Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA08

EFFORTS TO IMPROVE THE ASSEMBLY WORK OF SRF CAVITIES IN
THE CLEAN ROOM TO SUPPRESS FIELD EMISSION
Mathieu Omet†, Hayato Araki, Takeshi Dohmae, Hayato Ito, Ryo Katayama,
Hiroshi Sakai, Kensei Umemori, Yasuchika Yamamoto,High Energy Accelerator Research Organization, Tsukuba, JapanAbstract
Our main objective is to achieve as high as possible
quality factors Q0 and maximal accelerating voltages Eacc
within 1.3 GHz superconducting radio frequency (SRF)
cavities. Beside an adequate surface treatment, key toachieve good performance is a proper assembly in the cleanroom prior cavity testing or operation. In this contribution
we present the methods and results of our efforts to get abetter understanding of our clean room environment andthe particulate generation caused during the assembly work.
Furthermore, we present the introduced measures of documentation to support the planning, the performance, andthe analysis of the assembly work.

INTRODUCTION
Superconducting radio frequency (SRF) cavities [1] area state-of -the-art technology applied in many accelerators
around the world. The key performance parameters are theunloaded quality factor Q0 and the accelerator voltage
Eacc. Both parameters can be degraded by field emission(FE) [2]. Beside surface defects, particulate contaminations can be strong field emitters. Thus, the pollution of theinner cavity surface with particulates must be avoided. The
two sources of particulates are the environment (e.g. particulates in the clean room) and their generation during theassembly process. In the first case it is important to understand the cleanliness of the environment so that improvements can be implemented. In the second case the particulate generation mechanisms and the particulate movements
must be understood. In this proceeding we report on theeffort taken addressing these points. In addition, we reporton further measures taken to suppress FE as well as FE statistics of the five and a half years.

applied. The cavity is moved directly from the HPR standinto the clean room, where it is assembled. Afterwards thecavity is connected to a pumping station and leak checked.
Is this passed, the cavity baked e.g. at 120 °C for 48 hours[5]. In the last lest the cavity is moved to the vertical test(VT) stand and tested at 4 K and 2 K.

CLEAN ROOM SURVEY
A common method to evaluate the cleanliness of a clean
room is to setup a particle counter at different locations andheights and record the particle counts for certrain periodsof time. In the studies presented below we took a new
aproach utelizing different light sources, allowing theinspection of surfaces. In preparation of the studies described in the following, the ambient lights inside the clean
room were turned off. Since most of the clean room walls
have huge windows in them, there was still a certain levelof ambient light present.

Normal Light
For the study a strong spotlight was chosen. It was directed on different surfaces inside the class 1000 cleanroom. The inspection of the surface was done visually. For
documentation purposes pictures were taken. On mostwalls slight dust could be seen. The walls and windows
close to the entrance door showed significant contamination. In this area a hand-wash basin and an ultra-sonic bathis located. Also, the walls and windows in front of the highpressure rinsing (HPR) stand showed significant contamination (see Fig. 1). In this area the outer surface of the cavities is manually dry blown after HPR. On a table besidethe pumping stand dust and fibres could be seen. In thisarea the cavities are being baked using heater ribbons anda heater jacket.

CAVITY PREPARATION AND TESTCYCLE
After a new cavity was produced or received, the firststep is to perform an optical inspection of the inner surface
using the Kyoto camera [3]. After cleaning the cavity, it isannealed e.g. at 900 °C for 3 hours. This is followed by a
100 µm bulk electro polishing (EP) EP1 [4] and high-pressure rinsing (HPR) with ultra-pure water.
A typical cavity preparation and test cycle is as follows.
The cavity is optically inspected using the Kyoto camera.
If defects are found, they are locally grinded. After this thecavity is moved the tuning machine. On it the frequency
and field flatness are tuned. Furthermore, the cavity isstraightened, if necessary. In the next step, EP2 and HPR is____________________________________________† mathieu.omet@kek.jp

Figure 1: Window of air shower between STF class 1000
and class 10 clean room.

