D. Kato

H.A." Sakaue

K. Furuya

K. Motohashi

S. Kaga

S. Suzuki

T. Kenmotsu

Y. "Sakai

National Institute for Fusion Science (NIFS-Repository)

Hydrogen ion-solid surface interaction leads to for-mation of both positive and negative ions at the surface,and is of particular importance in wall conditioning ofplasma confinement devices. We have studied the fun-damental processes of beam-surface interactions by mea-suring the positive and negative ions scattered from thesolid surface by injecting low-energy (13 keV) light ionbeams. 1) Both positive and negative ions from molyb-denum, tungsten, vanadium alloy, and carbon nano wallwere observed. The intensities of negative ions formed onthese samples were the same order of magnitude. Thenegative ion yields from such high work function sur-faces of graphite, Si, and diamond are too large to beexplained by conventional surface charge transfer mod-els. We also found that the surface structure affected thereflection property. The results suggest that the surfacestructure is an important factor determining the inten-sities of scattered positive and negative ions. On theother hand, most experiments of the beam-surface inter-action have been performed at room temperature. Herewe report the surface roughness effect on the reflectionproperties at Si targets for a 1 keV H+ beam injection.Reflection from Si and carbon surfaces are measured forcomparison. Temperature effects of these materials arealso studied.We prepared a Si crystal, a rough-finished Si (a fewµm roughness), and a graphite (carbon plate) samples.The H+ beam energy was kept at 1 keV and beam cur-rent was 10-40 nA at the target throughout this series ofthe experiment. The reflected ions were measured by amagnetic momentum analyzer consists of a pair of mag-netic coils and a multi-channel plate set on a turntable.This analyzer system enables energy and angle resolvedmeasurements of reflected positive and negative ions withsingle scan.Reflection angle dependence of the reflected ion en-ergy and intensity at several incident angles were mea-sured for Si crystal, rough-finish Si crystal, and carbonplate. 2) FE-SEM images of the rough-finish SI crystaland carbon plate are shown in upper panels in Fig. 1.We define the incident and reflection angles with respectto the sample surface. The reflected ion intensity in theFig. 1: Reflection angle (β) dependence of the H+and H− ion intensity reflected from (a) Si crystal, (b)rough-finish Si, and (c) carbon plate. FE-SEM imagesfor rough-finish Si and carbon plate the are also shown.rough-finished Si was largely reduced compared to thatin the Si crystal as shown in Figs. 1(a) and 1(b). Theangular distribution of the rough-finished Si was slightlybroader than that of Si crystal at α = 10◦ and that ofthe graphite was much broader than the others. Figure1(c) shows result for the carbon plate (graphite). Thegraphite has a piled-up structure of small pieces horizon-tally. This result suggests that this structure has a simi-lar effect on the reflected ion intensity angle distributionas the diffuse scattering in the rough-finished Si. Thusthe structure and surface roughness affect on the scat-tering ion intensity and angle distribution particularlyat lower incident angles. We also studied temperaturedependence of the reflected ion intensity for these sam-ples. Temperature-dependence of the H+ ion intensityreflected from the Si crystal and graphite are measured.Both the Si crystal and rough-finished Si did not showclear temperature dependence, while reflected H+ ion in-tensity decreases and H− ion intensity increases slightlyas increasing the temperature of the graphite sample.1) M. Wada et al., Plasma Device and Operations 17,132 (2009).2) N. Tanaka et al., Rev. Sci. Instrum. 85, 02C311(2014).    Sputtering and backscattering processes of polycrystalline tungsten surfaces by ion impact have been studied by an optical emission spectroscopy. Since neutral atoms sputtered from the wall surface penetrate into the plasma across the magnetic fields and they play a role of cooling processes of the plasma, the clarification of these mechanisms is demanded from the point of the control of the fusion plasma. We have already reported as follows.1),2) (1) The mean velocities of excited tungsten(W) atoms in direction normal to the surface under the irradiation of krypton ion (Kr+) as a function of the perpendicular distance from the surface were almost independent on the incident beam energy. (2) They depended on the energy difference between the work function and the ionization potential of the excited state. In this work, we report about the mean velocities under the irradiation of Ar+ ion.  The experiments were carried out in the National Institute for Fusion Science (NIFS). The experimental procedure was same as the Kr+ cases. Briefly, we measured the intensity of light emission from sputtered atoms on W surfaces under the irradiations Ar+, as a function of the perpendicular distance from the surface. Using the analysis of decay curve, we estimated the mean vertical velocity component in direction normal to the surface. The intensity decay follows the well-known relation,  .       (1)  Where I0k is the intensity from a particular transition k at the surface (z=0), vԋis the vertical velocity component normal to the surface, Ĳk is the lifetime of the excited state. Figure 1 shows a typical optical emission spectrum obtained at z=0, in the wavelength of 380-440 nm. Even if projectiles were different from Kr+ cases, the intensity ratio of each peak in a spectrum was the same in essence. The excited W lines are classified into four types of transitions that consist of a combination of two upper and two lower states: 5d56pĺ5d56s (A, D, H, I), 5d56pĺ5d46s2 (K), 5d46s6pĺ5d56s (B, E, F), and 5d46s6pĺ5d46s2 (C, G, J). In addition, the backscattered Ar+ ions appear in the spectrum as broad peaks. It means that the backscattered Ar+ ions have large parallel velocity component. Although we also carried out the experiment with the incidence energy to 15 to 60keV, we were not able to find out a large difference in each spectrum except for the absolute intensity. Fig. 1. A typical survey optical emission spectrum obtained at z=0 mm, under the irradiation of Ar+. Fig. 2. The mean vertical velocity of various excited states of W as a function of the energy gap between the work-function Ef and Ip minus excitation energy Ees.   The estimated mean vertical velocities are almost same as the Kr+ cases. Therefore, we were able to confirm that the relationship of the energy gap between the work-function Ef and Ip minus excitation energy Ees is the same as the Kr+ case, as shown in Fig.2. The level-energy-dependent velocity suggests any inelastic processes play an important role in the excitation of sputtered atoms. The resonant electron transfer is considered to be one of the candidates of such inelastic collision processes.  1) K. Motohashi et.al.: Nucl. Instrum. Meth.B, 283,(2012)59.  2) Y.Sakai et al.: Research Report NIFS-PROC-91, 1,(2013). ¸¸¹·¨¨©§! A¦ kk kzII Wvexp0463§14. Plasma Atomic Processes Explored by Optical Emission Spectroscopy of Excited Atoms Sputtered from the Metal Surface by Ion BombardmentSakai, Y. (Toho Univ.), Furuya, K. (Kyushu Univ.), Kaga, S. (Toho Univ), Kato, D., Kenmotsu, T. (Doshisha Univ.), Motohashi, K. (Toyo Univ.),  Suzuki, S. (Toho Univ.), Sakaue, H.A.

Plasma Atomic Processes Explored by Optical Emission Spectroscopy of Excited Atoms Sputtered from the Metal Surface by Ion Bombardment

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Plasma Atomic Processes Explored by Optical Emission Spectroscopy of Excited Atoms Sputtered from the Metal Surface by Ion Bombardment

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