During the first trial, our langmuir probe told us that our electron beam intensity was fluctuating at 60 Hz.
This is a problem, because one of the main things we are trying to study is the way changes in beam intensity affect potential well depth, so we want a steady intensity. The frequency of the fluctuations suggest that the AC-powered hot cathode is to blame. I don’t totally understand the details of how a hot cathode running on AC 120v 60Hz translates to this waveform:
The important thing is to prevent it. To do that, I put a full wave bridge rectifier in the power supply. It converts the AC coming from the wall to DC
It has three essential components.
1) The bridge rectifier
This change the AC sine wave into a waveform expressed by the function abs(sin(x)):
Better, but still not steady DC.
2) The filter capacitor
This gets rid of the ripple. you could compare the capacitor to a bucket with a hole in the bottom. Even if I vary the rate at which putting water into the bucket, the rate at which it come out is always going to be more or less the same, provided that it is sufficiently large compared to the volume of water going in.
However, its impossible to get an absolutely perfect DC output with this setup, because the ammount of charge on the capacitor does affect the voltage at which the current comes out.
This 680 uF capacitor takes away enough of the ripple for our purposes:
3) An isolation transformer
Usually, the diode bridge and the capacitor would be enough, but our AC isn’t coming from the wall, its coming from a grounded auto transformer. this is a problem because the rectifier only works if the AC input is floating. A transformer with an equal number of primary and secondary wingdings accomplishes this without stepping the voltage up or down.
Nest step is to test it in the chamber.