Questions about the practical construction of a Linac

[Dennis P Brown]

Been dealing with a host of issues the last year and only now getting back to my electro-static accelerator (ESA) project relative to fusion. The unit itself is 100% completed, fully shielded for 0.5 MeV x-rays, and just lacking tune up issues to be addressed (as pointed out here and that person was correct - the accelerator tube needs mid- to low 10^-6 Pa to work; that was a bear to get even with a turbo and all KF fittings.) Do have to finish my detector system but that is mostly done - just needs wiring work.

Mr. Hoppis, any accelerator except a simple electron one is not a build it and it works exactly like you think project; especially as seen in articles and that video - by the way, that video has a serious issues with their electro-static lens design due to their lack of conduction between the focus tubes and collectors ring *(I now guess that part is to be designed later.) This design might work for all I know but looks like trouble. The classic SA design has a host of issues and that is rather simple compared to the video design.

Your idea for diffusion pump based vacuum will be hard to use in any ESA unless you use a cold trap (very cold) and that could be an issue on pumping speed. By the way, an ESA is not a Linac, which, as Carl Willis pointed out, requires very special RF supplies. The primary driving supply for a Linac is rather expensive unless you build it yourself and it is not an off-the-self item you will find on ebay. Building one is not a trivial issue and the mechanical issues in the project require very complex (precise) construction. I assume (limited budget) that an ESA is what you are considering.

Even metering the gas into the gun (I have had great success on my design holding vacuum) is difficult without very careful design. You can reference my system here if you want pictures - just search for Linear Accel. Shorting your Van de Graaf (VdG) can be an issue through the higher pressure gas leading up and into the gun via plasma feedback and was a simple issue I overlooked (amoung many - very good people here who catch mistakes very well!) until someone pointed that design flaw out in my pictures - missing stuff is all too easy and can kill a fully complete project. Then you will be lost on what is wrong and get side tracked. I did but that is what spending free time is about with complex projects ... .

Avoiding a VdG is a good idea in some ways but does require a big build for the high voltage source. Some have made Tesla coils - that is a special project in-of-itself, though. Yet a simple, complete and off the self VdG has a lot of merit and makes the project more simple.

Another issue is using hydrogen gas. That is great for creating protons but of no real use for neutron production; you will need deuterium gas for that. Also, for the target design/material, that is a complex problem and I and others here have written long discussions on that subject (such as a target needing a material containing deuterium and that target does not out gas and kill the vacuum when it is hit by the deuterons; the electrons that are freed from the target can back stream up the accelerator tube and that, in turn, can create serious issues of charge buildup on the tube. That then disrupts the proton beam ... and so on.) Depending on the current, water-cooling may be needed. I decided to add that just to be on the safe side rather than dealing with any possible issues later when my design was already built.

Radiation is a serious and possibly deadly issue if ignored; the beam could be up to 0.5 MeV (unlikely that high as a real flux but worse case is always what you design for.) Lead near the ESA should not be used - that would destroy the electro-static lens system unless extremely well design; the very least, it will bleed away charge in a manner that could compromise your ability to create a large electro-static field required to guide and accelerate the beam. Trying to then shield yourself from a distance using Pb would require massive amounts. Proper shielding is not a trivial issue. Solid concrete blocks were used in the SA article but that too can cause construction issues. Do buy dosimeters (ebay) and a good radiation detector – they can save your life.

Detectors for the beam or neutrons is last on the list of issues that you need to address for such a project to be successful. As I see you are doing, creating the high vacuum system is the best way to get a start.

I'm still at it three years later ... so, do allow yourself time. A lot of fun in the building and learning.

[Noah C Hoppis]

Dennis, It is true that it isn't a propper linac (more of a potencial drop accelorator).
I decided on a FW cockroft walton Multiplier do to stability of supply and the ability to supply each drift tube from a stage. The flanges would also be drilled on the rim to alow better gas purging.

[Dennis P Brown]

Keep us informed on your progress and as time permits, add photos; as someone who has gotten a lot of critically important and very useful information from all the people here this is a win-win for everyone.

