Linear deuterium Accelerator-Ion Gun Issue

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
User avatar
Dennis P Brown
Posts: 1704
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Dennis P Brown » Thu Jan 21, 2016 1:17 pm

Quick update: Started to clean & polish the dome - easier than I thought but still slow. The section I polished (1/4 of a half section of the dome) is a major improvement compared to the original surface - the dome was outside for many years and had been used for many other purposes (like a drum ...amazingly, very few dents and no scratches so the dome is in rather good shape to say the least.) After some polishing I can see my reflection, darkly (lol) but most areas of corrosion run just too deep to be removed even by mild chemical etching/cleaning but really, irrelevant. While the area polished is unimpressive compared to commercial domes the fact that this dome was "free" makes this dome ideal (even if a good bit larger than I wanted. Static loses could be an issue unless in very dry air so may have to address that issue.)

The belt is ready to install but realized I need to cover the rubber rollers with metal tape; forgot about that so need to address that problem - the reason for this is that the static charge created by the rubber roller and belt fiction effects could seriously interfere with the polarity of the power supply spray (i.e. the belt/roller may not create a positive charge so this causes negative charge build up on that section of the belt thus canceling part of the charged delivered by the spray system.) Could test to see if that is an issue but once the belt is installed, removing it would require cutting so I just need to address this issue first.

I was amazed to find 2 amp 30 kV diodes for under $3 a piece. So, ordered a few. Unfortunately, they are coming via a ultra slow boat from China and who knows when they will arrive ...should have paid for fast delivery ...lol. So, with my 40 kV caps (0.01 mF) and 7.5 kV NST (the system will have very, very little load so both the voltage and current will be near max values, I'd think ...not that the belt will draw even 0.1 ma at best.) I intend to both filter the NST output with three of those caps and then double the voltage (and it too will have a few caps to filter out the ripple) before spraying onto the belt. This is one of the few times a simple full wave multiplier and a NST can easily handle the load - a Van de Graaff requires very little current sprayed onto the belt to work well. I believe sources indicate that a good belt can, at best, carry just 0.001 milliamps per 50 sq. inches of belt area per sec passing the spray system. Guess I will see.

User avatar
Dennis P Brown
Posts: 1704
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Dennis P Brown » Fri Feb 05, 2016 12:15 pm

Finally received my 30 kV, 2 amp diodes yesterday so I guess I really must assemble my 25 kV, multi-milliamp positive voltage spray system this weekend. The system is based on a 60 ma, 15 kV NST, will have full wave diode rectification w/caps, and full wave voltage multiplier. The volt-multi will be under oil.

Also, got the metal tape so I really need to fix my Van de Graaff (VdG) rubber rollers and install the belt I made (10 cm width, over two meter total length); also, just do the work and finish polishing the VdG's dome.

Also, need to strengthen the motor support housing and improve its support base. Amazing how the details grow, and grow even with the VdG nearly fully constructed.

Still, the electro-static accelerator is 100% complete/tested so this is the only issue really holding me up from getting to the finish line; so time to start addressing this part of the puzzle ... .

DanielSandner
Posts: 11
Joined: Mon Jul 28, 2014 6:19 pm
Real name: DanielSandner
Location: Bavaria, Germany

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by DanielSandner » Fri Feb 05, 2016 5:47 pm

Hello,
Your Dome looks quite good and I can't wait to see first results.
But i don't understand why most of you use van de Graaf generators to gain high voltage. As the top is like a capacitor the Voltage can be described with U = Q/C. The higher the beam current, plus Corona discharge and so on, the lower the Voltage. If you want to measure the voltage with a spark gab, this won't be reliable because the Voltage is much lower if you have a constant current.
I'm a member of a German high voltage forum and someone build this: http://web367.login-102.hoststar.ch/kaskade_180kv.htm
The Ouptput is around 2mA and 180kV, with this high current you can also power more stages.
Compared to vacuum parts it was also pretty cheap, you can also reduce the capacity of the capacitors because you don't need a high current.
Maybe someone wants to build something like that, but also good luck with your van de Graaf.
kind regards, Daniel Sandner

User avatar
Richard Hull
Moderator
Posts: 11412
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Richard Hull » Fri Feb 05, 2016 8:37 pm

Almost no one here at fusor.net uses a van de graff to generate high voltage! Virtually 100% of us use formal plug-in-the-wall high voltage supplies capable of hundreds of watts at full voltage. The ion-gun forum, for some strange reason has people building accelerators, (of which, Ion guns are part), are using VDGs, yes! Thus far, none have been successful in doing fusion. I worry that such linear accerators in past literature have used large VDGs for HV sources and this is more or less a "hangers-on" phenomena.

