The higher heat transfer rates at the anode compared with those at the cathode can be explained by the physical phenomena occurring in free burning arcs. In plasma generators the superimposed forced convection may modify the picture somewhat. The heat transfer to the anode is due to the following effects: 1. Heat of condensation (work function) plus kinetic energy of the electrons impinging on the anode. This energy transfer depends on the current, the temperature in the arc column, the anode material, and the conditions in the anode sheath. 2. Heat transfer by molecular conduction as well as by radiation from the arc column.

The heat transfer to the anode in free burning arcs is enhanced by a hot gas jet flowing from the cathode towards the anode with velocities up **f. This phenomenon has been experimentally investigated in detail by Maecker (Ref. 1). The pressure gradient producing the jet is due to the nature of the magnetic field in the arc (rapid decrease of current density from cathode to the anode). Hence, the flow conditions at the anode of free burning arcs resemble those near a stagnation point.

it is apparent from the above and from experimental evidence that the cooling requirements for the anode of free burning arcs are large compared with those for the cathode. The gas flow through a plasma generator will modify these conditions; however, the anode is still the part receiving the largest heat flux. An attempt to improve the life of the anodes or the efficiency of the plasma generators must, therefore, aim at a reduction of the anode loss. The following possibilities exist for achieving this: 1. The use of high voltages and low currents by proper design to reduce electron heat transfer to the anode for a given power output. 2. Continuous motion of the arc contact area at the anode by flow or magnetic forces. 3. Feed back of the energy transferred to the anode by applying gas transpiration through the anode.