LET: Meetings: Minutes for 2005 June 21
Attending: Representatives from LEPP, FNAL, and SLAC
Linda Hendrickson of SLAC talked about steering feedback systems for the LET. Her studies are mainly of train-by-train feedbacks in the linac and beam delivery system, with 5 feedback loops in the linac, 1 loop in the BDS, and a collision feedback loop at the IP. The loops are tested using the KEK ground motion model (wavelike + diffusive) and incoherent jitter based on specifications in the TESLA TDR and the US Technology Options Study. The linac loops appear to have very good performance, though the final emittance is sensitive to the 50 nm of element jitter in the linac. The beam delivery feedback loop is very sensitive, especially to small errors in the determination of the beam energy ("golding" the feedback with the energy offset by 0.03% will result in huge spot size blowups when the beam is moved back to nominal energy and the feedback allowed to converge); it also has a very non-optimal layout of correctors and BPMs, driven by the low availability of desirable real estate in the BDS. BDS experts should look to see whether they can adjust the layout of bends, quads, and sextupoles in BDS to make better corrector locations available. Long range wakefields in the linac should be managable by some combination of a train-straightener (feedback that applies the same bunch-by-bunch correction to every train) and a true intra-train feedback at the end of the linac.
Jeff Smith of LEPP talked about static correction / steering algorithms for the main linac. He studied 3 techniques: ballistic alignment, Kubo's method of kick minimization, and dispersion free steering. In the absence of external fields, all 3 algorithms perform well, with BA somewhat better than DFS or Kubo's method. All 3 methods deteriorate somewhat in the presence of external DC fields comparable in magnitude to the Earth's magnetic field, with BA performing the worst in this case. It is expected that the RF cavities will be magnetically shielded down to about 2 uT residual field (which is a 95% shielding compared to the 54 uT nominal Earth field used in the study). Extending the shielding to cover magnets and drift spaces in the first 1.5 km of the linac almost entirely mitigates the effect of the Earth's magnetic field.
Next Meeting: Tuesday, 05 July 2005, 12:30 / 11:30 / 9:30.