Basic ion source theory

[Anson Tsang]

Gas flows through the ion source between the anode and the cathode. A positive voltage is applied to the anode. This voltage, combined with the high magnetic field between the tips of the internal and external cathodes allow a plasma to start. Ions from the plasma are repelled by the anode electric field. This creates an ion beam.

I found the theory of operation on wikipedia, but i am still a bit puzzled. First, where do you put the magnet, do you put it between the middle of the cathode and the anode? Also does the cathode have to be negatively charged, and the anode have to be positively charged? Finally, what is the use of a magnet? Wouldn't it melt? Thank all of you for reading this post, i am sorry once again if this question has been asked already.

[John Futter]

Anson
An ion source does not necessarrily need a magnet.
But in those ion sources that do use a magnetic field, this is to make the electrons describe a circular spiraling orbit giving a better chance of the electron hitting a gas molecule and therefore ionising it.

[Anson Tsang]

Anson
An ion source does not necessarrily need a magnet.
But in those ion sources that do use a magnetic field, this is to make the electrons describe a circular spiraling orbit giving a better chance of the electron hitting a gas molecule and therefore ionising it.

So I should first turn on the electromagnet, and after a certain period of time, I should turn it off

[John Futter]

No Anson
There are differeing types of ion source
to name a few
Penning
penning cold cathode
anode layer
PIG
magnetron

and there are many others

do a google search on these to find out the differences in the construction and the way they work

[Chris Bradley]

Anson - this and your magnetron post cause me to suspect that you are labouring under a misapprehension.

A 'regular fusor' does not need an ion source. Ions are initially created by an electron cascade ionisation process, most likely initially in the coronal region of the highest electric field gradients around the cathode. Once the plasma is 'struck', the ionisation cascade can then be sustained directly by the electric current passing through the fusor. The voltage may be pulled quite low during this startup phase, and the currents high. (This is the sort of operational mode that a neon bulb works in.)

As the pressure drops, this 'direct' electric current discharge effect will diminish and you will see a significant increase in the sustainable voltage across the device for a given current (viz. the impedance goes up markedly as direct discharge ionisation diminishes), but an ionisation process occurring at the shell will continue to support ion production (by sputtered electrons, due to the electron beams and fast neutrals striking the shell - this is the 'star' mode).

At this point, where the majority of ionisation is from the corona and sputtering at the shell, if the plasma momentarily extinguishes then it doesn't come back whilst the same electrical and pressure settings are sustained. This is because that shell sputtering process is self-sustaining, but for only so long as it is happening. If it stops for just a millisecond, you then need to 're-light' the plasma with a higher current and/or by raising the pressure.

Keeping the plasma lit in this 'self-sustaining' ionisation mode is a 'knife-edge' that you learn to push up to the highest voltages and lowest pressures, without causing the plasma to extinguish.

As the pressure drops further, you will see the plasma die back and the current drop off, so even the self-sustaining ionisation is no longer sufficient to keep the plasma alight. You will get a further increase in the possible applied voltage to sustain any plasma at all, and almost no current flowing. You might just see a corona around the cathode if you look very carefully or use a very long exposure camera to help you see it (which is like the very first phase, but that there isn't enough current to get the direct ionisation process running). At this point there is no real 'plasma' to speak of and you'll be well-below the typical 'fusor' discharge pressure. In this range of pressure, where the impedance is close to its maximum (viz. 'almost infinite'), the use of an independent ion source might be employed to replace the natural, self-ionisation processes in a 'regular' fusor that are no longer possible. These generated ions are then free to do what ions would normally do in the higher pressure, self-sustaining fusor. Some have a view that this is the pressure range that further 'fusor' research could usefully examine, because it pulls very little power (= very little current). I am not particularly convinced of that, because you suffer, proportionately, from low ion currents.

However, and in any case, when you are setting out, as you are, there is absolutely no need to implement such an ion source, but instead stick with the self-sustaining discharge mode pressure range. More complicated things can come later, sequentially, after making progress with self-sustaining discharges.

[Anson Tsang]

Thank you Chris for the insightful reply. Indeed I was a bit confused, but your post has replenished me will nesseaary information in order to build a fusor

[Richard Hull]

None to very little of what Chris has said will be truly understood or "owned" by you at a "core" level until you have struck and sustained your first glow in demo mode, followed by much experimentation by varying pressure voltage and currents with good reporting instruments for all three variables. You will become the critical computing and controlling engine in the closed loop system. Only then will much of what is explained sink deep within you and you will then have become a graduated "plasma cogniti".

Richard Hull