# Presentation: 804600

## SLAC HEP Activities:NLC and PEP-II Support 

- Andy Wolski, Christoph Steier,Marco Venturini, John Byrd,Daniel Lee, Kurt Kennedy
- DoE HEP Review
- February 2004

## LBNL collaborates closely with SLAC on HEP projects 

- Linear Collider
- LBNL leads the Damping Ring studies
- Lattice design and dynamics studies
- Studies of collective effects (electron cloud etc.)
- Oversight of technical and engineering issues
- Contribution to experimental program at KEK-ATF
- Contributes to activities of US Linear Collider Steering Group
- LBNL represented on Accelerator Subcommittee
- Contributed to review of US-sited LC options
- Providing information for ITRP
- PEP-II
- Improving present performance through lattice studies
- Validation of interaction region optics model
- Optics studies/analysis of closed orbit response matrix
- Planning for luminosity upgrade (2×1034 by July 2006)
- Low- lattice designs to benefit from reduced *
- RF system upgrade to allow stable beam at higher currents

## Previous lattice design was reworked to increase momentum compaction (by reducing dipole field) and lengthen the bunch.
Reduced strength of dipole field compensated by additional20 m of wiggler to preserve damping time.
Wiggler has strong nonlinear effects:
Detailed model needed to estimate impact on beam dynamics.
60 kW mean injected beam power requires ~100% injection efficiency.
Highly stable operation at high beam current needed:
Detailed studies of collective effects have begun.

- Reduced strength of dipole field compensated by additional20 m of wiggler to preserve damping time.
- Wiggler has strong nonlinear effects:
- Detailed model needed to estimate impact on beam dynamics.
- 60 kW mean injected beam power requires ~100% injection efficiency.
- Highly stable operation at high beam current needed:
- Detailed studies of collective effects have begun.

- NLC DR lattices revised in 2003; raised instability thresholds

## Developed new techniques for modeling wiggler effects

- Problem of modeling wiggler effects breaks down into two parts:
- Accurate representation of 3D wiggler field is needed.
- Effect of wiggler field on beam dynamics must be determined.
- Cylindrical mode expansion provides fast and accurate field fitting.
- Ideas for *fast* symplectic integration are being explored.
- Continuing collaboration with Cornell to benchmark new techniques:
- Effects of new wigglers on dynamics in CESR-c to be studied.

- Error using a 3D multipole field representation through azimuthal 14-pole term.

- Field profile of CESR-c 8 pole wiggler.

## Nonlinearities cause large tune shifts with amplitude 

- Large tune shift spreads beam particles  through a number of resonances
- Control tune shift as a way to make motion more regular? Improve dynamic aperture?

- Frequency map in tune space

- Vertical tune shift

- Horizontal tune shift

## Octupoles can be effective in controlling tune shifts

- Use two families of octupoles distributed through wiggler sections.
- Aim for a dynamic aperture of 15× injected beam size.

*Q**x*+2*Q**y*

- Dynamic aperture in co-ordinate space.
- 15× injected beam size is approximately
- 12 mm × 3.8 mm.

- Dynamic aperture in tune space, using octupoles to control the tune shifts.

## The wiggler also makes electron cloud more severe

- Electrons accelerated by positron beam generate high numbers of secondaries by impact with wall of vacuum chamber.
- Magnetic fields can confine the electrons and lead to high densities.
- Electron cloud drives beam instabilities.
- Collaboration with SLAC to research possibility of preventing build-up of the cloud using low secondary yield coatings.

- Simulated cloud density in TESLA wiggler.

- Cloud density vs chamber SEY in TESLA wiggler.

- Simulation results from Mauro Pivi (SLAC).

- TiN coatings produced at LBNL for analysis at SLAC.

- TiN coating on Al

- Gold coating on stainless

## Resistive wall wake field drives single bunch instability

- Threshold appears at a few times nominal bunch charge.
- Including other sources of impedance will lower the threshold.

- Longitudinal phase space at nominal, 8× nominal, and 12× nominal bunch charge.

- Growth of quadrupole mode amplitude with bunch charge.

## Resistive wall also drives multibunch instability

- Tracking simulations to determine growth rates include radiation damping and resistive wall wake field
- Bunch-by-bunch feedback can be used to suppress unstable modes.

- t = 0 ms

- t = 10 ms

## Recent experimental results at ATF support IBS theory

- Results obtained by KEK-ATF group using low emittance beam.
- Record low emittance of 4 pm, meets specification for NLC DRs.
- LBNL/SLAC contributed through studies of beam-based alignment.
- Reliable measurements obtained using advanced diagnostics.
- Calculations for present NLC damping ring lattice indicate IBS emittance growth will be within tolerable limits.

- Variation of bunch length, horizontal and vertical emittance with bunch intensity in KEK-ATF under various coupling conditions.

- Calculations of IBS bunch length and emittance growth in the NLC MDRs.  The red dashed lines show the design specifications.

