S-R1-2

Dynamic Behavior of Multi-Phase Structure Plasmas
in a Cathode Arc Spot

アーク陰極点近傍に形成される重相構造プラズマの動的挙動

Yoshihiko Uesugi, Yusuke Katada, Takeru Ito, Yasunori Tanaka, and Yoshihiro Yamaguchi1
上杉 喜彦，片田 優介，伊藤 毅，田中 康規，山口 義博 1
Kanazawa University and Komatsu Industries Corporation
Kakuma, Kanazawa, 920-1192, Japan and 1-1, 1Onomati-shinmati, Kanazawa, 920-0225, Japan
金沢大学 920-1192 金沢市角間，1 コマツ産機 920-0225 金沢市大野町新町 1-1

Dynamic behavior of multi-phase structure plasmas generated at the interface region between high current
density arc plasmas and hot molten cathode has been studied experimentally. Hot molten cathode spot wasobserved directly by color high-speed camera and dynamic evolution of the cathode surface temperaturefrom arc ignition was estimated from the RGB signals of color camera. In addition to the cathodetemperature measurement, the dynamic and unstable behavior of the cathode molten pool was seen as thehot spot rotation and, abrupt boiling and droplet ejection from the molten cathode surface.

1. Introduction
Study of the dynamic response of tungsten
surface to the transient and extremely highplasma heat load in type-I ELM’s and disruptions(>100MW/m 2) requires experimental approaches
different from those in steady state and low heat
flux experiments(<10MW/m 2 ). In the present
experiments, high current stabilized arc plasmas with
~GA/m2 is used as a high heat flux pulse and steadystate plasma source. The plasma heat flux onto thecathode surface is several hundreds MW/m2 in thesteady state and is several GW/m2 in the arc ignition
phase. These properties of high heat flux arc plasmasare very useful to study the transient behavior ofthe divertor materials during ELMs and disruptions
in fusion reactor complementally with other ELM/disruption experiments.
On the other hand, arc plasmas are usedin various devices, such as light sources, plasmaarc cutting, electric circuit breakers and ignitionplugs, etc. It is very important to understand thephysical and chemical interactions of the high heat
flux arc plasmas with the refractory metal cathodefor improvement of these industrial devices. In thepresent experiments dynamic interactions of highcurrent density arc plasmas with hot metal cathodeare studied using a plasma arc cutting device, which
can generate stable arc plasmas in a small fixedvolume[1] .
2. Experimental set up
Figure 1 shows a schematic view of theplasma arc cutting torch(Komatsu Industries)
employed in the experiments. Arc plasmas areFig. 1 Schematic view of the hot cathode of arc
discharges in the arc plasma cutting torch.
generated between water-cooled cathode made ofhafnium and a water-cooled copper target as an
anode. The arc plasma is erupted from the nozzle
outlet with the plasma gas. The plasma gas issupplied as swirl gas so as to fix the cathode spot at
the center of Hf electrode at the arc ignition phase.
The diameters of the Hf cathode and nozzle are
1.6 mm and 1.3 mm, respectively. In this study, aquartz window was installed at the nozzle in orderto observe the cathode surface directly. The Highspeed color camera can observe the cathode surface
through this quartz window. Two type of high-speed
color cameras(Keyence:VW-6000, VW-9000) areused. The max frame rates of VW-6000 and VW9000 are 24,000 fps and 230,000 fps, respectively.
3. Dynamic evolution of the hot cathode spot
Figure 2 shows images of the cathode hot
spot and arc current waveform after arc ignition toFig. 2 Temporal evolution of the hot cathode spot(top) and cathode surface temperature estimatedfrom the RGB pictures shown in the figure. The preprogrammed arc current is also shown.

steady state arc. These images are observed using
VW-6000. Rotation of local hot cathode spot heats
the Hf cathode surface gradually and forms the larger
cathode spot on the center of Hf electrode at t ~100
ms after ignition. The cathode spot area spreads overgradually from the fixed local cathode spot to thewhole cathode area. And the cathode emission area
becomes larger with increasing the arc current. Timeevolution of cathode surface temperature distribution
estimated from RGB signals of the pictures is also
shown in Fig.2. The temperature distribution showsthat the cathode surface temperature is between 3,700
K and 4,300 K. This temperature is higher than Hf
melting temperature(2,503 K) and lower than Hfboiling temperature(4,547 K)[2]. Therefore, it isclear that the cathode hot surface is melted during arc
discharges. Thermionic electron emission current
density J is calculated from the estimated cathodetemperature, supposing that whole cathode surface issolid Hf or solid HfO2. Thermionic electron emission
current IHf and IHfO2 are estimated to be ~160 A and40 A, respectively when the preset arc current is
Iarc=135 A, where IHf/ Iarc=1.2 and IHfO2/Iarc=0.3. To
estimate the themionic current J in detail, physicalproperties of Hf and HfO2 as hot liquid materials
are needed[2], but unfortunately not available at themoment.
4. Unstable droplet ejection from the molten cathodeUnstable phenomena of the molten
cathode pool, such as small droplet ejection
acompanying wavy motion of the molten cathodeand large dropelet ejection from the cathode, areFig. 3 Abrupt ejection of the cathode materials from themolten cathode pool after bubble formation and growth.

often observed when a new cathode with a flat
cathode surface is used at the first time. Unstableand dynamic behavior of the molten cathode likebubble formation and explosion is shown in Fig.
3. The bubble appears from the molten pool andgrows within ~0.2 ms, then evaporates intensively
due to the arc plasma heat flux, leading the bubble
explosion. After the bubble explosion, cathode spot
goes back to the quiet phase. These intensive anddynamic cathode phenomena can not seen in the arc
voltage and current measurement. So far we haveused the Hf cathode, which is normally used in arc
plasma cutting device. Now we have modifyed itto use tungsten as a cathode in spite of Hf. In near
future the interactions of tungsten cathode with thehigh curret arc plasma of hydrogen or helium will beshown elsewhere.
References[1]Y. Yamaguchi, et al., Journal of the Japan Welding
Society Vol. s29, No.1, pp. 010-017(2011) in Japanese.
[2]S.Izumi, Data book of metal, Maruzen Company(1984).