Production of square temperature waves in filaments of field emission microscopes operating at low temperatures
An important consideration in the measurement of activation energies of surface processes observed with the field emission microscope is the production of a square temperature wave. That is, the initial temperature rise to the final operating temperature must be sufficiently rapid so that uncertainties in time measurements can be eliminated. The temperature-current characteristics of a typical field emission microscope tungsten filament operating in a low temperature bath such as liquid nitrogen or liquid helium are derived from the well known solution of the heat conduction equation. The steady state current-temperature curves are of the form i = F + (B/ T) where B is negative for filament end temperatures above 50° K, zero at this temperature, and negative below. The steady state temperature-current curve for the filament of a tube operating at liquid helium temperature represents an unstable equilibrium. These predictions were confirmed experimentally with a 0.1 mm diameter, 2.5 cm long filament. A circuit capable of producing a filament temperature above 4.2° K to any desired temperature below 600° K in less than 0.3 sec will be described.