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Abstract:
Recently, a constricted attachment of an atmospheric pressure low-current argon arc in the centre of the
flat end face of a thoriated tungsten cathode was observed and spectroscopically analysed. Its diameter of
0.6mm and its length of the free standing part of 10mm are the typical dimensions of electrodes for
high-intensity discharge lamps. This paper gives a physical interpretation of the axially symmetric arc
spot by a simulation of its properties with a cathodic sheath model which takes into account a reduction
in the work function above a critical temperature of the cathode surface by a thorium ion current.
At first the optical observation and spectroscopic investigations are recapitulated. Then, an overview
is given on the essential elements which are needed to simulate the cathodic arc attachment on a hot
electrode. A simulation of a central cathode spot with these elements gives results which are far away
from the experimental findings if a constant work function φ is used. Therefore, a temperature-dependent
work function φ(T ) is introduced. This φ(T ) transitions from 4.55 to 3 eV above temperatures of the
order of 3000 K. With this emitter spot model a constricted arc attachment is obtained by simulation in
the centre of the flat end face of the cathode in accordance with experiment. For currents below
iarc,max ≈ 15.5A, two spot solutions with different cathode falls are found. They form a
current–voltage–characteristic consisting of two branches which extend from a turning point at iarc,max to
lower currents. For iarc > iarc,max, only a diffuse mode of cathodic arc attachment is obtained. It is shown
by a comparison with measured data for iarc = 7.5, 10, 12.5 and 15A that the solution with the lower
cathode fall is observed experimentally.
