Deuteron Ion Gun with a Penning Mod for use with a Linear Electro-Static Accelerator

[Dennis P Brown]

Hello. Hope all are having a good day recalling/honoring both our current and past Vets.

After John's excellent point about my not having resistors between my series of Einzel collector plates on the accelerator tube, I decided to remove them so they can be drilled and have 50 M-ohm resistors installed properly.

This made me realize that my ion gun had an issue even through I had added a unique mod to increase its efficiency. Looking over the design, I realized that my ionization path length between the anode and cathode was too short. Since this part of my accelerator is enclosed within the tube and sealed, this aspect of the gun cannot be changed. To help address this problem, I decided to add a powerful ceramic ring magnet I already had. This magnet will convert this gun into a type of Penning trap system.

See the attached Microsoft word drawing.

As can be seen, I use a hypodermic needle (about 25 micron ID) to allow deuterium to enter the ionization region. I exploit this design for one major and one minor reason.

The primary reason I use such a very long and very fine hollow needle is to not only allow the deuterium gas to enter the system more slowly but more critically, create a significantly higher local density right at the needle tip (I have a system to evacuate the supply system behind this needle to remove any air to a few millibar - so this will not be an issue for purging.)

A secondary reason but one that in hindsight, might prove an advantage, was I wanted the end of the needle to provide a vastly increased intense and focused electric field to better ionize the gas leaving this tube. With a higher local gas density combined with an increased focused electric field in this very region should get significantly more deuterium ionized within the active region between the anode and cathode.

By using a very small and very sharp needle, the electric potential at this location will be vastly increased (this trick has been used for decades to create higher voltages from these high gradient fields.) Since my maximum sustained permissible vacuum limits the amount of gas that can be allowed to flow into the accelerator tube, I decided to use the small diameter needle to permit this maximum volume of gas to be wholly located at the point of highest electric field - maximizing the number of molecules ionized. At least that is the theory.

To further aid this process, I have decided to add a ring magnet to force the ions created between the cathode and anode to develop a radical component in order to increase their probability ionizing more deuterium molecules. This should contribute to increasing my beam current (almost everything is about beam current in the accelerator since I want this device to produce a lot of neutrons for testing.)

The cathode will extend beyond my Van De Graaff dome so that the positive deuterium ions will be better shielded (i.e. not ‘see’ the huge VDG positive field and 'drift' down this tube) until outside this region. The anode/cathode system should also (at 25 kV potenial field gradient) allow the ions to be accelerated towards the cathode. This should impart enough momentum to enable the ions to be injected into the intense VDG electric field. These deuterons will then be raipidly accelerated towards the target (a Ti foil doped with deuterium.)

These are my current plans for the deuterium gun I developed for the accelerator. As I mentioned, I cannot change any components within the tube; however, if I build a future replacement tube, I would consider any possible ideas to improve the current gun.


Thank you.

As always, comments, ideas, and questions are welcomed.

Later Edit note: One issue I was thinking about with a magnetic field is that the ions will obtain some transverse momentum - this might not be a good thing for these ions relative to having the requirement of accelerating them along the axis of the tube. Transverse motion could cause issues for focusing the beam by the Einzel lens allowing some ions to diverge away. This factor may be too small compared to the field produced by the lens to cause problems.

So maybe the net overall effect of the Penning trap is it 'increases' the number of ions available, and this gain far exceeds any loss of ions due to transverse motion induced by this same field.

I really don't know whether this really matters (later, I could test this aspect of the system - removing the magnet will not be difficult at all.) Just a random thought.

[John Futter]

Dennis

You will not need a 25kV supply for the penning ion source
a 0-5kV supply should be ample
Why not use a positive supply so that the drift cathode is a terminal potential -easier maechanically

this might make the whole accel tube look a little more standard

[Dennis P Brown]

In such a short post, John, you ask some really interesting questions! I wish I had the background in E&M and/or experience with these systems to answer those questions in any manner other than 'that is how someone else did it.'

