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Current Applied Physics 13 (2013) 819e825

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Preliminary result of an advanced tangential X-ray pinhole camerasystem with a duplex MWPC on KSTAR plasma

Siwon Jang a, Sang Gon Lee b, Chang Hwy Lim c, Hyun Ok Kim c, Sang Yeol Kimd,Seung Hun Lee a, Joohwan Hong a, Juhyeok Jang a, Taemin Jeon a, Myung Kook Moon c,Wonho Choe a,*

aDepartment of Physics, Korea Advanced Institute of Science and Technology, 291 Gwahak-ro, Yuseong-gu, Daejeon 305-701, Republic of KoreabNational Fusion Research Institute, 113 Gwahak-ro, Yuseong-gu, Daejeon 305-333, Republic of KoreacKorea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of KoreadNotice Korea Ltd., 901 ho, K-center, 25, Simin-daero 248 beon-gil, Dongan-gu, Anyang-si, Gyeonggi-do 431-815, Republic of Korea

a r t i c l e i n f o

Article history:Received 20 November 2012Accepted 29 November 2012Available online 29 December 2012

Keywords:Tangential X-ray pinhole cameraDuplex MWPCKSTARDiagnostics

* Corresponding author.E-mail address: wchoe@kaist.ac.kr (W. Choe).

1567-1739/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2012.11.024

a b s t r a c t

An advanced tangential X-ray pinhole camera (TXPC) has been developed for KSTAR by utilizing a 2-Dduplex multi-wire proportional counter (MWPC) detector. The KSTAR MWPC employs a 2-D paralleltype readout system for high temporal resolution and adopts a duplex type for the capability of electrontemperature measurement via the multi-color method. This paper presents the performance test resultof the developed MWPC system utilizing a Fe-55 X-ray source. As a preliminary experimental result fromthe 2012 KSTAR campaign, the clear presentation of sawtooth activities and its frequency change, and 2-D plasma images during the vertical disruption event are given.

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1. Introduction

The tangential X-ray pinhole camera (TXPC) is a useful diag-nostic tool to measure soft X-ray emissivity from plasma in thetoroidal direction that enables 2-dimensional (2-D) imaging of thetoroidal plasma. The TXPC has been used to study various enrichedphysics on magnetohydrodynamic (MHD) instabilities, magneticisland dynamics, energy and particle transport, non-axisymmetricperturbations, small-scale poloidal MHD activities etc [1]. Sincethe first X-ray pinhole camerawas developed by von Goeler in 1990[2], it has been installed on many toroidal devices including TEX-TOR [1], NSTX [3], LHD [4], DIII-D [5], and EAST [6], which are basedon the scintillator in conjunction with fast CCD cameras [3e5], theabsolute X-ray ultraviolet (AXUV) solid state detectors [6], and thegas electron multiplier (GEM) detectors [7].

This paper discusses the development and preliminary results ofan advanced high-resolution tangential X-ray pinhole cameradeveloped for KSTAR by utilizing 2-D position sensitive duplexmulti-wire proportional counter (MWPC) detectors [8e10]. State-of-the-art parallel-type readout [11] and supporting electronics

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have been adopted to improve the time resolution of the diagnosticsystem. The 2-D duplex MWPC systemwith parallel readout type isnovel in fusion plasma research. The design and fabrication of theMWPC system are presented in Section 2, and the performancetests of the MWPC system with a Fe-55 X-ray source are presentedin Section 3. Preliminary experimental results from the 2012 KSTARcampaign are presented in Section 4.

2. Multi-wire proportional counter for X-ray detection

Themulti-wire proportional counter (MWPC) first developed byCharpak et al. [8] for high energy physics experiments consists oftwo cathode planes and an anode plane. Each plane has a set ofthin, parallel and equally spaced detectionwires. The anode plane issandwiched between two cathode planes, of which one set isperpendicular to the anode wires and the other is parallel to theanode wires for 2-D position sensitive detection [Fig. 1]. When anincident X-ray photon emitted from the plasma enters the gaschamber through a pinhole, it creates primary photo-electrons byphoto-ionization effect. These electrons are accelerated toward theanode wires by a high voltage applied to the anode. Due to thestrong local electric field near the anode wires, these electrons gainsufficient kinetic energy to create many secondary electrons bycollisions. This electron avalanche is collected by the anode wires

Fig. 1. Schematic of the duplex MWPC system for KSTAR TXPC.

