24P122-P
Dynamic behavior of hot cathode spot under high current DC arc plasma
大電流アーク放電陰極点における高熱流プラズマ-電極動的相互作用
Yusuke Katada, Takeru Ito, Yoshihiko Uesugi, Yasunori Tanaka, and Yoshihiro Yamaguchi
片田 優介，伊藤 毅，上杉 喜彦，田中 康規，山口 義博
Kanazawa University and Komatsu Industries Corporation
Kakuma, Kanazawa, 920-1192, Japan and 1-1, Onomati-shinmati, Kanazawa, 920-0225, Japan
金沢大学 〒920-1192 金沢市角間，コマツ産機 〒920-0225 金沢市大野町新町1-1
This paper describes the experimental results on the observation of the hot cathode spot in plasma arc
cutting torch. We observed the cathode surface phenomena using color high-speed camera. Use of RGB
signals of color camera enables us to estimate the cathode surface temperature. As a result, time evolutionof the cathode arc spot temperature from arc ignition to steady state can be obtained. In addition to thecathode temperature measurement, the dynamic behavior of the molten cathode spot was seen as the hot
spot rotation and, abrupt boiling and droplet ejection.

1. Introduction
The arc cathode is exposed directly by high heat
flux arc plasmas. In the case of plasma arc cutting,the heat flux to the cathode spot is estimated to be
several hundreds MW/m2 in the steady state arc.
The contact of such high heat flux plasmas with thecathode material can cause phase transitions fromsolid to liquid and from liquid to vapor
instantaneously. As a result, multi-phase plasmas
are generated near the cathode surface. The physicalphenomena on the cathode spot are muchcomplicated.
Today, arc plasma is applied to various
equipment such as plasma arc cutting, circuit
breaker and ignition plug. It is important tounderstand the physical and chemical phenomenaof the cathode spot for improvement of theequipment above. We observed the cathode surface
behavior during arc discharges using high-speed
color camera. Arc plasmas of arc cutting torch isuseful for cathode spot observation because
reproducible arc plasmas discharges easily and thecathode spot is fixed on the electrode stably.
2. Experimental set up
The plasma arc cutting torch(Komatsu Industries)
is shown in Fig.1. Arc plasmas generated between
water-cooled hafnium(Hf) electrode as a cathodeand a water-cooled copper target as an anode placedoutside the torch. The arc plasma is erupted fromthe nozzle outlet with the plasma gas(O2). Theplasma gas is supplied as swirl gas so as to fix thecathode spot at the center of Hf electrode at the arc
ignition phase. The nozzle works to shrink the arc
column with the surrounding cold supply gas. The
diameters of the Hf cathode and nozzle are 1.6 mm
and 1.3 mm, respectively. In this study, a quartz

Arc plasma

Nozzle(Cu)

Pressure sensor(Sunx:DPH-A02)+－PlasmagasAnode(Cu)Observation
Cathodewindow(Hf)High speed color camera(Keyence:VW-6000, VW-9000)

Fig.1. Schematic view of cathode spot observation

window is installed in this nozzle in order toobserve the cathode surface directly. Two types ofhigh-speed color camera (Keyence:VW-6000,VW-9000) are used. The max frame rates ofVW-6000 and VW-9000 are 24,000 fps and
230,000 fps, respectively.
3. Estimation of cathode surface temperature by
RGB signals of color camera
RGB signals of the color camera are obtained
specific values to the black-body temperature.
Theory of RGB intensity ratio to be observed is
calculated by Planck’s law. Calculated theory ofRGB intensity ratios is compared with RGB signalsof each pixel of high-speed color camera images.
As a result, cathode surface temperature distribution
can be estimated[1].
4. Experimental result
Time evolution of the cathode hot spot with arc
current waveform is shown in Fig.2. Just after arc
ignition, the cathode arc spot initially touches at theedge of the cathode, then rotates along the swirl gas
and moves to the cathode center. This local hot spot
rotation heats the Hf cathode surface gradually andExpansion of cathode spot

