Assuming that at the reduced pressures and temperatures of the cathode higher than 500ºC the convection can be neglected, the heat from the cathode can be lost only by radiation. At the equilibrium, when the cathode temperature is constant the heat lost by radiation must equal the head received from the ions impacting the cathode. Thus the power lost to the cathode is:
Code: Select all
2 Pk = S⋅Φ = S⋅σ⋅T where: S total area of the cathode σ Stefan Blozman constant 5.67e-8 W/m^2*K^4 T cathode temperature
The experimental setup is described in my previous posts, especially viewtopic.php?f=6&t=13921.
The pyrometer of Pyrolux II type with measurement range 700ºC to 2000ºC was installed on the tripod. A small flat mirror was installed above the fusor viewport to allow observation of the cathode through the pyrometer. The celling lights above the fusor were turned off during the measurements.
The pyrometer installed in front of the fusor.
The detail of the observation mirror
The measurements were conducted for two types of the wire cathodes. Both were constructed from three 35 mm loops of the stainless stell wire. One was made of 0.8 mm wire and other from 0.6 mm. The radiating area of the cathode was calculated as:
Code: Select all
2 Sk = 3⋅π⋅D⋅d where: D is the cathode diameter d is the wire diameter
First the measurements were made in air at 1.5e-2 Torr for different power settings with both cathodes. Then for the 0.8 mm cathode the measurement at the maximum power output of the power supply for different pressures were made. In all measurements the secondary side ballast resistor was not used. The primary side ballast was adjusted as needed. The voltage and current flowing through the fusor was also noted to allow calculation of the total power consumed.
The precent of power lost to the cathode vs total power. Air, pressure 1.5e-2 Torr. Continuous line 0.6 mm wire, dashed line 0.8 wire.
The percent of power lost to the cathode and total power vs pressure. 0.8 mm wire cathode. Continuous line - relative power lost to the cathode, dashed line - total fusor power.
What can be interesting - the thiner wire leads to higher power lost to the cathode. It may be attributed to the stronger electric field around the wires allowing easier ion interception. Lower pressures and higher power also seems to reduce the loses to the grid. It may be suspected, that the loses are smaller not due to higher power but the higher voltage resulting in higher ion energies, so plotting the power loss vs voltage may be interesting.