Re: The "wire" ion source - link
Posted: Wed Nov 30, 2011 7:09 pm
Good comments, Carl.
The additional information adds to fill in the picture. Do you have the pressure in the Druyvesteyn-Penning paper? The tests I ran were able to move out to right some distance past the Paschen minimum, until the DC supply ran out of voltage... (around 2kv).
The point regrding curve shapes, that you make is most important. These classic curves come with several important assumptions, that often are not realized in actual systems.
Our ionizer geometry was roughly cylinder-square. The grounded screen had a several mm open mesh, and it and the solid chamber walls were several cm away from the fine tungsten wire. So this was only somewhat like a coaxial cylinder setup, with a D/d ratio of maybe 100.
The ions travelled about 0.25 meter to the main electron beam channel, entering at about 45 deg angle, where the strong local cathode field drew them in on axis.
I was able to model (approximately) the paths of both ions and electrons using Simion 7.xx treating both as non interacting. (I also tried unsuccessfully to use Vector Fields for a more refined analysis, but the mesher choked repeatedly on the relatively complicated geometry of the ionizer screen. Simion's transparent screen is a very, very useful feature. David Dahl knew his stuff, when putting this software together.
One interesting result I remember getting was from an attempt to measure incoming ion energy. By adjusting the bias on a collector plate in the electron channel (cathode off) until current went to below 1 nA, a rough ion energy spectrum was developed. I'd have to go dig up the test results, now, but I recall ion energies (He) were pretty large, perhaps up to half the voltages in the ionizer.
This suggested the ions at 10-4 Torr pressures were fairly long lived, and could travel significant distances ( and around corners). This probably is not what happens at the higher pressures in the fusor. Again, as you say, it's mean free path.
We always ran with a positive wire ionizer, because the ions were for the SEE (secondary electron emission) gun application.
Our pressures had to be quite low, in order that main e gun did not itself go into glow discharge... (which did happen from time to time).
So much for the anecdotes....
Dave Cooper
The additional information adds to fill in the picture. Do you have the pressure in the Druyvesteyn-Penning paper? The tests I ran were able to move out to right some distance past the Paschen minimum, until the DC supply ran out of voltage... (around 2kv).
The point regrding curve shapes, that you make is most important. These classic curves come with several important assumptions, that often are not realized in actual systems.
Our ionizer geometry was roughly cylinder-square. The grounded screen had a several mm open mesh, and it and the solid chamber walls were several cm away from the fine tungsten wire. So this was only somewhat like a coaxial cylinder setup, with a D/d ratio of maybe 100.
The ions travelled about 0.25 meter to the main electron beam channel, entering at about 45 deg angle, where the strong local cathode field drew them in on axis.
I was able to model (approximately) the paths of both ions and electrons using Simion 7.xx treating both as non interacting. (I also tried unsuccessfully to use Vector Fields for a more refined analysis, but the mesher choked repeatedly on the relatively complicated geometry of the ionizer screen. Simion's transparent screen is a very, very useful feature. David Dahl knew his stuff, when putting this software together.
One interesting result I remember getting was from an attempt to measure incoming ion energy. By adjusting the bias on a collector plate in the electron channel (cathode off) until current went to below 1 nA, a rough ion energy spectrum was developed. I'd have to go dig up the test results, now, but I recall ion energies (He) were pretty large, perhaps up to half the voltages in the ionizer.
This suggested the ions at 10-4 Torr pressures were fairly long lived, and could travel significant distances ( and around corners). This probably is not what happens at the higher pressures in the fusor. Again, as you say, it's mean free path.
We always ran with a positive wire ionizer, because the ions were for the SEE (secondary electron emission) gun application.
Our pressures had to be quite low, in order that main e gun did not itself go into glow discharge... (which did happen from time to time).
So much for the anecdotes....
Dave Cooper