I can't explain the above propensity and have constantly noted there is no real acceleratory energy to be found in a VDG of amateur proportions. Still, everyone who desires to take a "whack at it" in their own particular manner should be allowed the thrill of the hunt and adventure into VDG land....and more power to 'em....literally!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Dennis P Brown
Posts: 1704
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Dennis P Brown » Sat Feb 06, 2016 7:42 pm

Hello Richard, and thank you both for the comments; also, thank you Mr. Daniel Sanders for the link (very nice system! and glad to get someone from Germany to comment!)

You are both correct - most Van de Graaffs are a joke in current supply (and why only voltage is generally advertised) and not the best way to go if one wants current! That is why I am building a monster VdG - to get the required current of 20 - 30 micro-amps at 200 - 400 kV. Since, any powerful voltage multiplier - whether a standard Vitter, or a Marx unit - are far easier to build than a VdG, to say the least(!) my issues are two-fold relative to that route:

First, I can build a monster VdG without too many issues -either cost or parts. That is, I have the massive dome (given to me for free), I routinely make VdG belts (rather easy and have two now ready for my new VdG), the motor/roller system was very small in cost and taken from a belt sander(!), and I had both the plexi-glass tube and the high voltage spray system parts so this whole unit will cost less than $100. Also, I have access to a machine shop and it is easy for me to make the required and rather unique parts any useful VdG often needs to enable the belt to run without issues.

The second part of this long explanation is more serious - frankly, I am afraid of any powerful (read high surge current) 30 kV+ power supply (much less those over 100 kV) since these are always lethal, behave in strange manners (arcs can jump further and towards things one would never guess) and I would need to work around that monster while it was charged! Not my idea of a home project! (lol.)

So, I decided to try a primitive high voltage and safe current device - ie: an over-sized VdG, for now. I do not need more than a few 10's of micro-amps to get 10^8/9 protons resulting in a narrow beam of 10^7 neutrons or so. Also, I can easily measure the VdG's current output and if it is too high, it is trivial to de-rate the VdG so it will remain safe to work around even if I have to over built the thing (seriously doubt that will ever be an issue (lol.))

In summation, I do not believe anyone should use a VdG for a proton (deuterium) electro-static accelerator to do fusion; however, I have a rather strange end goal to test a fusion system that requires a high flux-narrow beam of neutrons and this will meet that requirement; also, I have a secondary experiment as well relative to testing neutron detectors and again, this accelerator is ideal for that type of experiment (also, that is a safe flux to work with!) So, the trouble for me is worth it (also, I have spend less than $700 on the entire accelerator project, including all vacuum parts, to date.) This does not include the fun of building this stuff nor using this forum!

Jeroen Vriesman
Posts: 276
Joined: Mon Feb 07, 2011 9:08 pm
Real name: Jeroen Vriesman
Location: Netherlands
Contact:

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Jeroen Vriesman » Sat Feb 06, 2016 8:09 pm

How could this produce a "narrow beam of neutrons" ?

The neutron energy is much higher than the initial deuteron energy, so with beam on target you still have neutrons going in every direction? Or am I missing something?

If you do need more current, you might try to build a Felici generator.
http://lyonel.baum.pagesperso-orange.fr/felici.html

User avatar
Richard Hull
Moderator
Posts: 11412
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Richard Hull » Sun Feb 07, 2016 7:08 pm

Indeed, in D-D fusion, there is no beaming. The general assumption and published data proves this out. In D-D fusion the neutrons are emitted isotropically provided the energy used to accomplish this is well below the resultant fusion neutron energies. That said, once isotropically emitted, there are many environmental issues around the fusion event that can cause detection, at range, to appear anisotropic..... (scattering, absorption, etc.)

