Proposal for an RF roadmap towards Ultimate Intensity in the LHC

The LHC currently operates with 1380 bunches at 50 ns spacing and 1.4 1011 p per bunch (0.35A DC). In this paper the RF operation with ultimate bunch intensity (1.7 1011 p per bunch) and 25 ns spacing (2808 bunches per beam) summing up to 0.86A DC is presented. With the higher beam current, the demanded klystron power will be increased and the longitudinal stability margin reduced. One must also consider the impact of a klystron trip (voltage and power transients in the three turns latency before the beam is actually dumped). In this work a scheme is proposed that can deal with ultimate bunch intensity. Only a minor upgrade of the Low Level RF is necessary: the field set point will be modulated according to the phase shift produced by the transient beam loading, thus minimizing the RF power while keeping the strong feedback for stability and reduction of the RF noise.