Inputs from GG6 to decisions 2,7,8,15,21,27,34 V.Telnov Aug.24, 2005, Snowmass

D2. Beam and luminosity parameters For γγ we need beams with the geometric e-e- luminosity as large as possible. Additional requirements are connected with the disruption angle and beamstrahlung. After multiple Compton scattering the minimum energy of particles (which can give essential contribution to backgrounds) is about E~6 GeV. For such low energy particles the deflection angle in the field of the opposing beam. For N=2·10 10 and σ z =0.3 mm the disruption angle is about 10 mrad, which is still acceptable, but for σ z =0.15 mm it is too large. Also small σ z leads to coherent e+e- pair production at large ILC energies (in γγ case σ x is smaller). So, the decrease of σ z at fixed N is not possible for γγ. One can simultaneously decrease N, but the geometric luminosity should not decrease (emittances should decrease simultaneously). The distance between bunches 337 ns (100 m) is good for the photon collider. If it is reduced two times, than the distance 50 m is not enough for the loop around the detector. In this case one should have 2 laser bunches circulating in 100 m cavity. It means 2 times higher average power in the cavity, which is not desirable. So, present parameters are almost optimal for γγ, only the decrease of emittances is desirable.

D7: DR size and shape For γγ, the DR is preferable which gives smaller product of horizontal and vertical emittances. Smaller horizontal emittance allows smaller β x, so the decrease of ε nx by a factor of 2-4 is very desirable. Some decrease of ε ny will be also useful. As we understand, smaller then nominal emittances are possible by reducing the damping time with the help of wigglers. We appreciate steps in this direction.

D8: e + source type conv/undulator/compton GigaZ needs e+e- at 2E=90 GeV with good polarization of both beams and small energy spread (0.1%). The scheme with the undulator needs bypasses, otherwise the energy spread after deceleration is large enough. (See GG6 summary talk with refs to original talks) Below is response on D27, which is related to D8. Low energy running is necessary for GigaZ(e+e- at 2E=90 GeV), WW threshold (2E=160 GeV) and γγ →H(120) (electrons with 2E~200 GeV). All experiments need good emittances (good luminosity) and small energy spreads (for precision measurement at GigaZ and WW and for smaller chromo-geometric aberrations for γγ ). In the scheme with the laser positron source, according to Kubo-san the scheme with acceleration with full gradient and further deceleration gives smallest emittance dilution, but need more power. It is interesting also what happens with the energy spread in this scheme? Is such loss of power acceptable or better to make bypasses ? In the case of GigaZ and an undulator e+ source, if the beam passes the undulator and then decelerated, then the energy spread is about 0.3% (desirable 0.1%). It seems that the scheme with bypasses is better for this case (see D. Scott talk at GG6 or GG6 summary

D15: crossing angle Minimum crab-crossing angle for γγ is determined by the disruption angle and the size of the final quads. The horizontal disruption angle is about 10 mrad, E min ~6 GeV. During the Snowmass workshop B.Parker found very good design of the final quad which allows save removal of disrupted beams at the minimum crab crossing angle about mrad for L*= m, respectively. Obtaining of the final number needs some additional checks, but roughly it is 25 mrad. Note that the dilution of emittance due to SR in the detector field is small for this angle (see GG6 summary talk). So, it has sense in the baseline design to fixed the crossing angle compatible with e+e- and γγ.

D21:gamma-gamma upgrade path This decision is both political and scientific. My personal opinion is the following. First of all it is necessary in the near future to make some political decision on the photon collider. It is absolutely clear that this option is great and very natural at the linear collider. The incremental cost is small. The risk is small because the ILC can continue work in the e+e- mode. The decision is necessary now because the photon collider influences designs of many ILC system and all requirements should be taken into account now before beginning of the construction. Also people will not do any real work when the project is not supported, has no finances and there is alternative: e+e-. The optimum pass to the γγ may be the following. The ILC should have two IP with two detectors, one IP should allow crossing angle about 25 mrad and all other features necessary for γγ (lower emittances, special beam dump, place for the laser system, etc.). The corresponding detector should be specially designed for easy modification for the γγ mode (replacement of 100 mrad forward region). Both detectors start simultaneously the work with e+e- beams. People working on the γγ problems participate in e+e- experiments at the IP2 and simultaneously prepare upgrade for the γγ. After about 4 years of operation one of the detectors is modified for the γγ and the laser system is installed. This

upgrade together with adjustments may take years. It is not a problem if there are two IP and the first IP continues e+e- experiments. If, by some reason the laser system is not ready, then the second IP continues e+e- experiments. It is better, of course, to avoid such situation. The development and realization of the required laser system needs at least ten years. Before installation at the ILC it should be assembled and fully tested in a separate place. This program is not easy and needs attention, manpower and money. It can not be done only on enthusiasm. Therefore the photon collider should be considered as an integral part of the ILC program, get sufficient support, all participating HEP people should be members of the detector-2 collaboration from the start. Other possible scenarios are the following. 1) One detector, one or two IPs. It is difficult to imaging that e+e- people will agree to finish e+e- experiments, always will be some new ideas to measure something. Also modification of the detector, test runs can lead to years loss of the ILC operation time. 2) Two e+e- detectors with small crossing angle and one free IP with the large angle for the γγ upgrade. This scenario may be attractive for e+e-, but will be much more expensive.

D27: have bypass lines for low energy running? Low energy running is necessary for GigaZ(e+e- at 2E=90 GeV), WW threshold (2E=160 GeV) and γγ→H(120) (electrons with 2E~200 GeV). All experiments need good emittances (good luminosity) and small energy spreads (for precision measurement at GigaZ and WW and for smaller chromo-geometric aberrations for γγ). In the scheme with the laser positron source, according to Kubo-san the scheme with acceleration with full gradient and further deceleration gives smallest emittance dilution, but need more power. It is interesting also what happens with the energy spread in this scheme? Is such loss of power acceptable or better to make bypasses ? In the case of GigaZ and an undulator e+ source, if the beam passes the undulator and further decelerated, then the energy spread is about 0.3% (desirable 0.1%). It seems that the scheme with bypasses is better for this case (see D. Scott talk at GG6 or GG6 summary)

D34: L * In γγ experiment, the forward part of the detector will be changed therefore L * can be somewhat different. Smaller L * can give larger luminosity (smaller chromo- geometric effects), smaller effect of SR (shifted quad and the detector field compensate each other). But the crab crossing angle should be small. The size of the quad has minimum transverse size, therefore significant decrease of L * is not possible. At present the minimum β x is determined by chromo- geometric aberrations. This effect restricts the γγ luminosity. In order to make a final choice it is desirable to see how the geometric luminosity depends on L*. Related question. For obtaining zero vertical collision angle in e-e-, γγ case we plan to shift final quads. Should they be shifted mechanically or with the help of additional dipole coils?