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Distributed RF System (DRFS)
Author: Shigeki Fukuda
Outline
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Concept
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System description
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Components List in full beam scheme and low-power option
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Tunnel Layout
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Cost Consideration
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Availability Consideration for DRFS
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R&D Schedule
As an alternate cost-effective design in support of a single Main Linac tunnel design,
the Distributed RF System (DRFS) is to be investigated as part of the on-going cost
reduction studies. In basic DRFS, 750 kW Modulating Anode (MA) klystron feeds two
cavities (in case of low power option, four cavities). It has an advantage in its simple
HLRF system as well as in its flexibility when power re-adjustment is required by the
occurrence of cavity gradient degradation. Cost for large number of MA klystrons and
availability issues are main concerns to be studied and described later. R&D on this
DRFS has already started and prototype RF unit will be manufactured and tested in
STF-II.
System Description
In full beam scheme of ILC, 3 cryomodules with 26 cavities have thirteen 750-kW
modulated-anode (MA) klystrons. Each MA klystron feeds two cavities. For low-power
option a MA klystron feeds four cavities. In each klystron, power and pulse voltage are
supplied by a common DC power supply and MA modulator for 13 klystrons. Power
Distribution System (PDS) is relatively simple; a power from a MA klystron is divided
by half by a Magic-Tee and fed to two cavities. Circulator is not necessary since the
reflected powers from two cavities are cancelled. Other PDS components are a
directional couplers (DC) and flexible wave guide. In order to realize higher reliability, a
common DC power supply and a MA modulator have a backup DC power supply and a
MA modulator which are quickly turned on when the operating units are in fail. Power
and voltage distributed circuit has a high-voltage SW, which switches off the line when
over current failures are detected. A DC power supply has a bouncer circuit to
compensate the pulse flat top droop and this enables us the rather small capacitor bank.
Charger of the DC power supply comprises of several units of switching PS and they
enable us to increase electrical power by adding the number of the units. A common
heater power supply feeds filament power through the same line. Another high
reliability are aimed with employing the permanent focusing magnet for a MA klystron,
which eliminates the power supply. A MA klystron has operated by 65kV beam voltage
with micro-perveance of 1.2 and has a 60% efficiency. Conceptual block diagram is
shown in Figure 1.
Most advantageous feature for the DRFS is LLRF control. Vector sum control for many
cavities is not necessary and higher average accelerating gradient is expected. Simple
control system and high operability are possible when the some parts of the main linac
components are failed.
Figure 1 : Schematic Draw of DRFS System
Components List in full beam scheme and low-power option
In figure 2, configurations of rf units per 6 cryomodules with 52 cavities are shown in
the case of full beam scheme and low power option. In low power option, one MA
klystron feeds a power to four cavities as shown in the lower drawing of Figure 2 and
main components are listed. Pass from low power option to full beam scheme is straight
forwarded; increase the number of MA klystrons twice as much and elimination of
Magic-tees. Component list is also shown in Figure 2 upper.
Figure 2 : Component list in full beam scheme and low power option
Tunnel Layout
Single tunnel layout with 5.2 m diameter is assumed. Cryomodule is hanged down
from the top, so the structure for vibration suppression should be considered. RF
sources are almost equally distributed along to the tunnel and modulators, DC power
supply, LLRF rack units and other electrical devices are installed in the shielded place
to protect from the radiation exposure. More than 1 m2 air ducts are placed under the
floor. The drawing of Figure 2 includes the working space of the carrier girder of
cryomodule and klystron carriers. There studied another layout for the DRFS; one is the
layout that the cryomodule is placed on the floor in the 5.75 m diameter and the another
one that the cryomodule is placed on the shielding structure in the 5.2 m diameter.
Figure 3 Tunnel Layout of DRFS (Cross section view and 3-D drawing)
Cost Consideration
Since this distributed RF system requires large number of MA klystrons, cost
reduction is a key issue in order to realize the DRFS. The following considerations are
adopted for cost reduction.
1) Cost consideration for MA Klystron
The total of 9000 tubes will be manufactured during the construction period of 5 years
(1,800/year) and 400 tubes per year during the operation. In order to manufacture these
klystrons cheaply, cost cut-down efforts are considered as follows; no tube processing
(company proceeds up to tube baking and tube processing will be performed at the ILC
site), employing hydro-forming technique for manufacturing common parts of the tube,
exploiting auto tuning machine for cavity tuning, no ion pump (getter pump in stead),
no lead shield, introducing permanent magnet focusing (elimination of PS leads to high
availability).
2) Cost Study for Modulator and Power Supplies
In order to realize a large cost reduction, the scheme with one common MA modulator
and DC power supply for 13 klystrons is adopted. Another back-up will be introduced in
order to have a high reliability by switching to this in case of failures. Common filament
power supply for 13 klystrons is also introduced with back-up.
3) Power Distribution
Very simple power distribution system, which results in cost reduction, is possible in
this scheme; no circulator, a power divider employing Magic-T with a high isolation for
space saving, one phase-shifter with symmetric PDS between couplers or asymmetric
PDS with a phase-fixed waveguide for cost saving and 750 kW RF propagation in the
dry air without any extra ceramic window.
Taking into account the above mentioned considerations, the cost comparison of each
subsystem is as follows;
RF Source cost =65k$@1 RF unit cf. 23k$ (RF source target price)
Modulator cost =18.7k$@1 RF unit cf. 40k$ (RF source target price)
PDS cost =7k$@1 RF unit cf. 26.5k$ (RF source target price)
The resultant DRFS cost is almost the same with the corresponding RDR cost.
Availability Consideration for DRFS
We assumed 5 months operation (2500 hrs) and one month shut-down (maintenance)
in the ILC operation. The numbers of failed components after 5 months operation is as
follows when we calculate them using expected Mean Time Before Failure (MTBF):
DC PS: 32 (assumed MTBF of 50,000 hrs with total number of 650)
MA Modulator: 23 (assumed MTBF of 70,000 hrs with total number of 650)
MA Klystron: 192 (assumed MTBF of 110,000 hrs with total number of 8450)
This failure number of MA klystrons corresponds to 2.3% of the total. Since the present
RDR energy overhead is 3 %, these klystron failures don’t affect the ILC operation.
Klystron replacement will be conducted during the scheduled shutdown (every 6
months). If the scheduled maintenance scheme is not adopted, unscheduled
maintenance would be necessary when failed fraction exceeds the energy overhead.
Estimated failure fractions of MA modulator and DC power supply exceed the
allowable overhead. Back-up MA modulator and DC power supply will be introduced as
redundancy and these will cover the failed ones. Since it is very rare for two modules to
have failures at the same time, no down time will be expected. Failed MA modulators
and DC power supplies will be repaired during the scheduled shutdown time.
Klystron associated components are also simplified in order to achieve high availability.
The simplified scheme adopts no ion pump PS, no klystron focusing coil PS by adopting
permanent magnet focusing, and only one heater PS with back-up for 13 klystrons.
High flexibility in power re-adjustment, which is required when the cavity gradient
degradation occurs, is possible for DRFS LLRF since the number of feeding cavities
from one MA klystron is small. This results in high availability and lessens the energy
overhead.
R&D Schedule
Task force team has started R&Ds on the DRFS RF system. Prototype RF unit will be
manufactured in FY09 and further R&D will be continued. Prototype unit will be
evaluated in the S1 global test (FY10) and then installed in the buncher section of the
STF-II aiming for realistic operation of the system (FY11, 12). Realistic cost estimation
will be performed together with CFS team after fixing the scheme. Vibration issue of
cryomodule, caused by its location of hanging-down from the ceiling, will be studied too.