Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 THP57

EXPERIMENTAL RESULT OF HIGH-POWER TRANSMISSION
THROUGH 1.3 GHZ CERL INJECTOR PROTOTYPE COUPLER ∗Pragya Nama † 1 , 1 SOKENDAI, Japan
Ashish Kumar1,2 , Dai Arakawa2 , Kensei Umemori1,2 , Eiji Kako2 ,Hiroshi Sakai1,2 , Takako Miura1,22
KEK, Tsukuba, Japan
Abstract

Table 1: Main Specifications of cERL Injector Coupler

To get the experience of power transmission through highpower coupler, experiments were conducted at cERL test
facility of KEK. The transmission of high power, up to 20 kW
in CW mode at the frequency of 1.3 GHz, was achieved withthe cERL injector coupler’s prototype. The result of the RFtests showed that while transferring 20 kW power in CW
operation, the temperature rise at some parts of the coupler
is substantial, and a maximum temperature of 1830 𝐶 wasobserved at the inner conductor of the coupler. To realize
the transmission of higher power levels in CW operation, themodifications in the existing coupler design by introductionof some cooling mechanisms or a new coupler design isessential. RF simulations of the coupler test stand were also
performed and compared with measured S parameter results.
The results of the RF test are discussed and future plan of themodification for the new design is presented in this paper.

Parameter

Specification

Central Frequency
Conditioning power at test standOperational power at cryomodule
Q𝑒𝑥𝑡
Coupler type
Number of ceramic window
RF window type

1.3 GHz
40 kW, CW
10 kW, CW
106
Coaxial, Fixed1Disk

affixed to the inner conductors at locations anticipated toexperience higher temperature conditions. The details of theexperimental arrangement, the result of the RF test, and thefuture plan for the forthcoming design are discussed in thispaper.

Coupler Assembly

INTRODUCTION
Nowadays, the efforts to develop a 100 kW class highpower input coupler for a 1.3 GHz 𝑁𝑏3 𝑆𝑛 cavity-based accelerator, are underway at KEK. Currently, KEK has a stateof-the-art input coupler i.e., the cERL injector coupler which
can transfer 20-30 kW power in CW operation to the niobium cavity-based accelerators is available. However, since
𝑁𝑏3 𝑆𝑛 cavities are required to be cooled via cryocoolers
at 4K, this prerequisite necessitates a new and innovative
design for the input coupler.
In the series of development of this new coupler design,
it is essential to understand the limitations of the current design. The high-power RF test of the cERL injector prototype
coupler was planned to realize these constraints for the powertransfer beyond the established range of 20-30 kW. This aims
to identify opportunities for potential modifications in thegeometry or a new idea for future coupler development.
Table 1 outlines the principal characteristics of the cERL
injector coupler [1, 2].

In preparation for the high-power test, the coupler’s elements such as the inner conductor, outer conductor, anddoorknob were assembled. Given that the cold segment of
the coupler had already been assembled within the controlledenvironment of a clean room, only the warm part needed
to be assembled again. Figure 1 shows the cERL injector
prototype test stand after assembly at cERL facility of KEK.

EXPERIMENTAL CONFIGURATION
For this RF test, the coupler test stand was assembled
in the cERL facility of KEK. This particular coupler test
stand had undergone prior exposure to 1.3 GHz RF power atlevels up to 30 kW during continuous wave (CW) operation.
For the present investigation, the same methodology wasreplicated. However, additional temperature sensors were∗ WORK SUPPORTED BY SPECIAL RESEARCHER SCHOLARSHIP† pragya@post.kek.jp

Figure 1: cERL injector prototype coupler test stand.
The S parameter for the coupler test stand was measured
by network analyzer and compared with the simulated S
parameter result. Figure 2 shows the comparison between- 804 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 THP57

measured and simulated S parameters. For measured data,
the central frequency of first peak was 1.289 GHz having
bandwidth of 4 MHz and for second peak, central frequency
is 1.301 GHz having bandwidth of 2 MHz whereas for simulated case, the central frequency of first peak is 1.291 GHz
having bandwidth of 3 MHz and for second peak, central
frequency was 1.306 GHz having bandwidth of 2 MHz.
Here bandwidth was calculated for S parameter (S11) below-20 dB. Evidently, the measured S-parameter findings exhibit favorable concurrence with the simulated S-parametervalues, manifesting a frequency deviation of approximately
5 MHz.

