Interaction Region and Beam-beam: Status and Plans

1
Interaction Region and Beam-beam Status and Plans

• Tanaji Sen and John Johnstone
• FNAL

2
IR Layouts proposed

• Baseline layout with quads first and Nb3Sn/NbTiTa
  superconductor (LARP)
• Quads first with early separation dipole D0
  inside detector
• Dipoles first with triplet focusing (LARP)
• Dipoles first with doublet focusing (LARP)
• Combined function magnets with low field and
  large apertures.

3
Ratings of IR layouts

• Luminosity reach multi-parameter space but
  energy deposition critical
• RD time time required to develop the most
  critical hardware and to integrate it in the LHC
• Operational time time required to experimentally
  validate an upgrade scheme and to implement it in
  routine accelerator operation

Proposed by F. Ruggiero and W.Scandale
4
IR Upgrade Optics Repository

• Goal Rating of alternative layouts and to
  deliver a "forward looking" baseline scenario for
  the LHC Luminosity Upgrade by the end of 2006
• Repository at CERN
• (R. Tomas, F. Zimmermann)
• Webpage http//care-hhh.web.cern.ch/care-hhh/Sup
  erLHC_IRoptics/IRoptics.html
• Contains input lattice files, plots of apertures
  and optics functions for different dipoles
• first and quadrupoles first layouts
• Baseline layout, ß0.25m

5
IR Design Issues

• Requirements on magnet fields and apertures
• Optically matched designs at all stages
• Energy deposition
• Beam-beam interactions
• Chromaticity and non-linear correctors
• Dispersion correction
• Susceptibility to noise, ground motion emittance
  growth
• Closest approach of magnets to the IP (L)
• Impact of Nb3Sn magnets, e.g flux jumps
• .. All need to be considered in defining the
  luminosity reach

6
Optics solutions - LARP
Quads first
Dipoles first triplets
Dipoles first doublets
7
Impact of lower L Quads 1st vs Dipoles 1st

• Moving magnets closer to the IP allows smaller ß
  for the same ßmax
• Expect larger benefit with quads first optics
• Example,
• at L 19m,
• Quads 1st ß 0.2 m for a 25 increase in
  Luminosity
• Dipoles 1st ß 0.22 m for a 15 increase
  in luminosity

Quads 1st ßmax 9km
Dipoles 1st ßmax 25km
8
Flux jumps and chromaticity
Flux jumps have been measured in the model Nb3Sn
dipole magnets
Quads 1st
Dipoles 1st
Large fluctuations in the normal and skew
quadrupole components of HFDA04 dipole magnet are
observed. They are not necessarily complying
with the magnetic field symmetry (e.g. dipole)
and can produce fluctuations in allowed and
unallowed harmonics.
V. Kashikin
Qx
Qy
Q3
Q2b
Q2a
Q1
Q1
Q2a
Q2b
Q3

• Chromaticity change depends on the level of
  spurious dispersion in the IR.
• If Dx1cm at IP, then for
• ?b3 1unit, ?Q 2
• with dipoles 1st or quads 1st
• Nonlinear effects may be important

Changes in the multipole harmonics of the IR
quads could have a major impact on the beam,
e.g. changes in b3 (European) change the
chromaticity
9
Energy Deposition

• MARS simulations for Nb3Sn quads are in progress
  dependence on aperture, absorber thickness and
  material
• MARS simulations for an open mid-plane dipole
  (BNL) showed that with an additional absorber
  (TAS2), energy deposition could be mitigated in
  the dipoles.
• Scaling law for energy deposition (J.P.
  Koutchouk)
• Scaling law will be tested and refined with
  simulations. A reasonably accurate law will be
  very useful for testing different optics layouts

10
Beam-beam tune shifts

• Major motivation for the dipoles first optics is
  the smaller number of long-range interactions
• Quads 1st 30 long-range per IR with
  separations in the range 6 14 s
• Dipoles 1st 12 long-range per IR, separations
  constant at 9 s

• Footprint is larger for the quad 1st optics
  specially at 6s
• Do these wings in the footprint represent an
  adverse impact on the beam? Needs to be tested
  with simulations
• If wire compensation is shown to be effective,
  then this concern with the quads first optics
  could be mitigated.