Neutron beams are classically produced from intense neutron sources being forced to herd those few chance directional neutrons through a tunnel of shielding. The most intense used to be from a fission reactor's core through a hole in the containment.

Today there are a number ways to produce neutron beams in big science efforts that do not involve a fission reactor.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

User avatar
Dennis P Brown
Posts: 1704
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Dennis P Brown » Tue Feb 09, 2016 3:09 pm

Good to know - just assumed (without thinking) that momentum would be conserved but both of you are absolutely correct. I missed the nuclear reaction energy aspect/reaction of this whole process (lol.) Duh, as one will say after hitting one's head with the palm of their hand. Still, the Van de Graaf is built so, not ready (or willing) to change the high voltage field, yet. Will see how the VdG performs ...or doesn't ... .

User avatar
Dennis P Brown
Posts: 1704
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Dennis P Brown » Sat Feb 20, 2016 10:00 pm

Finally got back to the Van de Graaff (VdG) after other issues have taken me from the project. Finished the dome - had to remove a center "collar" that separated the two domes (and was used to hold each hemisphere to the other.) This collar just created an extra "seam" that would increase corona discharge without really adding much current "storage" area so I decided to remove it. Required machining new clamps to hold the two domes together but that wasn't very difficult. Also, finished the polishing and removing dents.

Another issue - started work on a cover for the top of the dome. The upper dome (like the lower one) has an 7.75 inch access hole in the top. Cut some Al plates to fit inside and remove the "hole" so the top of the dome is closed. The design I have started will allow easy removal/access of this cover so the inside of the dome can be quickly accessed. Handy to have that ability. Amazing how many details need to be addressed for a single VdG which is a side project but as I learned on the little commercial unit I had and modified, without all these extra's, any VdG unit is a problem to operate and not likely to run well.

Decided to fix a problem with the motor mount in the VdG so as to make belt tensioning easier but was more work than I thought (lol - always is ... .) Need to install the metal tape on the lower roller - not sure I need to worry about the upper roller relative to that need.

Still deciding on the caps to use for the positive high voltage spray - reluctant to install my rather big ones that can carry a lethal charge (but adding resistors to auto-discharge would make that safe but still, stuff fails and I don't want to add more dangerous issues to the system.) Also, can't make up my mind on how best to enclose that system.

Finally, mostly settled on the design for the change transfer systems for the voltage spray (base of the VdG) and pickup's in the dome. Looks like a return system, connected by a large series of resistors, is often used in bigger domes so I put one together - just need to install it in the dome. This will help "return" charge down the belt helping to further "charge" the upper dome positive.

User avatar
Dennis P Brown
Posts: 1704
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Linear deuterium Accelerator-Ion Gun Issue

Post by Dennis P Brown » Sat Feb 27, 2016 5:51 pm

Some good news on my accelerator project/fusor/ion gun - my Van de Graaf has passed its first (minor) milestone.

After realizing the motor mount needed another (!) degree of freedom (horizontal rotation - 5 - 10 degrees.) I made this change and also a few others: added another support rod to the motor mount (now a set of four); installed more flush allen bolt heads in the dome assembly and performed any extra counter sinking where needed; also, provided a lock for the upper roller stand (it can be rotated for adjustments but then must be locked in place).

After lengthen the belt I installed it and adjusted the tension using my vertical control of the motor/roller system.

I then ran the motor and the belt performed well but, of course, wasn't running true. So I made some minor adjustments but having all the small extra adjustment degree's of freedom (motor, rollers, column, and others) enabled me to quickly get the belt to run very smoothly and remain centered on both the lower and upper rollers.

I will next tackle the upper system of high voltage collector electrodes - a positive pick up array two inches below the top roller as it enters the dome and another two inches below the roller as the belt exits the dome. An expert in the field recommends this arrangement. I will also have a resistor loaded system that connects the dome to the inside of the belt on its exit so that reverse current flow can stabilize any downward current polarity issues (this allows better current loading of the belt/dome or so I have read.)

The project is moving along nicely after a long wait dealing with a host of related but slow to solve issues. If the upper electrodes work well I may attempt the lower array of electrodes for the high voltage spray system. But that may or may not be possible since I have a list of projects also waiting for tomorrow (lol.)

Post Reply