The vacuum system was installed to actively evacuate thecoupler test stand. The chiller system was connected to thewater cooling channels which cooled down the coupler’s
antenna during power transmission.

Figure 4: Arrangement of temperature sensors relative tothe magnetic field profile.

Final Setup
Figure 2: Comparison of simulated and measured S parameter.

Coupler Baking
Subsequent to the S-parameter assessment, the coupler’s
warm part was disassembled, and the cold part was bakedwith ribbon heaters. The configuration for the coupler baking
procedure is illustrated in Fig. 3. The surface temperature of
the coupler was incrementally elevated, reaching a range of100-120°C. After baking, the warm part was reassembled.

Figure 3: Configuration for the coupler baking process.

Figure 5 provides an illustrative depiction of the arrangement of the coupler test stand for the RF power test. In thissetup, a 30 kW, 1.3 GHz RF power source – specifically anInductive Output Tube (IOT) – transmit the power to the upstream coupler. The transmitted power from the downstream
coupler is judiciously absorbed by a dummy load.
The DC power supply was connected to the interlock system which further controls the operation of the IOT. Finally,
the interlock levels were set for vacuum conditions, power
parameters, arc occurrences, and water flow, collectively
safeguarding the coupler from any potential failures.

Figure 5: Schematic depiction of the coupler test stand interconnected with IOT and dummy Load. [Pf and Pb indicate
forward and backward power respectively; (𝑃𝑓0 , 𝑃𝑏0 ) signifies output power from IOT, (𝑃𝑓1 , 𝑃𝑏1 ) designates input
power to the upstream coupler, and (𝑃𝑓2 , 𝑃𝑏2 ) represents
transmitted power from the downstream coupler.]

RESULT OF HIGH POWER TEST

Connection of the sensors and chiller
A total of 17 thermocouples were affixed to the coupler
test stand based on the magnetic field profile evaluated by theCST simulations. Figure 4 shows the location of the thermocouples along with the magnetic field profile. Additionally,to avoid any potential failure resulting from discharges at
the RF window, three Arc sensors were installed.

The high-power test procedure commenced by subjecting
the coupler test stand to pulsed operation. The pulses were30 𝜇s, 100 𝜇s, 300 𝜇s, 1 ms (1% duty), 5 ms (2.5% duty),
10 ms (5% duty), 50 ms (25% duty), for power level up
to 25 kW. Subsequently, the RF test was performed in CW
operation, spanning power levels of 5 kW, 10 kW, 15 kW,- 805 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 THP57

and 20 kW. Figure 6 shows the variation in the vacuum levelwith input and transmitted power during CW operation.
A discharge event happened at the 20 kW power level– a rapid escalation in the vacuum level beyond the interlock threshold, prompting an automatic shutdown of the RFpower.

Figure 9 indicated that the temperature rise exhibits a linear correlation with the increase in power level, remaining
consistent until 20 kW during CW operation. This patternserves as a significant indication, underscoring that extending to the 100 kW level could result in a temperature ascentof approximately 800°C at the inner conductor. This, in turn,
could lead to the burned down of the coupler components
under the influence of RF power.

Figure 6: Fluctuations in vacuum level relative to input andtransmitted power during CW operation.
At each power increment during CW operation, sufficientwaiting time was allocated to observe the attainment of temperature saturation. The temperature profile at distinct locations within the coupler test stand has been represented inFig. 7, with the input power.
Furthermore, Figure 8 presents the peak temperaturereached at each specific location when operating at the 20 kW
power level during CW operation. The maximum temperature was observed at the warm part of the upstream coupleri.e., 1830 C was at the inner conductor of the warm partof the upstream coupler, whereas at the inner conductor ofdownstream coupler, the maximum temperature was 1520 C.

