Stabilization of the LHC single bunch transverse instability at high-energy by landau octupoles

When the first ramp was tried on Saturday 15/05/2010 with a single bunch of about nominal intensity (i.e. ~ 1011 p/b), the bunch became unstable in the horizontal plane at ~ 2 TeV. The three main observations were: (i) a “Christmas tree” in the transverse tune measurement application (with many synchrotron sidebands excited), (ii) beam losses (few tens of percents) in IR7, and (iii) an increase of the bunch length. This transverse coherent instability has been stabilized successfully with Landau octupoles. Comparing all the measurements performed during this first year of LHC commissioning with the theoretical and simulation predictions reveals a good agreement