Electron beam transport system

An electron beam transport system for controlling the position of two different electron beams, the electron beam transport system comprising:

• a main electron beam transport module defining a main path, the main electron beam transport module comprising a beam monitoring device disposed at a measurement position along the main path;
• a first input electron beam transport module defining a first input path, the first input electron beam transport module being arranged to deliver a first electron beam to the main electron beam transport module, the first input electron beam transport module comprising a first actuator for applying a perturbation to a transverse position of the first electron beam at a first actuation point disposed along the first input path;
• a second input electron beam transport module defining a second input path, the second input electron beam transport module being arranged to deliver a second electron beam to the main electron beam transport module, the second input electron beam transport module comprising a second actuator for applying a perturbation to a transverse position of the second electron beam at a second actuation point disposed along the second input path;
• and
• a controller operable to receive a signal from the beam monitoring device and to send control signals to the first actuator and the second actuator;
• wherein the electron beam transport system is operable to determine a first quantity and a second quantity for at least one control direction, the first quantity being indicative of a difference in a transverse position of the first and second electron beams in the control direction at the measurement point and the second quantity being indicative of an average transverse position of the first and second electron beams in the control direction at the measurement point;
• and
• wherein the controller is further operable to control the trajectories of the first and second electron beams in the control direction independently by implementing a first control loop that iteratively attempts to reduce the first quantity by using the first actuator to perturb a trajectory of the first electron beam, and a second control loop that iteratively perturbs a trajectory of the second electron beam using the second actuator such that the average transverse position of the two different electron beams moves towards a desired transverse position in the control direction.