As for the use of a positive supply, I did that at first (I have such a supply) but changed it after finding the original (Sci. Am. accel.) article and they used a negative supply (they give no reason why.) I assumed (?) that to ionize the deuterium it would only occur if I had the arrangement with the drift tube as the cathode. Again, I do not understand the dynamics of these systems.

So as I understand your point (and please, correct me if I am getting it wrong) the deuterium will ionize and still be accelerated in the correct direction if the drift tube is positive resulting in the reversing of the charge system in the setup I display in the drawing? (Sorry I do not display any pictures of the completed gun system since this device is rather small to see inside the VDG sphere. I do have an image of just the gun in the tube and this image is in my first Linear Accel. Post.)

As for the rather high voltage, again, I am just blindly following the method used in the article were they state that the 'gun' must be charged to at least 10 keV. As per my simple following previous work I restricted my sources to at least 10 keV for that reason not because I had any knowledge to do something else. Further, I upped the source to 25 keV simply because that was the only rating available for a battery operated system that also had a negative output (and I guess that is one reason they sell almost all positive sources in a large range of high voltages but only a single negative rated source - figures.)

I have a few low current battery operated 5 keV sources already (I assume, but have not confirmed that they are positive) so this would be an option.

To be truthful, I was puzzled by the requirement for 10 keV or higher by the writer since I realized that one of my 5 keV supplies would ionize any gas. I assumed (again, my lack of knowledge) that the higher voltage was required to accelerate the deuterons out into the VGF vastly larger (250 - 750 keV) field. Hence, I didn't mind upping the gun supply to 25 keV.

Any clarification or information on this subject would be appreciated!

An aside: In the diagram I display for my ion gun it is not fully complete - there is another 'electrode' between the current anode (needle) and cathode (long drift tube.) I did not include this since I was not using it.

This unused feature is a very narrow 'ring' (identical in diameter as the drift tube) and connected with a wire that runs out the back of the anode ceramic cap. It is possible I could charge this ring with a 5 keV positive charge and use the larger drift tube for something else (not sure what.)

Again thank you for your questions and I wish I understood these systems such that I could give E&M or technically based answers rather than "because that is how they did it.'.

[Dennis P Brown]

I am including a diagram and picture of the orginal deuterium ion gun system. I guess if I decided to switch the system so the deuterium supply source becomes the 'cathode' (neg.) and the small ring becomes the anode by using a 5 KeV positive source, this would create a gun more like what most here use. That would leave the current drift tube (labeled existing cathode in the MS word doc) as either unused or I could charge it (to a few keV negative voltage) in order for it to accelerate the deuterons from out between the new anode/cathode system.

I'm just guessing here but the long drift tube could maybe be used as a simple Einzil lens relative to the 'ionizer chamber' (The region formed between the Deuerium source and 'ring'.) Also, this 'lens' would accelerate the ions as well.

This idea of your's has merit - then if I add a magnet (to be centered only over the new anode/cathode system) and then if I use the longer 'drift' tube as both an Einzil lens/accelerating system this would make this complete assembly into a true PIG rather than the mod system I was proposing.

[Dennis P Brown]

To move the discussion along and clarify my points I am including a picture of a possible PIG for use with the system - this would address John's point about using a standard 5 keV positive excitation system for the deuterium supply. I could use the drift tube as an Einzel lens and as a method to inject these ions into the accelerator tube. The magnet would enable the lower voltage ionizer section of my setup to act as a Penning gun.

While all this may not be critical one way or another, I am trying to get a better handle on the differences between these systems and if any advantages could be gained using one gun design vs. another - this all could be just a mental exercise.

Continuing along this line, I realize that a gun for an electro-static accelerator is a very different beast compared to a fusor. The electro-static field created by the VDG can range from 0.25 MeV to 1 MeV and this places very different constraints on the system. A fusor’s voltage is very low compared to this and a 5 keV gun represents a kinetic energy reasonably close to a fusors operating voltage.

