E-Journal of Advanced Maintenance Vol.2 (2010/2011) 200-206
Japan Society of Maintenology

Future Bearing Inspection System using Portable Linac X-ray Source
Tomohiko YAMAMOTO1 and Mitsuru UESAKA2*12

Japan Atomic Energy Agency, 4002 Narita-chou, Oarai, Higashi-Ibaraki, Ibaraki, 311-1393 JapanThe University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki, 319-1188 JapanABSTRACT
We have developed an X-ray source using portable X-band (9.3-12 GHz) linear accelerator (linac) for bearing
real-time inspection. This system can generate pulsed-X-ray, therefore if X-ray repetition rate synchronizes withbearing rotation rate, we can get still images without stopping rotation. We succeeded in static imaging ofrotating equipment. We start to develop more advanced X-ray generators using portable linac and multi-beam
klystron (MBK). The advanced systems use higher electron energy linac and MBK as more stableradio-frequency (RF) source. We used 950 keV linac and 250 kW magnetron for X-ray imaging. However
oscillation is unstable because magnetron is self-excited oscillator. Moreover, its temperature and rotationinfluence its output RF (Radio-Frequency) power, electron beam current and X-ray intensity. For the purpose of
more stable operation and compactness of the system, we propose a multi-beam klystron (MBK) as its RFpower source. In this paper, we show the basic parameters of the system which stability is tolerable for bearing
condition based maintenance (CBM) in near future.
KEYWORDS

ARTICLE INFORMATION

Condition Based Maintenance (CBM), X-ray Source, X-band, Linac,
Multi-Beam Klystron (MBK)

Article history:
Received 31 January 2011
Accepted 25 February 2011

1. Introduction
Condition Based Maintenance (CBM) is one of the methods of maintenance for several kinds ofplants. Nuclear plants in Japan basically operate by Time Based Maintenance (TBM), but CBM isalso starting. Current methods of CBM are mainly vibration measurement and acoustic emission.
Recently, up to 300 kV X-ray tubes are also adopted for transmission inspection for thin (a few mm)
metal components on-site Small electron linac X-ray source of more than 950 keV is expected forthicker (more than 10 mm) components. The linac X-ray source operates for a few μsec at arbitrary
repetition rate up to 500 Hz. If the repetition rate is synchronized with the rotation frequency of apump, stacked imaging gives static image of the inner structure. If a single shot image taken ispossible by using a high sensitivity X-ray camera, even an image of randomly rotated bearing ball isavailable. Lifetime of bearings and pumps is so short that their maintenance and replacement are very
important in CBM. If small and stably operated linac X-ray source is used on-site, direct inspection ofinner bearing could be available for CBM.
In this paper, we describe the concept of the portable X-band Linac X-ray source for the purpose.

2. Components of Inspection System
2.1

Portable X-band linac

We can see a visible image using X-ray imaging at invisible space. We developed the portable
X-band linac for X-ray source. This resonant frequency is 9.4 GHz. The maximum electron beamenergy is 950 keV which can be used outside of radiation room under the Japanese law. Generated
X-rays are Bremsstrahlung so that it has broad energy spectrum [1-3].
*Corresponding author, E-mail: uesaka@nuclear.jp

ISSN – 1883 – 9894/10/11 © 2010/2011 – JSM and the authors. All rights reserved.

200T. Yamamoto et al./ Future Bearing Inspection Systemusing Portable Linac X-ray Source

We carried out experiments on single-shot imaging taking using the two X-ray detectors. We
show some imaging experimental results in Figs. 1 and 2. We used the flat-panel detector (FPD)
consisted of CsI and CMOS and High-gain Avalanche Rushing amorphous Photoconductor (HARP)
detector developed by NHK Science and Research Laboratory[4]. Table 1 shows the comparison oftwo experimental results. The HARP detector can get a more brightness images than FPD without
image correction because HARP detector is very high sensitivity. The image is obscure due toscattering and so-called buildup of low energy X-rays of less than tens keV in the targets. This effectis compensated by using thin copper filter to cut the low energy scattering X-rays. Actually, thecurrent linac has a problem of RF power oscillation in case of high electron current so that its X-ray
intensity is one thirtieth of the designed value. In order to solve the problem, we have changed thedesign of the accelerating structure from coaxial coupling to side-coupling. The improved 950 keV isgoing to be installed soon. Therefore, better spatial resolution and single shot transmission image aregoing to be realized very soon.
The rest subject is stabilization of X-ray yield by replacing the magnetron by the multi beam
klystron (MBK).

Fig. 1. Single shot image of practical bearing using 950 keV linac X-ray source and FPD

Fig. 2. Image using the HARP detector

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Table 1 Imaging Experimental Results

2.2

Multi Beam Klystron

Our X-band X-ray system uses the magnetron, which is a self-excited oscillator. It is 250 kW
magnetron used for radars, aiming at compactness and portability. However, it tends to beuncontrollable and so unstable. On the other hand, a klystron uses an RF power amplifier using input
RF. Its inner structure is depicted in Fig.3. Former and latter electrons emitted by the electron gun,consisting of the cathode and anode, are decelerated and accelerated at the input cavity so that theso-called velocity bunching occurs. The shorten electron bunch passes through and excites amplified
output RF field which goes to the linac. Power and phase controls are possible before and after theklystron and at other locations. Therefore, the stability is superior to the self-oscillated magnetron.

