Longitudinal (in)stability with batch injection

1. LSWG meeting The MD took place on May 8th, 6:00-10:00 T. Argyropoulos, P. Baudrenghien, C. Bhat, J. E. Muller, T. Mastoridis, G. Papotti, E. Shaposhnikova, D. Valuch A special mention for J.E. Muller and T. Mastoridis that have extracted and processed all the data presented here Many thanks also to the OP-crew on charge during the MD, for their kind assistance: R. Alemany Fernandez, R. Giachino, D. Jacquet, and A. Macpherson Longitudinal (in)stability with batch injection Presented by P. Baudrenghien BE-RF

2. Motivation • Long lasting dipole oscillations have been observed at injection in the LHC, with 50 ns bunch spacing, 1.5 ns 4-sigma bunch length, 0.5 eVs (at SPS extraction), and 1.2E11 p/bunch • During the MD we have studied these oscillations with four different fillings, varying the total beam current (keeping intensity per bunch constant), the number of bunches per batch and the distance between batches • We wanted to identify a coupled-bunch instability (function of total beam current and batch spacing) from a single-bunch instability,... or a measurement artefact.... LSWG meeting

3. Motivation (cont’d) • We also intended to vary the longitudinal emittanceof the injected bunch and the capture voltage but, due to the lack of time (4 hours allocated, 2+ hours of useful data gathering), this is postponed to the next MD • The MD took place at injection energy only • Beam parameters. Nominal: • Single bunch intensity 1.2E11p with some dispersion along the batch • Transverse emittances B1H 2.3 mm, B1V 1.9 mm, B2H 2.3 mm, B2V 2.5-2.7 mm • SPS adjusted to 1.5 ns 4-sigma length (0.5 eVs) LSWG meeting

4. Fill1: 12b + 36b + 36b at close spacing Anti-damping Damping • Intended to be used as a reference • First injected a pilot in bucket 1, followed by a first train of 12b starting in bucket 410 • We waited 15 minutes to let the oscillation decrease and take measurements, then injected a second train of 36b starting in bucket 991 (0.875 us between the two batches). • Again we waited 15 minutes and injected a third train of 36b starting in bucket 2061 (0.875 us between second and third batch) Left: Phase oscillation of the first and last bunches of the three batches for Fill1. Top: Amplitude of dipole oscillation (min, max, mean over all bunches of a given batch). Bottom: Bunch length from BQM averaged per batch. Evolution for Fill 1, Beam1. LSWG meeting

5. Fill2: 12b + 36b + 36b at large spacing • We wanted to see the influence of batch spacing: The filling was as in Fill1 except that the last two batches were spaced by half a turn: Pilot in bucket 1, 12 b train starting in bucket 410, 36b train starting in bucket 991 and 36b train starting in bucket 18811. • But the beam was not kept long enough to give useful data: not enough time to see any damping LSWG meeting

6. Fill3: 12b + 36b + 72b at close spacing • Influence of batch length • batches as in Fill1 but the last batch contained 72b: Pilot in bucket 1, 12b train starting in bucket 411, 36b train starting in bucket 991 and 72b train starting in bucket 2061 Damping Top: Amplitude of dipole oscillation (min, max, mean over all bunches of a given batch). Bottom: Bunch length from BQM averaged per batch. Evolution for Fill 3, Beam1. LSWG meeting

7. Fill4: 12b + 36b Parasitic during next MD setting-up Anti-damping Damping • We have two batches only: Pilot in bucket 1, 12 b train starting in bucket 410 followed by a 36b train starting in bucket 991 • Comparing to Fill3, we should see the possible influence of total beam current: 48b versus 120b • Beam 2 only Top: Amplitude of dipole oscillation (min, max, mean over all bunches of a given batch). Bottom: Bunch length from BQM averaged per batch. Evolution for Fill 4, Beam2. LSWG meeting

8. “Stable” length • For each bunch we follow the amplitude of the dipole oscillation and its length • The “stable length” is the reading when we switch from anti-damping to damping • There is no variation inside a bunch train • No variation between beams, batches, fills… Top right: Log[OscAmplitude] and bunch length function of time for one bunch. Left: Bunch length at the stab-instab limit, corrected for bunch intensity. All batches, all fills. LSWG meeting

9. Tentative conclusions (1) • The MD has confirmed the long-lasting dipole oscillations • It has revealed the existence of two regimes at injection: A first 5-10 minutes during which the dipole oscillation grows in amplitude, followed by damping with a time constant ~ 30 min • The bunch length evolution differs in these two regimes, fast during growth of dipole oscillation, and slower thereafter • We have measured fills with very different total intensity, number of bunches per batch and distance between batches. We see not much difference • Neither do we see a difference within one batch (head to tail) • This suggests that it is not coupled-bunch instability • The observations suggest a dependence on bunch length with a stab-instab threshold around 1.1-1.25 ns LSWG meeting

10. Tentative conclusions (2) • Broadband stability criteria • The RHS is proportional to • With 6 MV capture voltage in 2011 (instead of 4 MV in 2010), the mismatch leads to 1.2 ns long bunches, compared to the 1.5 ns (2010) • During the ramp we keep the bunch length constant but raise the voltage. That increases the threshold. LSWG meeting

11. Next MD • The next step is to complete the MD plans: Multi-bunch injections. • Vary the emittanceat injection (both shorter – 1.3 ns, 0.35 eVs -and longer SPS bunches – 1.7 ns, 0.7 eVs) • Vary capture voltage. From matched 3 MV to 8 MV. Big effect on length • Phase kick with stable beam • Synchronize b-by-b phase acquisition with each injection to see the initial phase pattern (transient beam loading in the SPS cavities?) • We need time to measure the very slow damping.... LSWG meeting

12. I thank you for your attention…. LSWG meeting