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?