Collimation Issues for the Phase 1 Insertion Upgrade

1
Collimation Issues for the Phase 1 Insertion
Upgrade

• R. Assmann, CERN, AB/ABP
• Acknowledgements to the colleagues in the LHC
  Collimation Working Group which worked out and
  presented most of the results shown here
• http//www.cern.ch/lhc-collimation
• Acknowledgements to the colleagues in AB/ABP and
  AT for discussions and help in understanding the
  upgrade ideas.
• CERN, Switzerland
• November 1st, 2007

2
1) Why Talk About Collimation?

• Collimation protects the machine aperture against
  damage and quenches.
• Any significant change in aperture must be
  revisited also from the collimation and machine
  protection view possible impact on protection,
  loss distribution, activation, quench
  limitations, experimental background.
• Critical issues The HERA insertion upgrade was
  heavily affected by unforeseen problems with beam
  loss and background after the upgrade. Loss in
  overall integrated luminosity with insertion
  upgrade.
• Goal of this talk Give collimation input to the
  ongoing discussions for the phase 1 triplet
  upgrade such that it will be fully successful.
• Note MP and dump issues only mentioned as far as
  collimation is affected ? for additional input
  see presentations at LUMI06 by B. Goddard and R.
  Schmidt.

3
System Design
Phase 1
Momentum Cleaning
Betatron Cleaning
Final system Layount is 100 frozen!
? Outcome of accelerator physics energy
deposition optimization
4
Multi-Stage Cleaning Protection
Beam axis
Beam propagation
Impact parameter
Collimator
Core
Particle
Unavoidable losses
Primary halo (p)
Secondary halo
p
p
Shower
p
Tertiary halo
Impact parameter 1 mm
p
e
p
Primary collimator
Secondary collimator
Shower
e
SC magnets and particle physics exp.
Absorber
Super-conducting magnets
Absorber
W/Cu
CFC
W/Cu
CFC
5
Functional Description

• Two-stage cleaning (robust CFC primary and
  secondary collimators).
• Catching the cleaning-induced showers (Cu/W
  collimators).
• Protecting the warm magnets against heat and
  radiation (passive absorbers).
• Local cleaning and protection at triplets (Cu/W
  collimators).
• Catching the p-p induced showers (Cu
  collimators).
• Intercepting mis-injected beam (TCDI, TDI, TCLI).
• Intercepting dumped beam (TCDQ, TCS.TCDQ).
• Scraping and halo diagnostics (primary
  collimators and thin scrapers).

6
Setting Strategy for Collimation and Protection
Elements

• Clear requirements for settingsLHC ring
  aperture sets scale aring? tight LHC
  apertureProtection devices must protect ring
  aperture aprot lt aring ? protect against
  injected beam take into account
  accuraciesSecondary collimators tighter than
  protection asec lt aprot ? avoid too much
  secondary halo hitting protection
  devicesPrimary collimators tighter than
  secondary aprim lt asec ? primary collimators
  define the aperture bottleneck in the LHC
  for cleaning of circulating beam!
• These conditions should always be fulfilled
  Not allowed to use protection devices (or warm
  aperture limits) as a single-stage cleaning
  system!

R. Assmann, Chamonix 2005
7
7 TeV Settings at (in sb ,d0, nominal b)
aabs 20.0 s Active absorbers in IR3 asec3
18.0 s Secondary collimators IR3 (H) aprim3
15.0 s Primary collimators IR3 (H) aabs
10.0 s Active absorbers in and IR7 aring 8.4
s Triplet cold aperture aprot 8.3 s TCT
protection and cleaning at triplet aprot 7.5
s TCDQ (H) protection element asec 7.0 s
Secondary collimators IR7 aprim 6.0 s
Primary collimators IR7 ? Canonical 6/7 s
collimation settings are achievable!
R. Assmann, Chamonix 2005
8
2) Insertion Aspects for Collimation

• Chamonix 2003 (R. Assmann)
• Only way to open collimator gaps Increase
  triplet aperture! Still true but lots of things
  understood since 2003!

9
2004 Addition of Tertiary Collimators

• Collimator settings defined with required
  hierarchy.
• Detailed loss maps for beam 1 and beam 2
  produced.
• Problem Halo losses in triplets (standard
  optics, 0.55m) up to 25 times above quench limit.
• Solution Addition of tertiary collimators
• Make use of large aperture until D1.
• Placed before D1 such that all losses are
  intercepted before the triplet is hit.
• Completely solves loss issue.
• Improves machine protection (allowing feasible
  requirements in local dump protection in IR6).
• Checked for experimental background in CMS and
  ATLAS Beneficial effect.