While my project is finished, clean up work and adding diagnostics is a pain but the instrument would be almost useless without these (diagnostics: a beam current display needs wiring, proton illumination screen (for focusing) needs alignment, a neutron detector built; clean up projects: anti-electron back stream grid also needs wiring, the removable target alienment system needs testing (works while under high vac), installation of accelerator potential needles, and VdG upgrades – HV spray, column potential rings.) Even with the accelerator completed, these secondary issues are still there. This assuming it really works, that is. If it does, confirming my radiation shielding covers all risk areas and is sufficient then becomes priority one before I do even my first experiment … . So, building the accelerator is only part of the job.

By the way, if you use a diffusion pump for your system, consider getting a cold trap that fits it (does not need liquid nitrogen.) An accelerator must get into the low 10^-6 torr as I’ve learned from people here. An untrapped DP will not do that and worse, will contaminate your accelerator tube with back streaming oil.

Good luck.

[Noah C Hoppis]

I know I'm going to get a lot of guff about this but here I go...
I was wondering if (since I am still in the accumulation phase and yet to acquire parts for the actual acceleration cavity) a 8" cyclotron would be a viable replacement (A, I am aware of the difficulty of constructing a cyclotron, and B, Yes I have spent too much time on Cyclotrons.net). I have seen little to no info on this topic on the web, but being oriented towards being the top of the pyramid (in the least egotistic way possible) I saw a cyclotron as the most difficult and rewarding thing possible. Cyclotrons being based around a large set of yolk shaped electromagnets, give me a distinct advantage over a glass PT accelerator because I can get steel, and a lot of steel. To me the idea of an atom traveling circularly is much more exiting than linearly. The system I picture would have a 2' by 2' by 4" wide yolk with 8" diameter faces and 12" diameter H20 cooled coils. from this point forth this should be about the practical construction of a 8" cyclotron vs. a potential drop accelerator.
(please don't say this is off topic, it is and I fervently apologize, but rather than make a whole new thread I would rather beg forgiveness)
-thanks!

[Dennis P Brown]

If you have a magnet that can cover a 8" diameter cyclotron, you should do this - if you think you can buy such a massive magnet off ebay, best of luck - the shipping would be a nightmare and handling that monster wouldn't be fun. Even a six inch magnetic face area would be rather large and heavy. The magnetic "Dee's" can be a little smaller but must be fairly close to the diameter of the cycltron. If the magnet is electro-magnetic, the power is not trival.

If you are suggesting winding your own ... that would be a project. The cost isn't steel but wire and copper cooling tubes just to name the most massive parts ... . Maybe you need to determine the required field and what it takes to generate that high a field that is uniform over that area. You should do that before going any further with a build it idea.

This has been covered before so I am puzzled that you haven't already read these posts on this topic.

In any case, I feel a linear electro-static accelerator (ESLA) is easier (in my opinon) because all parts are available on ebay. (But then, I have built one so I'm not neutral on the subject - my ES accelerator is currently in a test vacuum phase with all bell's and whistless's installed; it has just reached 2 * 10^ -6 torr in about 30 minutes. I might be able to test the deuteron gun in a few weeks. After that, finish the upgraded VdG and I could finally test the beast. Been over two years but that happens with projects and I started with the high vac system components.)

One thing I'd like to add - a linear accelerator is mostly about a high vacuum system and you are just about there - the machine work and minor electronics work is what any skilled HS student could do; also, nearly everything is available on ebay at reasonible costs.

A cyclotron is all about a massive and costly magnet, then issues like a high vac system and complex driver oscil. for the cyclotron's internal Dee's are then add on to the project. Consider carefully and look at what others have managed to construct.

Again, if you have access or own a large electro-magnetic, then I'd think that might be the way to go but even then, the remaining work is still more difficult than an ESLA. For a project to succeed you need the components or at least a way to get them; otherwise, a pile of expensive stuff will end up collecting dust in a storage room.

Finally, why do you want to build an atomic accelerator - either a electro-static, or true linear accelerator or even a cyclotron? If for nuclear reactions, then the type of energy you need determines the machine you should build; if for a neutron source (what I want it for) most the machines will do that if you have deuterium (gas and/or target). If I wasn't after building a neutron detector, I'd never have built the ESLA but rather a fusor. Hope this helps to clarify what you plan to do.