Penning sources are used for high end electro-static accelerators but the applied voltages and their arrangements are completely lacking even in the technical papers. So while this discussion is not germane to fusors directly, it does have applications to this device so I feel this discussion of my points is relevant.

For my application the given gun design may have a huge impact - or none at all. This is something I can test and for the magnet, is trivial. If I install duel high voltage power supplies in my 'Ion gun' dome, I could test a full PIG and convert back to a standard gun and play with the polarity of these devices, too. This might require dual batteries to prevent a serious interaction between the two HV supplies - that only experts here, I would think, could comment on that issue in any detail.

Unless there is a very good reason to try one method and ignore the others, this might be worth looking at - accelerator beam current at the target would be the final 'proof' of whether one system is superior or irrelevant or a disaster ... hopefully, not the later!

So if anyone wishes to comment relative to all or one of these approaches I would appreciate this - trying to understand these aspects of a gun is, to say the least, confusing for those of us with zero experience in this field.

[Chris Bradley]

Dennis P Brown wrote:
> The electro-static field created by the VDG can range from 0.25 MeV to 1 MeV

I like that you are actually doing stuff, but please see if you can improve your terminologies, it helps the discussion: A field is Volts/distance. Voltage is 'Volts'! 'eV' is energy.

Better, too, that you stick with the same title for the threads.

[Dennis P Brown]

Sorry - I try and make the titles relevant for searches relative to the topic contained within that part of the discussion at hand - I'll not change the titles any more.

Yes, sloppy units - again, sorry. Just so common when speaking of energies and potentials to ignore the differences and treat them on an equal footing since the later tends to create the former. I'll be more careful.

[Chris Bradley]

(Just to ask - could you please convert your .doc diagrams into a jpg image, then it will appear without having to download the file then find an application to open it.)

Just a few observations anticipating how well this set-up might work, or otherwise:

In regards a needle at the 'output' end of which you'll get ionisation from a flow of gas, it's a nice idea but I don't know if I would buy that it will help achieve anything more than a regular needle in a chamber:- Let's say the gas enters an electric field right at its tip, OK, so we get some ionised gas. But it will have little velocity and you'd have to be clear of the limiting magnetron condition to hope that a significant fraction of the ionised deuterium will be accelerated as far as your accelerating elements before it is just neutralised again. I guess 5kV should be fine (though depends on the mag field strength and distance to the 'gun cathode'). I would tend to anticipate that this set-up would actually achieve only a small ratio of accelerated ions to admitted gas... but I see no 'harm' in trying it this way...

I would actually have tended to think you would get most of the ionisation around the needle rather than at the end of it. Ions from a positive corona discharge (which is what I think you'd be effectively doing here) tend to get created in a rather odd way. There is a cyclic process that occurs in which ions are created which are expelled from the region around the conductor, leaving behind a charged region where electrons then re-ionise the gas creating ions which are expelled... etc... (Seems to be quite a fascinating, complex process, actually.) In practice, I've found this positive corona formation therefore tends to favour 'long' electrode structures, rather than 'pointy' ones, as the longer ones have more space around them in which the electrons can remain 'resident'.

Anyhow, with a magnetic field present, the electrons 'residence' period is extended so I think you will probably find you would get what visually appears to be a persistent positive-corona plasma clinging to the whole of the needle at what might strike you as surprisingly low voltages (because the electrons get such a long time to have a chance at ionising something). I typically begin to get a positive corona fired up somewhere from 650 to 1000V around rods up to 1mm in diameter, when immersed in a ~0.1T field at a few microns pressure.

[Dennis P Brown]

As soon as I can locate a converter, I will convert the diagrams into JPEG ... .

Ok, got it into PDF ... . That has a free downloadable viewer. And the site "CutePDF" offers a free converter that changes MS Word docs into PDF.