10
System Design
Phase 1
Momentum Cleaning
Betatron Cleaning
11
7 TeV Proton Loss Distribution
p losses inefficiency
G. Robert-Demolaize et al
12
Beam1 and Beam 2 Asymmetry
Beam1, 7 TeV Betatron cleaning Ideal performance
TCDQ
Quench limit (nominal I, t0.2h)
Local inefficiency 1/m
Beam2, 7 TeV Betatron cleaning Ideal performance
TCDQ
Quench limit (nominal I, t0.2h)
Local inefficiency p lost in bin over total p
lost over length of aperture bin!
13
Asymmetry IR1/IR5

• Beam 1 betatron halo losses on TCT left sides of
  IP
• IR1 5e-4 of total halo loss
• IR5 5e-6 of total halo loss
• Beam 2 betatron halo losses on TCT right sides of
  IP
• IR1 2e-5 of total halo loss
• IR5 3e-4 of total halo loss
• Halo losses in experimental insertions are
  asymmetric. Detailed losses depend on collimator
  settings, phase advance and halo phase space
  properties.
• Above settings assume IR2 and IR8 collimators
  present and at same setting as IR1/IR5 teriary
  collimators. We might open them and losses in
  IR1/IR5 will increase (small gaps not needed in
  IR2/8 trapped mode issues with higher b, small
  gaps in IR2/8).? In worst case increase losses
  by a factor 2.

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Interim Summary I

• Phase 1 collimation system fully shadows the
  IR1/5 at 7 TeV.
• Three components to reach this goal
• IR3/IR7 collimators set to shadow the triplet
  (depends on beta and triplet aperture) against
  the primary and secondary halo.
• Local tertiary collimators TCT (58 mm full
  maximum gap) to clean tertiary halo which would
  otherwise be 20 times above quench limit 4 per
  IR.
• Local copper absorbers TCL/TCLP to catch showers
  from IP before the arc.
• Insertion upgrade (ignoring cleaning efficiency
  limitations in IR7)
• An increased minimum normalized aperture in
  IR1/IR5 can in principle be used to open the
  IR3/IR7 collimators ? reduced impedance.
• Even if n1 is increased it is expected that the
  stringent aperture bottle-necks (including new
  and earlier bottle-necks) in IR1/IR5 must be
  protected with tertiary collimators (new types,
  more TCT 4 ? 6).
• Opening gaps can affect other issues (efficiency,
  protection, ).

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2003 Reduce Impedance with Increased Gaps!
Typical collimator half gap
104
103
102
LHC impedance without collimators
Transverse Impedance MO/m
10
factor 1.6
1
10-1
0 2 4
6 8 10
Half Gap mm
F. Ruggiero, L. Vos
2007 E. Metral ? stable beam with 50 larger
gaps from detailed analysis
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2) Limitation in Cleaning Efficiency

• As just shown, cleaning efficiency in IR1/5 is no
  issue any more.
• Fundamental limitation arises at the end of IR7
  (single-diffractive scattering tertiary halo)
  after betatron cleaning.
• Nuclear physics routine cross-checked between 3
  different programs.
• Independent studies at CERN, SLAC and IHEP agree
  on the predicted limitations.
• Outside experts usually think we are even
  optimistic
• Loss assumptions.
• Only partial errors included (loss of factor 2
  seen).
• Uncertainties in diffusion and halo models.
• SNS has just published a paper with 5-6 times
  higher fractional losses in magnets than
  predicted in simulations. Absolute losses locally
  up to 20 times above specifications!

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Preventing Quenches

• Quench limits of SC magnets given by design.
• Overall criterion for preventing quenches

Fractional Leakage Dilution Fractional
quench loss rate rate length limit (w/o BLM
threshold)
Minimize Minimize Spread losses inefficiency losse
s
Example 0.1 per s 1/5,000 1/(10 m) ? 2.0 10-8
(ms)-1
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Performance Prediction
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Uncertainties

• Many uncertainties do affect results
• Uncertainty in loss assumptions. Supported by
  external collimation review in 2004. However,
  some find the loss rates too low, other too high.
• Impact from imperfections Almost always drives
  efficiency down.
• Required margin from BLM thresholds Factor 3
  announced ? beam abort 3 times below quench
  limit.
• Impact from showers Dilutes losses ? gain
  somewhat.
• Quench limits Any reduction/increase in quench
  limit will reduce/improve the performance reach.
• We try to do the best possible simulations, given
  the available resources.
• Extensive cross-checks with other ongoing
  efforts in the field agreement on the predicted
  efficiency limitation (not a single published
  result with a different conclusion).
• Approach Being fully aware of the uncertainties,
  use the predictions to guide us towards achieving
  LHC performance goals.

20
7 TeV Proton Loss Prediction
Ideal case
G. Robert-Demolaize et al
With design orbit error
21
Peak Inefficiency Versus Gap
C. Bracco et al
Phase 1 upgrade possibility (n19)
LHC nominal
BLM threshold?
Quench limit
0 5 10
15 20
25
Aperture primary collimator s
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Other Possible Impacts from Larger Collimation
Gaps in IR7

• Machine Protection For example, in case of
  magnet trips (for example affecting x) with
  larger gaps (1) the first losses are seen later.
  (2) dx/dt when are seen is increased. ? R.
  Schmidt and MPWG.
• Momentum collimation gaps must be opened as well
  ? impact on abort gap population (synchrotron
  radiation) and abort gap cleaning.
• Increased impact of off-momentum beta-beat (if
  retraction distance between primary and
  secondary jaws is kept constant).

