Cyclotron Info

Builder FAQ

The questions people ask when they are thinking about building a small cyclotron, or have one half built, answered in plain language. Each answer stands on its own in its first paragraph and opens into the detail below it; every claim cites the Design Guide rule or the documented build behind it.

Start here if…

All the questions, in the order a build meets them

Part 1 Deciding and designing · planning and reading

Before You Build 8 questions

  1. Can an amateur actually build a cyclotron? Should I?
  2. What should count as a first success? Do I need to get the beam out of the chamber, or can I demonstrate acceleration with a detector inside?
  3. How do the particle, the energy, the magnet size, the field and the RF frequency fit together? Which of them can I actually choose?
  4. Could I accelerate electrons or other ions instead of protons? Would any of those be easier for an amateur?
  5. How do I know which requirements in a book or paper apply to my cyclotron? One says the magnetic field should be uniform, another says it should decrease with radius, and another says it should increase. Who is right?
  6. Can I copy a successful amateur cyclotron but change it to suit the parts I already have? Can I combine parts of different designs, or just scale one down?
  7. Do I need to simulate the cyclotron before building it? If a simulation shows one proton spiraling outward, how much does that prove?
  8. What should I read? The current literature seems hopelessly complicated and theoretical, and the old literature must be way out of date.

Part 2 Building it safely · subsystem choices and their checks

Before You Switch Anything On 5 questions

  1. What safety checks, expert review and local permissions do I need before I start testing the subsystems?
  2. Can the machine make radiation, or hurt me some other way, before I've even detected a beam?
  3. I've heard that switching off the magnet coils can produce a high voltage. How does that happen, and what protection do the coils and the power supply need?
  4. What should be grounded and what needs to float? I have a filament heater supply, a filament bias supply, RF drive, a biased collector plate going to an electrometer, a magnet supply, vacuum-gauge supplies, and exposed metal everywhere. Which parts can be tied together, which must be isolated, and what does "floating" actually mean?
  5. What are ground loops? How do I know if I have one, and how do I avoid them in my cyclotron?

The Magnet, and Staying in the Gap 7 questions

  1. The books make the magnet requirements sound beyond amateur reach: field uniformity, steel homogeneity, and so on. Some amateur builds look much simpler. How good does my magnet really need to be?
  2. The drawings show a flat spiral. What keeps the particles from wandering up or down and hitting the dee? Do I need "focusing"?
  3. Do I really need to map my magnet's field accurately? Is one reading in the middle enough, and which value goes into the frequency and energy calculations?
  4. How can my magnet's field have "harmonics" when the magnet runs on DC? I thought harmonics were an audio thing, like musical notes. Do I have to worry about them?
  5. How accurately do I need to line up the magnet, chamber, dee, ion source and collector? Which dimensions are really critical, and what should be adjustable?
  6. Can I use a permanent magnet instead of an electromagnet? It would save the power supply, the cooling and the coils.
  7. Can I build around a surplus magnet, like an old NMR or MRI magnet, or a stack of microwave-oven transformer cores?

Making Ions, and the First Few Turns 5 questions

  1. My filament is glowing. How do I know it's actually making ions? And what are the filament current, the bias voltage and the emission current each telling me?
  2. Where exactly should the filament or the source opening go? Is "at the center" good enough, and how do the first orbits avoid hitting the source?
  3. My ion source is on all the time, but the RF keeps reversing. How do the ions start at the right moment? Do I have to synchronize the source with the RF?
  4. How many turns should a proton make before it reaches my target energy? Fewer turns sounds easier, but isn't the whole point of a cyclotron to accelerate over lots of turns?
  5. What is "space charge", and when would it matter in a small cyclotron? Is it a problem in the source, during acceleration, or both?

Vacuum, Materials and Gas 7 questions

  1. What pressure should I aim for when the cyclotron is running? Don't I need more gas to make ions, but less gas to let them spiral around?
  2. Do I need a diffusion pump or a turbopump, or will a good roughing pump do?
  3. My pump is rated to reach a much better vacuum than I need. Why can't my chamber get there? What are pumping speed and conductance?
  4. Can I trust the pressure my vacuum gauge shows? Is that the pressure where the particles are travelling?
  5. What's the simplest practical way to control the gas flow and the chamber pressure? Needle valve, leak valve or mass-flow controller, and can I leave the setting alone once I find it?
  6. What materials can I use inside the chamber? Are ordinary plastics, adhesives, solder, rubber seals and workshop metals acceptable?
  7. How clean do the chamber and the plumbing really need to be? Which of the gloves, the special solvents, the deionized-water rinse and the bake-out do I actually need?

The Dee and Its RF 10 questions

  1. Why can't I just drive the dee with an RF amplifier, or step the voltage up with a transformer? What are the resonant circuit and the matching network actually doing?
  2. I keep seeing "cyclotron resonance", "RF resonance" and "impedance matching". Are these the same thing? What has to match what?
  3. Do I need two dees? What does a dummy dee do? Can the chamber lid serve as the RF return, or is that a different issue?
  4. Does my vacuum chamber itself have to be a resonant cavity? What is a resonant cavity, and how does it relate to the dee's resonant circuit?
  5. What is this "Q" people talk about? How do I measure it, how good does it need to be, and is higher always better?
  6. Should I use inductive or capacitive coupling to drive the dee? Which is easier to build, adjust and troubleshoot?
  7. How much RF power do I need, and how does that become voltage on the dee? Can I read the voltage off the power meter, or do I have to measure it?
  8. The literature says tens of kilovolts on the dee. Some amateur machines ran under a kilovolt. How is that possible, and what did it cost them?
  9. My RF circuit tunes up nicely on the bench. Why does it change when I close the chamber, pump down, turn on the source, or raise the power?
  10. People talk about RF "conditioning". What does that mean, and how do I tell conditioning from a fault that means I should stop?

Part 3 Running it and proving it · commissioning, troubleshooting, evidence

Finding and Keeping a Beam 6 questions

  1. What are all the knobs, and what does each one actually change? Which are operating controls, which are setup adjustments, and which can I leave alone?
  2. What can I test before the whole cyclotron works? How do I check each subsystem without already having a beam?
  3. How do I know my beam-current detector could detect the beam I'm looking for, before I use it to troubleshoot the cyclotron?
  4. How do I find a first beam when all these settings affect one another? Does everything have to be perfect at once before anything happens?
  5. It worked yesterday. Why doesn't putting every knob back where it was bring the beam back?
  6. I can detect acceleration near the centre, but not farther out. Is that a partly working cyclotron? How do I find where the particles are being lost?

Proving the Beam 4 questions

  1. How do I know my cyclotron is actually working? If I measure a current, how do I know it's accelerated particles rather than something else in the chamber?
  2. I'm feeding hydrogen into the chamber. Does that mean the accelerated particles must be protons? Why might I see more than one resonance peak?
  3. What are all these beam terms, fluence, emittance, dispersion, chromaticity, energy spread and so on? Which do I need to understand now, and which can wait?
  4. How should I document that my cyclotron is accelerating protons? What would someone else need to evaluate the claim?