# A Frequency Choice for the SCRF Proton Driver

G. William Foster

April 11 2004

This note summarizes the considerations for either standardizing on
TESLA-compatible (1300 MHz and 433 MHz) frequencies throughout the 8 GeV
SCRF injector linac, or on the SNS-compatible (402.5 MHz, 805 MHz, and
1207 MHz) frequencies assumed in the Design Study. Figures 1-3 show
block diagrams both options, for both 2 MW (ultimate) beam power and
0.5 MW (staged) beam power.

My conclusion after examining the trade-offs is that **the 1300 MHz
TESLA frequencies are preferred**, both from the point of view of a fast
start to the development program, and for the attractiveness of the
final machine. The conclusion doesn't depend heavily on whether a warm
(DTL) or a cold (SCRF Spoke Resonator) front-end linac is chosen.

The main implications for the development program for the 8 GeV Linac
are to begin development of a set of TESLA-compatible beta\<1 elliptical
cavities, to focus the development of the fast ferrite tuners at 1300
MHz, and to drop consideration of an early replacement of the FNAL 805
MHz linac front end.

**[Advantages of using the TESLA-compatible frequencies:]{.underline}**

1)  There are only two klystron types (433 & 1300 MHz) in the final
    machine, rather than three klystron types (402,805, & 1207 MHz) in
    the SNS-compatible design.

2)  No Klystron R&D program is needed. For the main 1300 MHz Multi-Beam
    Klystrons, TESLA has working prototypes and is in the process of
    qualifying three vendors (Thales, CPI, Hitachi). For the front-end
    433 MHz linac, several MW pulsed klystrons (e.g. Thales TH2120) also
    exist. Development cost for a new Klystron design is \~\$1M/vendor
    and takes 1-2 years. Cavity development is cheaper.

3)  There are fewer overall klystrons, since one 10 MW TESLA Klystron
    replaces two 5 MW SNS klystrons. This is a savings of 5 klystrons in
    the 2 MW design, or 2 klystrons in the 0.5 MW staged design.

4)  The long-term availability of the 5 MW SNS klystron is not clear,
    since the SNS upgrade plan is to replace the warm-copper CCL which
    uses the 5 MW klystrons with a SCRF section that does not use them.

5)  We don\'t have an immediate need for an RF coupler R&D program,
    since the conductively cooled TESLA (TTF3) coupler is adequate in
    both price and performance. For 805 MHz, we would have to design and
    test a conductively cooled variant of the (vapor cooled) SNS
    coupler, as well as a separate 1207 MHz coupler. The TTF3 coupler is
    not perfect, and the longer term, we may wish to continue RF coupler
    development.

6)  No immediate cryostat R&D is needed, since the beta\<1 cavities will
    fit into the same TESLA cryomodules as the beta=1 cavities. For the
    larger 805 MHz cavities, a new cryostat design is necessary.

7)  Ceiling height on the Klystron Gallery will be about 5 ft shorter.
    The TESLA Klystron height is 8.2 ft vs. 13 ft for SNS 805 MHz
    klystron. For an underground gallery the cost difference will be
    significant.

8)  Waveguide components, circulators, loads, ferrite tuners etc.are
    smaller & cheaper at 1300 MHz than at 805 MHz.

9)  There is some evidence that the larger 805 MHz cavities are harder
    to rinse than smaller TESLA cavities, and that this may be
    responsible for recent SNS cavity yield problems.

10) Frequency compatibility with A0 infrastructure may be an advantage
    for early tests

**[Advantages of the SNS-Compatible 402.5 MHz / 805 MHz / 1207 MHz
solution]{.underline}**

1)  Successful models of three out of four cavity designs exist: the SNS
    beta=0.83,0.61, and the RIA beta=0.47. Only one new cavity design
    (1207.5 MHz beta=1) would need to be created.

2)  Only three beta ranges (rather than four at 1300MHz) are required to
    cover the beta\<1 region, since the 6-cell 805 MHz cavities have a
    wider velocity acceptance than the 9‑cell 1300 MHz cavities. One
    fewer cryomodule type (plus spares) will be required.

