CERN
LHC Project Document No.
LHC-
CH-1211 Geneva 23
Switzerland
CERN Div./Group or Supplier/Contractor Document No.
AB/BT
the
EDMS Document No.
Large
Hadron
Collider
-
project
Date: 2004-09-27
Functional Specification
FUNCTION AND OPERATING CONDITIONS
OF THE TDI BEAM ABSORBER FOR
UPGRADED SPS BEAMS
Abstract
An adjustable, two jaw, beam absorber (TDI) is placed downstream of the MKI in each
of the two LHC injection systems, for setting up with low intensity and to protect the
superconducting machine elements in the event of MKI malfunctioning.
The Functional Specification of the TDI is given. It includes criteria for beam dilution,
circulating beam aperture, ALICE ZDC aperture, movement range, reproducibility and
precision, expected beam load conditions during regular operation, during setting up
and during MKI malfunction and beam induced RF heating.
After the first years of LHC operation, several problems have become apparent with
the present TDI design. In addition, the beam parameters required for the HL-LHC era
are significantly different to those initially specified. This document updates the TDI
Functional Specification in view of a system upgrade.
Prepared by :
Checked by :
Approval Group Leader:
Wolfgang BARTMANN TE/ABT
Chiara BRACCO TE/ABT
Brennan GODDARD TE/ABT
Verena KAIN AB/CO
Roberto LOSITO EN/STI
Stefano REDAELLI BE/ABP
O. ABERLE
V. BAGLIN
A. BERTARELLI
A. GRUDIEV
A. LECHNER
A. MASSI
M. MEDDAHI
R. SCHMIDT
J. WENNINGER
Volker MERTENS TE/ABT
Approval Group Members:
P. Collier, D. Forkel-Wirth, D. Missiaen, M. Nonis, M. Brugger, A. Siemko, M. Zerlauth,
E. Metral, M. Giovannozzi, B. Dehning, R. Jacobsson, A. Di Mauro, B. Gorini,
O. Brünning, L. Rossi, S.Chemli, Y.Muttoni
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History of Changes
Rev. No.
Date
0.1
16/05/12
Pages
Description of Changes
Draft version for comments
LHC Project Document No.
LHCPage 3 of 12
Table of Contents
1.
SCOPE .....................................................................................................4
2.
INTRODUCTION .......................................................................................4
3.
3.1
3.2
OPERATING CONDITIONS .......................................................................5
LHC FILLING VIA THE TRANSFER LINES TI 2 AND TI 8 .................................... 5
TDI POSTIONING ....................................................................................... 5
3.2.1
3.2.2
3.3
3.4
3.5
3.5.1
3.5.2
3.5.3
3.5.4
DIFFERENT SETTING TYPES ................................................................................. 5
PROTECTION SETTINGS ...................................................................................... 5
TDI PERFORMANCE OBJECTIVE .................................................................... 6
BEAM LOADING FOR ACTIVATION ANALYSIS ................................................. 7
THERMAL LOADING .................................................................................... 8
Direct beam load from secondary halo. ................................................................. 8
Direct beam load from bunches captured during setting-up. .................................... 8
Resistive losses. ................................................................................................. 8
Trapped RF modes.............................................................................................. 8
4.
4.1
4.2
APERTURE REQUIREMENTS AND CONSTRAINTS ......................................9
ALICE ZDC APERTURE ................................................................................. 9
CIRCULATING BEAM APERTURE.................................................................... 9
5.
5.1
5.2
5.3
5.4
5.5
5.6
DESIGN REQUIREMENTS AND CONSTRAINTS ..........................................9
MATERIALS ............................................................................................... 9
IMPEDANCE ............................................................................................... 9
VACUUM SYSTEM ..................................................................................... 10
BEAM SHIELDING ..................................................................................... 10
REQUIRED JAW MOVEMENT RANGE AND SETTING PRECISION ....................... 10
INTERLOCKS AND RELIABILITY .................................................................. 11
6.
REFERENCES..........................................................................................11
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1. SCOPE
This Functional Specification describes the TDI (Target Dump Injection) protection elements
[1]. It is intended as a stand-alone document, but is based heavily on the original Functional
Specification published in 2004 [2].
