The Electrode Erosion in a High Current Pulsed Arc

A.A. Bogomaz, A.V. Budin, M.E. Pinchuk and A.A. Pozubenkov, Ph.G. Rutberg
Institute of Electrophysics and Electric Power of Russian Academy of Science (IEE RAS)
Dvortsovaya nab., 18, St.-Petersburg, 191186, Russia.
rutberg@iperas.spb.su
Abstract. The results of erosion research are presented. It was found new type of erosion in the form of symmetrical
material ejection from the whole electrode butt-end surface on the current pulse falloff. It is connected with violation of
balance between the pressure above an electrode surface, which counterbalances magnetic force, and pressure in depth of
a superficial layer. The ejection exists in the limited range of the current and the current density and was registered in the
range of a current amplitudes 100-250 kA. The bottom border is caused by occurrence of a pinch-effect, the top border is
connected with a prevailing influence of vapor phase erosion. Transition from the erosion in a liquid phase to the vapors
corresponds to sharp growth of the electrode voltage falls.
Keywords: electrode erosion, high current arc, high pressure arc.
PACS: 52.58.Lq, 52.80.-s, 52.80.Mg, 52.80.Tn
At a research of processes of erosion in arc discharges the problem of ratio between liquid and vapor phases of a
erode electrode material plays a important role. The electrode material can erode as separate partiсles of an
electrode, as drops, as a vapour consisting of neutral and charged atoms of an electrode [1, 2]. The quantity of each
from these fractions depends on concrete conditions of discharge: current amplitudes, current density, discharge
duration and value of initial pressure, material and geometry of electrodes. For arcs of atmospheric pressure and
lower with current amplitude up to several kiloamperes this problem is analyzed in detail (for example in [3] and its
references). This paper is connected to a research of electrode erosion in the high and superhigh pressure discharges
at current amplitudes of 105-106 A. At this parameters earlier by us the form of erosion as a symmetrical ejection
with all surface of an electrode was registered. The results of experiments verifying detected effect and the
estimations of parameters of the ejection are presented below. Is shown, that the substance of the ejection stands in
vapour phase, not in liquid as was assumed earlier [4, 5].
EXPERIMENTAL RESULTS AND DISCUSSION
Parameters of experiments are: amplitude of a current 100 – 1500 kA, initial gas pressure 0.1 – 35 MPa, steel and
tungsten electrodes of 20 mm and 6 mm diameter accordingly, interelectrode gap 5 – 50 mm. The ejection is
registered in a range of current amplitudes of 100 – 200 kA.
The photo of the ejection from the surface of the cathode in discharge in hydrogen with amplitude of a current
125 kA is shown in a fig. 1a. On the fig. 1a the shadow photo with incomplete filtration of own radiation at a
wavelength 514.5 nm is presented. In a fig. 1b the photo of the ejection in the discharge in the air with current
amplitude of 180 kA is presented, where the ejection is observed from the anode surface in a difference from fig. 1a.
In many papers, concerning electrode erosion in discharges with current amplitude ≤105 A, are underlined, that
the main part of erosion occurs in a liquid phase [2, 6-8]. On our sight, as will be shown below, it is valid only at an
average current density on an electrode ≤105 А/см2. In these papers at solution of the thermal task for definition of
melting depth of electrodes the voltaic equivalent Ue is assumed equal 10-20 V [2, 8, 9]. It corresponds to actual heat
flow of energy on an electrode q = JUe and is close to the value near-electrode voltage drop U0 of ~ 10-20 V [1, 10].
In discharges of megampere range and at superhigh pressure the discharge channel is contracted by thermal
contraction and pinch-effect. In this case the current density j, average on a electrode surface, can achieve 106-107
A/cm2, and Ue – hundreds volt [11, 12]. The estimations show, that at such conditions a radiation gives the main
contribution in q [6, 7, 11]. In [11] the parameters of erosive plasma near to cathode and anode for discharge in
hydrogen with current amplitude Jmax = 380 kA were defined and is shown, that the Ue value is ~ 500 V, that
corresponds to maximum value q ~ 6×108 W/cm2. For the discharge in hydrogen at maximum pressure ~30 MPa and
Jmax = 1.5 MA [13] maximum values q is ~ 2×109 W/cm2. The energy flow on electrodes in this conditions becomes
comparable with flux of laser radiation moderate and high-power, which action on various metals is investigated
enough explicitly [14-16]. Thus, in the discharge the erosion jets are formed as well as for laser ablation and its
result to sharp increasing of voltage drops in near-electrode sheath [11, 12]. At q ≥109 W/cm2 [13] the discharge
burn in vapours of eroded.

CONCLUSION
1. The symmetrical ejection of substance from all surface of electrode butt-end was registered for cathode and
anode in hydrogen and air. It confirms a hypothesis about the uniform mechanism of such ejection for the anode and
cathode. It is connected with violation of balance between the pressure above an electrode surface, which
counterbalances magnetic force, and pressure in depth of a superficial layer [4].
2. For megampere range discharge the predominant erosion of electrodes is a vapour erosion. The sharp
increasing of a near-electrode voltage drop corresponds to the beginning of formation of intense erosion jets.
3. The substance in the ejected hemispherical shell is in a vapour phase. The ejection from all surface of
electrode butt-end exists in a limited range of amplitudes and density of a discharge current. The low boundary is
stipulated by pinch-effect, the high boundary is connected to predominant of erosion in a vapour phase.
ACKNOWLEDGMENTS
The work is partially supported by Russian fund of basic researches (grant № 05-02-16091-а, 05-02-16001-а and
№ 04-02-17527-а) and grant of the President of Russian Federation (grant № NSh-885.2003.2 and МК-2052.2005.8).
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