SPECTROGRAPHIC APPROACH FOR DIAGNOSING RF BREAKDOWNIN ACCELERATING RF STRUCTURES
H. Tomizawa1, T. Taniuchi1, H. Hanaki1, Y. Igarashi2, S. Yamaguchi2, A. Enomoto2
Accelerator Division, Japan Synchrotron Radiation Research Institute (SPring-8)
1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan1
High-Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan2

Abstract
The acceleration gradient of an electron linac is limited
by rf breakdown in its accelerating structure. We apply an
imaging spectrograph system to study the mechanism of
rf breakdown phenomena in accelerating rf structures.
Excited outgases from the surface emit light during rf
breakdown, and the type of outgases depend on surface
treatments and rinsing methods for their materials. To
study rf breakdown, we used 2-m-long accelerating
structures and investigated the effects when high-pressure
ultrapure water rinsing (HPR) treatment is applied tothese rf structures. We performed experiments to study
the outgases emanating from the surface of rf structureswith quadruple mass spectroscopy and imaging
spectrography of atomic lines. As a result, we could
observe instantly increasing signals at mass numbers of 2(H2), 28 (CO), and 44 (CO2), but not 18 (H2O) just after
the rf breakdown. We also conduct spectral imaging forthe light emissions from the atoms and ions in a vacuum
that are excited by rf breakdown. Using an accelerating
structure without HPR treatment, we observed the atomic
lines of outgases at 511 nm (Cu I), 622 nm (Cu II), and711 nm (C I). With HPR treatment, the atomic lines wereobserved at 395 nm (O I), 459 nm (O II), 511 nm (Cu I),
538 nm (C I), 570 nm (Cu I), 578 nm (Cu I), 656 nm (H:Lyman alpha), and 740 nm (Cu II). In additional surface
analysis, it was found that carbon is the most dominantelement, with the exception of copper, on the blackened
surface of the rf-conditioned accelerating structure
without HPR treatment. According to these experiments,
we understand that some components of the plasma canaffect a copper surface. We also provide aphenomenological review of our experimental results anda simple explanation of rf conditioning with rf breakdown.

1 INTRODUCTION
Since particle accelerators were first developed, they
have included rf structures to accelerate charged particles
due to the fact that accelerating rf structures aredemanded for higher-gradient acceleration. The
theoretical threshold of rf breakdown derived from thewell-known Kilpatrick criterion limits the maximum
accelerating field in rf structures. However, under thiscriterion, only hydrogen was taken into account as aresidual gas in rf structures. In reality though, many typesof gases may exist as residual gases in a vacuum due tosurface treatments and rinsing methods for their materials.
A range of contaminants and micron-sized particles may
remain on a surface following different treatments. Those
contaminants are field emitters, and outgassed due to therf breakdown in rf structures [1, 2]. According to theworks of J. Knobloch and P. B. Wilson, the less outgas
and the fewer the contaminating particles, the lower the rf
breakdown rate. They proposed the mechanism ofgenerating surface damage with a Helium-adding
experiment [1] and a simple model[2]. Their explanations
are as follows. An rf breakdown generates the outgas
plasma and copper vapour close to the copper surface.
This effect excavates a crater by surface heating andmelting due to an intense bombardment of the generated
plasma ions accelerated by the plasma sheath that
separates the body of plasma from the copper surface.
The process mentioned above is a mechanism ofgenerating surface damage with craters.
It has been reported that the high-pressure ultrapure
water rinsing technique (HPR) is very effective in
improving the field gradients for normal conducting andsuperconducting rf structures [3, 4] because HPRtreatment eliminates particle contamination on the surface,which is thought to be one of the causes of field emission.
We have a plan to apply this technique to the S-band 2-mlong disk-loaded accelerating structure (see Figure 1).
To study rf breakdown, we use 2-m-long accelerating
structures as tested rf structures. Microsecond-pulsed rf
power (average accelerating field of 45 MV/m at
maximum) is fed into these rf structures. We performedexperiments using quadruple mass spectroscopy andimaging spectrography of atomic lines to study theoutgases emanating from the surface of rf structures. Thisimaging spectrograph system is useful for observing
irreproducible phenomena such as rf breakdown.
Condensed gas and electrons have important roles intriggering the formation of aggressive plasma that acts
against the copper surface. In our work, excited neutral
and ionised gases in this plasma were observed as atomic
lines in the real accelerating structures. Additionally,using XPS, we analysed blackened copper surface of rfconditioned accelerating structure due to rf breakdown.
In this paper we report the phenomenological review ofour experimental results and a simple picture of rf
conditioning with rf breakdown.