The TDI consists of movable absorber blocks, to limit the vertical aperture, whose
essential purpose is to provide protection against malfunction during the injection process. A
TDI is installed at the two injection points of the LHC: IR 2 and IR 8.
The function, performance objectives and operating conditions in relation to the beam
parameters requested by HL-LHC [3] will be presented.
2. INTRODUCTION
The transfer of 450 GeV protons from SPS to LHC is made through the two beam transfer
lines TI 2 and TI 8 [4]. The beam is injected into the LHC in IR 2 and IR 8 (see Figures 1 and
2). The beam to be injected passes through 5 horizontally deflecting steel septum magnets
MSI with a total deflection of 12 mrad, and a vertically deflecting kicker MKI, consisting of 4
modules, with a nominal total kick strength of 0.85 mrad [5]. Uncontrolled beam loss
resulting from errors (missing, partial or wrong kick strength) in the MKI could result in
serious damage of the equipment in the LHC injection regions (in particular the
superconducting separation dipole D1), the triplet magnets near the ALICE or LHCb
experiments, or in the arcs of the LHC machine itself.
A movable 2-sided absorber TDI is installed about 70 m from the MKI, at a 90˚ phase
advance, in order to protect the LHC equipment. In addition, the TDI is occasionally used
during setting up and commissioning, and for some filling schemes where pilot intensity
bunches are steered onto the jaws when already circulating beam is over-injected with a new
bunch or batch.
Each TDI consists of two absorber jaws. The upper jaw intercepts injected beam which is
not (or not sufficiently) deflected by the injection kickers. The lower jaw intercepts the
deflected circulating beam in the event of a kicker mistiming. Other failure scenarios have
been explored in depth [7, 14] – the most critical is a flashover, or breakdown, in the kicker
tank which could lead to a grazing incidence beam.
TDI
Figure 1- View of the LHC injection regions in IR 2.
TDI
Figure 2- View of the LHC injection regions in IR 8.
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3. OPERATING CONDITIONS
3.1 LHC FILLING VIA THE TRANSFER LINES TI 2 AND TI 8
The LHC injection sequence can include the following stages:
1. Setting up of the injection process (commissioning). The TDI may be closed and the
injection kickers inhibited allowing pilot bunches to directly impact the TDI. This will
allow steering of the end of the injection lines and the septa channel. The upper jaw
catches injected beam not or not sufficiently deflected by injection kickers.
2. Injection with pilot in ‘relaxed’ settings (commissioning). The TDI is initially set at
some ‘relaxed’ settings for injection of a pilot or nominal bunch, to allow the
reference orbit to be established.
3. Setting up of TDI (commissioning). The TDI is adjusted with respect to the injected
beam, by adjusting the position and/or angle to intercept the beam, and recording
the beam loss. Automatic sequences can now be used for this, and positioning
accuracy and reproducibility are important.
4. Normal injection sequence:
a. TDI is set to ‘protection’ settings of about ±7 .
b. Injection with pilot bunch.
c. Injection of intermediate beam (12 or 24 bunches).
d. Injection of nominal batches, of up to 288 bunches. During this process the
pilot bunch is overinjected and dumped onto the lower TDI jaw. The TDIs must
be interlocked during the injection of intensity about 1012 p+.
e. At the end of the injection process and before the ramp the TDI is retracted to
the ‘high energy’ position, which may be the same as the parking position.
During the injection process it is required to first have a pilot bunch circulating in the
LHC. Higher intensity beam will systematically be over-injected which will mean the pilot
beam being dumped regularly on the TDI. In the case of a problem, it is also possible that
full intensity bunches or full batches could impact the TDI.
The TDI jaws are assumed to be moved in near to the beam only for the setting up of
the injection and for the injection process, and to be retracted otherwise.
3.2 TDI POSTIONING
3.2.1 DIFFERENT SETTING TYPES
The settings of the TDI jaws will have to cover:

Fully closed, at around ±1 mm for injection optimisation and possibly protection of
ALICE and LHCb during injection optimisation, with pilot bunches only.

Partially retracted (typically ±10 mm) when establish circulating beam etc.

Retracted to ‘high energy’ settings.

Parking position, usually during the preparation for injection.

