Failed Electron Detection

Struggling to detect an electron well recently. Doubled checked the electrical connections for the filaments, polywell, and langmuir probe today. Electrical setup was intact. More needs to be done to tease out what the issue is.

Entire system – capacitor bank discharge circuit on left and vacuum chamber on right.
Top view showing pumping system behind vacuum chamber and circuit. Two roughing pumps for extra fast pump down!

Detected a leak at the top flange of the diffusion pump the other day. Going to troubleshoot this later. Decided to prioritize crude electron detection under roughing pressures of 20mTorr. While mean free path of oxygen and nitrogen is around 3.5 – 3.7 mm at these pressures, electrons have a larger mean free path at these pressures and can concentrate in a cloud at the center of our polywell.

Pressure for crude langmuir probe tests. We could do better with this as always
View of filaments turning on inside chamber.
Electron emitting filaments illuminating polywell (red coils in background).
Opened chamber to check electrical connections… all good!

Filaments are in good condition too (no sputtering or coating and run only at low pressures), although they created a colorful iridescent coating on some tin foil around them. What is this from?

6 broken halogen bulbs serve as the thermionic electron emitters for our polywell. These should deliver a strong electron current due to the long filament length and emission area.
Aluminum around filaments is tinted many colors. What caused this to happen?

I believe it is an issue with the langmuir probe detection circuit. Its confusing how to bias the probe so electrons trapped in the polywell can be detected. When hooking the voltmeter up to probe directly I get no response from the electron well. There likely needs to be a follower circuit for the probe to properly bias it for attaining a signal on the voltmeter or oscilloscope.

Some papers I’ve read on langmuir probe theory briefly touch on follower circuits. These circuits have a variable voltage source for driving different currents and voltages on the probe over a period of time. With the polywell, we don’t have the time to make these measurements since it is run as a single shot device on the timescale of milliseconds. From what I can tell, langmuir probes would not work well with the polywell.

I have posted a question to clarify langmuir probe use on StackExchange: https://physics.stackexchange.com/questions/453224/how-to-measure-an-electron-plasma-with-an-electrically-biased-probe

I also contacted Mark Suppes for advice on crude langmuir probe measurements! Hopefully I get the detection figured out in the next couple days.

Some more pictures of todays work:

Work table needs cleaning…

We have two polywell models — a steel printed frame wrapped with 33 turns of small wire gauge. Coils on steel polywell have resistance of 2.9 Ohms, which limits currents to 5.1kA turns (units of amps x turns). We want stronger magnetic fields than this, so we will rewrap this frame with a thicker gauge soon.

Polywell models side by side

The white model is plastic 3D printed and can hold over 60kA turns. It’s wrapped with 14 gauge wire and has 60 turns on each coil. At max currents the plastic frame would likely explode from the opposing magnetic forces. We hope to print this model in steel eventually and get magnetic field pressures of > 1 Tesla on the ring axes.

Look at those thick coils!

Leave a Reply

Your email address will not be published. Required fields are marked *