Wehnelt Electrode Research Articles

Drift of amplification factor and its effects in a tri-electrode electron gun†

The drift of the amplification factor p. should induce drifts of the emitted current I a in a fixed-bias circuit and the beam energy in a self-bias circuit. The μ-drift due to the heat from the filament was measured, and also calculated from the I A-drift for an hour. The relative μ-drift thus calculated was 1-3% in 10 min in a conventional gun, and it decreased to smaller than 003% in 10 min in an improved gun, where thermal expansion of the Wehnelt electrode was compensated by the movement of the filament supporter. Energy of the beam was 20 kev. The initial emitted current I A(t= 0) was 198 μA in the conventional gun and 14.4 μA in the improved gun. The relative energy-drift should be proportional to the relative μ-drift and inversely proportional to (μ+ 1). Accordingly, the relative energy-drift should be 1.3 × 10−6; in 10 min in the conventional gun, while it should be smaller than 3 ×10−6 in 10 min in the improved gun, when μ is assumed to be 100 in both guns.

Triode characteristic of a tri-electrode electron gun†

A tri-oloctrode electron gun with a Wehnelt electrode and a hair-pin filament can be regarded as a triode (Munakata and Watanabe 1962). Hence its electrical characteristic should be specified with the triode constants ; amplification factor μ , mutual conductance gm and anode resistance rA (= μ /gm). The dependence of these constants upon geometrical parameters of the gun has been experimentally studied in the following regions : (1) the height h (0 to —1.6 mm) of the filament (0.1 mm φ-W-wire) tip from the Wehnelt electrode, (2) the diameter dW (1.5, 1.9 and 3.0 mm) of the hole in the flat-faced Wehnelt electrode, (3) the gap L (4— 12 mm) between the Wehnelt electrode and the anode, (4) the hole diameter dA (2.0, 2.4 and 3.0 mm) of the anode, (5) the accelerating voltage U (8.5 and 17 kv), (6) the anode current IA (≤ 200 μA) and (7) tho filament current IF (2.1-2.5A). In a space-charge-limited region, 1A is proportional to νw3.6±0.4, where Vw means control bias voltage referred to the cut-off bias volta...

New points of view in the design of electron guns for cylindrical beams of high space charge

The paper outlines the influences of the anode aperture, disregarded in the theory of the Pierce gun, which lead to an unsatisfactory functioning of such electron guns, particularly in the case of a high perveance. These influences are taken into consideration by imposing a rectilinear boundary on the beam, so that the space charge potential can be simply determined in the electrolytic tank. For this purpose, the anode is made tubular and sharp edged, and besides the Wehnelt electrode, an additional electrode of substantially the same potential is introduced, being mounted in the immediate vicinity of the anode aperture. Simple electrode forms and their favourable positions are given for a wide range of dimensioning. By the introduction of corrections, the dimensioning rules are extended to high perveance values. The magnitude of the current leaving the assumed boundary of the beam because of its initial thermal velocity is determined.

Behavior of the Wehnelt electrode in different gases: K. Fredenhagen.( Phys. Zeitschr., xv, ig.)

Behavior of the Wehnelt electrode in different gases: K. Fredenhagen.( Phys. Zeitschr., xv, ig.)