IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009 1495

Energy Spectra of Bremsstrahlung X-Rays EmittedFrom an FRP Insulator

Yutaka Tanaka, Yoshitaka Ikeda, Masaya Hanada, Kaoru Kobayashi, Masaki Kamada, Masashi Kisaki,Noboru Akino, Yasushi Yamano, Shinichi Kobayashi, and Larry R. Grisham

Abstract—Energy spectra of X-rays emitted from the surface ofa fiberglass-reinforced plastic (FRP) insulator were measured atthree different positions and compared with those of the vacuumgap between electrodes. Near the anode, the X-ray spectrum wasdominated by monoenergetic electrons. Near the cathode, thespectrum peak shifted to low energy as compared with that nearthe anode. This result showed that a large amount of low-energyelectrons was generated on the surface of the FRP insulator nearthe cathode.

Index Terms—Electrodes, flashover, vacuum insulation, X-raymeasurement.

I. INTRODUCTION

A NEUTRAL beam injector (NBI) is utilized for plasmaheating and plasma current drive in fusion applications.

On JT-60, a high-energy neutral beam is generated through theneutralization of D− ions, which are produced by negative-ionaccelerator. The accelerator is composed of grids with multi-apertures and large insulators [1]. Three fiberglass-reinforcedplastic (FRP) columns are used as insulators. Size of the FRPis 1.8 m in inner diameter and 31.5 cm in height. Althoughacceleration voltage was designed to be 500 kV, it was limitedto be 460 kV by breakdowns of the accelerator. Therefore,it is required to improve the voltage-holding capability ofthe accelerator. In our previous study, it was found that thevoltage-holding capability was correlated with the light in-tensity emitted from the surface of the FRP insulator in theaccelerator. Spectroscopic measurement of the light showedthat the light was originated by cathodoluminescence of theFRP [1]. These results show that voltage-holding capabilityof the JT-60 negative-ion accelerator is dominated by surfaceflashover on the large FRP insulator.

Vacuum surface flashover on ceramic insulators has beenstudied by many researchers. However, on an FRP insulator,the process of surface flashover has yet not been clarified. The

Manuscript received November 1, 2008; revised February 19, 2009. Firstpublished June 23, 2009; current version published August 12, 2009.

Y. Tanaka, Y. Ikeda, M. Hanada, K. Kobayashi, M. Kamada, and N. Akinoare with Japan Atomic Energy Agency, Naka 311-0193, Japan (e-mail: tanaka.yutaka@jaea.go.jp).

M. Kisaki is with the Graduate School of Engineering, Tohoku University,Sendai 980-8579, Japan.

Y. Yamano and S. Kobayashi are with the Graduate School of Science andEngineering, Saitama University, Saitama 338-8570, Japan.

L. R. Grisham is with Princeton Plasma Physics Laboratory, Princeton,NJ 08543 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2009.2020402

Fig. 1. Schematic view of (a) experimental apparatus and (b) X-ray spectrummeasurement.

surface-flashover process is strongly dependent on the energyof the electrons emitted from the cathode triple junction (CTJ).Therefore, the measurement of the electron energy is a keyin understanding surface flashover on FRPs and, hence, toimprove the JT-60 negative-ion accelerator. However, experi-mental results on the electron energy on the surface flashoverare minimal because direct measurement of electron energy isvery difficult.

To overcome this difficulty, the energy spectrum ofbremsstrahlung X-rays from an FRP insulator and the anodeelectrode were measured. Since electron trajectories are depen-dent on the state of the charge on the FRP insulator surface,the energy spectrum of the X-rays is measured for differentpositions on the FRP insulator by using a collimator. Thispaper presents the X-ray energy spectra emitted from the FRPinsulator for three different positions.

