Thoriated tungsten electrodes

FIGURE 1.21. Variation of emission current density with tungsten for pure and thoriated tungsten electrodes [1.93] according to Ref. 1.92. 

STATEMENT OF COMMISSION Vm ON HEALTH ASPECTS EM THE USE OF THORIATED TUNGSTEN ELECTRODES... 

Thorium oxides are found in Thoriated Tungsten Electrodes up to 4.2% (ISO 6848-WT 40 Electrode) . Thorium is radioactive and may present hazards by external and internal exposure. If alternatives are technically feasible, they should be used. 

Although thoriated tungsten electrodes are frequently nsed in industry to provide easier arc starting, greater stability, and reduced weld metal contamination, Crim and Bradley (253) found that exposures of workers during grinding and welding with these elecfrodes were below the DAC. 

Crim EM, Bradley TD. Measurements of air concentrations of thorium during grinding and welding operations using thoriated tungsten electrodes. Health Phys 1995 68 719-722. 

Apparatus. Plasma Jet. A section of the plasma arc gun is shown in Figure 1. A direct current arc was struck between the thoriated tungsten rod, which acted as the cathode, and the copper tube anode. Both electrodes were... 

Electrodes of 2%-thoriated tungsten are the most frequently used water-cooled nonconsumable electrodes. Water-cooled copper anodes have been widely used in experimental work. Figure 1 shows a typical plasma jet assembly. A reactor chamber may be of any configuration desired to accommodate different feeding and quenching devices. 

The electrodes were made from 2% thoriated tungsten helium-arc welding electrodes ( /16 inch x 7 inches), which are readily available (Philips Elmet Corp., Lewiston, Me., or Van Dresser and Hawkins, Inc., Atlanta, Ga.). The design of the electrode holders provides for easy removal from the chanber and for stable alignment of the cathode with the anode. 

Similarly, timgsten electrodes used in tungsten inert gas welding and filaments used in light bulbs and vacuum tubes are strengthened by dispersions of thoria (Th02) particles (thoriated timgsten). The presence of thoria in W also lowers work function and is used to improve the efficiency of electron emitters in x-ray tubes and other electron beam devices. 

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