RF measurements during floating MD in Week 40 3 rd of October 2012

1. RF measurements during floating MD in Week 40 3rd of October 2012 Participants: T. Argyropoulos, H. Bartosik, T. Bohl, J. Esteban Müller,H. Timko, E. Shaposhnikova CCC: G. Iadarola, Y. Papaphilippou, G. Rumolo Thanks to all SPS OP on shift LIU-SPS BD WG 25/10/2012

2. General • MD title: • Longitudinal set up of the 25 ns LHC beam with Q20 optics (nominal • ~1.25x1011 pb and high ~1.4x1011 p/b injected intensities) • MD aim: • Improve the bunch length distribution inside the batch (“U-shape”) • Improve the slope of the bunch lengths along the train • Achieve beam stability and acceptable beam parameters at flat top • Beam conditions: • 4 batches of 72 bunches • Intensities at injection: Np~1.25x1011 p/b and Np~1.45x1011 p/b • Varying parameters @SPS: • RF voltage amplitudes at FB (V200 and V800 - operation always in • double RF) • Controlled longitudinal emittance blow-up (Amplitude and scaling – always • on ) • Phase between the 2 RF systems at FT (first attempt to improve stability • by compensating for beam loading at 800 MHz)

3. Outline • Losses • Nominal intensities (~1.25x1011 at injection) • Optimization of the controlled longitudinal emittance BUP to • reduce the “U-shape” of the bunch length distribution inside • the batches at FT • Modification of the RF voltage (V200) to improve the slope of • the bunch lengths along the batch trains at FT • High intensities (~1.45x1011 at injection) • Improve stability: mainly by optimizing the BUP (amplitude, • scale) • Summary

4. Losses • Very good transmission for nominal intensities (~96 %) : not • affected by the changes in longitudinal parameters • Still low losses for the higher intensities (~6-7 %)

5. Nominal intensities (~1.25x1011 at injection) • Beam was stable (with the 800 MHz RF system and • controlled emittance blow-up) • Decrease of average bunch length along the FB for constant • RF voltage (3 or 4.5 MV) + slight increase (~1%) of capture • losses not observed with the voltage dips • Bunch length distribution at FT: • “U-shape” inside the batch • slope along the batch train • Example of the best conditions: • V200 = 4.5 MV • V800 = 0.45 MV • ScaleBUP = 0.93 • VBUP = 35 mV

6. U – shape of bunch length distribution Synchrotron frequency Distribution inside the bunch. Before BUP – V800/V200 = 0.1 • Due to beam loading the synchrotron • frequency distribution varies along the • batch • Different effect of the controlled BUP to • the bunches at the edges and the center of the • batch • Leads to the “U-shape” at FT • Reduce this by decreasing the scale • parameter in the BUP • Average of all acquisitions for different BUP scales: • V200 = 4.5 MV – V800 = 0.1V200 – VBUP = 30 mV • increasing the scale makes bunch lengths along the • batch more uniform • with scale=0.85 bunches a slightly unstable at FT • slightly better stability for scale=0.9 than 0.93 (mainly • dipole oscillations)

7. Slope along the batch train • Correlation of the V200 at FB with the bunch length slope along the batch at FT • V800 = 0.45 MV – VBUP = 35 mV – ScaleBU = 0.93 • Not significant improvement with the different voltage settings that were tried • Small difference in stability

8. Higher intensities (~1.45x1011 at injection) • Beam was unstable- with the 800 MHz RF system and controlled • emittance blow-up (same settings as in the low intensities) • Decrease of average bunch length along the FB for constant RF • voltage • Bunch length distribution at FT: “U-shape” inside the batch and slope • along the batch train still remain • Optimize the controlled emittance BUP: • increase the noise amplitude VBUP • lower the scale parameter • Limiting time with these intensities  only few acquisitions

9. Higher intensities (~1.45x1011 at injection) • Increase VBUP while keeping the same high scale (0.9) didn’t show any • improvement V200 = 3 MV – V800 = 0.45 MV ScaleBUP = 0.9 – VBUP = 35 mV V200 = 3 MV – V800 = 0.45 MV ScaleBUP = 0.9 – VBUP = 60 mV

10. Higher intensities (~1.45x1011 at injection) • Increase ScaleBUP while keeping the same high VBUP (60 mV) • improved stability • Still unstable with long bunches at FT V200 = 3 MV – V800 = 0.3 MV ScaleBUP = 0.85 – VBUP = 60 mV V200 = 3 MV – V800 = 0.45 MV ScaleBUP = 0.9 – VBUP = 60 mV Change of the ratio V800/V200 to 0.1 improved the situation but not significantly

11. Higher intensities (~1.45x1011 at injection) • Increase RF voltage at FB improved stability • Still some bunches are unstable with very long bunches at FT V200 = 3 MV – V800 = 0.3 MV ScaleBUP = 0.85 – VBUP = 60 mV V200 = 4.5 MV – V800 = 0.45 MV ScaleBUP = 0.85 – VBUP = 60 mV

12. Summary • Good transmission: • ~96-97 % for Np~1.25x1011 p/b • ~93-94 % for Np~1.45x1011 p/b • Stable beam always with the nominal intensities: • 800 MHz ON • controlled longitudinal emittance BUP ON • Decrease of average bunch length along the FB for constant RF voltage (3 or 4.5 MV) • not observed with the voltage dips • but slightly more unstable at FT • “U-shape” inside the batch : • improved with higher ScaleBUP • slope along the batch trainremains still •  further investigation is needed • Higher intensities • Beam unstable at FT • small improvement by optimizing the BUP • but bunches too long at FT •  More time is needed