The probe is a simple faraday cup 12 mm in diameter and 27 mm deep. The cup is installed in the grounded aluminum housing and insulated from it with mica. In front of the cup the biased suppressor grid is installed. To insulate the grid holder from the fuor shell and the farady cup housing the short glass tubes are used.
Construction drawing of the electron probe. The interface to the vacuum vessel is by the KF25 flange.
Components of the probe, cleaned and ready for assembly.
The complete probe. All joints are sealed with the Fraday's wax.
The probe installed on the fusor. The farafay cup is connected to the ground by 150 kΩ to allow the electron current to be observed with the scope.
The measurement circuit. The bias voltage is applied to the grid by the ZWN-42 3kV variable power supply.
Alternatively the electron current can be measured directly. The drop resistor is then removed and the Meratronik V623 microvoltmerer/nanoammeter is connected directly to the faraday cup.
Today I just made some brief tests. First I've checked the suppressor grid operation:
The typical trace with the suppressor grid grounded. Upper trace - faraday cup current, lower trace - total fusor current. The negative electron current peaks are perfectly aligned with the positive total current peaks. 0.5 ms/div, Ch. A 50 mV/div, Ch. B 2 V/div.
+55V applied to the suppressor grid. The fusor electrons are more or less blocked. Some negative corona current between the suppressor grid and the faraday cup can be observed.
Next I made some direct electron current measurements. All in air at 2.5e-2 Torr with the standard wire cathode, suppressor grid grounded.
Code: Select all
Uf If Ie
[kV] [mA] [μA]
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0.7 5.0 2.8
1.0 11.0 4.6
1.8 28.0 13.8
2.4 45.0 23.5
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