Longitudinal Stability with 3E11 p per bunch @ 4 TeV

1. LSWG meeting Longitudinal Stability with 3E11 p per bunch @ 4 TeV Parasitic observations during High Pile-Up floating MD, July 10th, 2012 P. Baudrenghien, T. Mastoridis

2. LSWG meeting What happened? • 2 bunches 3E11 p per ring in buckets 17821 and 27541 • 3 Fills with this intensity • Problems with longitudinal blow-up Beam 1 (OK for Beam 2) • In fill 2823, blow-up did not work • In fill 2824 blow-up was started manually late in the ramp and active at beginning of flat top • In fill 2825 blow-up worked OK • Problem was traced to an RF Controls issue: the FESA class has a “state-machine” that must be in the OFF state when receiving the START trigger. That was not the case. • Similar problem observed in May • A RESET is added in the Sequencer (Delphine and Michael)

3. LSWG meeting Fill 2823 Juan Esteban Muller Phase loop ON Phase loop OFF Bunch 1 and 2 Beam 1 • Left: Fill 2823 • 3E11 p • 1.38 ns at start ramp -> 0.71 eVs • 0.65 ns at 4 TeV -> 0.7 eVs • Dumped by OP • No clear sign of instability • Phase Loop ON • Right: RF MD June 20th • 2.4E11 p • 1.36 ns at start ramp -> 0.68 eVs • Signs of instability towards end of ramp when bunch length reaches 0.85 ns • Stabilizes at 1.1 ns, 4 TeV -> 1.9 eVs • Phase Loop OFF Phase Loop is necessary for stability during the ramp

4. LSWG meeting Fill 2824 unstable • Blow-up does not work in ramp and bunch length shrinks to 0.65 ns. No clear sign of instability but poor transmission • Blow-up manually switched ON towards end-ramp • When the noise spectrum intersects the beam response, the bunch length jumps to 1.4 ns and ~ 1 ns • From then on the shorter bunch is violently unstable

5. LSWG meeting Fill 2825 stable • Blow-up works OK, stabilizing bunch around 1.35 ns (target was set at 1.4 ns) • Small transmission loss in ramp (<2 %). (But large in squeeze) • The two bunches have similar evolutions (below)

6. LSWG meeting Conclusions • We have no single-bunch longitudinal instability during the 4 TeV ramp when blow-up works OK and Phase Loop ON • At 4 TeV we had (fill 2825) • 4 bunches 2.9E11 p per bunch • Length ranging from 1.28 ns to 1.36 ns -> 2.6 eVs to 2.9 eVs long. emittance • 12 MV RF -> 5.2 eVs bucket area • STABLE • When blow-up did not work the situation was more stable than during the RF MD with similar emittance/intensity • The reason is probably the presence of the Phase Loop (OFF during the RF MD) • This loop has no effect on quadrupole oscillations • It cannot damp single-bunch dipole oscillations in the presence of 2 bunches per ring • But it much reduces beam diffusion caused by RF noise, thereby preserving the longitudinal bunch profile as received from the SPS • “Gaussian” bunches are less stable [1]

7. LSWG meeting References [1] C. Y. Tan, A. Burov, Phase Modulation of the Bucket Stops Bunch Oscillations at the Fermilab Tevatron", Phys. Rev. ST Accel. Beams 15, 044401 (2012) [2] E. Shaposhnikova et. al., Longitudinal beam parameters during acceleration in the LHC", LHC Project Note 242, December 2000 [3] E. Shaposhnikova et. al., Loss of Landau damping in the LHC", Proc. IPAC 2011, 4-9 September 2011, San Sebastian, Spain [4] P. Baudrenghien et. al., Longitudinal EmittanceBlowup in the LHC", Proc. IPAC 2011, 4-9 September 2011, San Sebastian, Spain