Choice of frequency (1300 vs. 1207):

Summary opinion; I think it's probably true that there are enough
advantages of 1300 MHz over 1207 MHz to make the change in design
parameter. I.e., Piggy backing on the TESLA tube development, using
their input coupler as a baseline (or at least as a starting point to
minimize development time and cost), and copying their cryomodule.
However, I have two caveats:

(1) One needs to be sure that there are no significant beam loss
    problems created by bringing the 1300 MHz cavities down to the 87
    MeV point and by having the bigger step in frequency at the
    transition point. I'd do a few beam simulations to be sure that
    everything still looks good and there are not sizeable matching
    problems, beam loss and halo before I bricked up the opening.

(2) The other issue that should be considered is whether the existing
    402.5-MHz linac provides a unique ability to test components with a
    real beam in an actual accelerator environment. I think it might be
    wise to just list the hardware items that you might consider testing
    in the existing linac and how much you'd lose by changing
    frequencies. There might well be a challenge in the future with
    regard to this issue and it would be good to have a defense in hand.

Detailed comments:

Advantages of using TESLA compatible frequencies

(1) I agree that the fewest different klystron types the better.

(2) I think it makes a lot of sense to take advantage of the TESLA's
    successful multi-beam klystron R&D development program. However,
    it's not clear what are the true benefits since you're trading off
    klystron development, which probably in going down to 1207 from 1300
    isn't all that big or costly a deal, vs. cavity development. The
    costs quoted by JLAB are probably low and are not chiseled in stone.
    Things will come up at JLAB that will add to the cost burden,
    whereas, klystron manufacturers give fixed cost commitments.

(3) I also believe that fewer klystrons are better. I think a smaller
    body count means fewer chances for hardware failures. I really don't
    believe that there will be more failures with 10 Mw klystrons
    compared to 5 MW klystrons. In my experience anything above a few
    hundred kilowatts can, and usually does, have development problems
    and quirks that have to be ironed out, but once done will operate
    with equal reliability

(4) I doubt that SNS will ever replace the CCL. They have a lot of other
    improvement options that would be more effective use of the money.
    It would take something like a serious reliability problem (chronic
    water /vacuum leaks or arc downs and frequency detuning in the
    coupling cells) before replacement would ever get serious attention.

(5) I think it makes a lot of sense to use the TESLA input coupler since
    it seems to work, and that will result in a significant R&D savings.
    However, you still will want to have your own test stand for
    pre-conditioning of input couplers, and since that's the most
    expensive part of a coupler development program, I'd include an
    improvement program at some level to see if there are things that
    could easily done to get a better TESLA unit.

(6) I think using an existing cryomodule design, especially if you have
    access to the design drawings, would be a big cost and schedule
    factor for the construction project.

(7) There's no question that a lower tunnel height will save money.
    However I seem to remember that the structural engineers at SNS did
    claim that tunnel width was a big cost driver and a foot of tunnel
    width was more valuable to them than a foot of tunnel height. This
    could also be a site-specific issue since there was something about
    composition of the Chestnut Ridge soil that resulted in higher
    tunnel loading.

(8) WG elements are cheaper at 1300 MHz as long as you don't end up
    needing pressurized air or SF6 to hold the fields without arcing.
    The area of concern would be near the klystron and before the power
    splitters where you have to handle the full power.

(9) I don't think the size of the 805 KHz cavities is the problem, but
    rather, from what was presented at ASAC in February, the cleanliness
    of the rinse water system is. It seems that JLAB was using
    oil-sealed pumps and the QC wasn't always what it should have been.
    There was a SRF review committee that made a number of
    recommendations to SNS and JLAB on how to get more consistent
    performance, but a major recommendation seemed to be to just get
    someone dedicated to watching over production and making sure that
    the normal good handling practices are followed. They also
    recommended changing the oil-sealed pump.

Advantages of the SNS-Compatible 402.5 MHz/ 805 MHz/ 1207 MHz solution

(1) It's true that you would be able to use the SNS cavity designs at
    0.61 and 0.81 and probably the RIA cavity design at 0.47.

(2) Some kind of change will be needed at 1300, either another beta
    section or maybe fewer cells in the early sections.

(3) I'm not sure how big an advantage the actual SNS cost data at 805
    MHz is to the frequency decision. First, much of it extrapolates to
    other frequencies just as well. The modulators, water systems,
    interlocks and all that stuff are frequency independent. The bulk of
    the stuff that is in the waveguide system has very good scale
    factors that can be used over the frequency range of consideration.
    The cost estimates for the cold part of the linac will not be as
    valuable because you're going with a whole different cryomodule
    concept. It could also turn out that the SNS cost estimates could
    become a double-edged sword for whatever frequency is chosen.
    Suppose you come up with more cost effective solutions, or lower
    cost estimates because you expanded the range of manufacturers that
    could bid on the hardware, you'll find yourself in the position of
    overcoming skepticism that will come with the comparison of what
    will be perceived as "reality".

(4) I don't think lower waveguide losses are a very strong argument for
    basing a decision on frequency. Losses in circulators, ferrite
    tuners and various mismatched components will probably dominant
    losses.

(5) The ability to exchange parts with SNS would be an advantage, but
    isn't it a wash in the end because if you switch to 1300 MHz, you
    can trade parts with TESLA?

(6) The ability to test components, especially diagnostic equipment,
    with a beam in an accelerator environment is a big plus when it
    comes to commissioning. In my experience, half the problems with
    commissioning are caused by new hardware with a high rate of infant
    mortality that reduces beam physics time to next to nothing and
    prevents continuity in the commissioning program and the other half
    of the problems are diagnostics that don't work properly and mislead
    you when you're trying to interpret what's going on with the beam.
    Setting up the beam properties according to the physics design is
    almost trivial when the other stuff is working. You'd also be able
    to test a cavity and maybe even a cryomodule with a beam to see how
    everything performs as a system.

(7) I don't think having the documented SNS physic design is all that
    much of an advantage. Your "SNS-type" design has a different
    cryomodule design that results in different cavity and focusing
    element spacing and your cavity and real estate accelerating
    gradients are higher. Those differences are too big to directly use
    the SNS stuff. However, the computer codes that they are using are
    being validated as their commissioning progresses, and I believe
    that they are valuable for the FERMI design no matter what frequency
    is chosen.