Systems and processes for producing relatively uniform transverse irradiation fields of charged-particle beams

An adjustable aperture quadrupole, comprising:

• a first charge plate having a first charge;
• a second charge plate having a second charge and positioned generally opposite the first charge plate and offset therefrom by an adjustable distance;
• a charged particle transfer chamber positioned between the opposing first and second charge plates and having a size and shape for facilitating transfer of charged particles through the adjustable aperture quadrupole within a magnetic field formed through interaction of the first charge of the first charge plate and the second charge of the second charge plate, the characteristics of the magnetic field being responsive to the adjustable distance between the first charge plate and the second charge plate;
• and
• an adjuster coupled to at least one of the first charge plate or the second charge plate for selectively setting the adjustable distance between the first charge plate and the second charge plate to define the magnetic field within the charged particle transfer chamber through which charged particles travel.