODEL The recent improvements of the LHC impedance model include: (i) a re-evaluation of the geometric impedance of the collimators by theory and simulation; (ii) a refined resistive-wall model of the beam screens and the warm vacuum pipe, including the effect of the NEG coating for the latter and the effect of the longitudinal weld for the former; (iii) a theoretical re-evaluation of the impedance of the pumping holes; (iv) more details of the triplet region (tapers and BPMs); and (v) the broad-band and High Order Modes (HOMs) of the RF cavities, CMS, ALICE and LHCb experimental chambers. An important new result is that the contribution of the geometric impedance dominates the impedance of the tungsten collimators for all the considered gaps between the jaws, and the geometric impedance is not negligible with respect to the resistive-wall impedance of (relatively opened) CFC collimators (in IR6, or TCP/TCS at injection) (see Fig. 3 left) [6]. As concerns the current beam screens, the longitudinal weld has been modelized, using 3D CST [7] simulations, by a frequency dependent factor which can be more than 2 for the dipolar horizontal impedance (see Fig. 3 right) [8]. This result has been found to be very weakly dependent on the beam screen size.

Fig. 3 (Left) geometric transverse kick factor of current collimators (red) compared to the resistive-wall

component for CFC (blue) and tungsten (green) as a function of the half-gap in mm [6]. (Right) impact of the




Date: 19/11/2013

longitudinal weld on the resistive-wall impedance of beam screen (the factor G is defined with respect to the

same structure but without the longitudinal weld) [8].

A comparison between the “new” (updated) and the “old” (previous) impedance model is shown in Fig. 4, where it can be seen that the low-frequency part remains the same whereas the high-frequency one has been increased (by ~ 40% close to 1 GHz), which should explain a part of the factor ~ 2 observed in Fig. 2. The details of the various contributions (in percent) in each model can be found in Fig. 5 for the vertical dipolar impedance only. The impact of the geometric impedance is quite visible, as well as those of the pumping slots.

Fig. 4 Comparison between the “new” (updated) and the “old” (previous) impedance model.




Date: 19/11/2013

Fig. 5 Details of the various contributions (in percent) in each model for the vertical dipolar impedance only.

4. THE HL-LHC IMPEDANCE The first important modification during the HL-LHC era will be the change of the beta functions due to the ATS optics [8], as can be seen in Fig. 6. The beta functions will be much higher in IR1 and 5, but also in the arcs due to the very principle of the ATS optics.

Fig. 6 Vertical beta functions for the LHC and HL-LHC optics.




Date: 19/11/2013

The other changes will be: (i) possible Molybdenum (or Mo-coated) secondary collimators to improve the electric conductivity; (ii) different geometric impedance of new collimators with the double taper due to the BPM buttons (already after LS1); (iii) new triplet region with new apertures, tapers and BPMs; (iv) new broad-band and HOMs of CMS and ATLAS vacuum chamber transitions; (v) crab cavities and (vi) beam-beam wire compensators. Taking all this into account, a first estimate of the transverse impedance for HL-LHC is shown in Fig. 7 for the vertical dipolar impedance only, considering neither the crab cavities nor the beam-beam wire compensators, without and with Mo collimators for the TCS in IR3 and IR7, which are very efficient to decrease the total impedance. Taking into account both crab cavities and beam-beam wire compensators, it can be seen in Fig. 8 (a) that only the crab cavities have a significant (visible in the log-log plot) effect on the vertical dipolar impedance. The horizontal dipolar impedance and longitudinal impedance are depicted in Fig. 8 (b) and (c) respectively. The detailed contributions of the various components of the vertical dipolar impedance are shown in Fig. 9, where it can be seen that the main impedance contributors for HL-LHC are the collimators (resistive-wall and geometric effects), the pumping holes and the crab cavities. Finally, the highest impedance contributors among the collimators are revealed in Fig. 10 (left) for the LHC impedance model and in Fig. 10 (right) for the HL-LHC impedance model (with CFC secondary collimators). This analysis has been done in terms of single-bunch vertical coherent tune shift with Q’ = 0 and 1.7 1011 protons per bunch. It is seen that in both cases, the same collimators dominate due to the bad conductivity of the CFC.