Here is a site where students built the the lowest cost LA machine I have yet seen and it works.

http://www.ifpan.edu.pl/firststep/aw-wo ... neller.pdf

Best of luck.

[Noah C Hoppis]

Well I do have the advantage o knowing a place where i can purchase copper wire for only the price of copper. On the topic of other posts, I have in fact read them, and decided to post anyway because every situation is different and presents new problems. I haven't calculated much of anything and was and am still only toying with the idea. I'm basically looking for pictures and documentation of attempts at cyclotrons (I've already read the ones at coultersmithing, neill.org, and rutgers). Thanks for responding (and without comments on my butterfly attitude!)

[Noah C Hoppis]

Hello again! I have Cad drawings of the Accelerator now! does anyone know how you would go about calculating the turns of the beam in the cyclotron? (Sorry about the change of subject line but the previous was far too misleading)

[Carl Willis]

Hi Noah,

Very nice, detailed solid models.

I think your question was about how many revolutions a particle will undergo in the cyclotron. If so, the answer depends on the injection energy, dee voltage, magnetic field, and the amount of space available for orbits (the dee radius, or wherever your extraction system is placed).

The particle has to have some injection energy Emin in order to clear the ion source structure on its first orbit. If the approximation that q*INT(E*dl) = qV is good for the accelerating gaps, where q is the particle charge and V is the dee voltage, then the particle gets a steady and easily-calculable boost of 2qV every orbit. The dee or extraction hardware supports a maximum radius R and thus a maximum classical particle energy of Emax = (qBR)^2/(2m). B is the magnetic field, m the particle mass. So the number of orbits can be approximated by (Emax - Emin)/2qV. The assumption that the particle gets 2qV every orbit is optimistic because of transit-time considerations--the particle takes time to accelerate, during which the field is changing sinusoidally. And relativistic effects may be significant but probably aren't.

I hope you are a member of the Cyclotrons.net forum. The experts on small amateur cyclotrons are all over there!

-Carl

[Noah C Hoppis]

Oh yes I am indeed a member of Cyclotrons.net (the site is a bit slow for Items that will get discussed back and forth, so I prefer fusor for things of this nature)! I also am glad to hear half a year of practice at Solidworks is paying off! (though it may be naive) I tried to calculate the equations and ended up with a grotesque number around ~1.25*10^25; which is a bit odd for a number of turns of a beam! Are the units involved all SI units? (most importantly the mass (G?) charge (elementary charge?) and field(T?)). Again I am new to physics and have come to expect each value to be painstakingly drawn out in some thesis! I really appreciate the concern and help;
Thanks!

[Dennis P Brown]

If you really want the cyclotron, I will suggest that you read the very old Scientific American amateur science article on how some high school students built one (and the magnet) in the fifties – a google search should pull that up. A lot of good information and no physics equations. That said, you will really need to sit down and learn some of the physics – not knowing what energies will result from what field, and oscillator driver frequencies and so forth will create real design problems. Regardless, you also really need a plan of action or else this will just remain a though experiment that leads to a pile of equipment sitting in a room.

The first issue (if you are serious) and the one you still have not answered is what you want a cyclotron for? Without that, the design parameters cannot be set and you are just heading for problems – unless you just for a working cyclotron for its own sake.

The kids that made the one in the SA article did it for that very purpose but they had access to experts in the field for help, free access to big shops, and lots of people for free labor in a host of technical fields.

A one person attempt could be an issue (which, of course, money can go a long way to solving if one has deep pockets.) Simply offering computer drawings pulled from thin air is useless without the real numbers provided by the calculations on the particles (what you intend to accelerate), magnetic field strength/size, and oscillator energy/power all mixed in to determine a real world design so you can get started – not just have something to become wall art.

I would rather you approached this thinking realistic than waste your money – a small electro-static linear accelerator is vastly simpler because, frankly, I have built one (yet still not working!!!!) and know how what appears so simple can rapidly get complex and fail in ways that one could never think of until they bulid it - mine has cost very little money but lots of time. The whole reason I have spent so little is because I planed it out and have followed this plan closely - I've wasted little on anything that did not get me to the desired goal. That is the process that turns ideas into real world devices and not piles of useless (but expensive) equipment sitting in a room.