Fig. 4. Voltage response characteristics of the amplifier system at different inputvoltage.

S. Jang et al. / Current Applied Physics 13 (2013) 819e825820

while the positive ions move toward both sides of the nearestcathode wires.

As the cathode signals are collected, the position information ofboth X- and Y-wires can be extracted by proper readout methods.There are generally two readout methods for MWPCs: the delay-

Fig. 2. (a) Anode plane (2 mm between adjacent anode wires) an

Fig. 3. (a) Transmission curves for the first MWPC (with 76 mm Be filter, 200 mm C filter and 3and 14.18 mm thick Ar mixed gas). (b) Calculated photon detection efficiencies versus photonray photons.

line readout method [12] and parallel type readout method [11].Although the delay-line readout method, which uses a smallnumber of preamplifiers and discriminators with a delay line anda time-to-digital converter (TDC), is simple and cost-effective, itprovides low time resolution (typically less than 100 Hz). On theother hand, the parallel-type readout, where each individual

d (b) cathode plane (1 mm between adjacent cathode wires).

.88 mm thick Ar mixed gas) and the second MWPC (with 76 mm Be filter, 400 mm C filterenergy of the first and the second MWPC systemwith an assumed plasma emitting X-

Fig. 5. The developed duplex MWPC with parallel-type readout: (a) the front side and (b) the back side of the detector.

Fig. 6. Gain-voltage characteristics of the MWPC.

Fig. 7. Measured 2-D images of a wrench: (a) a wrench image

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cathode wire is associated with its own preamplifier, discriminator,and analog and digital electronic channel, is complex and expensivebut it offers high temporal resolution (as large as 100 kHz).Therefore, the parallel-type readout method is appropriate forstudying fast MHD activities and energy and particle transportphenomena inside fusion plasmas.

The KSTAR TXPC adopts the parallel-type signal readout methodfor fast temporal resolution and photon counting mode to measurethe incoming time and the pulse height of each X-ray photon for 2-D imaging of the plasma. Moreover, the KSTAR TXPC employs theduplex MWPC type for direct 2-D electron temperature measure-ment [Fig. 1]. The duplex MWPC consists of two identical MWPCswith different filter thicknesses (200 mm each) in series. Since thetwo MWPCs are positioned closely to each other, each corre-sponding channel of the two MWPCs shares the same line of sight,so that the corresponding two channels view the same electrondensity, electron temperature, and effective charge Zeff of the smallplasma volume. The only difference is the X-ray emission intensi-ties because of the different filter thicknesses and gas gap widths.

setup and (b) the measured 2-D image from the MWPC.

Fig. 8. Viewing coverage of TXPC on KSTAR: (a) horizontal cross-section, and (b) vertical cross-section.

Fig. 9. Layout of the KSTAR TXPC body.

Fig. 10. (a) MWPC signals from two channels of r/a ¼ 0.10 (black) and r/a ¼ 0.30 (blue), andcurrent is 600 kA (Shot 7640). (For interpretation of the references to colour in this figure

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The ratio of the two intensities can provide electron temperatureand its fluctuation by the two-filter technique [13] with high timeresolution provided by the parallel-type signal readout and a fastdata acquisition system.

The KSTAR 2-D duplex MWPC with parallel-type readout hasa 10 cm � 10 cm position sensitive area, connected to the printedcircuit boards for both the anode and cathode and 200 preampli-fiers. The anode plane has 46 independent gold-coated tungstensensing wires of 10 mm in diameter and 2 mm apart from eachother. The anode wires are stretched with equal tension and biasedby the same high voltage. To avoid excessive high electric fieldgradient at the edge, two 30-mm-diameter wires on each side of theanode plane are added. The whole set of anode wires is connectedin parallel to be applied with the same high voltage [Fig. 2(a)]. Thecathode plane has 100 independent sensing wires of 30 mm indiameter and 1 mm apart from each other. Since two wires areconnected together to get a large signal, the total number ofchannels of each cathode is 50 [Fig. 2(b)]. Toowide of a gap distancebetween the anode and the cathodemakes a long drift time for ionsand low spatial resolution. On the other hand, too narrow anodeecathode spacing increases the capacitance of the sensing wires andthe required maximum tension to sensing wires for the electro-static stability condition [14]. Therefore, we set the anodeecathode

(b) ECE signals (Channel 2 and Channel 40) during sawtooth oscillations. The plasmalegend, the reader is referred to the web version of this article.)

