### Minutes & Notes from Proton Driver RF Meeting March 16 2004

*(G.W. Foster)*

Subject: the 805 MHz fast phase shifter and related topics.

Attendees: Steve Hays, V. Kashikhin, Dave Wildman, Iouri Terechkine,
Pierre Bauer, Markus Huening, Ivan Gonin, Ding Sun, Gennady Romanov, GW
Foster, J. Kerby, Milorad Popovic, Temirgali Khabiboulline.

**[MINUTES]{.underline}**

1\) Marcus Huening presented a brief verbal status report on his
simulation results of the fast ferrite shifter in the 805 MHz section of
the linac. He used an ideal E-H tuner response but a finite response
time for the ferrite loaded stubs. The time response assumed for the
tuner was a 10usec fixed delay in the feedback loop, followed by a
300usec time constant in the response of the tuner. (Note this is
pessimistic compared to the target specifications for the tuners, a
100usec settling time for a 45-degree phase jump). Despite the sluggish
tuner response assumed in his simulations, he felt the results were
excellent. Marcus has promised to present actual viewgraphs with
equations and numbers next week.

2\) Dave Wildman reported on progress getting the Ferrite ordered. The
Trans-Tech order for the 510 material has not yet been placed because we
are negotiating how much to pay for expedited delivery. Dave put a req
from a 2^nd^ vendor (somewhere in NJ or PA?) into the system. This
material has B(sat) \~400 Gauss and a lower Curie temperature, but it
was cheap and relatively fast delivery. Four other potential vendors
have yet to reply.

3\) General discussion the simulation results ensued. The question is
whether we have reproduced the experimental results for the phase shift
and impedance of the Bell Labs tuner, and our existing YIG tuner, well
enough that we can confidently predict the phase shift and impedance of
a coax line using the ferrite parts we are ordering. Dave W. chose the
geometry (6:1 coax diameter ratio) by estimating the impedance assuming
that we will want to operate the ferrite close enough to the resonance
that the RF permeability would be in the range of \~2-5. For curves that
he had available for YIG at \~50 MHz, running this far from resonance
was enough to ensure Q's in the range of tens of thousands. Ding and
others are working on the simulations and should have results to present
next week. Meanwhile we will continue under the assumption that a tuner
with 5 rings (2.5" length of ferrite) will provide the +/-45-degree
(90-degree total) phase shift. This is consistent with a naïve scaling
from measurements of the the existing YIG tuner.

4\) Vladimir Kashikhin reported on magnetic simulations of the coil.
(See attached slides). The geometry he simulated was:

a)  a coaxial shorted stub \~6" long, with the final 2.5" filled with a
    ferrite cylinder with 3" OD, 0.5" ID, 2.5" length.

b)  the metal shell was 0.5mm thick stainless steel. Adding an addional
    20 microns of copper plating would increase the effective thickness
    to the equivalent of 0.8mm of stainless steel. During the simulation
    he realized that slotting the metal shell (as was done with part of
    the existing YIG tuner) would greatly reduce the magnetic shielding
    and speed up the tuner response. Iouri Terechkine had
    hand-calculated the shielding time constant to be \~25 usec, which
    Vladimir verified off-line. Since this is somewhat faster than our
    100usec response time requirement, the time for field penetration
    through the shell may not be a problem, even without slots. So the
    candidate coax shells are copper-plated stainless, or
    heavily-slotted copper.

c)  A copper drive coil 10cm long with roughly 200V x 100A drive
    requirement. The coil is pulsed with 1.5% duty cycle.

d)  A cylindrical flux return made from medium-frequency soft ferrite
    with Bsat \~0.4 T. The flux return had an end-cap on the shorted
    stub end.

