Other Beam Dynamics activities at CERN have been described in the previous two newsletters.
J.P. Koutchouk Jean-Pierre.Koutchouk@cern.ch
3mm F. Ruggiero Francesco.Ruggiero@cern.ch
The LHC insertion is now split into functional modules: the dispersion suppressor is separated from the low- section and has been extended into the first arc cell; the antisymmetry constraint has been dropped. With these provisions, the dispersion suppressor acts as an optical buffer and provides the required flexibility to implement the required tune splits.
The minimum (amongst 60 seeds) medium term dynamic aperture has increased to 9 sigmas, but is still clearly limited by the systematic part of the a4/b4 imperfection of the dipole (about 11 sigmas if they would be corrected by small coils at the end of the dipoles). Yet, no clear correlation could be established yet between dynamic aperture and either the average of over one turn nor the driving term of the 4th order difference coupling resonance, neither with detunings.
First tracking results show that the linear imperfections (closed orbit, beta-beating and linear coupling corrected to reasonable values) and the long-range beam-beam interactions at injection do not reduce the dynamic aperture.
A conjecture [Giovannozzi et al.] on the evolution in time of the dynamic apertures averaged over the (x,y) projection of phase space appears to fit well the LHC survival plots and could give hints to beam stability after very long times.
3mm E.Shaposhnikova email@example.com
CERN3mm The main beam dynamics activities over the past year have been connected with the preparation of the SPS as LHC injector and operation of the SPS as lead ion accelerator.
The most critical limitation to the single bunch intensity of the LHC beam in the SPS comes from the microwave instability. Scaling from previous ppbar data suggests that the nominal bunch will be at the limit of stability. Intense studies were done to replace the existing longitudinal broad-band impedance model of the SPS by a more realistic one. A new method using long single bunches injected with RF off allowed the dominant resonant impedances with high R/Q and low Q to be seen. The dominant sources have been identified as the septum magnets, (previously unknown), the travelling wave RF cavities and the vacuum ports, (main cause of the microwave instability). Ways of reducing these impedances are being studied. The consequences of reducing to raise the threshold are also under consideration. Experimental studies are under way to verify the use of "passive" superconducting 400 MHz cavities for compression of high intensity bunches. Controlled longitudinal emittance blow-up using phase modulation of the 4th harmonic of the RF frequency was experimentally studied. Reasonable particle distribution was obtained by static synchrotron frequency variation produced by the higher RF system inside the bunch.
Coasting and bunched beam longitudinal echos have been observed at 120 GeV.
Investigations of coupled bunch transverse instabilities on short batches, important for LHC beams, have started.
Following the first run with lead ions in 1994 the efficiency and beam structure at extraction were improved by the introduction of RF gymnastics on an intermediate flat top at 26 GeV and the reduction of noise in the system used for fixed frequency acceleration.