The excitation theory of arcs with evaporating cathodes

Most metals when used as the cathode of an arc discharge cannot reach temperatures sufficiently high to emit electrons thermionically. However, the temperatures are high enough to produce considerable evaporation. Mercury and copper are examples. The older theories suggest that here the current at the cathode is carried by electrons extracted by field emission, photo-electric emission or secondary emission by positive ions, or that the entire current is carried by positive ions produced by thermal ionization in the gas. All the theories are shown to be quantitatively inconsistent with observations. A new theory is suggested: electrons are released from the cathode by the impact of excited atoms. The electrons gain energy in the cathode fall and produce excited atoms in the dense vapour. The radiation from the excited atoms diffuses, mainly in the direction of the cathode, by successive absorption and re-emission in the vapour and is ultimately absorbed by atoms which strike the cathode. Positive ions are formed in the vapour by collisions between excited atoms, and by electrons colliding with excited atoms. The positive ions have three functions; their space charge provides the cathode fall in potential, they supply energy for evaporation and they transfer momentum to the evaporated atoms. The majority of the latter are back-scattered and in this way a vapour density is set up close to the cathode which is many orders of magnitude larger than elsewhere. An exceptionally high density, however, is a necessary condition for a low cathode fall and a high current density. The new picture is also consistent with the observed force on the cathode and the evaporation rate. The energy balance also supports the theory.

[1]  Allan C. G. Mitchell,et al.  Resonance radiation and excited atoms , 1934 .

[2]  R. Rompe,et al.  Zur Theorie der kathodischen Entladungsteile eines Lichtbogens , 1940 .

[3]  K. T. Compton,et al.  Potential Drop and Ionization at Mercury Arc Cathode , 1931 .

[4]  von R. Dorrestein.  Die auslösung von elektronen aus metallen durch metastabile atome, angewandt auf die messung von anregungsfunktionen , 1942 .

[5]  L. Tonks.  The Force at an Anchored Cathode Spot , 1936 .

[6]  A. E. Robson,et al.  An Explanation of the Tanberg Effect , 1957, Nature.

[7]  J. Slepian.  Theory of Current Transference at the Cathode of an Arc , 1926 .

[8]  E. Kobel,et al.  Pressure and High Velocity Vapour Jets at Cathodes of a Mercury Vacuum Arc , 1930 .

[9]  R. M. Robertson.  The Force on the Cathode of a Copper Arc , 1938 .

[10]  W. Lamb,et al.  On the Extraction of Electrons from a Metal Surface by Ions and Metastable Atoms , 1944 .

[11]  J. R. Haynes.  The Production of High Velocity Mercury Vapor Jets by Spark Discharge , 1948 .

[12]  A. E. Robson,et al.  Origin of Retrograde Motion of Arc Cathode Spots , 1954 .

[13]  P. C. Keenan,et al.  REVIEW: Resonance Radiation and Excited Atoms , 1934 .

[14]  S. S. Mackeown.  The cathode drop in an electric arc , 1929 .

[15]  R. Tanberg,et al.  On the Cathode of an Arc Drawn in Vacuum , 1930 .

[16]  A. E. Robson,et al.  Excitation Processes and the Theory of the Arc Discharge , 1955, Nature.

[17]  T. J. Killian.  The Uniform Positive Column of an Electric Discharge in Mercury Vapor , 1930 .

[18]  W. Penney.  The Theory of the Excitation of Atomic Mercury by Electron Impact , 1932 .

[19]  W. Blevin,et al.  Current Densities in the Cathode Spots of Transient Arcs , 1949 .

[20]  H. Bertele.  CORRESPONDENCE: Current densities of free-moving cathode spots on mercury , 1952 .

[21]  K. T. Compton.  On the Theory of the Mercury Arc , 1931 .

[22]  L. Tonks.  The Pressure of Plasma Electrons and the Force on the Cathode of an Arc , 1934 .

[23]  L. Tonks.  The Rate of Vaporization of Mercury from an Anchored Cathode Spot , 1938 .

[24]  H. Davy.  Elements of chemical philosophy , 1812 .