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IBS Mini-Workshop August 2007 Cockcroft Institute, Daresbury, UK IBS in the ILC Damping Rings

IBS Mini-Workshop August 2007 Cockcroft Institute, Daresbury, UK IBS in the ILC Damping Rings. Andy Wolski University of Liverpool and the Cockcroft Institute. The present ILC damping rings configuration. A range of different configurations were considered. PPA: 2.8 km, 5 GeV.

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IBS Mini-Workshop August 2007 Cockcroft Institute, Daresbury, UK IBS in the ILC Damping Rings

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  1. IBS Mini-WorkshopAugust 2007Cockcroft Institute, Daresbury, UKIBS in the ILC Damping Rings Andy Wolski University of Liverpool and the Cockcroft Institute

  2. The present ILC damping rings configuration

  3. A range of different configurations were considered PPA: 2.8 km, 5 GeV OTW: 3.2 km, 5 GeV OCS: 6.1 km, 5 GeV BRU: 6.3 km, 3.7 GeV MCH: 15.9 km, 5 GeV • Lattices were evaluated on: • beam dynamics • technical subsystem demands • availability • cost DAS: 17.0 km, 5 GeV TESLA: 17.0 km, 5 GeV

  4. Intrabeam scattering in the ILC damping rings • Intrabeam scattering was not expected to be a dominant effect, but was included in the evaluation for each lattice. • Extracting a low-emittance beam from the damping rings will be critical for production of luminosity. Any effect that can increase the emittance needs to be taken into account. • Most electron storage rings operate with larger emittances than are specified for the ILC damping rings: IBS is not a significant effect. • Synchrotron light sources typically have ~ few nm natural emittance and operate with around 1% coupling (to achieve reasonable beam lifetime). • The ATF has produced the world's lowest emittance beam (~ 4.5 pm vertical), and some nice measurements of IBS effects have been made. • The ILC damping rings will need to produce a vertical emittance more than a factor of two smaller than the ATF, which would make IBS stronger… • …but the beam energy will be much higher (5 GeV in ILC, 1.28 GeV in ATF), which will counteract the effects of smaller emittance.

  5. IBS calculations for the configuration studies • To calculate the IBS emittance growth, we applied a "standard" procedure: • Evaluate the equilibrium emittances ei at low bunch charge, and the synchrotron radiation damping times, ti. • Evaluate the IBS growth rates 1/Ti(ei) for the given emittances, averaged around the lattice. • Calculate the "new equilibrium" emittance from: • Iterate from step 2, until a true equilibrium is found. • For the vertical emittance, we need to take into account the fact that the IBS growth depends on how the emittance is generated. • Instead of the above formula, we use: • where k varies from 0 (ey is generated purely from dispersion) to 1 (ey is generated purely from betatron coupling).

  6. Calculating the IBS emittance growth rates • To find the equilibrium emittances, we need to find the IBS growth rates. • Our calculations are based on the Bjorken-Mtingwa formalism. • J.D. Bjorken and S.K. Mtingwa, “Intrabeam Scattering,” Part. Accel. 13, 115 (1983). • However, we weren't able to implement the full B-M formulae in such a way as to compute the equilibrium growth rates sufficiently quickly. • To complete the calculations in a reasonable amount of time, we used two different approximations: • Bane's approximation • Completely-integrated modified Piwinski • These approximations are valid for high-energy beams, in the parameter regime of the ILC damping rings. • We cross-checked the calculations based on the above approximations: • against "point" calculations of the IBS growth rates, using the fullB-M formulae; • against experimental data from the ATF.

  7. Bane's approximation • K. Bane, “A Simplified Model of Intrabeam Scattering,” Proceedings of EPAC 2002, Paris, France (2002).

  8. Completely-Integrated Modified Piwinski formulae • K. Kubo, S.K. Mtingwa, A. Wolski, "Intrabeam Scattering Formulas for High Energy Beams," Phys. Rev. ST Accel. Beams 8, 081001 (2005).

  9. Data from ATF were used for benchmarking the calculations • As we shall see in the following slides: • The Bane approximation and the CIMP approximation are in good agreement when applied to the ATF with ultra-low vertical emittance. • The Bane and CIMP approximations may overestimate the IBS emittance growth, compared with the SAD calculations (which are in good agreement with experimental data).

  10. Comparison of transverse emittance growth in ATF

  11. Comparisons of longitudinal emittance growth in ATF data at large ey show effects of impedance data at large ey show effects of impedance calculations include IBS & an empirical model of impedance calculations include IBS & an empirical model of impedance

  12. Results for ILC damping ring configuration options: horizontal emittance • Note: results shown are for 6 mm bunch length in each case. (Latest specification is for a bunch length of 9 mm.) Emittance growth for OCS lattice is approximately 20% at a bunch population of 21010.

  13. Results for ILC damping ring configuration options: vertical emittance • Note: we assume k= 0.5 in each case; i.e. half of the vertical emittance is generated by betatron coupling, and half is generated by vertical dispersion. Emittance growth for OCS lattice is approximately 10% at a bunch population of 21010.

  14. Results for ILC damping ring configuration options: bunch length

  15. Results for ILC damping ring configuration options: energy spread

  16. More recent results by Katsunobu Oide, using SAD for OCS6 lattice • Calculations are performed with random vertical sextupole misalignments sufficient to generate 0.2% emittance ratio. No corrections were applied. Error bars show standard deviation in results from 12 seeds of random errors. Bunch length is 6 mm. • Emittance growth is approximately 15% horizontally, and 3% vertically.

  17. Conclusions • Calculations using two different approximations to the IBS growth rates (Bane's approximation, and the CIMP approximation) are in good agreement with each other, but overestimate the IBS growth when benchmarked against data from the ATF. • For the ILC damping rings, assuming that half the vertical emittance is generated by dispersion and half by betatron coupling, the strongest IBS effects will be observed in the horizontal plane. • For the OCS lattice (closest to the present baseline), the horizontal emittance increases by about 20% at a bunch population of 21010 particles and an rms bunch length of 6 mm. The vertical emittance growth is approximately 10%. • The present bunch length specification is 9 mm, and operation with bunch population in the range from 11010 particles to 21010 particles is envisaged. • IBS should not prevent the specified extracted emittance of 8 um (normalised) being achieved, but some margin should be allowed in the design. • It is probably not desirable to reduce the beam energy in the damping rings below the present specification of 5 GeV.

  18. Open questions • There are more serious effects to worry about, so IBS is not a high priority for the ILC damping rings. Nonetheless, there are some interesting questions to answer, such as: • What is the reason for the discrepancy between our calculations and the ATF data? (Perhaps an inaccurate value for the Coulomb log…) • What will be the best value of k to use? We need more input from simulations of low-emittance tuning; and we should probably use a range of values for k. • What will be the impact of IBS during the damping process? We have calculated the equilibrium emittances in the presence of IBS, but the beam is extracted before it reaches equilibrium… • Could IBS affect the beam distribution, perhaps generating tails?

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