Questions about the practical construction of a Linac

[Noah C Hoppis]

I have recently become interested in Linacs and I have only found sparse information on the construction of a Linac, so I pose the question, how would one go about constructing a DC Linac on a relatively low budget? I get the concept of a Linac but the actual ingredients of a Linac have eluded me. I've decided on a Linac that is DC driven because I have no idea how to get my hands on a good RF supply. I also want to accelerate protons from striped hydrogen (would that even be practical?). I am not sure how to go about the actual acceleration cavity. Would it have many drift tubes or only an anode & cathode? Please help and don't just say 'you don't know what your getting into'. Thanks!

PS sorry if this is in the wrong category (I'm new)

[Carl Willis]

Hi Noah,

I have several years of career experience building and operating RF linacs, and a couple projects at home that are just ion sources coupled to potential-drop beamlines (I suppose this qualifies as a linac in some primitive sense).

The linear geometry requires more attention to charged-particle optics than a fusor, and this translates into more mechanical complexity during construction. I think it takes a good familiarity with standard vacuum fittings, feedthrough technology, and light machining to make a decent shot at it. Glass and ceramics, solders and brazes, and elastomer seals often need to be custom-worked to realize one's vision.

A linac typically consists of a beamline containing the accelerating structure at high vacuum, and an ion source that operates at a much higher pressure. The pressure difference between the source and the accelerating structure is maintained by differential pumping. Unlike a fusor, a fast vacuum system is really paramount for a linac that operates as I have described. Good vacuum engineering is crucial.

There are some accelerator projects described in the old Scientific American "Amateur Scientist" column that offer perspective on the simplest kinds of potential-drop accelerators, powered by Van de Graaff generators. These projects are worth looking at, but keep in mind that vacuum technology has changed radically (and for the better) since those ideas were tested. Also, some ideas that work acceptably with the low-power, low-current Van de Graaff designs are going to be inadequate for driving whole milliamps of current using modern switched Cockroft-Walton supplies. Gradient shaping by the use of external wires on a glass beamline is a prime example.

I hope that input is helpful.

-Carl

[Noah C Hoppis]

I suspected that a very hard vacuum was required
Would something like this (https://www.youtube.com/watch?v=PNJGqjB ... Q&index=16) be possible / in the realm of amateur feasibility? Also what would its capabilities be limited to if it was driven by a VM and supplied with protons? Thanks!

[Carl Willis]

The project shown in that video is a high-quality amateur project that follows the design conventions of the old Amateur Scientist column.

It is hard to differentially pump all but the tiniest ion sources with this arrangement, but that's probably just as well since the beamline cannot handle more than a few microamps and the power supply (a Van de Graaff) can't supply more than a few microamps without a debilitating drop in terminal voltage. You might consider some alternative arrangements that put the ion source at ground potential. For one thing, this opens the door to use some powerful ion source designs that require RF or auxiliary DC voltages. It also makes an easier job of differentially pumping the beamline right near the ion source aperture, and of supplying gas to the ion source. If ion source power supplies and gas supplies have to be floated at high potential because the ion source is located at the "hot" end of the beamline, then you have some real challenging engineering to figure out. The downside to putting the source at the grounded end is that the target is then at HV; for certain experiments that can be a nuisance.

When using powerful multiplier-based supplies that can source many mA of current at high potential, some other issues need to be considered. Backstreaming electrons and insulator charging are some problems. The beam emitted from the ion source needs to be well-defined and should not be allowed line-of-sight to any insulating surface. Secondary electrons created bv beam impingement in the accelerator structure or at the target should be minimized by the beam optics design, and then blocked from traveling upstream in the structure through the use of suppression voltages and magnetic traps. It is easy to put magnetic and electric traps at the target and at the ion source, and this is the minimum requirement for keeping the electron current under control. Inadequate control of electron current results in breakdown, damage to the ion source, and horrendous x-ray yields. Finally, you might think about supplying the focusing electrodes (drift tubes) directly from various points on the multiplier stack. Unlike the VdG designs, you can't reasonably get away with a corona divider when running with serious current.