Hf cathode area Rotation of arc spot

Arc current
[A]

150
100
50
023 ms

21 ms

2 ms

Arc plasma

165 ms

100 ms

250 msArc current increase

050

100
150
200
250
Time [ms]Fig.2. Time evolution of cathode hot spot was observed by VW-6000

Arc ignition (0 ms)

Temperature [103 K]
4.5
4500
4475
4450

Hf cathode area

4425
4400
4375
4350
4325
4100
4075
4050
4025
4000
3975
3950
3925
3900
3875
3850
3.53825
3800
3775
3750
3725
3700
3675
3650
3625
3600
3575
3550
3525
3500
3475
3450
3.0
3425
3400
3375
3350
3325
3300
3275
3250
3225
3200
3175
3150
3125
3100
2.5
3075
3050
3025
3000
4300
4275
4250
4225
4.0
4200
4175
4150
4125

2 ms

21 ms

23 ms

100 ms

165 ms

250 msFig.3. Cathode surface temperature distribution

forms the round cathode spot on the Hf electrode att ~100 ms after ignition. The cathode spot area
spreads gradually from the fixed local cathode spot
to the whole Hf cathode. Moreover, the cathodeemission area becomes larger with increase of thearc current. Figure.3 shows time evolution ofcathode surface temperature distribution estimatedfrom RGB signals of the pictures shown in Fig.2.
The temperature distribution shows that cathodesurface temperature is between 3700 K and 4300 K.
This temperature is higher than Hf melting
temperature (2503 K) and lower than Hf boilingtemperature (4547 K)[2]. Therefore, it is clear thatthe cathode surface is melted during arc discharges.
Thermionic electron emission current density J iscalculated from the estimated cathode temperature.
Hafnium oxide(HfO2) is formed on the Hf cathodesurface by oxygen plasma discharges. However
rates of Hf and HfO2 on the cathode surface are notknown. The J is calculated under the assumptionthat whole cathode surface is solid Hf or solid HfO2.
JHf and JHfO2 is calculated with work function ofsolid Hf and HfO2, and Richardson constant of Hfand HfO2, respectively. As a result, thermionicelectron emission current IHf(thermo) and IHfO2(thermo) areestimated about 160 A / mm2 and 40 A / mm2
respectively when arc current of power supply I(arc)is 135 A. Compared to the I(arc), IHf(thermo)/I(arc)=1.19
and IHfO2(thermo)/I(arc)=0.30 can be calculated.
Although the physical properties of the hot cathodespot have large ambiguity, such as cathode surface
irregularity and formation of oxidation layer, it may
say that reasonable value of I(thermo) is calculated. To
evaluate J in detail, physical properties of Hf andHfO2 as hot liquid are needed.

Cathodespot
0 ms

0.087 ms

0.13 ms

0.17 ms
0.30 ms
0.26 ms
Fig.4. Observation of the cathode material ejection
was observed by VW-9000 (The arc current is 150A)

However such physical properties cannot be
obtained at the moment.
Unstable phenomena of the molten cathode spot,
such as the wavy motion, and ejection of the moltenmaterials from the cathode surface, are often
observed. One is the ejection of droplet. Small
particles of molten material are dispersed abruptly
from cathode surface. The other is bubble formation.
The images of a large bubble formation in themolten cathode spot are shown in Fig.4. The bubble
appears from the spot and grows within ~0.1 ms,
then evaporates intensively, leading the bubble
explosion. After the bubble explosion, cathode spot
goes back to the quiet phase. These intensive anddynamic cathode phenomena can not be seen in thearc voltage and current measurement.
References[1] Y. Yamaguchi: Journal of the Japan Welding
Society Vol.29 No.1 p. 010-017(2011).
[2] S. Izumi: Data book of metal, Maruzen
Company (1984).