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Experiment 18 (06/06/05)- Ion triode

© Engineer Xavier Borg - Blaze Labs Research

The triode term usually refers to a type of vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electrical amplification device and a few types like the 13EM7 and 12AX7 still remain in production for a few dedicated amplification purposes. In a vacuum triode, the cathode to anode electron current is controlled by the grid voltage.

The ionic triode shown below, works in a similar fashion, with the difference that electric current flows using positive ionised air molecules (mainly Nitrogen) to pass the current instead of the conventional negatively charged electrons. Thus this device does not need a vacuum or glass envelope, but works in open air.

Ion triode
Ion triode experiment

The triode shown above has a 17cm corona wire, held 6cm above a rectangular aluminium grid measuring 29cm x 12cm, with 3mm grid cell spacing. Below the top or control grid, a second similar grid, the collector grid can slide up or down the vertical supports in order to vary the electric field strength between the two grids. Positive ions are generated by the top corona wire held at a high voltage relative to both grids. The lower grid is the ion collector/ destination and the control grid is set to the required positive voltage. The aim of this setup was primarily to detect how many ionised air molecules can make it through the control grid and hit the collector grid, since it was argued that when the ionised molecules travel through the equipotential control grid, with no electric field within it, the ions momentarily loose their driving force and become trapped and conducted all to the collector grid terminal, leaving no current to flow to the lower grid.

Test run #1 Both grids grounded

The first setup tested the triode for conduction to the collector grid with no electric field between the control and collector grid. The voltages in the setup were as follows:

Corona wire : +40kV
Control grid: 0kV
Collector grid: 0kV

V/d corona-control grid= 40/6= 6.66kV/cm
Current readings:

Corona current: 221uA
Control grid current: 215uA
Collector grid current: 6uA

The results indicate that with the control voltage at zero, and no electric field in between the grids, just 3% of the ions reach the collector grid. Blowing external air from top of the device increases the collector current by 2uA, indicating that the air flow generated by the ion triode (which is very similar to the Ionocraft design is reponsible for the few escaping ions which get swept down by the neutral air flow.

Test run #2 Equal V/d above and under control grid

In the second setup, the control grid was set at +10kV, with the collector grid grounded. The distance between grids was set to 15mm to get approximately the same electric field above and below the control grid. We kept the corona to control grid voltage difference and hence corona current the same as the previous setup. The voltages in this setup were as follows:

Corona wire : +50kV
Control grid: +10kV
Collector grid: 0kV

V/d corona-control grid= 40/6= 6.66kV/cm
Current readings:

Corona current: 220uA
Control grid current: 100uA
Collector grid current: 120uA

These results indicate that over 50% of the ions generated at the corona wire, have now been collected by the collector grid, after successfully passing through the control grid. Again, blowing from above changes the collected current by only a couple of uA, indicating that the electric field is the main driving force, and that the ions are not being trapped within the control grid. Also, equal V/d does not necessarily mean equal E-field gradient, as the top part is a wire to plane (non uniform field) and the lower part is a plane to plane uniform field.

Test run #3 Higher V/d below control grid

In the third setup, the control grid was set again at +10kV, with the collector grid grounded. This time, the distance between grids was set to 9mm to get a saturated current condition between the two grids. The electric field below the control field is now at a higher level than that above it. The voltages in this setup were as follows:

Corona wire : +50kV
Control grid: +10kV
Collector grid: 0kV

V/d corona-control grid= 40/6= 6.66kV/cm
Current readings:

Corona current: 380uA
Control grid current: 100uA
Collector grid current: 280uA

This setup was quite noisy so corona formation at the grids may have contributed to the measured currents. These results indicate that over 73% of the ions generated at the corona wire, have now been collected by the collector grid.

Test run #4 Floating control grid

First, with corona wire disconnected, I connected the supply between the 2 grids at 3cm apart. Verified that current was less than 0.1uA for voltages 0 to 15kV, so this means that no ionisation can happen at the control grid for E-field strengths up to 5kv/cm. The control grid was then left floating, and the supply connected between corona and lower grid. Corona to conrol grid spacing=6cm, grid to grid spacing=3cm. Theoretically, a single stage setup with a 12cm wide grid spaced at 9cm from the corona wire should consume 24uA. An hv probe was connected to the floating grid, and the supply voltage was increased from zero until the voltage on the floating grid was 15kV, at which point the supply voltage was 31kV. Current read on the lower grid at this point is 11.8uA, almost half the current which should have theoretically reached the grid with no floating grid.
Disconnecting the probe, the collector grid current when further down, to 7uA, resulting in a screening factor of 70%. Usually my 400Mega Ohms hv probe has no effect on the readings, but here we are talking about a floating grid, charging up at a current of a few uA.
Some crackling now occurs between the grids indicating that the voltage at the floating grid is now higher than that which was measured by the probe. This means that a big part of the measured 7uA are due to air breakdown due to the now higher E-field between the two grids.
It can be concluded that a smooth floating grid, will provide a very high degree of shielding from incoming ions, given that the E-field gradients around it are not enough to cause further air ionisation.

Shield response
Shield response trace

The above trace shows the shield response of the floating grid. Full shielding is achieved in just 1.5 milliseconds. The ion cloud will only be able to pass at its full density for the first 0.1ms

Shield charging
Voltage on floating grid as it is reached by ion cloud.

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