Minutes of the ICE section

64th meeting on Wednesday 12/09/2012 (08:40-10:30, 6/2-004)

1) Electron cloud instability (Alexey Burov): pdf

- Popper believed that between the two extremes of deterministic order (clocks) and unpredictable chaos (clouds) there are infinite nuances and most physical systems can be classified in the middle, closer to one or the other extreme. Electron clouds and their interaction with a particle beam could be quite an unpredictable system, that's why the best way to describe is to look for qualitative and rough estimates, rather than writing and solving complex systems of differential equations.

- The main effects of electron clouds are:

- Create a coherent tune shift due to their focusing effect,

- Generate a tune spread because of the strong nonlinearity associated with the electron cloud distributions,

- Be source of a wake field, which can be roughly described as a broad band resonator.

- According to the way these 3 ingredients interplay, and how they interact with the beam direct space charge, there can be different regimes for the onset of collective motion in a bunch:

- Weak HeadTail (WHT) instabilities of mode 0 could occur but they can be suppressed using the appropriate sign of chromaticity.

- Higher order WHT modes are damped because it can be seen that the electron cloud induced tune spread is larger than their rate. However, there can be a regime in which space charge compensates for the electron cloud detuning and higher order WHT modes may become unstable.

- Transverse Mode Coupling Instability also seems unlikely to happen, as the distance between modes with the electron cloud wake is much larger than the synchrotron tune. Space charge in this case will even stabilize further, if it is strong.

- However, if the electron cloud density is very high (electron cloud coherent detuning much larger than the synchrotron tune), we can enter a regime of Beam Break Up, which can only be stabilized by high chromaticity.

- W. Herr observed that these qualitative predictions could be confirmed by full self-consistent simulations of the interaction between a bunch and an electron cloud. G. Rumolo said that HEADTAIL simulations show that mode 0 shifts upwards with the electron cloud density, then the main spectral line jumps to a higher value when the bunch becomes unstable (without any indication of coupling between other modes when this happens) and finally returns on the main zero line, when the nonlinearities of the electron cloud are dominant and the bunch is again stabilized. He also observed that WHT of mode 0 could be seen with negative chromaticity in older simulations. G. Rumolo also observed that modeling the electron cloud wake as a broad band impedance-type wake is a very simplified picture, because it assumes that the response of the electron cloud to a passing bunch can be written as the convolution of the pulse response by the bunch distribution, which is not the case for the electron cloud, as the pulse response depends on the previous history of the electron cloud. Besides, the pinching effect of the passing bunch on the electron cloud also changes a lot the amount and the distribution of the tune spreads.  

2) Planning for the LHC scrubbing run (GR): pptx

- The LHC scrubbing run will take place on the 4-8 October 2012. The Secondary Electron Yield (SEY) on the wall of the beam screen in the arcs was lowered from the original value above two to about 1.52 at the end of 2011 thanks to the 25ns MDs. Injections tests of the 25ns beam in 2012 have revealed that it slightly receded to 1.65 (deconditioning) but then it quickly went down to 1.55 after about one hour of degraded 25ns beam in the machine, which is consistent with the fact that reconditioning is usually much faster than the first conditioning. The goal of the scrubbing run is to reduce the SEY even further (possibly below 1.5), collect more quantitative information of how scrubbing works in its "slow" part  (important for post-LS1), learn more on the difference between 25 and 50ns beams (also in terms of RF heating, instabilities, UFO) and make possible MDs using 25ns beams (e.g. long range beam beam) during the following MD block. The scrubbing beam should be injected in trains of 144 bunches or more, and with a current per bunch of 1.1-1.3e11 ppb. Scrubbing will be done at 450 GeV and, depending on how it goes, it will be then decided how many bunches can be ramped to 4 TeV during the first MD block, and with which filling pattern. During the scrubbing run, temperature and vacuum will need to be monitored on some specific equipment (collimators, TDI, MKI, BSRT, stand-alones, triplets). In particular, the scrubbing process could be limited by the MKI (temperature and pressure) as well as by the BSRT. The heat load in the arcs will be used, as usual, as a figure of electron cloud and efficiency of scrubbing. Other parameters that will have to be monitored during the scrubbing run are: 1) transverse emittances, lifetimes, beam oscillations and 2) bunch-by-bunch stable phase shift, vacuum, possibly electron cloud signal from BPMs. A meeting with all the parts involved (experts of vacuum, cryo, instrumentation, collimators, RF) will be called for next week to put in place all the necessary arrangements and strategies for both the scrubbing run and subsequent MDs with ramps to 4 TeV.

- W. Herr remarked that the polarity of LHCb must be positive because the limitations are at injection. Without a vertical crossing angle it is not possible to inject 25 ns trains with the "bad" polarity.

- F. Caspers underlined the strong limitation could coming from the BSRT, whose upgrade is presently under discussion to make it more robust.

3) Miscellaneous

- Deadlines and important dates for ICE.

- The next (65th) meeting will take place on 26/09/2012 => Agenda:

1) Summary of HB2012 WG-A on High-Intensity and High-Brightness Hadron Beams (EM).

2) AOBs.

- See preliminary agendas for the next meetings.

Minutes by W. Herr and G. Rumolo (sent by EM on 25/09/2012) => Many thanks!