Design of a Compact Power Distribution System for the ILC
1B.T. DU†,1, T. MATSUMOTO1,2, S. MICHIZONO1,2, T. MIURA1,2, F. QIU1,2, N. LIU1
The Graduate University for Advanced Studies (SOKENDAI), 240-0193, Hayama, Japan
2
High Energy Accelerator Research Organization (KEK), 305-0801, Tsukuba, Japan

Abstract
The Local power distribution system (LPDS) of the International Linear Collider (ILC) is constructed to transmit
RF power from the 10 MW klystron to 39 cavities. Eacheight or nine 9-cell cavities is assembled in one cryomodule. The variable hybrid is used to adjust the power dividing ratio due to the different required power of each cavity
and the variable phase shifter is used to compensate thephase drift caused by the variable hybrid.
More compact LPDS is expected to be integrated on thecryomodule decreasing financial cost. We re-design theshorter variable hybrid with a margin of power ratio of±25% and phase shifter of total phase range being 35° forcompensating hybrid and on-crest searching. Fixed phaseshifters are designed to adjust the phase difference between
adjacent cavities for beam acceleration. Simulated resultsof total compact LPDS can meet the requirements of ILC.

INTRODUCTION
The International Linear Collider (ILC) is a 250 GeV linear electron-positron collider, based on the 1.3 GHz superconducting radio-frequency technology. Figure 1 shows
the power distribution system (PDS) for the LC as described in the ILC technical design report (TDR) [1]. A
10MW multi-beam klystron is used as a power source todrive 39 Tesla type 9-cell super conducting cavities. Eachcryomodule has 8 or 9 cavities. Figure 2 shows the localPDS (LPDS) for ILC as described in the ILC TDR. EachPDS contains three LPDS. Each LPDS is fed 3.33 MW
from main variable hybrid to distribute power to 13 cavities
by three power dividers and 10 secondary variable hybrids.
Under the condition of equal power driving cavities, theaverage accelerating gradient is expected to be 31.5 MV/m.
The TDR indicate that this gradient would have a variationof ±20%, owing to the manufactured differences.
LPDS contains variable hybrids, variable phase shifters
and fixed phase shifters [2]. The variable hybrids are usedto adjust the power ratio to each cavities. The phase relationship between cavities drifts when the power ratiochanges. The variable phase shifters are used to compensate this phase drift and on-crest search. The geometricallength between adjacent cavities is 1326 mm, which is 5.75
λ0 (λ0 is wavelength in free space at 1.3 GHz). The fixedphase shifters can adjust the phase relationship betweencavities before using the variable phase shifters for beam
acceleration.

Figure 1: PDS for ILC in ILC TDR [1].

Figure 2: LPDS for ILC in ILC TDR [1].
Considering the LPDS from the ILC TDR, each variable
phase shifter can only adjust the phase of a single cavity.
The phase drift of each hybrid is accumulated for all subsequent cavities. This increases the maximal required
phase range of the variable phase shifter which, resulting
in its long geometrical length. Thus, the vertical length of
the LPDS is more than the diameter of the cryomodule.
Under this condition, the cryomodule can only be assembled in the tunnel first. Then, the LPDS is connected to thecryomodule. Assembly difficulty is generated and moretunnel space is required. The LPDS is expected to be totally
integrated on the cryomodule before the cryomodule is assembled in the tunnel. Additionally, the number andlengths of the waveguides is expected to decrease because
the total number of RF units and cavities of ILC being 236
and approximately 8,000 for 250 GeV respectively [3].
Figure 3 shows the LPDS in the Super-Conducting RFTest Facility (STF) at the High Energy Accelerator Research Organization (KEK), which applies superconducting technology for the ILC. During operations from January through March 2019, beam acceleration was demonstrated with nine cavities. The feasibility of LPDS was verified in the STF.

___________________________________________† baiting@post.kek.jp

Figure 3: LPDS in the STF.

To design the compact LPDS for the ILC, we first consider the possible model in which the hybrids and variable
phase shifters are on the main line of LPDS. This design
reduces the height of the LPDS. Then, we design a shorter
hybrid and shorter variable phase shifter based on geometrical, power distribution, and phase adjusting requirements.
Then, the total compact LPDS is simulated.

MODEL OF COMPACT LPDS
Figure 4 shows the model of the compact LPDS for left(upper) and right (below) directions. For the chain of fivecavities, four variable hybrids, five variable phase shifters,
four fixed phase shifters and 14 H-corners are used. Variable hybrids, variable phase shifters and fixed phase shifters
are arranged on the main line to reduce the vertical lengthof LPDS and to decrease the number of waveguides. The
distance between each cavity is 1,326 mm, which is 5.75
λ0.

Figure 5: Model of compact variable hybrid.
Figure 6 shows the simulated S parameter of the compact
variable hybrid. When the fin is moved from 0 to 14.7 mm,the S31 is changed from -9.95 to -2.57 dB. This range cancover the required S31 from -6.99 to -3.01 dB for the average power distributed condition, and S11 and S41 are bothless than -23 dB. For the power margin of ±25%, which
need the maximal S31 to be -2.04 dB, the fin must move
18.0 mm, and, S11 and S41 can only be < -19.3 dB. Figure
7 shows the simulated S21 and S31 phase of compact variable hybrid. When the power ratio is changed from the 25% condition to the +25% condition, the maximal phasedrift of S21 or S31 is 20.4°.

