LONGITUDINAL (IN)STABILITY WITH BATCH INJECTION 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 LSWG meeting The MD took place on May 8 th, 6:00-10:00 Presented by P. Baudrenghien BE-RF

Motivation LSWG meeting 2  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....

Motivation (cont’d) LSWG meeting 3  We also intended to vary the longitudinal emittance of 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  m, B1V 1.9  m, B2H 2.3  m, B2V  m  SPS adjusted to 1.5 ns 4-sigma length (0.5 eVs)

Fill1: 12b + 36b + 36b at close spacing LSWG meeting 4 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.  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) Anti-damping Damping

Fill2: 12b + 36b + 36b at large spacing LSWG meeting 5  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  But the beam was not kept long enough to give useful data: not enough time to see any damping

Fill3: 12b + 36b + 72b at close spacing LSWG meeting 6 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.  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

Fill4: 12b + 36b Parasitic during next MD setting-up LSWG meeting 7 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.  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 Anti-dampingDamping

“Stable” length LSWG meeting 8 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.  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…

Tentative conclusions (1) LSWG meeting 9  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 ns

Tentative conclusions (2) LSWG meeting 10  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.

Next MD LSWG meeting 11  The next step is to complete the MD plans: Multi-bunch injections.  Vary the emittance at 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....

I thank you for your attention… LSWG meeting