Fig. 3. Schematic view of klystron202

T. Yamamoto et al./ Future Bearing Inspection Systemusing Portable Linac X-ray Source

The size of the klystron system is governed by the voltage of the electron gun and its powersource. Therefore, to lower the voltage and increase the current keeping the power is very important.
In order to solve the subject, we plan to use a multi-beam klystron which has several electron gun toincrease the current and to decrease the voltage. This is the ultimate compact and stable RF source.
The important parameter of klystron is the perveance. The perveance is the beam current [A]divided the beam voltage [kV] power 3/2, shown by equation (1). The unit is [P(erveance)] andusually the micro perveance is used as,

K=I
V 3/ 2[ μP] .

(1)

RF power is shown in equation (2), where η is the effective factor and η has a scaling rule as,

P = ηIV = ηKV 5 / 2 ,(2)η ≈ 90 − 20 K [%] .

(3)

The low perveance attributes to good efficiency, as you see in equation (3). As we increase thecurrent, we can decrease the voltage following the equation (2).
The multi-beam klystron is a new type of klystron[5]. Most of commercial klystrons have asingle cathode. However, MBK has several cathodes. Furthermore, the perveance of the MBK isinfluenced to the single beam current because the perveance is decided by the single cathode.
Therefore, MBK can generate a high quality beam power.
Our target RF power is shown in red letters on Table 2. Our MBK is designed to be a eight-beamklystron. One beam power is 250 kW, therefore MBK beam power becomes 2 MW.
We have started the basic design of this MBK as shown in Figure 4. The beam tube length isabout 130 mm. The number of cavities is six. The beam voltage is 50 kV because this voltage isusually used by conventional klystron, which attributes to the low cost. We have to analyze a detailed
beam dynamics in the cavities by 2D MAGIC code and magnetic dynamics by 3D code.
Figure 5 shows the example of the calculation result. The electron beam energy is decreased inthe cavity. The RF power increases in this effect.
Figure 6 shows the example of the 3D cavity drawing. This image is 1/8 part of the single cavity.
Table 2 Comparison of magnetrons and klystrons203

T. Yamamoto et al./ Future Bearing Inspection Systemusing Portable Linac X-ray Source

Fig. 4. Schematic view of multi-beam klystron (MBK)

Fig.5. The example of calculation result

Fig.6. The example of 3D cavity drawing

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3. Future Inspection SystemWe have done some experiments of X-ray imaging on the laboratory base. We aim to use theX-ray inspection system using X-band linac and MBK. This system is a mobile inspection system atthe site as shown in Fig.7. This system can have separated linac part, RF source part and detector partand connect them to one system. These weights are less than about 30 kg, respectively. We set upthese parts at the inspection site. We can take apart this system after X-ray inspection.
This future inspection system is expected to be used at nuclear plants’ inspection for CBM.

Fig. 7. Mobile Inspection System

4. Conclusion
Our development flow chart is depicted in Fig. 8. We did basic experiments for the imaging ofrotating bearings. We have succeeded in getting static images of the rotating bearing by thesynchronization between the rotation frequency and X-ray generation repetition rate. We have started
to develop more stable X-ray inspection system using MBK. In future, X-band linac based X-ray
source is expected for CBM of rotating devices (bearing, pump etc.).

Fig. 8. Flow-Chart of Our Development

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Acknowledgement
A part of this research is funded by Japan Society of Maintenology. The authors would like tothank to Drs. K. Tanioka, and M. Kubota of NHK Science and Research Laboratory, Drs. T. Shiraishi,T. Kawai, and K. Miyakawa of NHK Engineering and Service for HARP experiment. Prof. Fukuda,and Dr. Yoshida in High Energy Accelerator Research Organization (KEK) advice the MBK design.

References[1] T. Yamamoto, Application of Portable High Energy X-ray Source for Nuclear Maintenance, Ph.D.
Dissertation, The University of Tokyo, 2009 (In Japanese)[2] T. Yamamoto, et al., Application of 950 keV Portable X-band Linac X-ray Source to Condition BasedMaintenance, Proceedings of COMADEM 2010[3] T. Yamamoto, et al., Development of 960keV X-Band Linac for Portable Nondestructive Testing System,
Proceedings of Japan Society of Maintenology 2010, (In Japanese)[4] K. Tanioka, High-Gain Avalanche Rushing amorphous Photoconductor (HARP) detector, Nucl. Inst. andMethods A, Vol.608, Issue 1, Supplement 1, pp.S15-S17 (2009)[5] A. Larionov, et al., Design of Multi-Beam Klystron in X-Band, Proceedings of the 27th Linear Accelerator
Meeting in Japan 2002206