Figure 4: Model of compact LPDS.

DESIGN OF RF COMPONENTS FORCOMPACT LPDS
For super-conducting cavities with very low power loss,
optimal tuning the reflected power from cavity is critical,
the minimal generated power ((Pg)min) is shown by Eq. 1[4]. Vcav is cavity voltage, proportional to accelerating gradient. Ib0 is the DC component of beam current. φb is thebeam phase defined between the beam current and the cavity voltage. Thus, (Pg)min is approximately proportional tothe accelerating gradient. From the ILC TDR, the average
accelerating gradient of a 9-cell cavity is 31.5 MV/m witha variation of ±20% caused by the manufactured differences. The input power of cavities should have a power
margin of ±25%.
 Pg min  Vcav  Ib0  cos b(1)
Figure 5 shows the model of the compact variable hybrid.
The length is 630 mm. Two metallic fins are moved symmetrically to adjust the power distribution. Two fixed metallic posts are set at symmetrical positions to reduce thereflection of the four ports [2]. Based on the power marginof ±25%, the variable hybrid should have the coupling
power ratio (S31) from -8.24 to -2.04 dB.

Figure 6: Simulated S parameter of compact variable hybrid.

Figure 7: Simulated S21 and S31 phase of compact variable hybrid.
The variable phase shifters are set to compensate thephase drift caused by the variable hybrids. Another 15° foron-crest searching is considered so that a total phase rangeof 35° is necessary for the variable phase shifters. Figure 8shows the model of the variable phase shifter. One metallic
fin is moved to change the transmitted phase [2]. The
length of the variable phase shifter is 300 mm. Figure 9shows the simulated transmitted phase and reflection
power of the variable phase shifter. When the fin is moved

from 0 to 38.5 mm, the phase is changed from 36.0° to 0.8°.
The total phase range is 35.2°, and the eflection is less than-33 dB.

phase difference between simulated and ideal values is 4.3°
and 4.8° for the left and right directions, respectively. Thiscan be compensated by the variable phase shifter. The maximal power deviation from the average power distributed
case, -6.99 dB for each cavity, is 2.7% and 3.2% for left
and right directions, respectively. Superposition of reflection in the LPDS will influence the power transmitted tothe cavities. With the input power of 1.3 MW, the maximal
electric field is 4.40 and 6.17 kV/cm for left and right directions, respectively. This is lower than 21% of the breakdown field in dry air of 30 kV/cm.

Figure 8: Model of variable phase shifter.

Figure 11: Simulation of compact LPDS.

Figure 9: Simulated transmitted phase and reflectionpower of variable phase shifter.
Under the average power distributed condition and withphase shifter set at the center position of its phase range,
the required phase of fixed phase shifter is determined tomeet the requirements of beam acceleration. Figure 11
shows the three different ways of shortening wide side andlengthening the wide side, and adding metallic posts fordesigning fixed phase shifters. A total of eight different
fixed phase shifters are designed for left and right compact
LPDS.

Each component will be manufactured and tested underhigh power by the resonant ring [5]. After the high power
test of the RF component, the compact LPDS will be constructed to test the variation of power and phase and thestability of the long period. This system is expected to betested with beam in the STF.

CONCLUSION
The compact variable hybrid, variable phase shifter andfixed phase shifter were designed to meet the requirementsof compact LPDS. Simulated results of complete compact
LPDS meets the ILC requirements. The compact LPDS
will be constructed after the high power test of each component using the resonant ring. The adjustable margin ofpower and phase will be tested with klystron. The feasibility of compact LPDS is expected to be tested with beam
operation in the STF.

REFERENCES[1] ILC,

http://www.linearcol-lider.org/ILC/?pid=1000895

Figure 10: Fixed phase shifters (3 types).

SIMULATED RESULTS OF COMPACT
LPDS
Figure 11 shows the simulation of the compact LPDS forleft (upper) and right (below) directions. Hybrids are set for
average power distribution, and variable phase shifters areset at the center position of its phase range. The maximal[2] S. Kazakov et al., “L-Band Waveguide Elements for SRF Application”, in Proc. 6th Particle Accelerator Society Meeting(PASJ’09), Tokai, Japan, Aug. 2009, pp.980-982.
[3] L. Evans and S. Michizono, “The International Linear Collider Machine Staging Report 2017”, Rep. KEK 2017-3, Oct.
2017.
[4] T. Schilcher, “Vector Sum Control of Pulsed Accelerating
Fields in Lorentz Force Detuned Superconducting Cavities”,
Ph.D. thesis, Phys. Dept., University Hamburg, Hamburg,Germany, 1998.
[5] B.T. Du et al., “L-Band Resonant Ring for Testing RF Windows for ILC”, in Proc. 29th Linear Accelerator Conf.
(LINAC’18), Beijing, China, Sep. 2018, pp.679-681. doi:
10.18429/JACoW-LINAC2018-THPO003