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.