A. Krasilnikov

M. Isobe

M. Sasao

National Institute for Fusion Science (NIFS-Repository)

§11. Fast Atom Measurement Using a Natural Diamond Detector on LHD Krasilnikov, A. (Troitsk Inst. for Innovating and Fusion Research) Sasao, M., Isobe, M. The specially developed NDD with very thin graphite contacts enabled its application to charge exchange atom spectrometry in an energy range of about 35 - 100 keY in experiments on the Large Helical Device. Fig. 1 shows the schematic view of experimental arrangement. NDD was used in pulse height analysis mode. Only charge exchange atoms produced counts in the energy range 35 - 100 keY ofNDD pulse height spectrum during LHD experiments with hydrogen and helium plasmas. The NDD was installed inside the midplane vacuum flight tube 8.25 m from the plasma axis with an observation line tangential to the plasma axis. Its cone of view value was equal to 0.470 with a diameter of 13.4 cm at the plasma axis. Therefore the energy distribution of only counter passing untrapped fast beam ions was studied. Almost no counts corresponding to fast charge exchange atoms were accumulated during beam injection only in co-direction. The solid angle of the NDD atom spectra measurements (1.2xl0-8 steradians) was detennined by a 1 mm diameter circular aperture installed in 15 cm in front of NDD which had a diameter of 2 mm. Such a small diameter aperture was chosen to provide count rates ::; 2x 105 counts/s over a wide range of LHD plasma parameters to reduce pile-up effects in using an EG&G Ortec 673 spectroscopy amplifier operating with a shaping time constant 0.25 J.1s. The energy resolution of the NDD used was previously measured with an 241 Am a-source and was equal to 5.9%. The energy resolution of this NDD for 100 ke V hydrogen atoms was estimated before to have a value of better than 12%. The energy resolution of these measurements was detennined primarily by the EG&E Ortec 142A preamplifier plus the EG&G Ortec 673 spectroscopy amplifier noise. Measured using a generator of standard pulses, the energy resolution was equal to 16 ke V. The hydrogen atom charge exchange spectra were measured by NDD in experiments with 100 ke V If> injection into hydrogen and helium plasmas with electron and ion temperature up to 1.5 keY. Spectra measured in experiments with very low (::; 5 x lOll cm-3) plasma densities demonstrate peaks of charge exchange atom energy distribution close to injection energy. In the case of helium plasma density 5 x 1012 cm-3 If> charge exchange atom flux onto detector is essentially higher then the previous case (::; 5 x lOll cm-3) due to increase of beam deposition . in a plasma, and the peak position moves to 65 ke V due to fast ion slowing down. Spectra measured during shots with plasma densities 1 - 2.5xl013cm-3 also illustrate the effect of beam ions slowing down in LHD plasmas. 150 Noticeable difference in the shapes of fast hydrogen atom spectra measured in cases of hydrogen and helium plasmas with densities higher than 1013 cm-3 can be explained by the energy dependence of corresponding charge exchange cross sections. Reduction of atom flux with increase rf plasma density higher than 1013 cm-3 is connected with charge exchange flux attenuation and with reduction of If> , HeO and He+ densities on plasma axis. In Fig. 2 are shown the time evolution of the fast neutral particles in the energy range of35-71 keY and 71-106 keY. After the tennination of the couter-injected beam, the couting rate is also decaying with the time constant rf aound 200 ms. The confmement time ofthe beam particles can be estimated. Ctr. beam BT: CW . ~ H.vJ~ Fig. 1 Schematic View of Experimental Arrangement --35.4 • 70.8 keV --70.8 - 106.2 keV 60 50 UJ 40 -C ::J 0 0 30 0 0 20 z 10 0 0 0.5 1 1.5 2 Time (s) Fig. 2 The time evolution of the fast neutral particles detected by the NDD. 

Fast Atom Measurement Using a Natural Diamond Detector on LHD

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