Experimental studies of IBS in RHIC and comparison

1. Experimental studies of IBS in RHIC and comparison with theories A. Fedotov, W. Fischer, S. Tepikian and J. Wei Collider-Accelerator Department, BNL November 2, 2006

2. Outline • The need for quantitative comparison of models with measurements • Dedicated studies of IBS • IBS during standard operational conditions

3. Goals for RHIC-II Present performance of the RHIC collider with heavy ions is limited by the process of Intra-Beam Scattering (IBS) within the ion beam. To achieve required luminosities for the future upgrade of the RHIC complex (known as RHIC-II) an Electron Cooling system is proposed. • The baseline of the heavy-ion program for RHIC is operation with Au ions at total energy per beam of 100 GeV/n. For such an operation, the electron cooling should compensate emittance growth due to IBS.

4. Intra-beam scattering (IBS) in RHIC A critical item for choosing appropriate parameters of the cooler for RHIC is an accurate description of the IBS. In addition to previous measurements of IBS growth rates in RHIC (W. Fischer et al., EPAC02) dedicated studies of IBS were done: 1. with Au ions in 2004 (J. Wei et al., ICFA Workshop HB2004). 2. with Cu ions in 2005 (A. Fedotov et al., ICFA Workshop HB2006). Also, several theoretical models of IBS were implemented and benchmarked within the BETACOOL code (JINR, Dubna, Russia).

5. IBS models For simulations of IBS we use BETACOOL code, which has various IBS models: by Piwinski, Martini, Bjorken-Mtingwa, Parzen, Wei, etc. • Benchmarking between various models for the RHIC parameters were performed. • Since for our studies we are concerned about the accuracy of IBS description we use either Martini or Bjorken-Mtingwa models for the RHIC lattice with all the derivatives of the lattice functions (without any approximation). For comparison with experimental data presented here we use Martini’s model.

6. Dedicated measurements • To ensure accurate benchmarking, collisions were turned off. In addition, h=360 rf system was used to avoid loss of particles from the bucket. • Six bunches of different intensity and different initial emittance were injected, which allowed us to test expected for IBS scaling with intensity and emittance. • Measurements of the bunch length were done using Wall Current Monitor (WCM). • Measurements of the horizontal and vertical emittance in each individual bunch were done using Ionization Profile Monitor (IPM).

7. Example of benchmarking for 2004 measurements with Au ions 95 % normalized emittance Approximate model with 50% higher growth rate measurements [mm] Martini’s model for exact RHIC lattice time [sec]

8. 2004 Au ions data Longitudinal growth: Measurements are in good agreement (within 10 %) with simulations (J. Wei et al., ICFA Workshop HB2004). Transverse emittance growth: IBS based on the full RHIC lattice underestimates transverse growth rates by 50%. Uncertainties and assumption in 2004 experiments: • Coupling was not measured. Assumed full coupling which divides horizontal rate in half • Uncertainty about dispersion in real lattice vs model • Uncertainty in vertical dispersion (we assume zero, measured dispersion is not zero) • Calibration of emittance values based on IPM bunch by bunch • IPM did not produce good fits in both horizontal and vertical planes at the time of measurements. These uncertainties were minimized in 2005 measurements

9. 2005: Cu ions At injection (11.2 GeV/n): yellow Ring – coupled (horizontal-vertical) blue Ring – decoupled At store energy (100 GeV/n, gt=23): measured DQmin: in both rings – full coupling. no measurements with decoupling were done

10. IPM measurements – worked well in both planes and both rings at store

11. Intensities [×109] of six bunches during measurements in Blue ring Cu ions [sec]

12. IBS in RHIC – measurements vs theoryExample of 2005 data with Cu ions. Simulations – Martini’s model of IBS for exact designed lattice of RHIC, including derivatives of the lattice functions. horizontal vertical Growth of 95% normalized emittance [mm] for bunch with intensity N=2.9·109

13. Two bunches with different intensities, Cu ions Growth of horizontal 95% normalized emittance [mm] for two bunch intensities N=2.9·109 and 1.4∙109 FWHM [ns] bunch length growth for intensities N=2.9·109 and 1.4·109

14. Transverse growth rate Cu ions 50% stronger rate used before for 2004 data Martini’s model for exact RHIC lattice IPM measurements [sec]

15. Results of 2005 analysis • Measurements with Au ions in 2004 gave very good agreement with IBS theory for the longitudinal growth rates but some disagreement for the transverse growth rate. • To understand disagreement we studied various effects, such as FODO lattice vs realistic RHIC lattice, average dispersion function, dispersion wave and measured dispersion, etc. • Our conclusion were that the disagreement observed is most likely due to the uncertainties in the 2004 measurements. • Measurements were improved for the 2005 studies. • The latest 2005 data for the Cu ions showed very good agreement with the IBS theory (Martini’s model) both for the longitudinal and transverse growth rates.

16. IBS for standard operational conditions Au ions – 2004 typical store

17. RHIC performance for Au ions – 2004 run Intensity loss Luminosity loss

18. BETACOOL simulations for 2004 “typical” parameters: 45 bunches, N=1e9, ein95%=15 p mm mrad <L>=4.2e26

19. 2004 run Simulations: 45% intensity loss in 7 hours. intensity Emittance: 50% growth in first 1.8 hours 100%(factor of 2) growth in 6 h. rms unnormalized emittance

20. Au ions 2004 run 2004 typical store 5-6 hours: <L>=5e26 (number used in W. Fischer Table) intensity loss: 40% Simulations: <L>=5.1e26 intensity loss: 40%.

21. Summary • We believe we have accurate models for IBS. • Simulations describe data reasonably well. However, since we had some puzzles for dedicated Au-2004 data it would be useful to repeat dedicated IBS measurements during 2007 run: • With fully coupled beams at store • With decoupled beams at store Many thanks to Ilan Ben-Zvi, Vladimir Litvinenko, George Parzen, Thomas Roser and many others from the Accelerator Physics group of RHIC for useful discussions and support.