Methods and apparatus are described wherein a charged beam in an enclosed conducting cavity in an accelerator is monitored for position, current, and energy. One method uses induced electric signals on non-intercepting conducting electrodes. Another method uses an intercepting and moving electrode than can be moved into the beam to different degrees to monitor the beam current and vertical profile at different radial positions. Non-intercepting electrodes are also used as part of a moving diagnostic probe to monitor properties of the beam at different radial positions. Another method uses the current in the leads to a power supply, a portion of this current being equal to the beam current. Another method uses the magnetic and electric fields from the beam that penetrates a non-conducting portion of the conducting cavity. Yet another method uses the radiation emitted during acceleration of the beam by the deflecting magnets that guide the beam.