[Noah C Hoppis]

Oh Yes the specs! the Diameter is 8", the RF supply is 4kV @ 1-20 MHz, the magnet has 8" pole faces @ .8 T. The first experiment will be measuring for a beam current of accelerated Protons, and the second is for the break down of Li by Protons into Alpha particles. The plan is to net get the magnet / magnet core (and in the latter case, then wind the coils), then purchase the parts for the acceleration cavity, mill the necessary parts in the summer / fall, get access to a TIG and weld the vacuum assembly together. Then final assembly and testing.

[Richard Hull]

Equations are cool things. They are SI, MKS and CGS independent in most cases where a system specific fudge factor has not been used. What is important is that every value you plug in must be in the same units system.

If you get a whacko result, go back and examine the unit system you used. Were all the variables you supplied in the same system?

Richard Hull

[Noah C Hoppis]

That's what I thought, the problem is the radius of the Dee is either 4" or ~10cm and it isn't ever balanced, but I assume it is cm and after using coulombs instead of elementary charge I got -0.5 turns of the beam (well at least its not in SI notation anymore!) the Lack of units is really screwy though, I still am betting it is some kind of mix of units!

[Carl Willis]

Elementary charge is a constant that has dimensions of charge in whatever unit system you choose. I recommend the MKS (SI) unit system. It's pretty much the standard.

So you have some specifications:

B = 0.8T (probably reasonable for a small cyclotron; this is already in MKS units)
m = 1.67E-27 kg (you mentioned your particles would be protons)
q = +1e = 1.602E-19 C (elementary charge)
V = 4kV assumed to be rms, so Vpeak = 1.414*4kV = 5700 V
R = (8"/2) = 4" = 0.102m

Emin isn't very influential in this situation, but I will assume it is 5 keV = 8E-16 joules (SI unit of energy) just for the hell of it.

Run the numbers. The first thing you find (if you didn't already have a handle on it) is that Emax is 5E-14 joules. Often this is reported in electronvolts instead; it's about 320 keV. Next you wanted to know approximate number of orbits:

(5E-14 - 8E-16)/(2*1.602E-19*5700) = 28 orbits

Emax is a good solid number you can take to the bank, as long as max radius and mag field strength are well-defined. The calculated number of orbits is just approximate and likely to be too small chiefly because of the transit time effect.

If you did this math using units of inches, slugs, foot-pounds, statcoulombs, etc., you would still get a right answer, strictly speaking. It would still be dimensionally correct. But you would have trouble contextualizing it. Best to stick with the same SI units that everyone else uses, and maybe convert to electronvolts for beam energy since that is in widespread use too.

-Carl

[Rich Feldman]

Noah,
Not just because of units issues, I urge you to run your numbers using a spreadsheet calculator program. If you don't have one, get one and learn how to use it. It's a great way to document your units and formulas, and facilitates debugging and changing variables.

Here's a freshly whipped example. Blue cells are the only numerical inputs, and three of them are physical constants you can look up.
I have included some unit conversions, as always NEVER in the same cell (calculation step) as a physics formula. Cutting corners invites mistakes. Remember, cells don't cost much at all. In fact, I make a habit of using separate formula cells to convert between SI values (hertz, farad) and practical I/O units (megahertz, microfarad).

The example has some sloppiness and minor mistakes. I should have given a B value of 8894 gauss, to match the 13.56 MHz ISM band. As if your flux uniformity could justify four significant figures.

-Rich

[Noah C Hoppis]

So I should need about 1.0e-6torr, correct? Wow 320 keV is better than I was expecting! I assume the next step is to start contacting local universities in search of cheap / free stuff from there labs!

[Dennis P Brown]

The bottom of the 10^-6 torr should work fine; getting there with a diffusion pump will require a good cold trap.

[Noah C Hoppis]

That much I assumed; I am wondering how I should extract the beam of protons out of orbit to a target without destroying the beam quality. I think a magnet may be necessary maybe NdFeB's or ceramics, because I picture a hole in the wall of the chamber, and it would just chop the beam up right? any suggestions?

[Carl Willis]

You might want to pitch it as a collaborative effort, rather than just a scrounging effort.