-Carl

[Noah C Hoppis]

I was wondering about that! I knew a VDG wouldn't supply enough current and the drift tube geometry would adversely affect te ability of the diff pump to remove all the gas. you mentioned the design not working with a VM, was it simply the lack of supply to each drift tube that wouldn't work, or the whole thing failing utterly due to geometry? I've also heard that a value of ~1G ohm between each drift tube would be sufficient, correct? The comment on electrons back flowing, I was thinking of putting a ring magnet from a Magnetron out of a microwave around the Ion source to stop electrons from running rampant, would that work or would more be required? (sorry for all the questions!)

Thanks!

[Daniel Firth]

I've been planning an electrostatic LINAC as well. Here's everything I've managed to dig up:

1959 Amateur Scientist Article, http://xa.yimg.com/kq/groups/10603698/7 ... rotons.pdf

1971 Amateur Scientist Article, http://xa.yimg.com/kq/groups/10603698/1 ... erator.pdf

Website detailing John Cockcroft and Ernest Walton's nuclear transmutation experiment:
http://homepage.eircom.net/~louiseboyla ... walton.htm

Fred Niell's site: http://niell.org/linac.html

Segment from "Stephen Hawking's Universe" featuring Fred Niell and his LINAC.
http://www.youtube.com/watch?v=6dnEchmSYzk

Andrew Seltzman's accelerator: http://www.rtftechnologies.org/physics/linac.htm

Particle Accelerators, by M. Stanley Livingston: http://archive.org/details/ParticleAccelerators

Introduction To Experimental Physics, by Wiliam B. Fretter: http://archive.org/details/introductiontoex032616mbp

Ions, Electrons, And Ionizing Radiations, by James Crowther: http://archive.org/details/ionselectronsand030780mbp

John Cockcroft and Ernest Walton published their findings by submitting them to the Royal Society. These articles were then published
in a journal known as "Proceedings of the Royal Society."

Experiments with High Velocity Positive Ions
http://rspa.royalsocietypublishing.org/ ... l.pdf+html

Experiments with High Velocity Positive Ions. (I). Further Developments in the Method of Obtaining High Velocity Posistive Ions
http://rspa.royalsocietypublishing.org/ ... l.pdf+html

Experiments with High Velocity Positive Ions. (II). The Disintegration of Elements by High Velocity Protons
http://rspa.royalsocietypublishing.org/ ... l.pdf+html

Experiments with High Velocity Positive Ions. (III). The Disintegration of Lithium, Boron, and Carbon by Heavy Hydrogen Ions.
http://rspa.royalsocietypublishing.org/ ... l.pdf+html

Experiments with High Velocity Positive Ions. (IV). The Production of Induced Radioactivity by High Velocity Protons and Diplons
http://rspa.royalsocietypublishing.org/ ... 814df7cc22

Experiments with High Velocity Positive Ions. (V). Further Experiments of the Disintegration of Boron
http://rspa.royalsocietypublishing.org/ ... 814df7cc22

Experiments with High Velocity Positive Ions. (VI). The Disintegration of Carbon, Nitrogen, and Oxygen by Deuterons
http://rspa.royalsocietypublishing.org/ ... 814df7cc22

[Richard Hull]

It is important to remember that any glass beam line or any metal beamline that works at over 60kv will produce "horrendous" and deadly x-radiation at the target end. Carl mentioned this near the end of his last post.

A linac is a cool device, but as Carl also notes, it requires a special diligence in assembly and operation that the fusor does not. A well constructed linac can do a lot of beam-on-target fusion and make lots of neutrons, but with an attendant x-radiation that must be dealt with in a manner the average fusioneer never has to worry about.

Richard Hull

[Carl Willis]

Using a resistive divider or a corona divider is limited to applications where the beam current is very small and the power supply of very low power. If you use a resistive divider to supply focusing voltages and some breakdown or beam mis-steering or blowup occurs (a very routine occurrence), it is possible for the full supply voltage to appear across one or more gaps. The resistors typically cannot survive this unless the HVPS is low-powered, and even if they survive, they may be unable to sufficiently regulate the focusing voltages to get control of the beam. Finally, resistors just waste energy. The lower the divider resistances (and the lower the beam current), the better the regulation of the voltages supplied to the focusing electrodes, but at the expense of power wasted as heat. In a multiplier, you often have access to all points along the cascade and can supply the focusing voltages directly without having to run current down an external divider.

