Progress of the Nonlinear Collimation System

1
Progress of the Nonlinear Collimation System

T. Asaka A. Faus-Golfe J. Resta López
D. Schulte F. Zimmermann

2
Outline

• The CS of a LC
• Nonlinear CS for LC
• Basic scheme
• State of the art
• Scheme using 21 skew sextupoles
• A Nonlinear CS for CLIC
• Previous design Energy-Betatron Collimation
• New designs Energy Collimation

3
The Collimation System of a LC

• reduce the background in the detector by
  removing particles at large betatron amplitudes
  or energy offsets, which otherwise would be lost
  generating muons near the IP or emit synchrotron
  radiation photons in the final doublet
• withstand the impact of a full bunch train in
  case of machine failure
• not produce intolerable wake fields that might
  degrade the orbit stability or dilute the
  emittance

The CS for a LC must fulfill
4
The Collimation System of a LC
only linear elements
Standard Linear System
quads and bends
Energy
Betatron
5
NonLinear CS for LC Basic Scheme
Deflection at the nonlinear element
6
Nonlinear CS for LC Basic Scheme

• Nonlinear elements used are skew sextupoles or
  octupoles

• Higher-order multipoles (decapoles, dodecapoles,
  ) are not useful because they do not penetrate
  to the small distances needed
• N. Merminga et al., SLAC-PUB-5165 Rev. May
  1994

7
Nonlinear CS for LC State of the art

• Scheme with skew-sextupole pairs for nonlinear
  collimation only in the vertical plane
• N.Merminga, J. Irwin, R. Helm and R. Ruth,
  SLAC PUB 5165 Rev. (1994)
• Tail folding octupoles in the NLC final focus
  system
• R. Brinkmann, P. Raimondi and A. Seryi,
  PAC2001, PAC 2001 Chicago
• A Magnetic Energy Spoiler (MES) for the TESLA
  post-linac collimation system
• R. Brinkmann, N. J. Walker and G. Blair,
  DESY TESLA-01-12
• (2001)
• Scheme with (21) skew sextupoles for CLIC
  A. Faus-Golfe and F.
  Zimmermann, EPAC 2002, Paris

8
Nonlinear CS for LC Scheme
using 21 skew sextupoles
Energy and Betatron collimation

• Optics design

Increase sx,y at spoiler
Orthogonal IP phase collimation
Cancellation of geometric aberrations
9
Nonlinear CS for LC Scheme
using 21 skew sextupoles
The Hamiltonian
The deflection
10
Nonlinear CS for LC Scheme
using 21 skew sextupoles
Position at the downstream spoiler
Position at the downstream spoiler w/o skew
sextupole
11
Nonlinear CS for LC Scheme
using 21 skew sextupoles
Beam size at the spoiler
Gaussian momentum distribution
Uniform flat momentum distribution
New Formulas!
average momentum offset
12
Nonlinear CS for LC Scheme
using 21 skew sextupoles
Beam size at the spoiler for Gaussian
Beam size at the spoiler for Uniform flat
13
Nonlinear CS for LC Scheme using
21 skew sextupoles

• For spoiler survival

14
Nonlinear CS for LC Scheme using
21 skew sextupoles

• The achievable value of Dxs is limited by the
  emittance growth ?(?ex) due to SR in the dipole
  magnets

7
1.0 x 10-19 m
f fraction of the initial emittance
I5 radiation integral
15
Nonlinear CS for LC Scheme
using 21 skew sextupoles
In addition to energy collimation the sextupolar
deflection also yields a collimation for
horizontal or vertical amplitudes at collimation
depth (units of s) of
Alternatively we can collimate (in the other
betatron phase) using the linear optics
16
Nonlinear CS for LC Scheme using
21 skew sextupoles

• By combining these equations we could
  collimate in both betatron phases and in energy
  using a single spoiler. Alternatively, we could
  choose either linear or nonlinear collimation in
  one phase.