The coil waveform is pulsed at 10 Hz with a 0.5msec rise time to 1.25kG,
a 1msec "flat-top", and a 0.5msec fall time. During "flat top" the field
in the ferrite slews under the control of the feedback loop between
1 kGauss and 1.5 kGauss, at a maximum slew rate of 2.5Gauss/usec
(inductively limited). Assuming the phase shift is linear with field
(not true) and that the 2.5" ferrite length provides 90 degrees of phase
shift, this meets our target of a 45-degree phase shift in 100 usec.

The uniformity of the bias field in the Ferrite is about +/-10%, with
the major defect being a drop-off in the field strength in the center of
the ferrite at the end away from the flux return end cap. Vladimir
believes that this could be substantially improved with additional
end-compensation turns in the solenoidal coil, which he will attempt.

Iouri asked what the uniformity requirement for the bias field was. He
pointed out that the radial component of the B-field was not important
for RF dissipation, since it is also perpendicular to the RF magnetic
field in a coax geometry. It seems the main requirement is that the
entire volume of the ferrite be above resonance, to avoid local volumes
of the ferrite with high RF losses.

5\) Steve Hays brought a power supply module in common use in the
Accelerator Division, that comes withing spitting distance of meeting
the specifications for driving the bias coil on the tuner. It is a
switching module which shares a common bulk supply with a number of
identical modules. Many of these module are currently being built for
NUMI, where they are housed in chassis containing 12 modules sharing a
common bulk supply. Two bias supplies are required per cavity, so a
total of 192 supplies (16 chassis) would be required for all 96 cavities
in the 805 MHz Beta\<1 section of the linac. The module itself costs
\$1000, and when the bulk supply, controls, and cable are included a
reasonable cost is \$2000/channel (\$4000/cavity). This is far less than
the power supply cost estimate obtained from AFT.

The major changes required are to speed up the response by eliminating
the slew rate limiter circuit, increasing the switching frequency, and
altering or abolishing the output filter. Various additional controls
will be required such as an output used to inhibit the RF pulse when the
ferrite is biased below resonance, and a pulse duty cycle limiter. Steve
also guessed that some additional filtering would be required to prevent
the RF from coming up the cable and disturbing the power supply control
circuit. A two‑conductor‑plus‑shield power cable is generally used with
these supplies.

The maximum output current of the existing modules is 60 Amps DC, but
Steve believed it should be possible to increase this above 100A level
given the 1.5% pulsed duty cycle required for the bias coil. He felt
that it would be best if the coil design were optimized for a peak
voltage in the range of +/-150V to match the typical bulk supply
voltage.

6\) Test stand time and requirements:

At present we are concentrating on 805MHz fast tuners required for the
96 beta\<1 cavities in the baseline design. The linac test stand that we
have been using can provide 12 MW at a 0.15% duty cycle (100 usec x
15Hz), whereas for the tuner we need 0.6MW at 1.5% duty cycle (1.5msec x
10Hz). So we can do peak-power testing (where most of the problems are
felt likely to occur) but not average power testing. Dave W. pointed out
that we can kind of fake high average power testing by biasing the
ferrite close to resonance where the heating will increase. This can be
combined with pulsing the coil at a higher duty factor than the Klystron
can provide.

Testing at the full average power will require access to a 1 msec x
0.6 MW pulsed source at 805MHz. Currently these exist at SNS, JLAB, and
LANL. Ultimately the most convincing system test would be to connect the
ferrite tuner to an SCRF cavity and actually do resonance control. This
argues for JLAB or possibly SNS.

Milorad indicated that (although he cannot speak officially) the Linac
Klystron test stand will likely be used for high priority testing of the
replacement Klystrons in \~4 months when they arrive, but may largely be
available until then. Two possible locations exist to set up the tuners:
either the linac gallery where we have been operating so far, or the
Muon Collaboration test area which also connects to microwave plumbing
from the test stand. The advantages of using the Muon test area is that
it is much less crowded, that RF safety is less of an issue. The muon
test area will be very convenient once we start to set up a more
complete mockup including the RF power split and multiple stations of
tuners.

*(see also Vladimr Kashikhin's slides from the meeting)*