H. "Yamanishi

H. Miyake

K. Matsuoka

M." Isobe

National Institute for Fusion Science (NIFS-Repository)

Distance from CHS (m)Fig. 2 Comparison of weekly measured dose betweenRMSAFE and two integrating dosimeters.Jan.2001--e--TLD-----RPLD·······RMSAFEWN IA IeW8 we WfJut Sep.2000............"""-A-b' B······-c-o-D-"-F···"F-GlOS.'"' 104C\l.)> 103\l.)-'->10"{}jc\l.) 101(/)0"'0"'0 10°\l.)C(1) 10'(/)..0010 210 310170C) 60l/)0"0 50"C ........C)40"5E, 30>UV):::i. 20>-32Q) 10Q)0Apr.Fig. 1 Relationship between observed dose and distancefrom CHS 7 events which were detectedthe dose greater than 0.005 mSv in the CHS torushall.RMSAFE in the CHS torus hall were consistent with theresults of integrated dose measurements bythermoluminescent dosimeter (TLD) and byradiophotoluminescent dosimeter (RPLD), andfurthermore, the general decreasing tendency of theobserved dose with the distance from the CHS torus wasclearly seen, though detailed radiation patterns might bedependent on source plasma and other conditions. Theseresults support our conclusion that RMSAFE is able tosuccessfully detect burst X-rays.§ 1. Burst X-Ray Detection Using RMSAFEWhen a burst X-ray event was detected in the torus hall,increases in the count were sometimes observed even atThe function of the radiation monitoring systemRMSAFE (Radiation Monitoring System Applicable toFusion Experiments) is well verified to detect burstradiation, that is, radiation generated suddenly andexplosively. When an increase in 50 ms integrated countfrom a radiation monitor, which is recorded and updatedevery 10 ms in the system CPU, is encountered to exceed apre-set level, RMSAFE recognizes it as an outbreak ofburst radiation and alters its recording mode so that theburst event data is saved in a specified file. In this study,we detected X-rays arising from Compact Helical System(CHS), a high-temperature plasma experimental device, inorder to verify that RMSAFE is able to detect radiationbursts successfully and accurately.Because the CHS does not make use of asuperconducting coil, the magnetic field rises and fallsduring each plasma experiment. The typical pattern of themagnetic field during a plasma shot is an increase for 0.8s, followed by a steady level for 0.6-1.2 seconds, andfinally a decrease for 0.8 s. The electrons in the plasmavacuum vessel are accelerated by this varying magneticfield. When the hydrogen gas in the vacuum vessel is oflow density, the accelerated electrons become run-awayelectrons free from collision, which then do finally hitsomewhere in the vacuum vessel, producingbremsstrahlung X-rays. These X-rays were used in thepresent study to test the burst detection capabilities ofRMSAFE.posts WN, WB, lA, WC, IC, WF. Increases in the dose ofradiation due to X-rays from CHS were observedconcurrently at various observation points in severalplasma shots (see Fig. 1). Approximately 90 events wereobserved in the fiscal year 2000. Doses greater than 0.001mSv were observed in the torus hall in 35 events. Theweekly accumulated values of radiation dose observed byYarnanishi, H., Miyake, H., Matsuoka, K., Isobe, M.408

Burst X-Ray Detection Using RMSAFE

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