10/08/04gek/ITRP/korea1 ITRPGEK Pre Meeting 6 thoughtsAug Both technologies can be made to work and either would lead to a linear collider with the specified performance in energy and luminosity, although not necessarily in the specified time frame. Fully functional detectors can be built for either technology. Two experimental areas should be provided from the start and neither of these should be at zero degrees. The main distinguishing features are the risks associated with each technology. There are other factors that have also to be taken into account in making the decision, these are a)the potential for upgrading both integrated luminosity and energy, b) the potential the chosen technology has for other branches of science and industry and c) the symbiosis between the technology and future higher energy machines. It is the weighting that is put on the above criteria that determines the choice.

10/08/04gek/ITRP/korea2 Qualitative Comparison of the two technologies Electron Source, polarized and unpolarized:COLD = WARM Positron Source, unpolarized:COLD < WARM Positron Source, polarized:COLD = WARM Damping Ring:COLD < WARM Linac: Initial Alignment:COLD > WARM Operation, ie integrated luminosity:COLD > WARM Reliability:COLD > WARM Upgradeability in Energy:COLD < WARM Upgradeability in Luminosity:COLD > WARM Beam Delivery System:COLD = WARM Experimental Conditions:COLD = WARM Application to other areas of science and technologyCOLD > WARM Symbiosis with possible future higher energy LC’sCOLD < WARM Summary:5 COLD > WARM 4 WARM > COLD 4 COLD = WARM

10/08/04gek/ITRP/korea3 Discussion of technology comparisons: Electron Sources, unpolarized and polarized: Although there are detailed differences between the sources required by the warm and cold options, both appear to be well within current technology. Positron Source, polarized: Both require some R&D but similar, should be feasible Positron Source, unpolarized: This is desirable for fast tuning and possibly operation. The cold option seems to have significantly more challenging specifications, eg 3 orders of magnitude more positrons in a bunch train than those in a single SLC bunch, while the warm option bunch train requires less than 2 orders of magnitude. Damping Rings: Damping rings for warm option are both well understood and v. similar to existing machines. Damping rings for cold option not similar to existing machines, therefore relies on simulation, in principle OK, but vacuum, electron cloud? In warm option damping rings in separate tunnels so tuning can be performed independently of installation of linac (at least for electrons?). In cold option, to be independent of installation of linac, requires more complications.

10/08/04gek/ITRP/korea4 Discussion of technology comparisons, cont.: LINAC: –Initial Alignment: The smaller aperture, lower tolerances larger number of moveable components and greater sensitivity to vibration of the warm option makes both the initial alignment and the operation of the warm option more delicate than the cold. The temperature stability is far better for the cold than the warm, but the relationship between the cold position of the accelerating structure and the outside world, for surveying, needs to be absolutely secure. –Operation, integrated luminosity: The points made above, together with a larger intrinsic L, smaller number of klystrons and klystron failures, and probably fewer trips and less damage to the accelerating structure should make the cold machine more stable and easier to operate at close to optimum luminosity. Clearly the integrated luminosity does not only depend on the Linac. –Reliability: This is largely covered by the above, but the sheer number of RF structures, water circuits, precision movers, quads and klystrons as well as the smaller tolerances seems to disfavour the warm.

10/08/04gek/ITRP/korea5 Discussion of technology comparisons, cont.: LINAC cont.: –Upgradeability in Energy –There are two paths to upgrading the energy beyond 1 GeV in the warm technology, firstly by reducing the luminosity and secondly by replacing the accelerating structure with structures able to sustain greater than 75MV/m, eg tungsten irises. This second possibility depends on more R&D and cost considerations. In the case of the cold technology, there is little room for increasing the energy with the present cavity design, there might be a possibility with a new cavity design. This would require serious R&D. The upgrades in energy of both technologies by changing the accelerating structures might be part of the upgrade from 0.5 to 1 TeV, ie half the machine would be equipped with the higher gradient structures. For this to happen, much R&D needs to be done. The most straightforward way for the warm machine would allow a 30% increase in energy for a loss of about a factor of 10 in luminosity.