-Carl

[Doug Coulter]

I have to second this - X rays are real and are a hazard, though I wouldn't declare the amount and energy of the ones a "regular" fusor creates as harmless, myself - which is why I spent all that time and money on lead and lead glass, along with detectors to ensure my personal safety. Even with these measures, at my op position the X rays are roughly 10x the background count - on a simple pancake geiger tube that isn't particularly low energy X ray sensitive compared to say, a plastic or NaI scintillator.

In fact, back in the day when color TV's got to 35-40kv ultor voltages, there was quite the hue and cry about the copious emitted "low energy" X rays, and the industry went to leaded glass face-plates - even with lower beam currents than fusors use. Whether that was self-panicking or a real issue is left to those wiser than myself.

I will also note that there are limits due to the Z of whatever things are hitting - the K lines only go up so far for a given Z, almost no matter the input energy. This was brought home to me in spades when I experimentally coated my fusor tank walls with Pd - while it doubled my neutron output and Q, it also much more than doubled the X ray output due to the high Z of the Pd. Changing this to Ti (which also holds D) cut the X rays way down, leaving the neutrons and Q still higher than plain stainless steel walls.

In fact, with the Ti coating, X rays were less than with the plain stainless steel - even with higher neutron output.

[Noah C Hoppis]

I expected some X-rays but that sounds pretty gnarly, I guess all that Pb shot will come in handy...

Carl,
I guess this implies corona rings will not be required? So a multiplier of about 200kV distributed down 8 drift tubes should provide proper reaction of an Lithium target with protons?

[Carl Willis]

Hi Noah

Corona rings have an important function: They prevent breakdown of the air by distributing charge on a surface with sufficiently-low electric field on it. The field strength is proportional to potential and inversely proportional to the local curvature radius of the charged surface, and in dry air at standard conditions, fields much above about 20 kV / cm are to be avoided. This principle, and the 20 kV / cm value, are a good starting point when figuring out the radii and separation of corona rings and terminals, but exact geometry, polarity, air pressure, humidity, and even the metals and construction techniques of the terminals all figure in.

Electrical breakdown in vacuum is also of concern. Values exceeding about 100 kV / cm are problematic. Sometimes one finds sharp edges around apertures in beamlines, or in the Houskeeper seals encountered in glass-metal construction, or the metallized seals in ceramic-metal construction. A terminal like an external corona ring can often shield these sensitive features, in addition to doing the job of preventing corona per se on the air side.

It's hard to qualify what a "proper" reaction is, since that is a subjective idea critically depending on your own criteria and detection capabilities. The Li-7(p,a) reaction is well-known for its high Q-value (it is strongly exothermic and has no energy threshold) and high cross-section even at low energies. It's a good choice among a variety of possible low-energy reactions. But you will have to plan for how to detect it.

-Carl

[Noah C Hoppis]

So, from what I've read, the breakdown of lithium would produce alpha particles, which could be measured with the humble geiger counter, correct me if you se any inherent problems, but the geiger tube could be submerged in the vacuum with the target angled to project the beam at it. My only question is whether the tube could stand the vacuum, and if there are any ways to get the alpha particles through the wall of the chamber without compromising it?

[Carl Willis]

Hi Noah,

You have identified some of the most important challenges in detecting the alpha particles.

Thin-window Geiger tubes are pressurized and will burst if subject to vacuum. With careful engineering (perhaps a support lattice on the vacuum side of the window), you may be able to solve that problem. But Geiger counters have other issues too: they are sensitive to ALL radiation that reaches them. That includes x-rays from the accelerator, prompt gamma rays from various (p,g) reactions, and the normal range of background cosmic rays and emissions from naturally-occurring radioactive material. It may be impossible to distinguish alpha particles from the target using a Geiger tube in the presence of all this competition. Some other kinds of detectors permit better discrimination of the alpha particles, such as surface-barrier detectors, scintillators, and proportional tubes. Some thin-window Geiger tubes can be carefully operated at reduced voltage to behave somewhat like proportional tubes. Having the ability to determine the energy spectrum of detected particles would be a real advantage here, since with that information, there would be little doubt about the origin of the particles in the (p,a) reaction.