2 EXPERIMENTAL SET-UP
2.1 Tested Accelerating structure
We performed experiments using the high-power test
stand at KEK. We chose the two S-band 2-m-accelerating
structures with the following fabrication processes:
A. A high-precision turning lathe with a diamond byte
machines the accelerating structure’s disks andcylinders.
B. The electroplating fabrication method is applied to fix
the disks and cylinders together.
C. High-pressure ultra-pure water rinsing (HPR) is
applied.
Note that these accelerating structures have a crescentshaped cut opposite the waveguide iris (see Figure 1) tocorrect the asymmetry of the electromagnetic fields incoupler cavities.
For comparison, we prepared two different processed
accelerating structures with or without HPR treatment( process C). One of the accelerating structures was rinsed
while moving vertically (60 mm/min), orbiting (6.5 rpm)
and spraying the pipe with nozzles that jet high-pressure
ultra-pure water under optimal conditions [5, 6].
Following to the HPR process, a scroll pump immediately
dried the structure, and valves were attached in a cleanroom. After that, it was evacuated sufficiently by a turbo
molecular pump and inserted into a high-power test stand.
It is not exposed to the atmosphere after assembly in theclean room.

Figure 1: Schematic drawing of the accelerating structure

2.2 Imaging Spectrograph System
2.2.1 Spectrometer and I.I.-camera
The imaging spectrograph monitor consists of threeparts: a spectrometer (SPECIM; ImSpector), an image
intensifier (SWAROVSKI; MK500) and a CCD-camera(SONY; Model XC-7500).
The spectrometer was used as an imaging spectrographthat captures a line image and disperses it to a spectrum ina direction orthogonal to the line image. This optics isbased on the Prism-Grating-Prism (PGP) technique,which makes a direct-sight imaging spectrograph inwhich light passes through a straight optical axis. The
above configuration offers a combination of several
beneficial features:
An astigmatism-free image is available even with a short
focal length.

High-efficiency holographic volume diffraction grating
and a low f-number produce high throughput.
No reflective surfaces on the optics. The throughput isimmune to polarization.
After an image converts to a spectral line image, theimage intensifier (I.I.) intensifies the light on the image
more than 104 times, making it possible to take aspectrograph of a light emitter with low brightness. The
CCD camera then changes intensified spectral line images
into video frames with a repetition rate of 30 pps.
All image data were recorded on digital videotapes,
because rf breakdown is unpredictable. Each frame
contains the line pixels in one dimension (spatial axis)
and the spectral pixels in the other dimension (spectralaxis), providing full spectral information for each line
pixel. It was possible to take spectral information over theentire visible region (380 - 780 nm). The absolute
wavelength was calibrated with a He-Ne laser (633 nm)
and a mercury lamp (shortest spectral line: 404 nm),while the resolution and geometrical resolution of thisspectrograph system were 8.9 nm and 2.4 nm/dot,
respectively, over the measured spectral region. Note that,
if two spectral lines are separated by more than 8.9 nm,
the wavelength accuracy of the spectral peaks is 2.4 nm.

Figure 2: Spectrometer and I.I.-camera for the imaging
spectrograph2.2.2 Data analysing System
The system of data acquisition and analysis was
specially developed for this imaging spectrograph. The
vertical and horizontal axes of imaging spectrograph(upper right in Figure 3) represent spatial position andwavelength, respectively. This system enables us toanalyse raw video data with a Windows-XP computer(CPU: 2.8 GHz-Pentium4, RAM: 1 GB, Video captureboard: Matrox CORONA2), and in addition, it can takedata directly form a CCD-camera or a video player andanalyse them in real time. It automatically searches video
frames for rf breakdown phenomena. The frames are
displayed on the screen in the “Random Access” section(see the centre-right in Figure 3). The selected frameswith rf breakdown are then sorted into two different
groups: “Luminescence Frame” and “Projection Frame”.
Those frames belonging to the “Luminescence Frame”

group are frames that have one pixel that is brighter than
the threshold level. The frames belong to the other group,“Projection Frame”, are those that have an integrated
pixel brightness along the vertical axis greater than thethreshold level. Both thresholds are determined bybackground noise on the video images. In this experiment,
most of the “Luminescence” and “Projection” frames
were full of radiation noise and light emission in the rf
breakdown phenomena, respectively. We chose the framesof the “Projection” group and studied their spectra.

Figure 3: GUI (on Windows XP) of the data analysing system
for imaging the spectrographbecause the surface current densities at these locations are
10 times higher than other surfaces [6]. We often
observed some blacked area damaged by rf breakdown inthis crescent-shaped cut.
The main difficulty to utilize this optics is how thehomogenised laser profile transports toward the cathodesurface with focusing. Even if the whole wave front oflaser does not reach on the cathode at the same time, thelaser spot on the surface should be in the depth of a focus.

Figure 4: High-power test stand at KEK: The camera system isinstalled downstream to take spectrographs of light emitted due
to rf breakdown.

