1. 1.Energy reach  High power klystrons and modulators for X-band still need development and industrialization, and thus brings risk. But klystrons of similar type have been constructed in past.  X-band structure breakdown: not fully understood at fundamental level, but considerable working experience; I do not rate this as a huge risk. Structures condition with time (SLAC linac experience). Very long time operation not tested.  SC cavity dark currents: my reading of the Tesla data to date is that some cavities, extrapolated to electropolished surfaces and 35 MV/m may initially show dark current above the stated goals. Annealing the emitters will occur and will reduce this somewhat, but it remains a worry.  Cold rf coupler performance remains some risk. They must be made totally immune to catastrophic vacuum failure. Major risks Grannis 1

2. 2.Luminosity (emittance preservation):  Wakefield control: about 10x more difficult for X band (BNS damping energy spread difference); studied to considerable degree in ASSET, NLCTA, and claimed to be under control. TRC concluded emittance growth control plan was feasible.  Alignment of structures and cavities: cold is more forgiving, but alignment is harder to achieve (where cavities are inside cryomodule, so don’t have structures serving as BPMs). This is a big risk for both, but more so for warm.  Final focus alignment: a big deal for both. More sensitivity to offsets in cold due to larger disruption parameter; instabilities comparable.  Damping rings: large dogbones for SC are not yet well understood  Electron cloud effects are a worry for both SC and X-band, but somewhat more difficult for X-band.  Positron production with undulator for SC is untried and risky.  Commissioning: SC needs electron linac in operation for high current positrons; beam based alignment more delicate for X band due to smaller tolerances. 2

3. 3.Cost risks:  Both have large extrapolations in key component cost reductions, so both are vulnerable. Warm has larger number of individual components. The estimates of learning curve slopes are uncertain.  For X-band, klystrons, SLED systems,modulators, structures all require large cost reductions (factors of 2-6).  For SC, cost reductions needed for Nb sheet, for cavity and cryomodule assembly, damping ring vacuum are quite large (factors 2-6).  The high risk technical subsystems (rf, cavities) are 34% of warm TPC and 30% of cold TPC (US cold/warm). The rest is civil construction, injectors, instrumentation, BDS, engineering, management common to both. 3

4. 4.Schedule risks:  XFEL serves as learning ground for many SC LC systems – klystrons, couplers, cavity preparation and cryomodules (but not at 35 MV/m). Does not address damping ring, positron production, final focus risks.  Commissioning for X band can proceed with more elements in parallel; SC high current e + requires the e - linac.  Probably the largest schedule risks are associated with the learning curve for industrialized components, component delivery (budgets), and the political process that governs the project. 4

5. 5.Operability risks:  Machine protection is not well developed for either technology and is of great importance. Catastrophic loss of beam can destroy the accelerator structures. For warm the bunch interval (and natural time constants) are shorter and thus catastrophic beam loss is more worrisome. For cold, the need to prevent any contamination of the SC cavities is a severe risk, even if accidents are unlikely, since the consequences are so dire. The couplers present some vulnerabilty.  Beam based alignment and maintaining the gold orbit without stopping for special runs is a key challenge for both; higher precision needed for warm, but more BPM information exists from structure BPMs. However, tuning for BDS is the biggest issue and is similar for both.  Time stamping energy deposits in detectors is very useful for control of beamstrahlung photon interactions, and is simpler in cold. However the detector people claim that either technology is workable. 5

6. 6.Other risks:  Effect on HEP community: If cold, lose the synergy with 2-beam acceleration. Impacts CERN’s reaction to LC adversely. If cold, there may be a diminishment of KEK & SLAC engagement. There is competition for expert manpower in DESY due to XFEL, (TESLA group of 2 DESY scientists now) but would gain some important added expertise (Jlab, Argonne, FNAL …) If warm, lose some fraction of effort by FNAL; DESY role would be diminished; less connection with non-HEP accelerator (light sources, proton linacs etc.)  Risk for getting government approvals; building portions of the 1 TeV upgrade in the initial baseline machine will be hard to sell. Unfilled tunnel in warm seems to give vulnerability. To lesser extent, building 35 MV/m SC cavities but operating them at 24 MV/m may be a problem. 6

7. I believe that to a considerable extent, the technical risks for either machine are comparable, though quite different. Both machines are extremely challenging. But I think we know enough to expect that for both, the technical challenges can be met. Making the choice of which set of challenges to focus on will allow a concerted attack. (You get to pick which set of really hard problems you attack!) The enlarged pool of talent and ideas will help. The more difficult risks to assess are those related to politics and funding, and those that connect to the aspirations of those in other physics disciplines, the wider scientific community and the public. Our wisdom should extend to understanding how these wider issues play out; they could well be the deciding factor. 7