More Thoughts on Vertical Arcs in ILC LET

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More Thoughts on Vertical Arcs in ILC LET

• PT
• 05-July-2005

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Reminder VARC optics
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Issues from Last Discussion

• Seryi configuration make the straight segments
  more nearly equal
• Reduce beam energy at final VARC
• Better for all SR effects
• Leave room for BDS expansion at multi-TeV
  energies (!)
• Limit critical energy of SR in final VARC
• 5.0 MeV international limit for food irradiation
• Leads to negligible activation from neutrons
  ejected via Giant Dipole Resonance

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Seryi Configuration

• Straight segment length constraint from DR
• Want 7.5 km straight section for dogbone
• Cant overlap first 245 GeV of linac
• Want 250 GeV linac and DR in separate PPS zones
• Harder if DR is downstream of 250 GeV linac
• Harder if gradient is higher
• Design philosophy
• Assume 250 GeV linac in upstream half of tunnel
• VARC1 goes at 250 GeV point
• VARC2 goes 7.5 km downstream VARC1

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VARC parameters
Assumes 990 GeV acceleration 3 km BDS (ie, 1
TeV CM configuration)
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SR issues

• Critical energy scales with E3?
• VARC2 in 40 MV/m case will have largest critical
  energy
• Central bend critical energy 4.86 MeV
• Emittance growth scales with E6?5
• VARC2 in 30 MV/m case will have the largest SR
  emittance growth in vertical!
• 0.375 nm 1.8 of DR extraction (20 nm)
• Still unacceptable at 3 TeV CM
• Need to go to gradient bends and lengthen VARC2
• Now have space to expand VARC2 for multi-TeV

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Optical Bandwidth

• VARC has uncorrected 2nd order dispersion
• Leads to emittance growth, esp. for off-momentum
  beams
• Lattice may be chromatic as well
• Off-energy particles have large transverse
  amplitudes
• Test measure emittance BMAG (mismatch factor)
  as a function of beam central momentum
• Use full nominal energy spread 130 MeV
• Consider 2 design central momenta
• 45.6 GeV (z-pole)
• 250 GeV
• 1.62 mrad VARC (largest bend angle)
• Consider also post-VARC amplitude as a function
  of momentum error

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45.6 GeV/beam Bandwidth
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250 GeV/beam bandwidth
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Vertical Oscillation from Energy Error
High energy may be MPS issue
Low-energy probably OK
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Error Sensitivity

• Power supply stability
• Assume 5 quad PS
• 4 QFs with independent PS for tuning ?y
• All other QF/QD quads in series
• Assume 2 bend PS strings
• 1 string for half-strength bends
• 1 string for full-strength bends
• Magnet Matching on strings
• Quad and bend rotation
• Absolute and not stability
• Present system has no SQs for ?x control

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Power Supply and Magnet Matching Sensitivity
Emit Growth vs Bend PS Jitter -- worst case out
of 100 seeds shown 250 GeV beam, 1.62 mrad VARC
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Quad Rotations
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Bend Rotation
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Conclusions

• Seryi configuration for VARCs looks attractive
• SR effects okay up to 500 GeV/beam
• Need lattice upgrade for multi-TeV LC
• Bandwidth is OK at 250 GeV beam energy
• Bandwidth somewhat narrow at 46 GeV beam energy
• Orbit for particles far above design energy may
  be a problem low energy looks okay
• Emittance sensitivities pretty conventional
• Except maybe matching of bend magnets
• Wont need skew quads to tune ?x