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October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA08

D Light
In a further study we evaluated the usability of anotherlight source: the ViEST D Light Type F provided by Sin
Nippon Air Technologies (SNK) [6]. The spectrum of thislight source is distributed around violet, reaching also into
the ultraviolet region. It allows the observation of contaminants and stains with fluorescent components. It is used incombination with dedicated set of googles for diffuse reflection suppression.
In a first test a with dust contaminated surface was observed with a stationary camera. Pictures were taken fordifferent positions and angles of the light source. The visibility of the dust was very similar for all light positions.
In a second test another contaminated surface was observed. Many bright dots were seen, which were interpreted to be dust. The surface was clean blown, wiped withalcohol, and clean blown again. After this only very few
bright dots remained. This was interpreted as a confirmation that we indeed can observe dust using the D light.
In the following 564 different surfaces in the following
areas were investigated: COI changing room, COI class1000 clean room, COI class 10 clean room, STF changingroom, STF class 1000 clean room, and STF class 10 cleanroom.
In the changing rooms at both facilities a lot of dust andstains were found. Also, the walls of both air locks with airshowers were dusty. Furthermore, on most of the surfaces
in all clean rooms dust particulates were visible. Prominentplaces were tools and tool trays. Beside this, on surfaces ofin shelfs stored items and on surfaces of devices such as
pumping stations and ultra-sonic baths, dust particulates
were present.
From this study it was concluded that cleaning of certain
surfaces was necessary. After cleaning (alcohol wiping andclean blowing) of e.g. windows within the STF class 1000
clean room, no dust particles were visible (see Fig. 2).

STUDY ON PARTICLE GENERATIONDURING ASSEMBLY
The goal of this study was to learn about the particulate
generation, distribution, and dynamics during an assembly
procedure of a 9-cell TESLA-type cavity. It was conductedtogther with employees from SNK. A green laser emmitter
was placed in the STF class 10 clean room [7]. The laser
beam as fanned out and transmitted through an area ofinterest. Particulates falling or floating through the laser
beam scattered the light. This is recorded by a narrow-bandvideo camera. In order to allow the laster operation, allwindows of the surrounding class 1000 clean room wereshaded. This measure improved at the same the signal tobackground/noise ratio.
The in the following descibed tests are only a selectionof a larger number of tests performed.
In one configuration the faned out laser beam was passedby just in front and below the input coupler port. The port
was closed with a bling flange and an O ring. During therecoding the flange, the port, and the surrounding are wereclean-blown with an ion gun. At the same time also a handheld particle counter was used. It showed in total less than10 particles. On the video the scattered light of many
particulates were seen (see Fig. 3 for a single frame). Most
particluates move quite fast and have a downwards
trajectory.

Figure 3: Scattered light of particulates passing throughfanned out laser beam.
Figure 2: Window of air shower between STF class 1000
and class 10 clean room: a) before cleaning, b) after cleaning.
It was not feasible to clean the entire clean rooms withall items in them. Thus, it has to be avoided to whirl up dust
from e.g. behind shelfs or torque wrenches, etc. Furthermore, in the clean room stored items need to be cleaned
before usage.

In the next step the blots holding the blind flange werereleased. During this work the scattered light of manyslowly moving particulates were recorded. Most of themwere floating with both upwards and downwardsmovements.
After removing the blind flange a L-shaped vacuum pipe
connected to a vacuum valve was assembled to the coupler
input port. The video recorded during the tightening of thebolts showed that many particulates were produced andreleased. These were floating in the air with a mostly
downward direction. But also particulates floating upwards
were observed.
From this study we could conclude that the hand-heldparticle counter only gives a rough idea of how many- 185 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA08

particulates are actually present in the working area. Its
small inlet nozzel is not capable of sucking in enough airto measure particulates in a larger volume. We furthermore
could confirm that it is imperative to clean the working
area properly. Also slow and careful movements can avoid
the release of particulates from the clean wear surface.

ASSEMBLY WORK PLANNING,
DOCUMENTATION AND ANALYSIS
Ensuring a consistently high assembly quality is essential not only in cavity mass production, but also during thecavity R&D process. To this end we follow the steps described in the following.

Work Planning
Typically, a week before the assembly process, a work
plan detailing the procedure is written. It is then discussed
within the workgroup and afterwards with the supportingtechnical staff. The work plan is uploaded to the cavity’selectronical logbook, so that everyone always has access toit. This way the work plan is also accessible via a tablet PC
in the STF class 1000 clean room. In case of uncertainty
during the assembly it is always possible to check on thepreviously decided plan.

Video Recording
In case of a special or unique assembly processes, videosof the assembly are recorded. These are taken by either acamera placed in the class 1000 clean room pointing
through a window into the class 10 clean room or by ahead-mounted camera. The recorded videos are stored andshared by a group-internal cloud storage. This way every
group member can review the entire assembly process.
Typically, an edited version of the video, showing only theessential assembly steps, is shown and discussed during agroup meeting. Especially the videos from the headmounted camera are very valuable, since they show themost detail.

