S. Masuzaki

National Institute for Fusion Science (NIFS-Repository)

§51. Langmuir Probes for the LH D Masuzaki, S. In order to measure the edge plasma parameters, two types Langmuir probes will be installed in LHD. One is the fast scanning probe and the other is the attached probes on the divertor tiles. The fast scanning probe (FSP) will be installed from the lower measurement port as shown in Fig. 1, and measure the profiles ·of electron density, tem-perature and potential, and their fluctuations. The gate valve of the FSP is located inside the vacuum vessel, at the top of a tube (1.8 m of length, 197 mm inner diameter). All parts of the FS P are put in a tube. This metal sealed gate valve is developed for the FSP. Its hole ($ 39 mm) is in the center of the gate valve. The plasma facing surface is water cooled to reduce the temperature rise by the radiation from the plasma. There are two types of moving parts. The fast part is driven by a compressed air cylinde~ and the moving length is 0. 6 m. The slow part is driven by two hydraulic cylinders, and the moving length is 1.3 m. The feature of this hydraulic cylin-der is that the rod is fixed and utilized as a linear way, and the cylinder part moves on this rod. The moving test of the fast part was done. It took about 0.3 sec for one way moving. As shown in Fig.1, the FSP can reach the last closed flux surface (LCFS). In the case of right position in Fig. 1, the slow part must stay in divertor leg to measure near the LCFS. It is difficult because of the high heat load, so this measurement is only possible in low additional heat-ing power operation In the case of the left position in Fig. 1, the FSP can reach the LCFS near the null point only by the fast move. So the first FSP will be installed in this position. In a local island divertor (LID) operation, the FSP can measure the plasma parameters in-side the rnln = 111 is-land easily. The probe tip is replaceable, and the measurements us-ing multi electrodes type probes or multi grid electrostatic en-ergy analyzer are in Fig. 1. The fast scanning probe ( FSP) in the UlD. Two positions of the FSP are indicated. the planning stage. The attached probes on the divertor tiles (APs) areutilized to measure the plasma parameters just in front of the divertor tiles. The measurement of the particle and the heat flux profiles on the divertor tiles using IR TV is one of the main purposes of the APs. The insertion of Fig. 2 shows the first design of the AP. The APs are exposed to the particle and heat flux to the divertor tile. So the material for APs must be choosen suitably. The materials of the probe tip, electric insulator and feedthru are C/C compos-ite, boron nitride (BN) and molybdenum, respec-tively. The C/C composite has good heat conductiv-ity (390 W/mK), and BN has also relatively good heat conductivity (60 W/mK). Molybdenum has the thermal expansion of - 5 x 10-6 /K which is nearly equal to that of graphite. The feedthru is screwed in the probe tip. The support plate is screwed on the divertor tile, and it fixes theAP. An active cooling is not applied to APs, so the APs are cooled by mainly heat conduction to the active cooled divertor tile through the electric insulator and radiation. The heat load test was performed using electron beam in the ACT device. Two APs are attached on real size divertor tile. Figure 2 shows the result of the test. In this case, the temperature difference beween the divert:or tile and AP is about 200K for the heat load of0.75 MW/m2 corresponding to the forecasted heat load in 3 MW heating steady state operation of the LHD. The interesting point is that theAP tempera-ture decreases immediately to the tile temperature after terminating the electron beam. It means that the h~eat conduction between tile and AP through the BN electric insulator is good. The probe tip di-ameter is 6 mm in the first design. Now smaller tip is under design to obtain the fine spatial resolution. The APs' signals are taken out using the MI cables which are laid under the divertor tiles. 700 600 - - -0.75 MWirJi-:- - - - -pro e t1p 0.7 - 0.6~ 0 500 ~ -Q) 400 0.5 s '- ~ :J «1 300 0.4-; '- :::J Q) plate electric insulator (BN) 0. feedthru (Mo) 0.3 ;;:: E -200 tile temp. m 2 robe temp. 0.2 ~ 100 0.1 0 0 0 200 400 600 800 time (s) Fig. 2. The result ofheat load test using electron beam per bnned in the ACI' device. insertion: first design ofattached probe on the divertor tile. 57 

Langmuir Probes for the LHD

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