-Carl

[Noah C Hoppis]

In my quest for avoiding total bankruptcy, I stumbled upon the original idea of using a phosphor to detect the alpha emissions, I could coat the inside of a window of the chamber with said phosphor and remotely record for any light, however, I'm not sure where to go for an alpha florescent material.

[Rich Feldman]

Noah C Hoppis wrote:
> In my quest for avoiding total bankruptcy, I stumbled upon the original idea of using a phosphor to detect the alpha emissions, I could coat the inside of a window of the chamber with said phosphor and remotely record for any light, however, I'm not sure where to go for an alpha florescent material.

Cockroft and Walton eventually put scintillation screens on opposite sides of their lithium target. Simultaneous flashes of light on both screens further cemented their previous conclusion (see below) that the bombardment produced pairs of alpha particles.
http://www.nature.com/physics/looking-b ... index.html
"We have employed the same method to examine the effect of the bombardment of a layer of lithium by a stream of these ions, the lithium being placed inside the tube at 45° to the beam. A mica window of stopping power of 2 cm. of air was sealed on to the side of the tube, and the existence of radiation from the lithium was investigated by the scintillation method outside the tube. The thickness of the mica window was much more than sufficient to prevent any scattered protons from escaping into the air even at the highest voltages used.
...
"The brightness of the scintillations and the density of the tracks observed in the expansion chamber suggest that the particles are normal a-particles. If this point of view turns out to be correct, it seems not unlikely that the lithium isotope of mass 7 occasionally captures a proton and the resulting nucleus of mass 8 breaks into two a-particles, each of mass four and each with an energy of about eight million electron volts.The evolution of energy on this view is about sixteen million electron volts per disintegration, agreeing approximately with that to be expected from the decrease of atomic mass involved in such a disintegration."

[aka47]

All of the copied in text translates to (In lay persons terms):-

Coat a bit of view port with ZnS (Ag) and watch for it sparkling.

Bottom line though, is that if you haven’t got dark adapted eyes (Not trivial) and enough flux use a PMT.

Trying to do the work of Rutherford, Cockroft & walton without the fortitude to avoid loosing it via sensory deprivation. Is certainly not trivial. Doing the same and still turning in a valid result. is even less trivial.

There have been some stunningly poor results turned in by folk who thought that watching scintillation screens after being sat in complete darkness for stupidly long periods of time, were a good scientific method.

Don't do it.

Use technology or leave it alone.

[Noah C Hoppis]

You took the word out of my mouth, I was just about to ask about zinc sulfide, so I take it that a camera focused at the scintillator with the brightness up would circumvent the beta and gamma particles as well as X rays and only give me the alpha particles in the form of computer enhanced flashes?

[Richard Hull]

It all sounds trivial, but a good alpha only detector does use a ZnS:Ag screen of usually opaque mylar, aluminum or beryllium that is hyper thin attached to a PMT. The intensity of the flashes in this arrangement will not generally yield any useful alpha energy data due to a number of physical considerations.

For alpha energy data that is reliable for spectro work you need a PIPS detector as Carl has noted.

Richard Hull

[Noah C Hoppis]

Now I'm wondering how you would supply hydrogen to this, Up till this point I thought I was going to run the gas through a store bought brass ball valve (purged with acetone and the grease replaced with diff pump oil) running the gas through a medicinal metal needle in the chamber attached to the 400kV which would break down the Hydrogen into protons which it would accelerate. I'm questioning if this would kill my vacuum or not and whether I need to separate the Hydrogen by any other means.

[Rich Feldman]

Noah C Hoppis wrote:
> Now I'm wondering how you would supply hydrogen to ... the 400kV which would break down the Hydrogen into protons which it would accelerate. I'm questioning if this would kill my vacuum or not and whether I need to separate the Hydrogen by any other means.

Been answered many times, starting with Cockroft and Walton. I see we are already in the Ion Gun forum. How many threads, aside from this one, have you read here? Here's one that just caught my eye: viewtopic.php?f=12&t=5043#p32351

[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.