I want a crude way to confirm the existence of an electron cloud in a moderate vacuum of 2 mTorr. In my set up, filaments emit electrons into a magnetic trap which concentrates them (similar to an electron beam). From my understanding, one can electrically bias a small wire immersed in the electron cloud and observe voltage changes relative to the biasing of the probe. This is similar to using a constant electrical bias on a Langmuir probe to determine plasma floating potential.

I am confused on how to positively bias the probe so it will attract electrons in the electron cloud and give a voltage & current response signaling the existence of the electrons. If I connect the positive end of a battery to the probe, the probe will not have a positive bias due to the lack of a closed circuit between the negative and positive terminal of the battery. No electrons will be attracted to the wire in this case.

I have a fundamental misunderstanding of how to positively bias a wire in this case. How can I get electrons from the electron cloud to induce a voltage or current on the probe so I can read it with a voltmeter or oscilloscope?

  • $\begingroup$ You cannot "measure a plasma", the same way as you cannot "measure a solid" or "measure a gas". That doesn't make sense. You want to measure some property of the plasma. Which property is that? Conductivity? $\endgroup$ Commented Jan 10, 2019 at 3:35
  • $\begingroup$ Right. I should specify this. I want to measure the floating potential of the electron plasma, and see some sort of induced voltage or current on the probe based on the electron density present in the plasma. Basically, I want a crude way to confirm an electron plasma is present in my vacuum chamber. $\endgroup$ Commented Jan 10, 2019 at 3:41
  • $\begingroup$ A bunch of electrons in a magnetic trap are not a plasma - plasmas are electrically neutral with equal numbers of free electrons and ions. To measure if there is charge in the trap, you should measure the properties of the trap. $\endgroup$
    – Jon Custer
    Commented Jan 10, 2019 at 14:07
  • $\begingroup$ Yes, in this case I am trying to measure a dense region of electrons with no ion population so it is not a plasma. I edited my question to clarify this misnomer. I already know properties of the trap (magnetic field strength, symmetry, etc.). I am asking how I can get a signal of charge in the trap with an electrically biased wire! $\endgroup$ Commented Jan 10, 2019 at 15:35
  • $\begingroup$ Neutrality is not necessary for a system to exhibit the hallmarks of a plasma (e.g., Langmuir waves). As long as there are enough electrons for the Debye length to be smaller than the trap size, it is a plasma. To wit, "non-neutral" plasmas are an active area of research. $\endgroup$
    – Endulum
    Commented Jan 10, 2019 at 16:20

1 Answer 1


The answer to this is actually much simpler than I anticipated. For a crude measurement of electron potential, all I need to due to do is attach the Langmuir probe to the ground end of voltmeter. I can ground the other voltmeter cable to the vacuum chamber to read a negative voltage potential of the electron cloud. Basically, the Langmuir probe acts as an extension of the voltmeter and measures the electrons with respect to ground. No need to bias the probe or induce currents on it!

Got this detection method to work today.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.