What is the time estimated to change the energy and re-establish stable operation by steps of ~1% (threshold scan), a few%, or more than 10%? Comments by Giorgio Bellettini ITRP meeting 5, CalTech, June 28, 2004

Cold at  s>300 GeV For  s >±0.5% the process will be the same for small as for large energy changes, as long as  s >300 GeV. Linac: --tune the lattice to the new energy --change the RF gradient (~instantaneous) --re-steer a low intensity beam to check and tune the orbit --ramp to full Linac specifications. “Setting the Linac to the new energy should take ~1h”. BDS: --rescale the magnetic fields of the BDS. However, magnets will have to be cycled to fix and hold the very precise tune. Despite extensive automatisation of the feedback system L will have to be optimized by continuous tuning. “Experience” indicates that this will take ~one day. Tuning Linac sounds easy, tuning BDS will be as for warm.

Cold at  s<300 GeV Below  s~300 GeV the luminosity will drop because of the reduced production efficiency of positrons by the e - beam+undulator system below beam energy of ~150 GeV. By using alternatively full energy pulses to produce positrons, and reduced energy pulses for physics the loss in L would be exactly a factor of 2. “For extended running at low energies one might decide to make some hardware modifications” to restore maximum L. At the Z the event rate is still very high and the factor of 2 loss would be acceptable. A scan at 200<  s<300 would suffer.

Warm, 1% steps scan Experience: a ±1% 2-step scan around the Z was made at SLC. It took ~8h to set stable operations and a few days to complete the scan. For the LC the estimate is also a few hours. Time is dictated by a) Linac: setting a new energy point in the energy feedback system which is currently used for energy stabilization in the Linac b) BDS: adjust and stabilize the new magnetic fields. This will require cycling the magnets several times. More, maximizing L at the BDS will be a delicate process, and “problems can be encountered”. “Experience shows that L should be maximized in a few shifts”. It is not as simple as feeding a new data table to an on-line computer and then adjusting a few knobs, but it looks feasible to me. Adjusting the BDS is the same problem as for cold.

Warm, large steps scan. Same as for small steps, except that: a) If beams are polarized, the spin rotators at the exit of the damping rings would have to be re-optimized to correct for the different spin precession in the beam delivery system. b)The positron capture efficiency downstream of an undulator depends on energy, but it would remain optimized if the undulator is at 150 GeV and if collision energies below  s = 300 GeV are obtained by decelerating the beams in the second part of the Linac. c)Adjustment of collimators in the final focus might be required. These operations would be needed only at a mature stage of the LC, when one will have learned a lot on how to optimize it. Therefore I would not worry too much now. Operations a) and c) would also be needed for cold.

Conclusion The difference between cold and warm boils down to the energy stabilization problem in the Linac, which seems more delicate for warm.