II. EXPERIMENTAL SETUP

A schematic view of experimental apparatus is shown inFig. 1(a). The sample FRP insulator was installed in a largeepoxy tube used as a vacuum chamber [2]. Inner diameter andheight of the epoxy tube were 630 and 1000 mm, respectively.A turbo molecular pump, whose pumping speed was 500 L/s,

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1496 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009

Fig. 2. Flashover voltage as a function of flashover number. FRP lengths were10, 30, and 100 mm.

was used to evacuate the chamber. Background pressure inthe chamber was 6 × 10−4 Pa. The sample FRP insulator wassandwiched between a pair of Rogowski electrodes made ofoxygen-free copper. DC high voltage was applied from a dcpower supply, which had a capability to generate 300 kV. Acurrent-limiting resistor of 200 kΩ was connected in series to ahigh-voltage line to suppress breakdown and flashover current.Dark current was measured by a shunt resistor of 2 kΩ.

The sample FRP insulators, which had a diameter of 40 mmand lengths of 10, 30, 100 mm, were tested. The emitted X-rays were detected by a cadmium telluride detector, which wasplaced at 1200 mm from the sample, as shown in Fig. 1(b).Electrical pulses were generated by X-ray photons on thedetector. Energy of the X-ray was analyzed using these pulsesby an MCA (SEIKO EG&G MCA7600). To avoid the deadtime of the MCA due to high X-ray flux and to enhance thespatial resolution of the detector, a collimator was placed justin front of the detector. The material and dimensions of thecollimator are lead and 3 mm in diameter and 10 mm in length.For three different heights, the energy spectra of the X-ray weremeasured.

III. FLASHOVER VOLTAGE CHARACTERISTIC

Surface flashover characteristics of the FRP insulator wereexamined for three different lengths: 10, 30, and 100 mm.Fig. 2 shows flashover voltage as a function of the number offlashovers. For a length of 100 mm, the initial flashover voltagewas about 120 kV. Flashover voltage gradually increased andsaturated at about 180 kV after 40 flashovers. This resultshows that voltage-holding capability increased 50% by theconditioning. On the other hand, in the case of 10 and 30 mm,flashover voltage decreased after several flashovers. This isprobably due to damage of the FRP surface, since high darkcurrent was observed.

Maximum voltages were plotted as a function of the FRPlength, as shown in Fig. 3. For comparison, breakdown voltageof a vacuum gap was also obtained in the same facility andplotted. Voltage-holding capability for the FRP insulator wasclearly lower than that of vacuum gap. With a 10-mm length,the flashover voltage was less than 1/3 of the vacuum break-down voltage. Curve fits shown in Fig. 3 were found using aleast-square method. Flashover voltage for the FRP insulatorvaried as V = 18d0.51, where V was in kilovolts and d was inmillimeters.

Fig. 3. Comparison of the flashover voltage (FRP) and the breakdown volt-age (GAP).

Fig. 4. Flashover photos obtained for (a) FRP of 100 mm in length and appliedvoltage of 123 kV and for (b) FRP of 10 mm in length and applied voltage of48 kV. Lines indicate surfaces of the electrodes and the FRP insulator.(c) Visible-light spectrum.

To understand the surface flashover on the FRP insulator, thelight emitted by the surface flashover and its spectrum weremeasured. Fig. 4(a) shows a photo of the surface flashoverobtained from an FRP of 100 mm in length and with an appliedvoltage of 123 kV. Lines in the photograph indicate the positionof the electrodes and the FRP insulator. Blue light was clearlyobserved in the broad area of the FRP surface. This light is verybright at the contact point between the FRP and the anode.Similarly, blue light was observed in the case of an FRP oflength 10 mm and an applied voltage of 48 kV, as shown inFig. 4(b). In this case, the light was emitted from right and leftsides of the FRP, and intensity of the light was stronger thanthat obtained from an FRP of length 100 mm. Electrodes areconnected by this light. To make clear the difference betweenboth flashovers, these lights were analyzed by a spectrometer,as shown in Fig. 4(c). There were sharp spectral lines anda broad spectrum. A broad spectrum was observed in therange of 300–700 nm. A similar spectrum was observed ascathodoluminescence at 300–800 nm when the FRP insulatorwas irradiated by a 3-keV electron beam [3]. Therefore, thisresult shows that electrons hit the FRP, and cathodoluminescentlight from the FRP is emitted. In addition to a broad spectrum,sharp spectral lines of C+, C2+, O+, and H0 (CII, CIII, OII,

