Electron current measurements

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Maciek Szymanski
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Electron current measurements

Post by Maciek Szymanski »

To complete the grid loss measurements I've decided to build the electron current probe, and do some measurements. In my opinion the electrons play the key role in the simple, non-gunned fusor, And I'd like to find some more about their currents and energies.

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.

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Construction drawing of the electron probe. The interface to the vacuum vessel is by the KF25 flange.


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Components of the probe, cleaned and ready for assembly.


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The complete probe. All joints are sealed with the Fraday's wax.


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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.


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The measurement circuit. The bias voltage is applied to the grid by the ZWN-42 3kV variable power supply.


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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:


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B888A17A-5714-4319-B3D0-49466E6D47F9.jpeg (15.62 KiB) Viewed 3911 times
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.


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17A567A5-EB37-43F5-97BC-A3E5F8990FD2.jpeg (16.04 KiB) Viewed 3911 times
+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]
     -------------------------
       0.7     5.0       2.8
       1.0    11.0       4.6
       1.8    28.0      13.8
       2.4    45.0      23.5
     -------------------------
       
Surely I've to do the math to calculate the relationship between the currents, and do much more measurements. But basically I'm quite hapy with the probe, as is seems to be quie useful tool.
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John Futter
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Re: Electron current measurements

Post by John Futter »

Maciek
The suppressor grid has to be negative with respect to the faraday cup

typically at the voltages we use 0-400 volts negative will suppress secondary electrons so that you get a true indication of ion current

nearly 20 years ago I patented a self suppression scheme using 400 volt transil diodes (P6KE series)to self generate the required bias
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Maciek Szymanski
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Re: Electron current measurements

Post by Maciek Szymanski »

But I'm measuring the electrons, not ions. I think, that the positive grid decelerates and collects the electrons incoming at the drift speed. I have not calculated the drift speed yet, but I'm supposing it to be much below the total cathode voltage at 2,5e-2 Torr and sigle kVs.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
John Futter
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Re: Electron current measurements

Post by John Futter »

Yes
but your electron current is a sum of the actual ion current + whatever secondary electrons are emitted from your cathode
Secondary electron could be as much as 2-6 times the actual ion current
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Maciek Szymanski
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Re: Electron current measurements

Post by Maciek Szymanski »

I do not expect any ion current in the collector located in on the fusor shell. But the negative polarity makes sense - it should decelerate the incoming electrons (so at the certain level the electron current should drop to zero) and repulse the secondaries back to the collector. I think it will be visible as the slight increase of the electron current when the secondaries are suppressed and then the gradual reduction when the incoming electrons are screened. I have check it!
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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Maciek Szymanski
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Re: Electron current measurements

Post by Maciek Szymanski »

I've made some measurements with the negative biased grid. What it interesting for the moderate bias voltages (in few hundreds of volts range) it worked as expected - first a light increase of the electron current than the drop. I have assumed that the current at this maximum is the "real" collector current. But when the voltage is further increased the collector current starts to rise again while staying negative. This surprised me a bit as I expected to be able to stop the incoming electrons with enough negative bias. The only explanation I have for now is that the negatively biased grid accelerates the ions generated in the anode region and those accelerated ions knock out electrons form the grid causing the negative collector current. Seems possible to me at the voltage in 1 kV range.

For other hand I have compared the electron current with the grid loses I have measured earlier. It is very hard to scale the collector current to the total electron current. But it may be assumed that the relation of the shell loss is linear to the ratio of the probe current to the total currentthe hypothetical loss curve may be obtained by just reversing the collector current curve and scaling it, taking that the Fusor power = grid loss (ion current) + shell loss (electron current).

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The upper curve is the ratio of the electron probe current to the total current (left axis). The lower continuous curve is the percent of the power lost to the cathode measured with the aid of the pyrometer (right axit). The dashed curve is the same loss obtained form the electron current. The absolute values are assumed but the character of the curves seems to fit quite well.

Other observation worth noting is that increase of the pressure increases the total current, but reduces the electron current.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland
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Re: Electron current measurements

Post by Richard Hull »

The fusor is a heat engine following logical expectations. Loss total in the device = ion loses + electron current losses. These effectively sum to equal input power to a degree and heat all components involved via collision with shell and grid. Only one part in a billion of the energy input is converted to fusion energy with the introduction of low pressure deuterium gas. Never pretty or efficient, but it does fusion.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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