10/08/04gek/ITRP/korea6 Discussion of technology comparisons, cont.: –LINAC cont.: –Upgradeability in Luminosity This was addressed by the warm proponents by saying that there was headroom in their quoted parameters, so that if they all attained their “theoretical” limits the luminosity could be considerably higher. My impression is that the quoted luminosities are already optimistic. The real question is not whether it is possible to increase the maximum luminosity by 20-30%, but which technology has the potential of delivering substantial factors higher integrated luminosity. It should be noted that very few (no?) accelerators have been built that have not attained their design energy, essentially at turn-on, but very few have attained their design luminosity within 5 years. Some (many) have been down by at least an order of magnitude. THE ENERGY VERSUS LUMINOSITY DISCUSSION SHOULD NOT BE BASED ON A POTENTIAL 30% INCREASE IN ENERGY VERSUS A 30% HIGHER LUMINOSITY, BUT A 30% INCREASE IN ENERGY VERSUS A POTENTIAL ORDER OF MAGNITUDE HIGHER LUMINOSITY

10/08/04gek/ITRP/korea7 Discussion of technology comparisons, cont.: Beam Delivery System: These are complex multielement systems that require excellent vibration stabilization and compensation. The two BDS can be made to be rather similar. The spot sizes at 500 GeV (Sigma(x)/sigma(y)) are 554/5.0 and 243/3.0 for the cold and warm options, before pinch. These are both very challenging, the cold being slightly less demanding. Experimental Conditions: Although there have been earlier claims that the bunch structure of the cold machine makes the readout of the vertex detector questionable, these now seem to be groundless with the availability of new types of CCD’s. Main concerns are probably backgrounds from synchrotron radiation and at higher energy muons. Excellent detectors for both technologies should not be a great problem. They should neither be as complex or demanding or expensive as the general purpose LHC detectors.

10/08/04gek/ITRP/korea8 Discussion of technology comparisons, cont.: Applications to other areas of Science and Technology: The development of SC RF has had applications in the field of modern accelerators, where it is used extensively. Warm X-band is also used eg for radar systems, but these don’t seem to require high power/gradients. My perception is that there are likely to be more application for the SC RF technology, both cavities and klystrons in the future than for the equivalent warm structures and klystrons. There is another factor which should be addressed, that is energy conservation and efficiency. One way of measuring this is cost, but this depends on energy cost, if as is not unlikely energy costs increase far faster than inflation then differential operating might become important. Also, there may be a “political” premium in building the most energy efficient machine.

10/08/04gek/ITRP/korea9 Discussion of technology comparisons, cont.: Symbiosis with possible future higher energy LC’s: The leading candidate for the next generation higher energy LC is CLIC. It is clear that many of the lessons learned from a warm LC, can be of direct interest for CLIC. There are large extrapolations from present machines (mostly SLC), to the LC, whereas the extrapolations between an X-band warm machine and CLIC would be much smaller, especially if the frequency of CLIC were reduced. This similarity is in several quite critical parameters, eg cavity material and structure, bunch structure……… This is not true, or at least much less so for the cold machine.

10/08/04gek/ITRP/korea10 Summary: Based on these considerations, I would at this stage vote for a cold machine. The easier upgrading of the energy by ~30% does not in my mind compensate for the greater difficulty in operating the machine and obtaining integrated luminosity within factors of those quoted. Note; Although SLC started about 2 years before LEP, essentially all the Z- pole physics came from LEP, nor in the end was pushing the energy to its very highest value productive! However, my present position is a very finely balanced one. I am still open to arguments, either that I am wrong in some of my qualitative comparisons or that I have ascribed the wrong weight to some of them, or both or that there are other factors which I have not adequately considered. It should be stated again that I believe that both technologies can be made to work and produce useable and useful accelerators. In my mind it is essential that we make a decision and that this decision is accepted by the whole panel.