• If we opt for nonlinear betatron collimation,
  the other phase could also be collimated by
  installing a pre skew sextupole with a phase
  advance of p/2 in front of the first skew
  sextupole in a non dipersive location.

17
A Nonlinear CS for Previous design
Energy-Betatron Collimation
Normal sextupoles for chromaticity correction
Tracking studies show strong residual
aberration
18
A Nonlinear CS for
Energy Collimation

• Main changes compared to previous CLIC design
• collimation only in energy
• linear energy collimation in horizontal plane
• 1st skew sextupole is only to increase vertical
  spot size at the spoiler
• increase the overall fraction of the system
  occupied by bends, decrease bending angle until
  SR effect is reasonably small
• keep b-functions as regular as possible to avoid
  need of chromatic correction

19
A Nonlinear CS for
Energy Collimation
1st optics solution
No bends between the skews
20
A Nonlinear CS for Energy
Collimation
Beam, optics and collimation parameters
21
A Nonlinear CS for
Energy Collimation

• There is however a problem
• collimation system is not that short (2.8 km)

• To make further progress..
• fill as many of the cells with weak bends
• increase the dispersion at the spoiler
• increase the bending angle (I5 lt 1.0 x 10-19)
• increase the cell length

22
A Nonlinear CS for Energy
Collimation
2nd optics solution
Bends between the skews
23
A Nonlinear CS for Energy
Collimation
Beam, optics and collimation parameters
24
A Nonlinear CS for
Energy Collimation
Goal Compare the performance and collimation
efficiency of the nonlinear system with those of
alternative linear designs for CLIC
25
A Nonlinear CS for
Energy Collimation
See J. Resta López talk

• Ongoing work
• Collimation survival
• install perfect spoiler perform simulations
  with MAD and PLACET
• T.Asaka, J. Resta López Characterization and
  Performance of the CLIC BDS with MAD, SAD and
  PLACET ELAN (2005)
• consider real spoiler with scattering, install
  absorbers, optimize absorber locations, run BDSIM
  or SIXTRACK or MARS simulations (linear system
  already contains spoilers and absorbers)
• Drozhdin et al, Comparison of the TESLA, NLC
  and Beam Collimation system performance CLIC
  Note 555 (2003)
• Chromatic properties Luminosity performance
  Beam size at the spoiler vs sextupole strength
  average momentun off-set

26
A Nonlinear CS for
Energy Collimation
Outlook Optics with bends between the skews
shows better performance from the collimation
efficiency point of view but there is no complete
cancellation of the geometric aberration and the
luminosity is very poor
Further work New optics with no bends between
the skews to avoid the luminosity degradation
keeping good collimation efficiency
27
A Nonlinear CS for Energy-Betatron
Collimation

• Optics design

A. Faus-Golfe and F. Zimmermann, EPAC 2002,
Paris
28
A Nonlinear CS for
Energy-Betatron Collimation

• A weak pre-skew sextupole
• 1st skew sextupole
• Single spoiler
• 2nd skew sextupole

?µp/2
?µp/2
Does not include chromatic correction !

?µp/2
29
A Nonlinear CS for Energy-Betatron
Collimation
Old Formulas !
Beam, optics and collimation parameters
30
A Nonlinear CS for Energy-Betatron
Collimation
Normal sextupoles for chromaticity correction
Tracking studies show strong residual
aberration
31

A Nonlinear CS for Betatron
Collimation

• Optics design

• The LHC momentum spread is 2 orders of magnitude
  smaller than in CLIC, cannot be exploited for
  widening the beam during collimation
• Emittance growth from SR is insignificant, not
  constrain in the design of the collimation system
• The geometric vertical emittance is about 3
  orders of magnitude larger than in CLIC

32

A Nonlinear CS for Betatron
collimation

Multiparticle tracking shows encouraging results
LHC Collimation group
33
A Nonlinear CS for Betatron
collimation
Beam, optics and collimation parameters
34
A Nonlinear CS for Betatron
collimation

• Further work
• include spoiler between two skew sextupoles
• optimize the collimators gap to reduce impedance
  budget