Gap across zero to block aperture (typically -/+5 mm)
3.2.2 PROTECTION SETTINGS
The TDI is positioned with respect to the circulating beam. At IP2 and IP8 at injection
there are crossing and separation bumps, which produce a closed orbit at the TDI with both
an offset and angle with respect to the unperturbed orbit (see Table 1).
 Point 2: beam 1 injected, vertical crossing, horizontal separation
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
Point 8: beam 2 injected, horizontal crossing, vertical separation
Further, the position of the jaws should also take into account the beam divergence
(orbit ANGLE AND ALIGNEMNT ERRORS). Orbit distortions with respect to the central beam
axis will also be present. There will also be alignment errors, and mechanical tolerances on
the angle. Finally, setup measurements with deliberately applied angles are used to check
the actual alignment with respect to the beam [5 CB IPAC12]
To cater for these requirements a maximum angle of the jaws with respect to the
central axis of around 2 mrad should be allowed. An asymmetric offset of the two jaws is
clearly needed and in the limit an individual jaw position will need to cross nominal beam
axis.
Appropriate interlocks on jaw positioning during the filling process should be anticipated
to ensure the jaw positions and gap.
Table 1. Nominal vertical beam positions and angles at TDI in IR2 and IR8. To update (or
delate!)
IR2 – beam 1
IR8 – beam 2
Nominal y [mm]
3.7
-1.0
Nominal y’ [mm]
0.05
-0.03
y
-0.45
0.45
Nominal upper jaw [mm]
7.7
3.0
Nominal lower jaw [mm]
-0.3
-5.0
3.3 TDI PERFORMANCE OBJECTIVE
Protection of LHC equipment from damage
A fault on the MKI could result in the whole injected batch being mis-steered [ref].
Wrongly injected beams with large excursions before they arrive at the collimation
insertions in the LHC, which is in any case not designed to provide phase coverage for single
pass failures.
The damage level for LHC equipment for fast and localised losses is estimated to be
around 20 J/g [5] Secondary and scattered particles must not cause damage to local
equipment, in particular D1 [6]. During such events the TDI should not itself be damaged,
either in terms of the jaw material, coating, vacuum system, positioning system or in other
functional ways like integrity of the impedance shielding.
Detailed energy deposition simulations are therefore required with the new beam
parameters and any proposed new TDI design, to evaluate the energy deposition
Quench protection
The quench level for fast and localised losses is estimated to be about four orders of
magnitude lower than the damage level, at around 5 mJ/g [8] for 450 GeV.
For the expected load cases during routine injection, the D1 and downstream magnets
must not quench. This includes impact of pilot bunches on the TDI, plus the load from
uncaptured beam kicked onto the TDI, and also must cover the continuous load from the
secondary beam halo when the lifetime is low (limit about 6 1010 p+ / s). For a DC load, at
450 GeV the quench limit is taken to be 40 mW/g [8]. Anway given by retraction and
collimation setup.
Quenches can be permitted for rare failures, e.g. MKI flashover at grazing incidence, or
MKI sweep due a timing error. The load cases and design objective are shown in Table 2.
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Table 2. Summary of load cases and required performance objectives. Note that these are
all at 450 GeV; after the injection process the MKI must be interlocked “off”.
Load Case
MKI Kick
[%]
protons on TDI
[Upper/Lower jaw]
Minimum
objective
Setting-up pilot
0 (inj, beam)
1010 [U]
No quench
0.5% uncaptured beam
100 (circ. beam)
3.6 x 10
11
Halo during low lifetime
0 (circ. beam)
6 x 10
/ s [U and L]
Timing error (1)
100 (circ.)
288 x 1.7 1011 [L]
No damage
0 (inj.)
288 x 1.7 10
No damage
0-100 (circ.)
(MKI rise during gap)
Timing error (2)
10
[L]
No quench
11
[U]
No quench
<288 x 1.7 10
11
[L]
No damage
100-0 (inj)
<288 x 1.7 10
11
[U]
No damage
1 MKI module off
75 (inj)
1 MKI module flashover
75-125 (inj)
(MKI rise during beam)
288 x 1.7 10
11
[U]
No damage
288 x 1.7 10
11
[U or L]
No damage
3.4 BEAM LOADING FOR ACTIVATION ANALYSIS
For the TDI load, 200 days of LHC physics per year and 4 LHC fills per day at 450 GeV are
assumed.