2.3 Experimental Setup in High-power test
stand
We constructed a test stand for high-power tests on theaccelerating structures (Figure 4). A klystron with amaximum peak rf power of 45 MW was used, and a
SLED-type pulse compressor to generate a high-power rf
pulse was installed in the stand. The pulse width andrepetition rate of the rf pulse were 4.0 µsec and 50 Hz,
respectively, and the klystron output power level was
gradually increased. The reflection of the rf power fromthe accelerating structure, or waveguides, and the vacuum
pressure of the ion pumps (IP in Figure 5) were used as aninterlock to stop the rf power. The vacuum pressure of theaccelerating structure was maintained at around 1⋅10-6 Pa
during rf conditioning.
We installed a quadruple mass spectroscopy (QMS in
Figure 5) and an imaging spectrograph system (S in
Figure 5) to study the outgases released from the surfaceof accelerating structures during rf conditioning. We
measured the outgases due to rf breakdown with aquadruple mass spectroscopy (ABB Extrel: Type MAX200) fitted with four 19-mm diameter mass filter poles;
such a large pole diameter was chosen for its higher
sensitivity.
We additionally installed the imaging spectrographcamera to directly view the crescent-shaped cuts in the
coupler cavities. We supposed that these were some of themost likely locations for rf breakdown to frequently occur,

Figure 5: Rf breakdown diagnostic set-up for accelerating rf
structure at KEK (KLY: klystron, BH: Bethe hole coupler, IP:
ion pump, W: rf-window, SLED: SLED-type pulse compressor,
PE: Penning vacuum gauge, SC: screen monitor, FC: Faraday
cup, QMS: quadruple mass spectrometer, AM: analyser magnet)

3 EXPERIMENTAL RESULTS
3.1 Spectrographs of the accelerating structure
without HPR
The initial test was carried out in the accelerating
structure but without HPR treatment. After the rf
conditioning, an average accelerating field of 40 MV/m
was obtained after 1.0⋅108 shots (542 hours). Also, thehigh-power test was terminated due to a time limit. The
system experienced 4559 trips due to interlocks with
VSWR, which measures rf reflection due to rf breakdown

in the accelerating structures. An imaging spectrographand the light spectrum of light emission on an rf
breakdown are shown in Figure 6 and 7, respectively.
Figure 7 clearly shows the atomic lines from outgassed
atoms at 711 nm (C I), and evaporated copper at 515 nm(Cu I) and 622 nm (Cu II).

respectively. Figure 8 shows the atomic lines at 395 nm(O I), 511 nm (Cu I), 570 nm (Cu I), 578 nm (Cu I), 740
nm (Cu II), and the most dominant lines at 459 nm (O II)
and 656 nm (H: Lyman alpha). In Figure 9 just 538 nm(C I), 656 nm (H: Lyman alpha) and 740 nm (Cu II) wereobserved; after rf conditioning (elapsed time: 677 hours),
we observed only these three lines. On examining rf
breakdown over three frames, we found that the 740-nmline glows first, and then the 656-nm-line glows in thenext frame.

Figure 6: Spectrograph of light emission of rf breakdown in theaccelerating structure without HPR treatment (After rf
conditioning; elapsed time: 631 hours; average accelerating
field: 30 MV/m)
Figure 8: Spectrograph of the accelerating structure with HPRtreatment (During rf conditioning; elapsed time: 307 hours;
average accelerating field: 42 MV/m)

Figure 7: Light spectrum of light emitted due to rf breakdown

3.2 Spectrographs of the accelerating structurewith HPRThe next test was carried out using the accelerating
structure with HPR treatment (average water pressure: 5.0
MPa). After the rf conditioning, an average accelerating
field of 45 MV/m was obtained after 6.4⋅107 shots (356
hours). Although this gradient was limited by the klystron
output power. The system experienced 1894 trips due tointerlocks with VSWR. Figures 8 and 9 show two
imaging spectrographs at elapsed times of 307 (during rf
conditioning) and 677 (after rf conditioning) hours,

Figure 9: Spectrograph of the accelerating structure with HPRtreatment (After rf-conditioning; elapsed time: 677 hours;
average accelerating field: 45 MV/m)

3.3 The results of analysis on outgases andsurface of the accelerating structures
As a result of the outgas analysis, we could observe
instantly increasing signals at mass numbers of 2 (H2), 28(CO), and 44 (CO2), but not 18 (H2O) (see Figure 10) justafter rf breakdown in the accelerating structures. The
mass spectrum without rf power is not significantly
different form the spectrum before rf breakdown.
In addition, an XPS surface analysis was performed onblackened areas of the crescent-shaped cuts in the couplercavities (without HPR treatment), and it was found thatcarbon (graphite) was the most dominant element, except
copper, on the blackened surface of rf-conditioned
surfaces.

