An apparatus for generating plasma, comprising: a plasma discharge tube substantially transparent to microwave energy, the plasma discharge tube having a longitudinal axis; a microwave cavity surrounding the plasma discharge tube; a waveguide (272A, 372) coupled to the microwave cavity for guiding the microwave energy into the plasma discharge tube such that the plasma is generated in the plasma discharge tube; a conductive coil wound around an outer surface of the plasma discharge tube, the conductive coil comprising an electrically conductive material, wherein, in operation, an electric field of the microwave energy induces an electric current in the conductive coil, the electric current affecting power absorption in the plasma discharge tube; characterised by a controller for adjusting the angle between the electric field of the microwave energy and the longitudinal axis of the plasma discharge tube; a rotational coupling device for coupling the waveguide to the plasma discharge tube, the rotational coupling device allowing the waveguide to be rotated with respect to the plasma discharge tube to adjust an angle between the electric field of the microwave energy and the longitudinal axis of the plasma discharge tube, such that power absorption in the plasma discharge tube is adjustable via the rotational coupling device to a predetermined profile with respect to the longitudinal axis of the plasma discharge tube; and at least one of an electromagnetic field sensor for generating a first signal indicative of electromagnetic field intensity in the plasma discharge tube or a temperature sensor for generating a second signal indicative of temperature in the plasma discharge tube, the controller controlling the angle between the electric field of the microwave energy and the longitudinal axis of the plasma discharge tube based on at least one of the first and second signals. a plasma discharge tube substantially transparent to microwave energy, the plasma discharge tube having a longitudinal axis; a microwave cavity surrounding the plasma discharge tube; a waveguide (272A, 372) coupled to the microwave cavity for guiding the microwave energy into the plasma discharge tube such that the plasma is generated in the plasma discharge tube; a conductive coil wound around an outer surface of the plasma discharge tube, the conductive coil comprising an electrically conductive material, wherein, in operation, an electric field of the microwave energy induces an electric current in the conductive coil, the electric current affecting power absorption in the plasma discharge tube; characterised by a controller for adjusting the angle between the electric field of the microwave energy and the longitudinal axis of the plasma discharge tube; a rotational coupling device for coupling the waveguide to the plasma discharge tube, the rotational coupling device allowing the waveguide to be rotated with respect to the plasma discharge tube to adjust an angle between the electric field of the microwave energy and the longitudinal axis of the plasma discharge tube, such that power absorption in the plasma discharge tube is adjustable via the rotational coupling device to a predetermined profile with respect to the longitudinal axis of the plasma discharge tube; and at least one of an electromagnetic field sensor for generating a first signal indicative of electromagnetic field intensity in the plasma discharge tube or a temperature sensor for generating a second signal indicative of temperature in the plasma discharge tube, the controller controlling the angle between the electric field of the microwave energy and the longitudinal axis of the plasma discharge tube based on at least one of the first and second signals.