[Enthalpy]

A cyclotron for 500MeV protons isn't that big: D=6.6m H=2.9m on the appended sketch - unless beam focussing brings other constraints.

115kA per small lukewarm copper coil consume only 59kW together, while accelerating the protons for 1mol/year neutrons draw some 1.2MW at the plug. Bigger coils would reduce the 59kW or let increase the gap.

730t Fe cost ~1M€, as little as 100kW over 5 years.

A smaller cyclotron needs more than 1.8T, hence without iron, and the consumption jumps. 4T at the centre need 2×2.2MA. Pure Al at 20K and 60kOe resists 76pΩ×m, so each coil dissipates 52kW at 20K, and the cooler evacuates 5.6MW: as expensive in a month as iron costs once. The power is only proportional to the induction since the dimensions vary too. 6T at the rim limit the coils' size, which I didn't check as it won't improve much the consumption.

Maybe superconductors do that better, I can't tell. The unshielded field remains badly dangerous.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Some γ sources suggested here respond immediately to the proton impact. Since a magnetic or electric field can steer the proton beam of few MeV, one can steer the γ source. This enables 2D or 3D γ tomography similar to X-ray tomography, and more uses.

Little power can rotate an aggregate with permanent magnet that deflects the beam so its impact makes turns. Combined with a rotating partial ring of detectors or an immobile complete ring, this makes already Computer Tomography (CT) scans with γ rays, as one slice or many.

Electronic steering is faster and more diverse, up to MHz frequencies limited by the drive power. The impact can for instance sweep an emitting surface. The combination with detectors covering a receiving surface makes 3D images with less irradiation dose than successive slices.

Cooling the target(s) can become easier.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Permanent γ sources bring inconveniences
  Bbc
that's why I suggest a switchable one.

[Enthalpy]

I extended Helmholtz coils to have a significant length in an other thread
  30 Jan 2023 - 13 Feb 2023 on chemicalforums
and suggested coils of cold aluminium for MRI apparatus here on 21 Aug 2022
  chemicalforums
If such coils use no magnetic path, the long coils computation applies to them, whether they comprise permanent, warm, cold or superconducting magnets.