Figure 10: Experimental results of Q-mass: The datum (red solidline) was taken at the same time as the imaging spectrographshown in Figure 8. The datum (blue solid line) is the mass
spectrum before this rf breakdown. The mass spectrum without
rf power is not significantly different form the spectrum before
rf breakdown.

copper surface of the accelerating structure could become
almost oxygen-free, Since an oxidized layer is insulator,
ion bombardments can be concentrated on the oxidized
layer. Consequently, after rf conditioning, oxygen lines
were not observed.
Comparing the two accelerating structures with different
processes, we found that the structure using the HPRprocess reached 45 MV/m with an rf-conditioning period
1.5-times shorter, a lower rf breakdown rate, and a lower
dark current than the one that didn’t (see reference [6] formore details). Furthermore, the results indicate that the
HPR treatment and rf conditioning process help eachother to remove contaminants from the surface.
We observed blackened areas damaged by rf breakdown
in these crescent-shaped cuts of the rf conditioned
accelerating structures with and without HPR treatment.
Therefore, we investigated elements of a blackened areaon the surface of an rf conditioned accelerating structure
without HPR treatment as an example. As a result, themost dominant element, with the exception of copper, on
the blackened surface was carbon (graphite). We
supposed that carbon ions in plasma recombine withelectrons on the copper surface due to the much higher
electron density and low electron temperature in thematerial than in the vacuum. Fortunately, graphite is aconductor. Therefore, if the surface is ideally smooth, itcannot emit a field to initiate rf breakdown.

5 ACKNOWLEDGMENT
4 SUMMARY
We performed experiments to study the outgases
emanating from the surface of rf structures by employing
quadruple mass spectroscopy and imaging spectrographyof atomic lines. This spectrographic method proved to beparticularly useful for observing irreproducible
instantaneous phenomena such as rf breakdown.
Generally, in both of the accelerating structures, we could
observe increasing quantities of H2, CO, and CO2
molecules immediately after the rf breakdown. Thesemolecules could either emanate from the surface or be
generated in recombination plasma induced by rf
breakdown. According to the spectrographs of thisrecombination plasma, we mainly observed atomic lines
of evaporated copper. Using an accelerating structure
without HPR treatment, we observed that the atomic
carbon line was the most intense, but no oxygen was
detected. On the other hand, with HPR treatment, theatomic oxygen and hydrogen (the most intense) lines
were observed in the early stages of rf conditioning; infact, micro-sized carbon dust particles were present.
According to our experimental results, HPR treatment
can remove dust particles, although it causes different
contaminant layers to form on the copper surface of theaccelerating structure. One of these layers could be a thinoxidized layer, because an oxygen line glows in the earlystages of rf conditioning, but not after rf conditioning.
This indicates that by the end of rf conditioning, theThe authors wish to thank the members of the KEKB
Injector for their help. We also acknowledge Dr. K. Saito
for advice about the HPR process. In particular, we offer
our deepest thanks to the members of the development
team at KAWASAKI STEEL Techno-research
Corporation. Messrs. S. Moriya, T. Akiyama, H. Fukuda
developed and calibrated the imaging spectrographsystem.

6 REFERENCES[1] J. Knobloch, “Advanced Thermometry Studies ofSuperconducting Radio-Frequency Cavities”, Ph.D.
thesis, Cornell University, August 1997.
[2] P.B. Wilson, Proceedings of the 20th Int. Linear
Accelerating Conference, 21-25 August 2000,
LINAC2000, Monterey, pp. 618-620.
[3] K. Saito, H. Miwa, K. Kurosawa, P. Kneisel, S.
Noguchi, E. Kako, M. Ono, T. Shishido, and T.
Suzuki, Proceedings of the 6th Workshop on
Superconductivity, 4-8 October, 1993, CEBAF,
Newport News, USA, pp. 1151-1159.
[4] M. Yoshioka, H. Matsumoto, Y. Takeuchi, K. Saito, H.
Akiyama, E. Tanabe, K. Nishitani, H. Miwa and T.
Suzuki, Proceedings of the 17th International Linac
Conference, 1994, Tsukuba, Japan, pp. 302-304.
[5] Y. Igarashi, S. Yamaguchi, A. Enomoto, T. Oogoe, K.
Kakihara, and S. Ohsawa, Proceedings of the 21th Int.

Linear Accelerator Conference, 19-23 August, 2002,
Gyeongju, Korea, in printing.
[6] Y. Igarashi, S. Yamaguchi, A. Enomoto, T. Oogoe, K.
Kakihara, S. Ohsawa, H. Tomizawa, T. Taniuchi, and
H. Hanaki, Proceedings of the 2003 Particle
Accelerator Conference, 12-16 May, 2003, Portland,
Oregon, in printing.