Well, measuring beam current isn't as simple as I thought. Secondary electron emission is a real problem and can throw off measurements by an order of magnitude. I thought this was only a problem with measuring electron beams, but... Secondary electron emission is likely the reason behind the blue glow in your photographs of the ion gun in action.
Anyways, a faraday cup is not that complicated - in theory. Sure, you can bust a gut over the design and implementation, but for our needs? The beam currents we are likely to produce in our wildest dreams shouldn't require water cooling, so I'm hoping for the best (as always). I have some very good drawings and design notes in my ion sources handbook, so I'm going to take a crack at it.
As a plus, though, faraday cups are used for accurate beam alignment. It'll be a darn useful instrument, none the less.
In the meantime, for the curious:
Horribly expensive and high precision faraday cups from Kimball Physics (but nice explanation)
Ion beam current measurement
Re: Ion beam current measurement
Hal -
the key to measuring ion currents correctly is to use only enough voltage to get the saturation level of ion currents. The amount of electron current necessary to accomplish this will depend on the details of the configuration of your particular ionizer.
I have used the ion current in sealed electron beam tubes as a relative measure of the internal pressure. One lights the filament, applies some positive potential to an electrode a convenient distance away, so as to get a good electron current flowing. A negative potential to capture ions is applied to a second electrode somewhat farther away from the cathode.
In all the cases I have studied, the ion current increases up to about 100 volts electron energy. The actual current flow in the system we controlled with the filament temperature.
The ion extraction voltage also shows a saturation level. The saturation voltage is indicated when further increases in Cathode to ion collector voltage ( voltage on the Faraday cup in the arrangement you are considering) produce no significant increases in ion current. We found in our case, microamps of electron current produced nano-amps of ion current, with pressures in the 10 -7 to 10-9 Torr, quite a bit lower than pressure people here are considering.
Somewhere, I have a simple write up deriving the expected ion currents. Our apparatus, produced currents that were within about 1 order of magnitude of the theoretical values. We considered that acceptable since the are several factors that are uusually not known... such as ionization efficiency ( electrons through per ion produced), effective ionizing volume ( the volume of the ionizer that actually is passing electrons, recombination rates and etc....
A starting point, of course, is to assume ionizing efficiency is 100 %, and that the effective volume of ionization is the volume between the filament (or cathode) and the anode. Assuming pressures are low enough that the mean free path is much longer than the dimensions of the ionizer and the path to the ion collector are also quite reasonable for a first guess.
By keeping all voltages as low as possible, you tend to minimze, if not entirely avoid, secondary ionization problems. Use of metals that form oxides on the surface (which one doesn't?, and soft glasses) can be problematical for secondary emission.
But... and this is an important "but", you need another potential more positive than the anode in order for the secondary emission to cause any significant net current changes. (If the cathode is near zero volts and the anode is at +100v, secondaries produced by incoming electrons at the anode, will generally return to the anode, since the potential well favors that.
Ions produced by electron impact will fall away to the negative collector. Electron beam crosswise to the ion flow is generally a good design.
So scale up should be possible here. For quite good vacuums, you should be able to get an effective 1000:1 or lower, electron current to ion current ratio - mA of electrons a 100 volts should give uA or more ion currents. Thus you would need amperes if you want mA of ions.
Personally, I'd settle for a much smaller number at the start to work out the method.
Just some thoughts.....
Dave Cooper
the key to measuring ion currents correctly is to use only enough voltage to get the saturation level of ion currents. The amount of electron current necessary to accomplish this will depend on the details of the configuration of your particular ionizer.
I have used the ion current in sealed electron beam tubes as a relative measure of the internal pressure. One lights the filament, applies some positive potential to an electrode a convenient distance away, so as to get a good electron current flowing. A negative potential to capture ions is applied to a second electrode somewhat farther away from the cathode.
In all the cases I have studied, the ion current increases up to about 100 volts electron energy. The actual current flow in the system we controlled with the filament temperature.
The ion extraction voltage also shows a saturation level. The saturation voltage is indicated when further increases in Cathode to ion collector voltage ( voltage on the Faraday cup in the arrangement you are considering) produce no significant increases in ion current. We found in our case, microamps of electron current produced nano-amps of ion current, with pressures in the 10 -7 to 10-9 Torr, quite a bit lower than pressure people here are considering.
Somewhere, I have a simple write up deriving the expected ion currents. Our apparatus, produced currents that were within about 1 order of magnitude of the theoretical values. We considered that acceptable since the are several factors that are uusually not known... such as ionization efficiency ( electrons through per ion produced), effective ionizing volume ( the volume of the ionizer that actually is passing electrons, recombination rates and etc....
A starting point, of course, is to assume ionizing efficiency is 100 %, and that the effective volume of ionization is the volume between the filament (or cathode) and the anode. Assuming pressures are low enough that the mean free path is much longer than the dimensions of the ionizer and the path to the ion collector are also quite reasonable for a first guess.
By keeping all voltages as low as possible, you tend to minimze, if not entirely avoid, secondary ionization problems. Use of metals that form oxides on the surface (which one doesn't?, and soft glasses) can be problematical for secondary emission.
But... and this is an important "but", you need another potential more positive than the anode in order for the secondary emission to cause any significant net current changes. (If the cathode is near zero volts and the anode is at +100v, secondaries produced by incoming electrons at the anode, will generally return to the anode, since the potential well favors that.
Ions produced by electron impact will fall away to the negative collector. Electron beam crosswise to the ion flow is generally a good design.
So scale up should be possible here. For quite good vacuums, you should be able to get an effective 1000:1 or lower, electron current to ion current ratio - mA of electrons a 100 volts should give uA or more ion currents. Thus you would need amperes if you want mA of ions.
Personally, I'd settle for a much smaller number at the start to work out the method.
Just some thoughts.....
Dave Cooper
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Re: Ion beam current measurement
<heh> Interesting stuff. I just love this forum.
Many thanks.
And I do agree that start low and build up. uA are far easier to deal with than mA. I don't even want to think about ampere of ion beam current. Yi.
But I do wonder about the voltages. From what I've been reading and from what I've seen playing around with simion, the design potentials have a big impact on focussing. Don't you have to stay within the band of the design to get decent results? If you go too low, then wouldn't the beam be completely unfocused and severely distort results?
Many thanks.
And I do agree that start low and build up. uA are far easier to deal with than mA. I don't even want to think about ampere of ion beam current. Yi.
But I do wonder about the voltages. From what I've been reading and from what I've seen playing around with simion, the design potentials have a big impact on focussing. Don't you have to stay within the band of the design to get decent results? If you go too low, then wouldn't the beam be completely unfocused and severely distort results?
Re: Ion beam current measurement
You're right, this IS very interesting "stuff".
Generally, the potentials (and dimensions for that matter) all scale proportionally. You can make a design larger or smaller with higher or lower voltages and still retain the focus.
I would just recommend modelling the approximate dimensions and when you get the desired beam profile, record those voltages. They will be what you need.
Dave Cooper
Generally, the potentials (and dimensions for that matter) all scale proportionally. You can make a design larger or smaller with higher or lower voltages and still retain the focus.
I would just recommend modelling the approximate dimensions and when you get the desired beam profile, record those voltages. They will be what you need.
Dave Cooper