Deepening of Floating Potential for Tungsten Target Plate on the way to Nanostructure Formation

Abstract

Deepening of floating potential has been observed on the tungsten target plate immersed in high-density helium plasma with hot electron component on the way to nanostructure formation. The physical mechanism is thought to be a reduction of secondary electron emission from such a complex nano fiber-form structure on the tungsten surface. Tungsten material is very important in terms of fusion reactors, especially plasma-facing component. Helium defects are the concerns when employing the tungsten for the divertor target and/or the first wall since helium is the fusion product and would be contained by around 10% of scrape-off layer plasmas in fusion devices. Recently the nano fiber-form structures have been identified on a variety of tungsten surface irradiated by helium or helium/deuterium mixture plasmas [1,2].The surface characteristics of thus formed tungsten plate would change compared with the flat non-damaged surface, for example, the radiation emissivity [3], the sputtering yield [4], the heat conduction [5], the discharge property [6] and so on. In this report the secondary electron emission (SEE) property will be discussed in relation to the floating potential which is important with respect to the impurity releases through physical sputtering and plasma heat flux.Nano fiber-form structured tungsten surfaces have been formed in a newly developed linear plasma generator AIT-PID [3,7] as shown in Fig. 1. In this device the high density helium plasmas have been produced with a hot electron component. The plasma density exceeds 1 × 10 18 m −3 and the bulk electron temperature (T c ) is about 4 eV, while the hot electron component with the fraction of roughly 8% has a temperature (T h ) of up to 40 eV. Such a two electron temperature plasma gives a deep floating potential ensuring a high ion impact energy which is important to form nanostructured tungsten surface. The floating potential is around −40 V with respect to the vacuum chamber, and the ion incident energy to the tungsten is 45 eV considering the plasma potential of around +5 V which is unchanged during the exposure. Such a high sheath voltage can be explained by the numerical analyauthor's e-mail: takamura@aitech.ac.jp sis on the floating condition that the electron flux balances with the ion one, as shown in Fig. 2, which indicates that the value of 45 V is a little smaller than the theoretical prediction assuming a complete Maxwellian distribution for both electrons. Figure 3 shows the time evolutions of the floating potential and the tungsten surface temperature observed with a radiation thermometer with a fixed emissivity of 0.43. The drop of surface temperature comes from an increase of radiation emissivity and associated target cooling assuming a constant plasma heat flux onto the target, which was discussed in [3]. The floating potential changes rapidly from −40.0 down to −48.5 V during the period of large change in surface temperature, meaning a certain correlation with nanostructure formation.

Recovery of Tungsten Surface with Fiber-Form Nanostructure by the Argon Plasma Irradiation at a High Surface Temperature

“…author's e-mail: takamura@aitech.ac.jp surface temperature of 1420 K. The tungsten is made with cold-worked powder metallurgy. The helium plasma condition in AIT-PID (Aichi Institute of Technology -Plasma Irradiation Device) is similar to that in the previous report [4]. The experiment of temperature excursion in helium plasma [5] has shown a shortening and a fattening of originally long and thin nano fibers when the surface temperature increases up to 1600 K for a few minutes.…”

Modified Power Transmission Factor of Tungsten in Plasmas with Hot Electron Component

“…where φ work is the work function of the employed metal, γ ion is the ion-induced electron emission coefficient and γ e is the SEE one on the W surface due to energetic electrons, while γ e from bulk electrons may be neglected In fact, a virgin W has a fairly high SEE [13] and a very large ion-induced electron emission [14] including the kinetic as well as Auger effects, while the nanostructured W suppresses those electron emissions due to deep valleys associated by the formation of fiber-form nanostructure [7,15,16]. Each electron emission from metal surface extracts the thermal energy of the metal corresponding to, at least, work function, 4.5 eV for W.…”

Characteristics of the compact plasma device AIT‐PID with multicusp magnetic confinement

“…One of the PWI studies is the fiber‐form nanostructure formation on a tungsten (W) surface [5–11]. This is one of the He defects on W at a relatively low surface temperature, i.e., below about 1500 K. Another He defect is the formation of bubbles/holes on the W surface at a surface temperature higher than 1600 K. Preliminary results on such PWI studies in the AIT‐PID have been reported [12].…”

“…We can recognize a forest‐like appearance made of very thin fiber‐form W. Many researches have started using the present facility of AIT‐PID. The suppression of secondary electron emission was demonstrated by utilizing the presence of the hot‐electron component in this device [9]. The recovery of W surface with fiber‐form nanostructure has been tried in the Ar discharge plasma in AIT‐PID [10].…”

“…However, we do not have much experience in the environment of high‐power condition [3], nor has there been sufficient characterization of tungsten as the plasma‐facing material. For example, the formation of bubbles and holes at high surface temperature [4] and the arborescent nanostructure at relatively low surface temperature are the typical morphologies of a damaged surface that may have weak thermal resistance in terms of impurity release by surface melting due to lost thermal conduction to the deep bulk, easily triggering unipolar arc [5–11] and dust formation.…”