Three types of losses at the TDI are considered:
1) Regular losses from halo load
The TDI is set at about ±7 σ. A worst-case assumption is made that the average beam
lifetime during the injection process is 25 hours, and that each injection lasts 30 minutes.
The average intensity is assumed to be 50% of the peak. Set at 7 σ the TDI can intercept
20% of lost particles [9] – on average this is assumed to be 10%, as the LHC intensity
increases from zero to the maximum over the injection period.
The total number of particles lost per year at the TDI is then:
200 x 4 x 2808 x 2.5 x 1011 x 0.5 x (1-exp[-0.5/15]) x 0.1
= 9.2 x 1014 p+ / y
2) Regular losses from ‘setting up’
From setting up with pilot bunches it is assumed that 2 pilot bunches impact each TDI for
every fill.
The total per year is then :
200 x 4 x 2 x 1010
= 1.6 x 1013 p+ / y
3) Occasional losses from failures
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During LHC filling of the order of a few serious failures per year with the full beam of
7.22·1013 protons are expected to be the upper limit for the transfer lines. Assuming 10
such failures as an upper limit, including problems with abort gap keeper etc., the total on
the TDI is then:
10 x 288 x 2.5 x 1011
= 7.2 x 1014 p+ / y
Based on the above, 1.7 x 1015 protons/year per TDI is taken as a safe estimate for the
upper limit for regular losses spread through the year from scraping, setting up and
accidents. 7.2 x 1013 protons (one full batch of 288 bunches) should be taken as the figure
to calculate cooling times for an instantaneous accidental loss following a failure.
3.5 THERMAL LOADING
There will be a thermal load from direct beam impact, and also from RF heating from the
circulating beam. The impedance of the TDI absorber has to be taken into account, as do
the losses from trapped RF modes in the TDI tank. The TDI performance (vacuum, time in
beam) should not be limited by the expected heating effects.
3.5.1 Direct beam load from secondary halo.
The TDI is set at ±7 σ. A worst-case assumption is made that the average beam lifetime
during the injection process is 5 hours, and that each injection lasts 30 minutes. The
average intensity is assumed to be 50% of the peak. Set at 7 σ the TDI can intercept 20%
of lost particles – on average this is assumed to be 10%.
The average direct beam load per second per TDI jaw during filling is then :
2808 x 2.5 x 1011 x 0.5 x (1-exp[-1/5 x 60 x 60]) x 0.1
= 1.9 x 109 p+ / s
If all the energy is deposited in the TDI (very unlikely), the heat load is then 280 W
(140 W per jaw), and the total energy deposited in 30 minutes is 500 kJ.
3.5.2 Direct beam load from bunches captured during setting-up.
During setting-up low-intensity bunches may be repeatedly directed onto the TDI. Assuming
as a worst case bunches of pilot intensity, the average direct beam load per second for the
upper jaw during filling for a 20 s period between injections is then :
1010 x 0.05
= 5 x 108 p+ / s
If all the energy is deposited in the upper TDI jaw, the heat load is then 37 W.
3.5.3 Resistive losses.
With the present design the resistive heat load for the jaws in the injection position with the
half gap of 4.56 mm is estimated to be about 900 W per jaw for 25 ns beam case
(2808x2.2e11) and 1150 W per jaw for the 50 ns case (1404x3.5e11). For the retracted
position with half gap of 55 mm the loss is much lower: 95 W and 120 W for the 25 ns and
50 ns cases, respectively [10]. The loss can be significantly reduced in the new design if
adequate coating of the absorber blocks with 10 um of Cu or 100 um of Ti is implemented
as specified in [15]. This needs careful checking in simulation, including also sensitivity
analysis e.g. for surface coating degradation, impact damage etc. Adequate cooling of the
absorber block is mandatory.
3.5.4 Trapped RF modes.
The heat load from trapped RF modes in the TDI tank must be taken into account. The heat
load depends on the mechanical design and beam structure. In the present design with the
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half gap set to 8 mm it is estimated to be of about 1 kW for the case of two 25 ns beams
(2 x 2808 x 2.2e11) circulating in TDI. It is about 40% lower for the case of two 50 ns
beams [17]. It is much lower for the retracted position of the jaws. 50 % of this is lost in
the jaws and 40 % on the Cu beam screen. Detailed simulations must be made to estimate
the loads for a particular design, and in addition adequate margins based on experience to
date taken into account in the cooling and mechanical design.