TANAKA et al.: ENERGY SPECTRA OF BREMSSTRAHLUNG X-RAYS EMITTED FROM AN FRP INSULATOR 1497

Fig. 5. Typical time evolution of (a) dark current, (b) visible-light intensity,(c) gas pressure, and (d) X-ray intensity during FRP conditioning. Appliedvoltage was 180 kV.

and Hα) were also observed. These spectra originate from thedischarge of the gas which is desorbed by electrons hitting theFRP, because the surface of the FRP insulator is covered withepoxy resin composed of carbon, oxygen, and hydrogen. Asharp Cu+ spectral line (CuII), which is observed only withan FRP length of 10 mm, shows that copper vapor originatingfrom electrodes is produced by the flashover. Although thesesharp lines show the discharge of the desorbed gas, the pressurein the chamber was < 7 × 10−4 Pa during the surface flashoverand much less than the pressure required for a glow discharge(> 10−1 Pa). The discharge of the desorbed gas indicates thatthe pressure on the FRP surface is > 10−1 Pa.

IV. X-RAY EMISSION FROM FRP

Energy of electrons hitting FRP can be estimated from theenergy spectrum of the X-ray. However, they were very difficultto measure during the surface flashover because the timescaleof the flashover was on the order of a microsecond. Therefore,the X-rays were measured during the vacuum conditioning ofthe FRP insulator because X-rays were emitted continuously.Fig. 5 shows a typical time evolution of dark current [Fig. 5(a)],visible-light intensity [Fig. 5(b)], gas pressure [Fig. 5(c)], andX-ray intensity during FRP conditioning [Fig. 5(d)]. Theapplied voltage was 180 kV, which was 2–3 kV less than theflashover voltage. The FRP length was 100 mm. Sharp spikeswere observed in the dark current, indicating the generation ofan electron avalanche. It repeated at intervals of about 100 s.Visible light was observed, and gas pressure increased togetherwith the dark current. This light emission is considered to be acathodoluminescence and/or a microdischarge, which is similarto a flashover. X-ray intensity measured at ∼1 m from the FRPsample also increased together with dark current. Since thesecharacteristics are similar to surface flashover, the process of

Fig. 6. (a) X-ray spectra measured near the anode (H = 24 mm), betweenelectrodes (H = 52 mm), and near the cathode (H = 81 mm). (b) X-rayspectra normalized by (1). For comparison, X-ray spectrum of a gap wasplotted.

X-ray emission is expected to be the same as that of surfaceflashover.

The energy spectra of the X-rays were analyzed by pulse-height analysis. To avoid the dead time of an MCA, the incom-ing flux to the MCA was limited by the collimator. The useof the collimator also allowed measurement with high spatialresolution because the solid angle of the detector was signif-icantly reduced. Therefore, the energy spectrum of the X-raycan be measured for different positions on the FRP insulator.X-ray spectra were measured near the anode (H = 24 mm),between the cathode and the anode (H = 52 mm), and nearthe cathode (H = 81 mm), where H is the height from theanode, as shown in Fig. 1(b). Applied voltage and FRP lengthwere 180 kV and 100 mm, respectively. Fig. 6(a) shows X-rayphoton count per 1 s. Measurement time of each X-ray spectrumwas 200–600 s. Dead time was less than 1 s, which wasnegligibly small as compared with the measurement time. Theenergy spectrum of the X-rays had a peak at an energy of lessthan 100 keV. Near the cathode, it is seen that the spectrum peakshifts to low energy as compared with the peak of the othertwo spectra. The peak energy may be determined by theelectron energy. Then, the shift indicates the difference in theelectron energy. Maximum energy of the spectrum was less than180 keV, which corresponded with the maximum energy of theelectrons. Near the cathode, the number of X-ray photons waslower than that in the other positions. Since X-rays are inducedby hits of electrons emitted from the CTJ, the difference inX-ray photons is due to the difference in the number and energyof the hitting electrons.