You need climate-controlled floorspace rated for the weight of the cyclotron, and provisions for its significant power and water requirements. You might benefit enormously from the expert and physical resources of a university physics department (including design mentoring, a competent machine shop, and possibly an existing radiation machines license, because unlike fusors, home cyclotrons have gotten people in trouble with their state radiation control departments).

I think your pole size and field strength are best left undetermined until a suitable core is available. Unfortunately, this is probably an item of opportunity, rather than something you can reasonably specify up-front. Even more than a shiny set of SolidWorks models, having a lead on a core tells possible collaborators that this is a serious project with a good chance of success.

-Carl

[Carl Willis]

Do you really need to "extract" the beam? Why not just shove something into it, which is a far easier proposition for starters. A radial linear motion feedthrough with a target on it can be moved into the beam to intercept particles at various energies depending on radius. It is a valuable diagnostic, answering questions about where your beam is and isn't, and what its energy is. There are reasons to extract a beam into a beamline to a distant target, for example to get PMT-based detectors away from the magnet, or to debunch the RF on it. But why not just cross that bridge when you come to it?

-Carl

[Noah C Hoppis]

OK. I have already contacted the University of Washington's Accelerator & high energy physics department (when I called the front office guy said,"OK I know just the guy to call" if you can believe it!). I guess It may be almost impossible to construct this thing alone, Though I am not quite willing to relinquish this thing openly to the community yet, however the idea of donations / mentor-ship are still very attractive. It seems that in Washington state it is only defined as an accelerator if by 246-229-0010,"any machine capable of accelerating electrons, protons, deuterons, or other charged particles in a vacuum and of discharging the resultant particulate or other radiation into a medium at energies usually in excess of 1 MeV". does this mean if I stay under 1MeV I am legal?

[Carl Willis]

That's an easy question for the Washington State Health Department. It all depends on how the individuals running the joint interpret their administrative rules. If you call or email them and ask if a proton cyclotron with beam energy less than 1 MeV is a "particle accelerator" under the regulatory definition, they will tell you their thoughts on the subject. The wording of your state's rules seems to leave a lot to their discretion.

-Carl

[JakeJHecla]

Talk to Mike Brennan: Mike.Brennan@doh.wa.gov, (360) 236-3253. He checked over the NWNC fusor.

[Noah C Hoppis]

I still wonder how I am going to extract the beam so I can do anything with It, any suggestions? It is true I do plan to use a PMT/scintillator based detector much to the chagrin of others...

[Rich Feldman]

Noah C Hoppis wrote:
> That much I assumed; I am wondering how I should extract the beam of protons out of orbit to a target without destroying the beam quality. I think a magnet may be necessary maybe NdFeB's or ceramics, because I picture a hole in the wall of the chamber, and it would just chop the beam up right? any suggestions?

Noah C Hoppis wrote:
> I still wonder how I am going to extract the beam so I can do anything with It, any suggestions? It is true I do plan to use a PMT/scintillator based detector much to the chagrin of others...

A few thoughts, grouped by category.

1. The problem has been solved many times before.
2. Search and ask questions on the amateur cyclotron forum.
3. Investigate known cyclotrons using the miracle of the Internet.
4. Consider metal foil windows, which can withstand atmospheric pressure and transmit most of the beam.

5. As Carl said, you can have a motion feedthrough to explore your proton beam within the chamber. A fluorescent screen might be handy, but maybe you can simply measure the DC current on a conductive target. Even in the presence of a strong RF field. Your experimental target for atom-smashing can go inside the vacuum, with resulting alpha particles or consequent scintillation photons passing through windows.

6. After the last accelerating pass, if your beam reaches the edge of your "uniform B field" it will curve less, which might be geometrically convenient.

7. Back to your first idea. If a strong local magnetic field is to divert the beam, it needn't employ permanent magnets. You can get similar field strength change (on the order of 1 tesla) with a steel structure that locally bypasses / shields the large electromagnet's field. For example, if the beam passes into the end of a mu-metal tube in the cyclotron plane, it will proceed without curving. I have not studied beam focusing issues.

8. This is purely speculative now. Your beam comes in bunches at the RF frequency. You could have a small but intense pulsed electromagnet, reaching full strength only at the phase that counts, perhaps reducing its power and cooling requirement.