Typically, grinding on all irises of the cavity to be assembled and tested is performed. The goal is to remove all surface defects. This is followed by an EP2. Only in rare casesnew surface defects are seen after the EP.
As a further measure to reduce field emission, the ion
gun in the STF class 10 clean room was replaced. Before
the Simco Top Gun was used. Now the Keyence SJ-L005G
is used.
Beside this the scroll pumps at the pumping station in theSTF class 1000 clean room and at the STF vertical test
stand have been replaced with dry pumps.

FIELD EMISSION STATISTICS
Improving the assembly method, required tools and devices is a continuous and lengthy process. To check theoverall effectiveness of our efforts, cavity vertical test datafrom January 2017 to August 2022 was evaluated. The key
parameters checked were the gradient, on which field emission occurred (onset), and the maximal acceleration gradient possible. In both cases only data for the final π-mode
measurement at 2 K were included. Three different typesof cavities have been tested, namely single cell, 3-cell, and9-cell cavities. Since all three types pose different kind ofchallenges, simply due to their different geometries, they
were evaluated separately.

Single-cell Cavities
The number of single-cell vertical cavity tests evaluatedis 112 (see Fig. 4). 75 tests (67.0%) did not show any fieldemission. For the remaining 37 tests (33.0%) the average
onset gradient is 27.3 MV/m. The spread of both, onset gradient and maximal acceleration gradient is quite large. In
both no real trend can be seen.

Analysis of Assembly Work
After the assembly process very divergence of the original work plan is noted down. All recorded pictures and videos are reviewed. Using all available materials, a work report is compiled, typically in form of a PowerPoint presentation. This report is then presented and discussed within
the weekly group meeting. Based on this, improvements
for future assemblies are decided. All reports are accessible
on an internal meeting webpage.

FURTHER MEASURES TO REDUCE
FIELD EMISSION

Figure 4: Maximum accelerating gradient (green bars) and
onset gradient (red triangles) over time for VTs of single
cell cavities.

3-cell Cavities

Beside the optimization of the assembly process, wehave also introduced further measures to reduce field emission.

The number of single-cell vertical cavity tests evaluatedis 14 (see Fig. 5). 7 tests (50.0%) did not show any fieldemission. For the remaining 7 tests (50.0%) the average
onset gradient is 22.7 MV/m. Both, for the onset gradientand the maximal acceleration gradient a clear positive
trend can be seen.

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Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROA08

Figure 5: Maximum accelerating gradient (green bars) and
onset gradient (red triangles) over time for VTs of 3-cellcavities.

9-cell Cavities
The number of single-cell vertical cavity tests evaluatedis 52 (see Fig. 6). 11 tests (21.2%) did not show any fieldemission. For the remaining 41 tests (78.9%) the average
onset gradient is 28.8 MV/m. Although the spread is quite
large, for both, for the onset gradient and the maximal acceleration gradient, a positive trend can be seen from May
2019 until January 2022. From February 2022 the trend
was not followed. The reason is most likely the new cavity
R&D performed in that period.

Figure 7: Comparison of single cell, 3-cell, and 9-cell cavity tests: a) percent of tests showing FE, b) average onset
gradient, c) radio between onset gradient and maximal accelerating gradient.

CONCLUSION
The suppression of field emission in SRF cavities is acomplex undertaking. The two key aspects are to remove
surface defects and to avoid contamination of the inner surface. For the first aspect an appropriate approach is irisgrinding followed by EP. The latter aspect is influenced bymany factors, e.g. the cleanliness of the environment, theparticulate production when using tools, the particulate
production within devices, and the assembly quality. Weare continuously evaluating and improving our procedures
and tooling to avoid or mitigate possible surface contamination.

ACKNOWLEDGEMENTS
The authors thank all the technical staff, which supported various aspects of the efforts to suppress field emission.

REFERENCES

Figure 6: Maximum accelerating gradient (green bars) andonset gradient (red triangles) over time for VTs of 3-cellcavities.

Comparison
With the number of cells per cavity, the number of vertical tests showing field emission is increasing (see Fig. 7 a)).
At the same time the average onset gradient as well as theratio of the onset gradient and the maximum accelerating
gradient decrease with the number of cells per cavity (seeFig. 7 b) & c)). The reason for this is most likely the increased inner surface area as well as the complexity of thecavity shape. E.g. single-cell cavities have less ports (only2) compared to 9-cell cavities (6 ports plus complicatedstructures such as the higher order mode (HOM) pickups).

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