Bombardment of field-emission cathodes by positive ions formed in the interelectrode region

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Abstract
Ions formed by the electron stream close to a field-emission cathode point return to the cathode. These ions can cause emission fluctuations by local work function changes, and erode the emitting surface by sputtering. An analysis of these effects with hydrogen as the residual gas over the range of tip radii from 500 A to 5000 A. which operate at voltages from I0O to 1000 volts, shows that for a given electron current the ion current to the cathode for the smaller cathode is about 16% that of the larger. However, the sputtering rate of the smaller cathode is three orders of magnitude less than for the larger cathode, mainly due to the rapid full-off of sputtering rate with ion voltage in the region below 150 volts. The lives of the cathodes, assumed to end when the radius of the cathode has been eroded by 10%, tend to be about the same, due to the lesser volume of material that has to be removed from the smaller cathode. Cathode lives may be extended by ensuring that the residual gas is hydrogen, which produces the lowest, sputter yield ion. using applied voltages as close to the threshold for sputtering as possible, and by using an array of smaller cathodes, at lower current per tip rather than a single larger eathode.

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