Follow up from last meeting

The updated version of the proton parameter table attached to the indico page of the last meeting was discussed. A remark that the longitudinal emittance calculated after filamentation at SPS injection assume 4 MV in the 200 MHz and 0.4 MV in the 800 MHz cavities. An additional remark about potential well distortion will also be included at the bottom of the table.

Update on SPS coupled bunch instabilities - Simulation studies

Joel presented the status of the simulation studies on the SPS longitudinal coupled bunch instabilities in view of the LIU goal of reaching 2.4e11 p/b before scraping. The main contributions to the SPS longitudinal impedance model used for these studies are the 200 MHz and 800 MHz cavities, the kickers (mostly MKP), and the vacuum flanges. The effect of the feed-forward and the one turn delay feedback in the machine is taken into account by a reduction of the main harmonic of the 200 MHz cavity impedance (26 db for the LIU scenario). The studies concentrated on the following points:

• The RF upgrade will raise the instability threshold to about 2.1e11 p/b due to larger longitudinal emittance that can be accelerated. The planned impedance reduction by shielding the QF flanges is expected to increase the instability threshold at flat top by about 10% compared to the threshold expected with the RF upgrade only. This is however still slightly below the required 2.4e11 p/b (before scraping).
• The 200 MHz cavities have several HOMs, of which the one at 630 MHz has been identified to be critical for longitudinal instabilities of the LHC beam. To reach the intensity target with sufficient margin, this HOM should be damped by at least a factor 3 compared to the present situation. Simulations have shown that the coupler for the 938 MHz transverse HOM actually increases the longitudinal HOM at 630 MHz. Removing the load of the 938 MHz transverse couplers increases the longitudinal stability threshold by almost 10%. The study on transverse damping requirements is ongoing. Another possibility for minimizing the shunt impedance of the 630 MHz resonance being studied is the spreading of HOM peaks from the 6 cavities in frequency, which is equivalent to damping. A factor 3 damping is achieved by a frequency spread of about 4 MHz. A modification of the probe geometry of the existing 630 MHz HOM couplers is also under study. An eventual new coupler design has to be finalized by the end of year to be ready for the installation during LS2. Finally, it has been observed in simulation studies that a shift of the HOM by +10 MHz has a significant effect on beam stability, raising the threshold by almost 50%. The sensitivity of the instability threshold on the exact frequency of the HOM is confirmed by the instability theory for a full machine (rigid bunch oscillations).
• Updated studies on the effect of the 800 MHz cavities on the instability threshold were also presented. At flat top, an increase of the threshold by about 40% is observed in simulations when increasing the 800 MHz voltage from 10% to 15% of the 200 MHz voltage. For an accurate assessment of the instability thresholds, the bunch-by-bunch phase shift due to beam loading in the presence of the feedback and feed-forward needs to be taken into account. The instability threshold appears to be quite sensitive to the phase shift. The optimal voltage ratio between the 200 MHz and the 800 MHz RF systems during the ramp will be studied in future simulations. These studies need to include the controlled longitudinal emittance blow-up applied in operation.
• MD studies planned for 2017 include measurements of coupled bunch and single bunch instability thresholds, and calibration studies of the 800 MHz phase with respect to the 200 MHz phase.