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The sums of the cathode sheath and anode sheath voltages, , for atmospheric-pressure arcs in helium between non-thermionic electrodes of Fe, Cu and Ti, have been measured to be 20.4, 18.2 and 21.4 V respectively. These values are significantly higher than the values of in , argon and air, which all have a lower ionization potential. The values of strongly depend on the plasma ionization potential, but are almost independent of electrode work function, which suggests that cathode sheath ionization rather than field emission is a principal cathode process for electron formation.

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