## Quality of work on DRs recognized by NLC MAC

- From NLC MAC Report June 2003:
- “The Committee would like to congratulate the damping ring team for the excellent work and the important recent achievements.”
- “The Committee is pleased with the good progress the Berkeley group has made with the design of the new lattice.”
- From NLC MAC Report December 2003
- “Study of the damping ring physics has made considerable progress since the last review.”
- “The stated plans for continued study of damping ring physics appear to be very complete ... The Committee believes it is entirely appropriate to place heavy emphasis on minimizing the production of secondary electrons by finding a suitable coating for the chamber interior.”
- “Recent results of emittance measurements at ATF are particularly impressive and important.”

## PEP-II: Luminosity > 2×1034 cm-2s-1 by July 2006

- Recently achieved 7.4×1033 cm-2s-1
- CDR (1993) specified 3×1033 cm-2s-1 [30 fb-1 per calendar year].
- Total delivered so far: 173 fb-1.
- Luminosity improvement will be achieved by a variety of means:
- Better reliability.
- Better control over lattice / tuning for optimum IP parameters.
- Increased current (bunch charge and number of bunches).
- Modified IR to reduce *
- Modified lattices / increased RF to reduce the bunch length.
- At SLAC’s request, LBNL is already contributing to:
- Support of existing systems, e.g. bunch-by-bunch feedback.
- Modeling of present lattices to improve tunability.
- Studies of options for low  lattices to reduce bunch length.
- ... and we are exploring other opportunities to support PEP-II
- Review of RF options and work needed to allow stable beam with higher beam current and RF voltage.

## Expertise with proven modeling techniques applied to PEP-II

- LOCO developed by J. Safranek (now at SSRL) applied successfully to many machines, including the ALS.
- Measure a closed orbit response matrix, and adjust parameters in a lattice model to reproduce the measured matrix.
- An effective technique for identifying optics errors (focusing, coupling).
- Now being applied to PEP-II, with some challenges:
- Interaction region has complex design.
- Large quantity of data involved (matrices with ~108 components).

- Measured response matrix in HER.

- Measured - fitted model response matrix.

## Phase advance in fitted model agrees well with direct measurement

## Fitted model identifies focusing and coupling errors in HER

## Zero crossing angle makes modeling IR complicated

- PEP-II Interaction Region presently modeled as sliced solenoid.
- >100 slices of length ~ 5 cm.
- Design trajectory through solenoid modeled by individual misalignments on solenoid slices and vertical kicks.
- Quadrupole and dipole fields inserted as kicks between slices.

## Simplifying the IR model requires new maps

- An accurate model of the IR is essential for optimizing luminosity.
- Fitting IR magnet parameters to measured data would be helped by using a simplified (not sliced) model.
*MARYLIE* is being used to integrate the trajectory through a detailed field model, and to verify the maps being used.

- Horizontal trajectory through IR.

- Vertical trajectory through IR.

## A new LER lattice: reducing  is challenging

- Reasonable low  solution for HER achieved by raising phase advance in arcs from 60o per cell to 90o per cell.
- Present LER arcs have 90o phase advance per cell.
- Raising phase advance to 120o in LER reduces  ~ 30%, and has a severe effect on the dynamic aperture.
- Other options require new magnets, for example:
- One option is to add a new quadrupole every two arc cells.
- Momentum compaction can be reduced from 12.4×10-4 to 4.8×10-4
- Natural emittance also reduced (from 24 nm to 16 nm).

- Present LER arc cell.

- Possible new LER arc cell.

## Low  LER may be possible with hardware changes

**10**************

**10***********x***

- Studies in simplified lattice show reasonable dynamic aperture.
- 3-fold symmetry, constructed from modified arcs.
- No errors included.
- Next step is to include IR and adjacent straights.
- Other options are still being explored, for example:
- Low  insert based on long dipole chicane.

## Increasing beam current requires RF upgrade

- In heavily beam-loaded large rings, the fundamental mode of the accelerating cavities becomes the dominant driver of beam instabilities.
- RF feedback is used to reduce beam impedance by factor 100.

- Existing system is at the edge of stability.
- RF trips are the primary reason for beam loss.

## LBNL has significant expertise in relevant RF systems

- Increased luminosity calls for:
- Factor of 2 increase in the beam currents.
- Factor of 2 increase in the RF voltage.
- Reduction in momentum compaction.
- All the above increase the strain on the low-level RF.
- LBNL has strong skills in critical areas:
- LBNL is providing RF systems for SNS.
- LBNL can contribute accelerator physics expertise for modeling and experimental studies for RF systems in PEP-II upgrade.
- Issues identified for possible initial studies:
- Evaluate options for high power RF upgrades.
- Redesign of low-level RF system using modern technology.

## Summary

- Quality of work and progress with NLC Damping Ring studies is recognized by NLC Collaboration and MAC.
- Work on wiggler has led to development of advanced techniques for modeling field and dynamics.
- Collaboration with ALS group has led to implementation of powerful techniques for studying nonlinear dynamics.
- Electron cloud is recognized as one of the most serious issues for the damping rings, and a clear path has been defined for R&D leading to a solution.
- A significant contribution is being made towards improving PEP-II luminosity.
- Expertise in use of modeling techniques is being applied to improve tuning and optics of present lattices.
- Strengths in nonlinear design and optimization are being used to find solutions for lower momentum compaction lattices, to benefit from reduced * at the interaction point.
- Skills in RF technology are a good match to critical systems in the luminosity upgrade.