Figure 9: Saturated temperature at distinct location withpower level. [IC: Inner conductor, DK: Doorknob, OC:Outer conductor, U: upstream, D: Downstream]

By following the preceding RF test, it was interesting tosee the effectiveness of enforced conventional air cooling.
Therefore, a fan was introduced to facilitate the coolingprocess during the transmission of 15 kW power in CW
operation as shown in Fig. 10. This experimental setup
aimed to validate the potency of the cooling condition.
Subsequently, Fig. 11 shows the trend of the temperature
with and without the fan operation. The temperature difference with and without fan operation was 19°C a the innerconductor and 27°C at the outer conductor of the upstream
coupler and this clarify that the cooling was effective evenat the inner conductor.

Figure 7: Temperature profile at distinct locations of the coupler with input power. [IC: Inner conductor, DK: Doorknob,
OC: Outer conductor, U: upstream, D: Downstream]

Figure 8: Maxmium temperature at distinct locations at20 kW power in CW operation.

Figure 10: Configuration of coupler test stand incorporating
cooling fan system.

- 806 -Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 THP57

Figure 11: Temperature profile with and without fan operation during 15 kW CW RF transmission. [IC: Inner conductor, DK: Doorknob, OC: Outer conductor, U: upstream, D:Downstream]

Figure 13: Schematic model to effectively cool the outer andinner conductor of warm part.

FUTURE PLAN
To actualize power transmission reaching up to 100 kW for
the Nb3 Sn cavity-based accelerator system, a strategic roadmap has been devised to address two primary challenges:• Mitigating Heat Load through Superconducting
Coating: To mitigate the heat load exerted on the cavity, the plan is to apply the coating of a superconducting
material, specifically MgB2 (𝑇𝑐 = 39K), onto the RF
surface of the cold segment’s outer conductor. andmaintaining the temperature of this region below 30K.
Visual representation of this plan is furnished in Fig. 12.

Figure 12: Schematic to illustrate the coupler’s outer conductor coated by superconducting material MgB2 to mitigate
heat load to the Nb3 Sn cavity.
• Enhancing Cooling Efficiency: A concurrent initiative involves a more comprehensive redesign of thewarm portion’s inner and outer conductor. This redesign entails augmenting the dimensions to accommodate an increased number of water cooling channels.
Additionally, a cooling jacket for the outer conductor isproposed to further enhance heat dissipation efficiency.
Figure 13 shows the schematic model for this plan.
Through these strategic interventions, it is envisaged thatthe envisaged power transmission threshold of 100 kW canbe achieved with an advanced thermal management approach, ultimately advancing the capabilities of the Nb3 Sn
cavity-based accelerator system.

SUMMARY
The RF power test of the prototype cERL injector coupler
was conducted in pulse and continuous wave (CW) operation in KEK’s cERL facility. Temperature rises linearlywith the power level where the peak temperature of 183°C
was observed at the inner conductor of the warm part of thecoupler during 20kW CW operation. Extrapolating fromthe observed linearity, it can be inferred that this inner conductor could potentially reach temperatures nearing 800°C
during 100 kW CW operation. This substantial temperaturerise serves as a limiting factor, effectively restricting powertransmission capabilities beyond the 20 kW.
A more sophisticated thermal design, coupled with a robust cooling strategy, becomes a paramount necessity inorder to surmount this temperature-related constraint. Thisendeavor seeks to devise an approach that can effectively
address thermal challenges, thereby unlocking the potential
to extend the power transfer threshold up to the envisaged
100 kW mark.

ACKNOWLEDGEMENT
The author would like to thank the RF group members
for their help and support, and the iCASA group members
for the discussion. This work is supported by ”SOKENDAISpecial Researcher Scholarship (Field-specific type)”.

REFERENCES[1] Kako, E., Noguchi, S., Shishido, T., Watanabe, K., Yamamoto,Y., ”High power tests of CW input couplers for CERL injectorcryomodule”, IPAC 2012 - International Particle Accelerator
Conference, 2012, pp. 2230–2232.
[2] Kako, E., Kondo, Y., Noguchi, S., Shishido, T., Watanabe, K.,Yamamoto, Y., Hitomi, H., Sennyu, K., ”Construction of injector cryomodule for CERL at KEK”, IPAC 2012 - International
Particle Accelerator Conference, 2012, pp. 2239–2241.

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