4. APERTURE REQUIREMENTS AND CONSTRAINTS
4.1 ALICE ZDC APERTURE
The ALICE ZDC in IR2 requires an enlarged aperture towards the inside of the ring, for
spectator protons deflected in D1. The aperture of any intervening vacuum element
between D1 and the ZDC, like the TDI, should provide an opening of at least 6 mrad
measured from the centre of the D1 magnet, towards the inside of the ring only. At the
extreme TDI location this means an opening of around 120 mm must be ensured from the
beam axis, when the absorber block is retracted in the ‘high energy’ or parking position.
4.2 CIRCULATING BEAM APERTURE
The TDI design must provide sufficient opening for the non-injected circulating beam.
According to the normal LHC aperture conventions [11], the n1 at the TDI should be greater
than 7.0. For these calculations the separation and crossing bumps are also to be included.
5. DESIGN REQUIREMENTS AND CONSTRAINTS
5.1 MATERIALS
The choice of all materials for the mechanical design must fulfil the general constraints
of thermal, mechanical, vacuum, radiological and environmental specifications for the LHC.
A list of the chemical composition of all component parts must be established to enable
activity prediction and tracking of potentially radioactive parts. Materials shall be chosen to
minimise induced activity, and approved by RP experts.
5.2 BEAM COUPLING IMPEDANCE
The vacuum elements must have a low electrical resistivity in order to have low
transverse machine impedance. The surface resistance Rs of the material used for the beam
pipes and the beam shielding and surrounding the LHC circulating beam should be smaller
than 3.9 10-11●d3 , where d (mm) is the chamber inner diameter or smallest dimension
[16]. This scaling is to be applied for the distance from the surface to the circulating beam:
d/2 > 40 mm. For smaller distances in particular for the absorber blocks dedicated
simulation of impedance is mandatory taking into account geometry and materials of the
blocks and the coatings. A good electrical contact between the absorber blocks with metallic
coating, beam screens, beam pipes and the tank should be envisaged to allow a smooth
conduction of the beam image currents. For the same reason, the number of gaps between
absorber blocks for thermal expansion must be minimized, in ideal case down to zero. The
new design must be submitted to the impedance team for approval, at the design phase.
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5.3 VACUUM SYSTEM
5.4 The design of the beam vacuum must be compatible with the ultra high vacuum system
of the LHC ring, as described in [12]. Particular attention must be paid to the choice of
material and cleanliness. The TDIs must be in-situ bakeable to at least 250 deg, and
better to 300 deg during 24h. The TDIs are in experimental insertions which means that
sector valves at the extremities of the tanks need to be foreseen, and possibly between
any segmented tanks. The TDIs are subjected to electron cloud, particularly during the
LHC commissioning phases. Being inside experimental insertions, an anti-multipacting
coating of the blocks and of the beam shielding must be foreseen to minimise electron
stimulated gas desorption. Appropriate pumping speed must be made available to
guarantee a reduced background level to the experiments.
5.5 After accidental high intensity beam impact, the vacuum pressure should return to
normal in a reasonable time (of the order of 1 hour), such that the operational efficiency
of the LHC is not affected, and that the pressure in the adjacent sectors, in particular the
experiments, does not degrade. In the case of impact by a pilot bunch, the vacuum
pressure rise must be small enough that no interlocks are triggered.
5.6 BEAM SHIELDING
The vacuum chamber or beam-shield (beam screen) geometry for the LHC circulating
beam should be smooth to minimise beam–induced geometrical wake-fields. General
guidelines should be the following: all cavities, however short, should be shielded whenever
feasible and the angle of transition between different chamber cross-sections should not
exceed 15º. This tapering angle should also be applied to the extremities of the jaws. The
cross-section of the beam pipes and beam shielding should be kept as small as possible
however not going closer than 50 mm to the circulating beams (adequate material choice
should be made according to scaling in section 5.2).
Pumping slots with a surface area of up to 20% of the pumping shields are acceptable
in terms of longitudinal and transverse impedance provided the slots have rounded corners
and their major axis is in the beam direction. The slots should be located as far from the
beams as possible.