23
Interim Summary 2

• Cleaning efficiency in IR7 is predicted to be the
  driving limitation from LHC collimation, more
  severe than impedance.
• Reducing collimator-induced impedance issue will
  not affect the overall limit (limitations must
  not be added, only the bigger counts).
• Small gaps are required to maximize cleaning
  efficiency in IR7.
• Gaps can still be opened but will lead to a
  decrease in the allowable intensity and such to a
  decrease in performance.
• Efficiency must be addressed in addition to
  impedance.
• With this in mind, an increased aperture in IR1/5
  and the phase 1 upgrade are fully supported
  from collimation, as it will provide the basic
  possibility of opening gaps after the phase 2
  upgrade.
• Other side effects of increased gaps must not be
  forgotten!

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3) Off-Momentum Beta Beat

• Off-momentum beat beat mixes up the 6D phase
  space and can corrupt collimation performance
  (secondary collimator becomes a primary
  collimator for off-momentum particles).
• Studies by C. Bracco and myself complete for
  nominal optics.
• Presentation by C. Bracco next Monday in LCU
  meeting.
• Here, first glance at preliminary results on
  upgrade optics that we got from R. de Maria.

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Upgrade optics nominal setting (TCP_at_6s,
TCSG_at_7s....) Horizontal Collimators
Allowed phase (yellow area) space should always
be defined by primary collimators (red lines). It
is not below d1e-3! Could lead to break-down
of cleaning efficiency for off-momentum particles!
C. Bracco, R. Assmann
26
Upgrade optics nominal setting (TCP_at_6s,
TCSG_at_7s...) - Vertical collimators
Secondary collimators become primary collimators!
27
Upgrade optics nominal setting (TCP_at_6s,
TCSG_at_7s....) IR7 collimators retraction
Nominal retraction
Secondary collimator becomes primary collimator
28
Upgrade optics new setting(TCP.IR7_at_9s,
TCSG.IR7_at_10s,TCLA.IR7_at_13s) IR7 collimators
Retraction
Nominal retraction
Secondary collimator becomes primary collimator
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Interim Summary 3

• We see noticeable but still acceptable effects
  from off-momentum beta-beat for the nominal LHC
  optics 30 loss in retraction.
• Preliminary results show much stronger effects
  for upgrade optics, up to the point where
  multi-stage cleaning is corrupted.
• Importance will depend on longitudinal beam
  lifetime, synchrotron losses and equilibrium beam
  distribution in off-momentum.
• Remedy Use larger retraction ? increased
  aperture would be used (at least partially) for
  increased retraction (reduced gain in impedance).
• Here we only show primary and secondary
  collimators. All collimator families must
  analyzed for this effect.
• Preliminary results further work needed and
  optics might be adapted to minimize impact.

30
4) Input and Proposals

• The upgraded triplets should include fixed masks
  for the incoming beam. This provides the best
  possible triplet protection and cleaning. These
  fixed masks should allow for easy exchange in
  case of damage.
• The upgrade optics should be compatible with the
  58 mm full aperture of the existing tertiary
  collimators, which are located between the D1 and
  the D2 magnets.
• In case that requirement 2) cannot be fulfilled,
  limited local background control for ATLAS and
  CMS must either be abandoned or new tertiary
  collimators must be developed, tested and
  constructed. This affects up to eight tertiary
  collimators installed in IR1 and IR5.
• The goal of providing room for opening primary
  collimators in IR7 to 9 s is supported (50
  increased collimator aperture as indicated by
  studies from E. Metral). This corresponds to n1
  10.5 (up from n1 7) if we just scale n1.
• For ultimate intensity a factor 2 increase in gap
  (compared to today) is needed to remove the
  impedance limit. This means n114. Stick to
  nominal in the next slides.

31
Input and Proposals continued

• To respect the specified hierarchy of collimator
  and protection settings minimal available IR
  aperture after upgrade must be 2.4 s to 3.6 s
  larger than the assumed setting of the primary
  collimators. Required 12.6 s (this corresponds
  to n1 10.5). If we take a constant 3s increase
  of all gaps 11.4 s. Will provide less
  reduction on impedance!
• The feasibility of any reduction in available
  aperture in adjacent magnets to the triplets must
  be checked with full evaluations of beam halo,
  beam loss maps, energy deposition, impedance and
  experimental background. The placement of
  additional tertiary collimators might be
  required.
• The showers from the IP must be tracked into the
  arc to verify that the foreseen TCL/TCLP
  collimators are still fully adequate.
• The performance reach of the LHC after the
  insertion upgrade must be calculated, taking into
  account also performance limitations in other
  areas of the machine. This concerns in particular
  the predicted limitations due to cleaning
  efficiency.

32
Input and Proposals continued

• If it is assumed that the betatron collimators
  are opened after a triplet upgrade, then various
  secondary effects must be considered. In
  particular, it might be required to open also the
  momentum collimators with resulting effects on
  losses of off-momentum particles (synchrotron
  radiation losses), abort gap cleaning etc.
• The compatibility of an additional
  triplet-induced off-momentum beta-beat with
  collimation must be verified.
• All warm and cold aperture limits must be in the
  shadow of machine protection collimators,
  independently of the fact that warm elements
  cannot quench!