Instabilities fills 7303 7304 Xavier and Benoit for

Instabilities fills 7303, 7304 Xavier and Benoit for the LHC instability monitoring team 15 oct 2018

overview

Fill 7303 • Instability started in B 2 H at flat top just before the tune change on the first 12 bunch-train. • Ramp had finished 15 to 20 s before the onset • Octupole current was decreased to 280 A (15 to 20 s before the onset) as in physics before the TS • Radial mode 2 • Azimuthal mode most likely 0 • Rise time ~5 s • B 2 H emittance of the first 12 bunches was very large already at injection (4 mic) tunes Beam energy octupoles BBQ amplitude B 2 • The instability propagated to almost all bunches after the tune change. More plots in appendix Instability onset

Fill 7304 very similar pattern • Instability started simultaneously in B 1 H and B 2 H at flat top at the start of tune change on two single colliding bunches (number 1805). • Very good correlation between the growth of both bunches • Octupole current was decreased to 280 A (90 s before the onset) as in physics before the TS • Azimuthal mode most likely 0 • Rise time ~4 s • The instability propagated to almost all bunches after the tune change. More plots in appendix tunes Beam energy BBQ amplitude B 1 and B 2 octupoles Instability onset

Recommendation? • Linear coupling should not be the relevant parameter here. • Since fill 7303 does not show coupled instability between colliding pairs, head-on beam is also not believed to be the main driver. • Since fill 7304 has standard emittance, emittance difference is not believed to be the main driver • Absence of long range for 75 bunches could make the difference in octupole threshold with respect to a full machine • Recommendation: keep octupoles high (as they are since fill 7305) until we are back to full machine.

• Recommendation applied: indeed no instability anymore with full machine, despite we came back to the same settings in fill 7310

• 600 bunches: the 3 separated indivs (810, 1704, 2595) for beam 1 blew up at the start of the squeeze sequence with octupoles at 395 A (instability with radial mode 2). • Back to full machine: instabilities at injection are back in B 1 V with octupoles at 65 A, but not much else to mention so far. • Coupling measured: ok.

Last fill of my watch - not much on B 1 H (apart from the desired UFO blow up) - blow up towards the end of injection on 3 bunches for B 2 H (the first one early on is likely the usual desired UFO blow up): - blow up at injection in B 1 V (one unexpected on the 12 bunch train, but a lot more towards the end of filling). To be checked if linked to the decrease of octupoles in the sequence - blow up at injection in B 2 V (towards the end of filling). To be checked if linked to the decrease of octupoles in the sequence. Georges noted that the preramp was very long this time.

Instabilities at injection

Fill 7303

Some activity on beam 2 H before tune change?

Hump buster B 1 H and B 2 H Some excitation in B 2 H before the tune change

Some excitation in B 2 H before the tune change

Headtail monitor (postmortem) All bunches unstable (INDIVs and 12)

2 nodes, bunches not in phase radial mode 2, as usual for loss of Landau damping with impedance excitation and chromaticity of 10 to 15

ADT activity monitor Started on the first 12 bunches

First bunches significantly blown up already at injection (also in beam 1)

Less in V plane

Fill 7304

Fill 7304 Emittance is more homogeneous than for fill 7303

Hump buster

Rise times B 1 H and B 2 H for the same bunch and same time stamp B 1 H bunch 1805 B 2 H bunch 1805 Colliding pair 1805 is the first to become unstable and is synchronized ( rise time: 3. 6 s (B 1) and 4. 2 s (B 2)