To compare the spectra clearly, the spectra were normalizedby total number of the X-ray photons integrated by the energy,i.e., as follows:

Nor. X-ray(E) =X-ray(E)

∫ 180keV

40keV X-ray(E) dE(1)

1498 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009

where the energy range of the integration was selected from 40to 180 keV. To exclude the thermal noise of the detector at lowX-ray energy, the minimum energy taken for the integration waslimited to 40 keV. Fig. 6(b) shows the normalized X-ray spectra.The X-ray spectra were also compared with that of the vacuumgap without the FRP. Near the anode, the spectrum was similarto that of the vacuum gap. Since the X-rays from the vacuumgap are expected to be induced by the monoenergetic electronscorresponding to the voltage difference between the electrodes,the X-ray spectrum of the FRP insulator near the anode isdominated by the monoenergetic electrons. Near the cathode, itwas clearly seen that the spectrum peak was 10–20 keV lowerthan that near the anode. This shift suggests that the low-energyelectrons induce the X-rays near the cathode. Since the X-rayphotons are proportional to the square of electron energy [4],a large number of low-energy electrons are generated near thecathode. Therefore, it is concluded that X-rays are emitted fromthe FRP surface near the cathode. The generation of low-energyelectrons suggests that the electrons emitted from the CTJ hitthe surface before full energy acceleration. These results showthat the surface flashover on an FRP insulator is characterizedas low-energy electrons hitting the FRP and the discharge of thegas desorbed by the electrons.

In this experiment, it was indicated that the X-ray energyspectrum of the FRP insulator was different from that of thevacuum gap between the grids. By measuring the X-ray energyspectrum emitted from the JT-60 negative-ion accelerator, theorigin of the breakdown, namely, the dominant process of thebreakdown, can be identified. This could lead to the improve-ment of the voltage-holding capability of the JT-60 negative-ionaccelerator.

V. SUMMARY

Energy spectra of the X-rays emitted from the surface ofan FRP insulator were measured at three different positionsand compared with those from a vacuum gap. Near the anode,the X-ray spectrum of the FRP insulator was dominated bymonoenergetic electrons. Near the cathode, the X-ray spectrumpeak was lower than that near the anode. This result showed thata large number of low-energy electrons were generated. X-rayswere emitted by these electrons from the surface of the FRPinsulator near the cathode.

ACKNOWLEDGMENT

The authors would like to thank other members of JT-60UNBI Group for their valuable discussion. They would also like

to thank Dr. T. Tsunematsu and Dr. N. Hosogane for theirsupport and encouragement.

REFERENCES

[1] M. Hanada, Y. Ikeda, M. Kamada, K. Kikuchi, M. Komata, K. Mogaki,N. Umeda, and K. Usui, “Correlation between voltage holding capabilityand light emission in a 500 keV electrostatic accelerator utilized for fusionapplication,” IEEE Trans. Dielectr. Elect. Insul., vol. 14, no. 3, pp. 572–576, Jun. 2007.

[2] K. Watanabe, M. Mizuno, Y. Ohara, M. Tanaka, K. Kobayashi,E. Takahashi, and T. Ueda, “DC voltage holding experiments of vacuumgap for high-energy ion sources,” J. Appl. Phys., vol. 72, no. 9, pp. 3949–3956, Nov. 1992.

[3] Y. Yamano, M. Takahashi, S. Kobayashi, M. Hanada, and Y. Ikeda, “Sur-face discharge related properties of fiberglass reinforced plastic insulatorfor use in neutral beam injector of JT-60U,” Rev. Sci. Instrum., vol. 79,no. 2, p. 02A 524-1, Feb. 2008.

[4] R. D. Evans, The Atomic Nucleus. Melbourne, FL: Krieger, 1985, p. 615.

Yutaka Tanaka, photograph and biography not available at the time ofpublication.

Yoshitaka Ikeda, photograph and biography not available at the time ofpublication.

Masaya Hanada, photograph and biography not available at the time ofpublication.

Kaoru Kobayashi, photograph and biography not available at the time ofpublication.

Masaki Kamada, photograph and biography not available at the time ofpublication.

Masashi Kisaki, photograph and biography not available at the time ofpublication.

Noboru Akino, photograph and biography not available at the time ofpublication.

Yasushi Yamano, photograph and biography not available at the time ofpublication.

Shinichi Kobayashi, photograph and biography not available at the time ofpublication.

Larry R. Grisham, photograph and biography not available at the time ofpublication.