3)  Cost data exist for the 805 MHz components, althought they would
    still have to be extrapolated to 1207MHz in the back 7/8 of the
    linac.

4)  Waveguide losses are lower for the larger 805 MHz waveguide.

5)  Compatibility and parts interchange with SNS is possible for the
    first 1/8 of linac \[but no parts exchange with anyone for the last
    7/8 of the linac\].

6)  Frequency compatibility with existing FNAL linac front end. This
    would be useful only if the warm front end gets replaced
    off-project, or if we want to do early beam tests in the linac
    gallery or muon area.

7)  Accelerator Physics are well documented for the 805 MHz SNS design.
    The 1300 MHz design will have a smaller RF bucket area and a smaller
    beam iris. Petr Ostromov\'s quick look at a point design did not
    uncover any problems, however.

**[Non-Issues]{.underline}**

1)  53 MHz Main Injector frequency compatibility is irrelevant since the
    beam will either be asynchronously chopped, or adiabatically
    rebunched at 53 MHz in the MI.

2)  Front end frequency compatibility with existing designs is also a
    wash. The 402.5 MHz SNS front end is not a useful starting point due
    to the high costs arising from its high duty factor. The AccSys
    RFQ/DTL runs at 425 MHz and would have to be frequency tweaked for
    either 402.5 or the TESLA-compatible 433 MHz. The SCRF spoke
    cavities for RIA would have to be tweaked in either case.

**[Comments on the Cavity Development Program]{.underline}**

In the SNS-compatible approach, only one new cavity design (1207.5 MHz
beta=1) would need to be created. However high-power testing of this
cavity could not take place until the 1207.5 MHz klystron development is
completed.

In the TESLA-compatible approach, four new beta\<1 elliptical cavities
must be developed. Informal discussions with JLAB personnel indicate
that their development cost of these cavities would be in the range of
\$500k total. If this is even approximately true then it is much cheaper
to develop new cavities than new Klystrons. It may be expedient and
propitious to spread this design & prototype work among several labs.

Evaluation of the SCRF Spoke Resonator option for the front end can
proceed at a more leisurely pace, since we already have an acceptable
warm DTL alternative. The key developments are measurement of the of the
gradients achievable in pulsed mode in cavities operating somewhare near
400 MHz, and the development of an economical pulsed RF system, possibly
including the fast ferrite phase shifters.

It is conceivable that the spoke resonators will end up attractive
enough to bite into the lower beta ranges of the elliptical cavities,
possibly reducing the number of new elliptical designs to three or even
two. So, the elliptical cavity prototype program should start at the
high-beta designs and work downwards. It will not be upsetting if we end
up with overlapping ranges of prototypes in which either spokes or
ellipticals are feasible.

![](media/image1.wmf){width="9.45in" height="6.35in"}

Fig. 1 - Design Study Baseline with 2 MW beam power and "SNS-compatible"
402.5 MHz, 805 MHz, and 1207.5 MHz klystrons and cavities. The 2 MW beam
power represents the

![](media/image2.wmf){width="10.0125in" height="6.7125in"}

Fig. 2 - "SNS-compatible Staged RF Power Design" with 0.5 MW beam power
(upgradeable to 2 MW) and 402.5 MHz, 805 MHz, and 1207.5 MHz klystrons
and cavities. A superconducting front end linac (shown in Fig. 3) is
also possible with these frequencies.

![](media/image3.wmf){width="9.75in" height="6.525in"}

Fig. 3 - "TESLA-compatible Staged RF Power Design" with 0.5 MW beam
power (upgradeable to 2 MW) and a spoke resonator front end. TESLA 1300
MHz multibeam klystrons drive elliptical cavities in both the beta\<1
and beta=1 sections. The front end linac can operate at either 1300/4 =
325 MHz (shown), or else at 1300/3 = 433 MHz (which may be preferable
due to the existence of commercially available Klystrons at that
frequency).