Fig. 11. (a) Soft X-ray signals from r/a ¼ 0.10 (black) and r/a ¼ 0.30 channels, respectively. 2-D images (b) before the crash (at 2.610 s) and (c) after the crash (at 2.622 s). (d) Theimage obtained by subtracting (b) from (c), demonstrating the energy relaxation in the direction of the minor radius of the plasma.

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spacing at 2.4 mm for high spatial resolution, and the anode andcathode planes were fabricated by stretching the sensing wireswith a set of calibrated weights more than the required maximumtension. For 2-D plasma imaging, one cathode plane is perpendic-ular and the other is parallel to the anode wires.

For soft X-ray photon energy selection, Be filters and C filters arepopularly used. The Be filter has a lower cutoff energy than thecarbon filter of the same thickness, but it is costly and more fragileover a small impact. Therefore, we chose 200 mm thick C filters for

the first and the second MWPC, respectively. Fig. 3 represents thecalculated result of the transmission curves for the first MWPC andthe second MWPC, and the photon detection efficiencies versusphoton energy of the first and the second MWPC system [15]. Forcalculating the photon detection efficiencies, the plasma isassumed to have electron temperature and density profiles with2 keV and 6 � 1019 m�3 maximumvalues at the center, and Zeff of 2.As a result, the calculated cutoff energy of the first MWPC is 4 keVand that of the second MWPC is 5.3 keV for 10% X-ray transmission

S. Jang et al. / Current Applied Physics 13 (2013) 819e825824

[green horizontal line in Fig. 3(a)]. The ratio of the total integratedX-ray emissions of the first and the second MWPC is 2.11. Theefficiency of the MWPC system depends on incident X-ray photonenergy, C filter transmission, gas pressure, gas gap width, andcomposition of the gas mixture. The MWPC operates with a mixedgas of 78% Ar, 20% C2H6, and 2% CF4 at atmospheric pressure. Thedistance between the first C filter and anode wires of the firstMWPC is 3.88 mm and that of the second MWPC is 14.18 mm,respectively.

The cathode wires are instrumented with independent ampli-fier systems that are mounted on the MWPC chamber. The currentsensitive Plessey amplifier (SL560) limits the signal bandwidth to300 MHz with low noise. Each amplifier system consists of twoSL560 amplifiers for high amplification factor. The total amplifiersystem for 200 channels is comprised of 20 amplifier boxes; eachamplifier box consists of a set of 10 amplifier systems. The amplifiersystem works on 9e12 V. The result of the performance test of theamplifier system is shown in Fig. 4 where the voltage responsecharacteristics of the amplifier system at different input voltage aredemonstrated. The input voltage range is from 100 mV to 1300 mVusing a function generator and a charge source with a 12 V bias

Fig. 12. 2-D images of the soft X-ray emissions during the vertical displacement event of theThe over-plotted curves are magnetic flux surfaces obtained from RT-EFIT.

voltage. The output signal increases as the input voltage increasesuntil the output signal becomes 1 V, and then it saturates. Thefabricated amplifier systems are connected closely to the MWPCchannels to guarantee high signal to noise ratio. Depicted in Fig. 5 isthe layout of the constructed duplex MWPC with the parallel-typereadout amplifier system. The data acquisition of the 200 channelsis performed at 100 MS/s per channel with the flash ADC system(100 MS FADC from Notice Korea Ltd.).

3. Performance test of the duplex MWPC system based onparallel type readout

A performance test of the developed duplex MWPC system hasbeen carried out with a Fe-55 radioactive X-ray source that emits5.9 keV K-alpha X-rays. It was located at 70 cm away from thedetector to ensure uniform radiation to the sensitive detector area.A high voltage was applied to the anode plane, from 0 V to 1750 V,to check the gain-voltage characteristics of the MWPC, and themeasured gain-voltage characteristics of the MWPC are given inFig. 6. The pulse height of the incoming X-ray photons starts at1475 V and increases exponentially until 1750 V, which means it is

plasma at (a) 5.021 s, (b) 5.039 s, (c) 5.047 s, and (d) 5.055 s, respectively (Shot 7886).

S. Jang et al. / Current Applied Physics 13 (2013) 819e825 825

in the proportional region. The detector becomes unstable over1800 V causing spontaneous sparking signals. From these results,we set the stable operating voltage at 1500e1650 V for bothMWPCs, under which a high signal to noise ratio of the X-raysignals was observed, with no sparks.