Bellows, required for thermal expansion compensation and alignment, must be shielded
with an impedance of 0.1 m. The electrical resistance of the contacts if any should be
< 100 .
Adequate cooling of the beam shielding should be implemented taking into account the
beam induced heat load described in section 3.5.
Finally, beam shields should not interfere with the aperture requirements.
5.7 REQUIRED JAW MOVEMENT RANGE AND SETTING PRECISION
The following values are based on the ~0.5 mm vertical beam sigma at the TDI location and
a setting of ±7 , and do NOT include the aperture requirements from the ALICE ZDC.
Table 3. Summary of jaw movement ranges and precisions.
Value (mm)
Nominal opening between jaws (±7 )
8.0
Minimum opening between jaws
2.0
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Maximum opening between jaws
60
Nominal individual jaw positions (no bumps)
±4.0
Maximum individual jaw positions
Minimum individual jaw positions (across axis)
±30
*
 4.0
Surface flatness
±0.10
Single jaw position knowledge precision
±0.10
Single jaw setting reproducibility
±0.05
The upper (+) jaw must move to a position -4 mm below the axis; the lower (-) jaw to a
position +4 mm above the axis.
*
5.8 INTERLOCKS AND RELIABILITY
The TDI is a critical safety element for the LHC and must be fully integrated into the
machine protection system, with appropriate interlocks for its positioning as a function of
machine mode. The positioning and interlocking must be done with high reliability; a
separate specification concerning the interlocking of the injection protection elements will
detail this information. An interlock on the gap between the jaws should be available based
on a direct mechanical system.
6. REFERENCES
[1]
B.Jeanneret, Collimation schemes and Injection protection Devices in LHC, Proceedings of
11th Chamonix workshop - LEP-SPS performance, CERN-SL-2001-003-DI, p298-300,
2003-01-19.
[2]
S.Bidon et al., Steel septum magnets for the LHC beam injection and extraction,
Proceedings of EPAC ‘02, Paris, France, p 2514. 2002.
[5]
L. Ducimetière et al., Design of the Injection Kicker Magnet System for CERN's 14 TeV
Proton Collider LHC, Proc. IEEE Pulsed Power Conference, Albuquerque, USA, July 10-13,
1995.
[5]
O. Brüning, J. B. Jeanneret, Optics constrains imposed by the injection in IR2 and IR8,
CERN/LHC Project Note 141 1998.
[6]
P.Sala, S.Peraire, Conceptual Optimisation of the TDI and TCDD Protections for LHC
Injection Lines, AB-Note-2003-059-ATB, CERN 2003.
[7]
V.Kain et al., The expected performance of the LHC injection protection system,
Presented at EPAC ’04; 9th European Particle Accelerator Conference 2004.
[8]
B.Jeanneret et al, Quench levels and transient beam losses in LHC magnets, LHC-ProjectReport-44 CERN 1996.
[9]
R.Assmann, private communication.
[10] N.Mounet, Privat communication, E-mail from 26/06/2012 (I think we can add a better
refference that an e-mail).
[11] J.B. Jeanneret, R. Ostojic, Geometrical Acceptance in LHC version 5.0 , LHC Project Note
111, Sept. 1997.
[12] J.M.Jimenez, Vacuum Requirements for the LHC Collimators, Functional Specification,
EDMS LHC-LVW-ES-0004, 2003-12-08.
[3]
I. Collins et al., Beam screens for LHC arc magnets, Functional Specification, EDMS LHCVSS-ES-0001.00 rev 0.2, 2002-02-01.
LHC Project Document No.
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[14] O. Brüning et al., Impact of and Protection against Failures of the LHC Injection Kickers,
CERN LHC-Project-Report-291, 1999.
[15] F.Ruggiero and L. Vos , "General impedance-related guidelines for the collimator design",
, 2006.
http://impedance.web.cern.ch/impedance/documents/PG_GuidelinesForImpedanceFromR
uggieroVos.pdf
[16] L. Vos , "Engineering specs for Cu coating of warm chambers",
http://impedance.web.cern.ch/impedance/documents/PG_Cu-coating-thicknessLSS_CDR_No06%20(2).pdf
[17] A. Grudiev, Private communication. (a better reference will be available)