# Presentation: 390308

## Comparison of Beam-Beam simulations and experiments in RHIC

- V. Ranjbar and T. Sen, FNAL

## RHIC Experiments and Simulations

- Several experiments were conducted at collision energy in RHIC
- April 5th – 1 bunch per beam, interaction at nominal location
- April 12th – 1 bunch per beam, interaction at IP6
**May 3****rd**** - ~10 bunches per beam, interaction at nominal location**
- May 24th - ~10 bunches per beam, interaction at nominal location, tune scan. Octupoles used to increase nonlinearities.
- May 30th - ~10 bunches per beam, interaction at nominal location, operate near 0.75. Octupoles on.
- We consider the May 3rd since setup most closely resembles simulations.

| Qx,Qy | (0.69,0.68)
(0.72,0.73) |
| --- | --- |
| Qx’,Qy’ | 2,2 |
| B.B. Par | 9.77E-3 |
| rms bunch length | 2 nsec |
| x  , y  | 15 mm-mrad |
| Intensity | 2 – 1.5E+11 |
| rms momentum deviation | 3.11E-4 |

## RHIC resonance lines

- Tune footprints with sextupoles and single parasitic interaction at (1) 3 separation, (2) 10  separation. Blue beam base tunes = (0.68, 0.69). The closest resonances are the 3rd, 6th and 10th order resonances but the footprint is clear of these resonances at both separations.

- 10th Order

- 3rd and 6th Order

## BBSIM Predictions 

- Tune footprint *
- Average Tune shift **
- Relative power for average tune.**
- Emittance growth *
- Lifetimes / Loss rate **
- Diffusion rates *
- *Difficult to compare with actual experiment
- **Directly compares with experiment

## 3.36 Sig

- 1.6 Sig

- May 3rd Experiment Moving Yellow beam Q= .69,.70

## Example of Tune shift and power comparisons in RHIC

- Simulated Results show change in tune of 10^-3 down
- And increase in vertical signal and decrease in horizontal signal

## BTF done at 4 Sig separation  Qx=0.6917, max= 8.313, Qy = 0.6964, max=209 

- BTF done at 4 Sig separation  Qx=0.6917, max= 8.313, Qy = 0.6964, max=209

## BTF done at  2 sigma Qx= .6905, max= 8.34, Qy=.6959, max = 330.84

- - Qx went down by 1.2E-3 and Qy by 0.5E-3.
- - Power went up by 8 units in vertical and .5 in horizontal

## Example of Loss Rate vs Beam separation Comparison in RHIC

## 1.6 Sig

- 3.34 Sig

## Emittance Evolution

- The vertical emittance growth was less than 0.2 % in BBSIM about 20%
- In the data.

## Conclusions from RHIC Experiments

- Tunes:
- BBSIM simulations show that both the vertical and horizontal tunes in the blue beam move down by about 0.001 when the separation is decreased from 4 to 2 sigma by moving the yellow beam. Observations: Measuring from 4. sigma to 2. sigma the horizontal tunes went down by 0.0005 and the vertical tunes went down by 0.0012. In the horizontal plane the measured tune variation was probably within error of the tune measurement since measurements made at intermediate sigma separations yielded lower tune measurements. The magnitude and direction of the tune change agreed with BBSIM predictions in the vertical plane.
- Power in the Tune signal
- BBSIM finds the power halves in the horizontal plane while the power doubles in the vertical plane again when the vertical separation is reduced from 4 to 2 sigma. Observations: The peak power in the horizontal plane went up from 7.66 to 8.23 and the peak power in the vertical plane went up from 245 to 253. The power in the horizontal plane went in the opposite direction predicted by BBSIM. The power in the vertical plane matches the direction of BBSIM prediction but not the size (assuming the BTF units are linear). However here again the power fluctuated at each measurement while moving the yellow beam from 4 to 2. sigma, reaching a maximum of 330 at 2.8 sigma.
- Emittance growth
- We predicted that emittance growth in the horizontal should be greater than the vertical and should increase as separation decreases. What we observed was that the emittance growth was larger in the horizontal plane and that the size of growth seemed to increase as separation decreased. However the Schottky emittance measurement during the last step returned a drop in emittance which is not believable which calls into question the reliability of this method.
- Experimental Lifetime Estimates:
- Problematic since there was not enough time to sit and fit a lifetime cure at each separation. However we do have max loss rates which should correlate with lifetimes.