In the stable operating voltage range, the spatial uniformity andthe 2-D image test of the MWPC were performed. The uniformitytest was done using the Fe-55 source, located at 70 cm from thedetector, at 600 s illumination time and 1650 V bias voltage. Thedata sheet of the average pulse heights of the 5.9 keV X-ray photonsof each channel obtained by the uniformity test is used as therelative response calibration factors of the MWPC. In addition, the2-D image test was performed using a wrench mounted in front ofthe MWPC with 600 s illumination time and 1650 V bias voltage.The clear wrench image presented in Fig. 7 indicates that theMWPC operates well at the optimized conditions.

4. Preliminary experimental results of tangential X-raypinhole camera from KSTAR plasma

Fig. 8 illustrates the spatial coverage of the TXPC on KSTAR. It isdesigned to cover 100 cm in the radial direction and �50 cm withrespect to the mid-plane in the vertical direction. The spatialresolution of the TXPC depends on the pinhole size and the distancebetween the pinhole and the detector. It is about 2 cm for bothvertical and horizontal directions under the current design witha 2 mm diameter pinhole. As shown in the layout of the TXPC bodydepicted in Fig. 9, the main components are a gate valve, a 76 mmthick and 3 cm diameter Be window as a vacuum boundary,a ceramic breaker, and a pinhole adapter. The Be window wasfabricated by diffusion bonding to support the 1 bar pressuredifference between the vacuum side and the gas detector chamber.The thickness of 76 mmwas chosen based on the stress test of 1 barpressure difference for a 3 cm diameter Be window. The pinholeadapter can accommodate different size pinholes from 0.3 mm to2 mm in diameter. The selection of the appropriate pinhole sizedepends on the spatial resolution and the intensity of the soft X-rayemission from the plasma. The constructed TXPC body and devel-oped MWPC were installed on Port B of the KSTAR vacuum vessel.

The soft X-ray emissions from two channels of the first MWPCduring the sawtooth oscillations of the KSTAR plasma (Shot 7640)are shown in Fig. 10(a). At each sawtooth crash, a rapidly decreasingX-ray emission at the r/a ¼ 0.10 channel and subsequentlyincreasing X-ray emission at the r/a ¼ 0.30 channel are clearlyobserved. The phase and frequency of the oscillations are inexcellent agreement with the ECE signals shown in Fig. 10(b). Achange in the oscillation frequencies from 24 Hz to 7 Hz is alsoobserved. The 2-D soft X-ray emissivity images before and after thesawtooth crash are presented in Fig. 11(b) and (c), respectively.The central hot core cools down and at the same time, the heat fromthe core spreads radially out, showing the larger bright area afterthe crash. This is confirmed by the image obtained by subtracting

(b) from (c), demonstrating the energy core energy relaxation[Fig. 11(d)].

Fig. 12 depicts 2-D images of soft X-ray emission from the TXPCduring the vertical displacement event of the plasma (Shot 7886).The magnetic flux surfaces obtained from RT-EFIT are over-plottedin the pictures. Since the 2-D images from the TXPC are toroidallyline-integrated, the brightest spot in each X-ray image looksslightly stretched toward the inboard side of the plasma withrespect to the magnetic axis. Nevertheless, the overall positions arein excellent agreement.

5. Summary

The MWPC-based tangential X-ray pinhole camera (TXPC) hasbeen successfully developed for KSTAR with the capability of hightemporal and spatial resolution and with strong neutron radiationhardness. The constructed MWPC utilizes the parallel type readoutsystem for high temporal resolution and adopts the duplex type forthe capability of electron temperature measurement. The perfor-mance test with a Fe-55 X-ray source indicates that the MWPCoperates well under optimal conditions.

As the preliminary results from the 2012 KSTAR campaign,sawtooth oscillations, 2-D images of the core energy relaxation, anda change in oscillation frequency were observed. 2-D X-ray emis-sion images during the vertical displacement event of the plasmaare also clearly demonstrated. These results are in excellentagreement with ECE diagnostics and RT-EFIT results.

Acknowledgments

The authors thankMr. Y.S. Kim (NFRI) for supporting installationof TXPC on KSTAR, Dr. S.S. Ryu (Notice Korea Ltd.) for providing theFADC program, and Ms. J.Y. Park (KAERI) for supporting fabricationof MWPC. This work was supported by the National R&D Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science, and Technology (Grant No.2012-0005923).

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