The third method was, to our knowledge, successfully applied for the first time by C. Sheer and co-workers (Ref. 2). The purpose of the present study is to study the thermal conditions and to establish an energy balance for a transpiration cooled anode as well as the effect of blowing on the arc voltage. Gas injection through a porous anode (transpiration cooling) not only feeds back the energy transferred to the anode by the above mentioned processes, but also modifies the conditions in the arc itself. A detailed study of this latter phenomenon was not attempted in this paper. Argon was used as a blowing gas to exclude any effects of dissociation or chemical reaction. The anode material was porous graphite. Sintered porous metals should be usable in principle. However, technical difficulties arise by melting at local hot spots. The experimental arrangement as described below is based on the geometry of free burning arcs. Thus, direct comparisons can be drawn with free burning arcs which have been studied in detail during the past years and decades by numerous investigators (Ref. 3).

Figures 1 to 3 show photographic and schematic views of the test stand and of two different models of the anode holder. The cathode consisted of a 1/4'' diameter thoriated tungsten rod attached to a water cooled copper tube. This tube could be adjusted in its axial direction by an electric drive to establish the required electrode spacing. The anode in figure 2 was mounted by means of the anode holder which was attached to a steel plug and disk. The transpiring gas ejected from the anode formed a jet directed axially towards the cathode below. Inflow of air from the surrounding atmosphere was prevented by the two disks shown in figure 2. Argon was also blown at low velocities (mass flow rate **f) through a tube coaxial with the cathode as an additional precaution against contamination of the arc by air. The anode consisted of a 1/2 inch diameter porous graphite plug, 1/4 inch long. The graphite was National Carbon NC 60, which has a porosity of 50% and an average pore size of 30 This small pore size was required to ensure uniformity of the flow leaving the anode. The anode plug (Figure 2) was inserted into a carbon anode holder. A shielded thermocouple was used to measure the upstream temperature of the transpiring gas. It was exposed to a high velocity gas jet. A plug and a tube with holes in its cylindrical walls divided the chamber above the porous plug into two parts. This arrangement had the purpose to prevent heated gas to reach the thermocouple by natural convection. Two pyrometers shown in figure 1 and 2 (Pyrometer Instrument Co. Model 95) served for simultaneous measurement of the anode surface temperature and the temperature distribution along the anode holder. Three thermocouples were placed at different locations in the aluminum disk surrounding the anode holder to determine its temperature.