Date: 19/11/2013

Fig. 7 First estimate of the vertical dipolar impedance of HL-LHC compared to LHC. Crab cavities and beam-beam wire compensators are not included. The impact of the replacement of the CFC secondary collimators

(TCS) with Molybdenum ones in LSS3 and LSS7 is clearly visible in the small box.

Fig. 8 (a) First estimate of the vertical dipolar impedance of HL-LHC compared to LHC, including both crab

cavities and beam-beam wire compensators, (b) horizontal dipolar impedance of HL-LHC and (c) longitudinal impedance of HL-LHC.




Date: 19/11/2013

Fig. 9 Detailed contributions of the various components of the HL-LHC vertical dipolar impedance: (left) real

part of the impedance and (right) imaginary part.




Date: 19/11/2013

Fig. 10 Highest impedance contributors among the collimators for the LHC (top) and HL-LHC (bottom)

impedance models.

5. FUTURE PLANS / CONCLUSION / RELATION TO HL-LHC WORK




Date: 19/11/2013

The LHC impedance model has been refined to better take into account several geometric contributions (collimator taper impedance, pumping holes, tapers and BPMs in triplets, HOMs in RF cavity and experimental pipes). The model is the same as the previous model at low frequency, whereas a significant increase (by ~ 40%) is observed around 1 GHz, which should explain a part of the factor 2 observed between the measured and simulated transverse coherent tune shifts. A first HL-LHC model has been built, taking into account the same contributors as for the LHC, plus some additions (additional BPMs in triplets, crab cavities, wire compensators, HOMs in experimental pipes). The HL-LHC impedance is not dramatically higher than the LHC one and Molybdenum-coated secondary collimators could significantly decrease the total impedance. Furthermore, special caution should be given to devices in high beta regions, as well as unshielded elements. The next step will consist in freezing the HL-LHC impedance model as it is now and study all the possible intensity limitation studies as a function of chromaticity, transverse damper gain, Landau octupoles current, etc., as well as the effect of a possible second harmonic RF system (800 MHz). The next (second and final) milestone is on 01/05/2014, when a first estimate of the intensity limitations should be provided. In parallel, during the next six months, one will continue and improve the impedance model (including an estimate of its accuracy), which will be updated in June 2014 to re-study (if needed) during the summer all the intensity limitations to be able to release the final report foreseen on 01/11/2014.




Date: 19/11/2013

REFERENCES [1] N. Mounet et al., Transverse Impedance in the HL-LHC era, 3rd Joint HiLumi LHC-LARP Annual Meeting, Daresbury, UK, 11-15/11/2013. [2] B. Salvant et al., Heat Load from Impedance on Existing and new Hardware in the LHC era, 3rd Joint HiLumi LHC-LARP Annual Meeting, Daresbury, UK, 11-15/11/2013. [3] E. Métral et al., Present Understanding of the Instabilities Observed at the LHC during Run I and Implications for HL-LHC, 3rd Joint HiLumi LHC-LARP Annual Meeting, Daresbury, UK, 11-15/11/2013. [4] LHC Design Report (Volume 1, Chapter 5): http://ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html. [5] G. Rumolo and F. Zimmermann, Phys. Rev. ST AB, 5 (2002), 121002. [6] O. Frasciello et al., Geometric Beam Coupling Impedance of LHC Collimators, to be published at IPAC’2014, Dresden, Germany, June 15-20, 2014. [7] http://www.cst.com. [8] C. Zannini, Electromagnetic Simulation of CERN accelerator Components and Experimental Applications, PhD, Ecole Polytechnique, Lausanne, 2013-03-11: CERN-THESIS-2013-076.pdf. [9] S. Fartoukh, Achromatic Telescopic Squeezing Scheme and Application to the LHC and its Luminosity Upgrade”, Phys. Rev. ST AB, 16, 111002 (2013).


http://ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html

http://ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html

http://www.cst.com/



Date: 19/11/2013

Annex: Glossary

Acronym Definition

ATLAS, CMS, ALICE and LHCb

The 4 experiments in the LHC

ATS Achromatic Telescopic Squeezing BI Beam Instrumentation BPMs Beam Position Monitors CFC Carbon Fiber Composite HL-LHC High Luminosity Large Hadron Collider IR Interaction Region LS1 Long Shutdown 1 (2013-2014) LSS Long Straight Section NEG Non Evaporable Getter RF Radio-Frequency TCP Primary collimators TCS Secondary collimators


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