A radiation generator may include an elongate generator housing having a proximal end and a distal end, a target electrode within the elongate generator housing at the distal end thereof, a charged particle source within the elongate generator housing at the proximal end thereof to direct charged particles at the target electrode. A plurality of accelerator electrodes may be spaced apart within the elongate generator housing between the target electrode and the charged particle source to define a charged particle accelerator section. Each accelerator electrode may include an annular portion having a first opening therein, and a frustoconical portion having a base coupled to the first opening of the annular portion and having a second opening so that charged particles from the charged particle source pass through the first and second openings to reach the target electrode.