Figure 1

The experimental $I\mathrm{\text{−}}V$ characteristic of a $12.5\mu \mathrm{m}$ diameter tungsten filament operating at low temperature in the vapor phase of helium. The dense set of data points is for constant voltage operation, and is a steady state set of points for slow variation of the voltage. The vertical dotted line is the path for a constant current mode (data not shown). These data were for a thoriated filament with a lower work function than pure tungsten. The dashed curve is the path that would be followed for a filament operating in vacuum. $V_C$ and $I_C$ are the critical voltage and current for a dramatic behavioral change where the filament becomes hot and starts emitting electrons.Reuse & Permissions

Figure 2

The calculated temperature of a $25\mu \mathrm{m}$ diameter tungsten wire in the vicinity of the peak temperature as a function of current through the wire. The different curves are for different heat transfer coefficients between the wire and the helium, using values 4 times larger and 4 times smaller than a selected standard value. The inset shows the calculated temperature profile of the wire for currents of 140 to 220 mA, in steps of 20 mA. Note the sharpening of the profile as the peak temperature increases.Reuse & Permissions

Figure 3

The calculated voltage-current characteristic as the current or voltage is kept fixed and the Nusselt number transitions from high to low value, shown in the inset. In this case it was varied by a factor of 3. The main characteristics of the experimental curve (see Fig. 1) are nicely reproduced in this model. This simulation is closer to what we expect for the filament operating in the vapor phase. For operation in the liquid phase of helium a much sharper transition (current flyback) is observed when the vapor sheath is formed.Reuse & Permissions