An example particle accelerator includes a magnet to generate a magnetic field, where the magnet includes first superconducting coils to pass current in a first direction to thereby generate the first magnetic field, and where the first magnetic field is at least 4 Tesla (T). The example particle accelerator also includes an active return system including second superconducting coils. Each of the second superconducting coils surrounds, and is concentric with, a corresponding first superconducting coil. The second superconducting coils are for passing current in a second direction that is opposite to the first direction to thereby generate a second magnetic field having a magnetic field of at least 2.5 T. The second magnetic field has a polarity that is opposite to a polarity of the first magnetic field.