ß 0.25m optics
11
Round vs Elliptical Beams at the IP

• Dipoles 1st triplets vs doublets
• Doublets produce elliptical beams at the IP
• Expect different beam-beam behavior

J. Qiang

• Elliptical beams have a larger footprint
• Emittance growth from the head-on collision only
  is larger with elliptical beams strong-strong
  simulation
• If elliptical beams are worse, doublets will
  have to be dropped

12
RHIC beam-beam experiments

• Motivation Test of wire compensation in 2007
• Determine if a single parasitic causes beam
  losses that need to be compensated
• Experiments in 2005
• At injection energy - found strong effects at
  separations 6s.
• Experiments in April 2006
• 2 experiments done so far April 5th, April
  12th
• Local orbit bump through IP6 1 interacting
  pair of bunches at a time, Several pairs of
  non-interacting bunches

13
Beam-beam experiments at RHIC
W. Fischer

• 2nd Study April 12th
• Done at IP6 with small ß
• No beam lifetime changes from vertical
  separation (lifetime changes above are from
  tune changes due to orbit changes)
• But small ß

• 1st study April 5th
• Done at nominal parasitic location
• Beam lifetime responds to vertical separation
• But vertical separation ? 4s

14
Simulations for the RHIC experiments

• Motivation Tests and improvements of codes,
  predictions of observations in 2006 and of wire
  compensation
• Four groups
• FNAL V. Ranjbar, T. Sen
• SLAC A. Kabel
• LBL J. Qiang
• University of Kansas J. Shi
• Website http//www-ap.fnal.gov/tsen/RHIC
• for information exchange and results

15
Beam-beam simulation results
BBSIM (FNAL)

• Chromaticity sextupoles, synchrotron
  oscillations, random noise included
• Tune footprints, dynamic apertures, diffusion
  coefficients and lifetimes
• estimated from diffusion
• Lifetimes show a linear dependence on the beam
  separation

16
Simulation results (contd.)
Emittance growth
Losses
U Kansas
LBL
Predict qualitative change at separations lt 5s
SLAC
17
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18
Summary

• Several new optics layouts proposed. Repository
  at CERN.
• Proposal to rate IR layouts on luminosity reach,
  RD time, operational time. Work towards defining
  a baseline scenario.
• Most optics require quads with apertures gt 100 mm
• Energy deposition will be key in defining
  luminosity reach
• Beam-beam simulations will help in deciding
  between round and elliptical beams.
• Beam-beam experiments started at RHIC. Initial
  results from beam-beam simulations by several
  groups
• Wire compensator designed

19
Plans

• IR Design
• Develop matched solutions at all stages,
  determine correction schemes towards the baseline
  report for the upgrade
• Energy Deposition
• Optimize magnet design, determine absorber
  (TAN, TAS) requirements and validate scaling laws
• Beam-beam experiments at RHIC in 2006
• explore vertical separation at nominal
  long-range interaction point with better
  sensitivity of beam loss measurement
• Beam-beam simulations
• RHIC detailed comparisons with data from
  2006 experiments.
• IR optics compare different layouts and
  sensitivity to imperfections
• Wire compensator
• Install both units in RHIC during 2006
  shutdown.
• First tests with beam in 2007

20
Backups
21
Features of the doublet optics

• Symmetric about IP from Q1 to Q3, anti-symmetric
  from Q4 onwards
• Q1, Q2 are identical quads, Q1T is a trim quad
  (125 T/m). L(Q1) L(Q2) 6.6 m
• Q3 to Q6 are at positions different from
  baseline optics
• All gradients under 205 T/m
• Phase advance preserved from injection to
  collision
• At collision, ßx 0.462m, ßy 0.135m, ßeff
  0.25m
• Same separation in units of beam size with a
  smaller crossing angle FE v(ßR/ ßE) FR 0.74
  FR
• Luminosity gain compared to round beam

Including the hourglass factor,
22
Chromaticity contributions

• Inner triplet and inner doublet dominate the
  chromaticity
• Anti-symmetric optics upstream and downstream
  quads have opposite
• chromaticities
• Symmetric optics upstream and downstream quads
  have the same sign of
• chromaticities

23
Chromaticity comparison
ß 0.25m Complete Qx Insertion Qy Inner Qx Magnets Qy
Quads first Dipoles first triplets Dipoles first - doublets -48 -99 -105 -48 -96 -121 -44 -82 -103 -44 -82 -112

• Including IR1 and IR5
• Q (dipoles 1st with triplets) - Q (quads
  1st ) gt 99, per plane
• Q (dipoles 1st with doublets) - Q
  (dipoles 1st with triplets) gt 31, per plane

24
Energy deposition in open mid-plane dipole
Optimized dipole with TAS2 IP end of D1 is well
protected by TAS. Non-IP end of D1 needs
protection. Magnetized TAS is not useful.
Estimated field 20 T-m Instead split D1 into D1A
and D1B. Spray from D1A is absorbed by
additional absorber TAS2 Results (N. Mokhov)
Peak power density in SC coils 0.4mW/g,
well below the quench limit Dynamic heat load to
D1 is drastically reduced. Estimated
lifetime based on displacements per atom is
10 years