An apparatus (200,200A) for generating plasma, comprising: a plasma discharge tube substantially transparent to microwave energy, the plasma discharge tube having a longitudinal axis; a conductive coil wound around an outer surface of the plasma discharge tube, the conductive coil comprising an electrically conductive material; a microwave cavity surrounding the plasma discharge tube, a waveguide (272, 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, the waveguide having a rectangular cross-section defining a narrower side of the rectangular cross-section and a wider side of the rectangular cross-section, a first cross-sectional axis of the rectangular cross-section being parallel to the wider side of the rectangular cross-section and a second cross-sectional axis of the rectangular cross-section being parallel to the narrower side of the rectangular cross-section, the waveguide (272,372) being arranged such that the second cross-sectional axis, and therefore in use the microwave electric field, forms a predetermined angle with respect to the longitudinal axis of the plasma discharge tube such that, in use, the microwave electric field induces an electric current in the conductive coil, the electric current affecting power absorption in the plasma discharge tube, characterised by the predetermined angle being selectable such that power absorption in the plasma discharge tube is accordi ng to a predetermined profile with respect to the longitudinal axis of the plasma discharge tube; and by an actuator (318A, 318B) for receiving a signal to adjust the predetermined angle by rotation of the waveguide about the longitudinal axis of the waveguide. a plasma discharge tube substantially transparent to microwave energy, the plasma discharge tube having a longitudinal axis; a conductive coil wound around an outer surface of the plasma discharge tube, the conductive coil comprising an electrically conductive material; a microwave cavity surrounding the plasma discharge tube, a waveguide (272, 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, the waveguide having a rectangular cross-section defining a narrower side of the rectangular cross-section and a wider side of the rectangular cross-section, a first cross-sectional axis of the rectangular cross-section being parallel to the wider side of the rectangular cross-section and a second cross-sectional axis of the rectangular cross-section being parallel to the narrower side of the rectangular cross-section, the waveguide (272,372) being arranged such that the second cross-sectional axis, and therefore in use the microwave electric field, forms a predetermined angle with respect to the longitudinal axis of the plasma discharge tube such that, in use, the microwave electric field induces an electric current in the conductive coil, the electric current affecting power absorption in the plasma discharge tube, characterised by the predetermined angle being selectable such that power absorption in the plasma discharge tube is accordi ng to a predetermined profile with respect to the longitudinal axis of the plasma discharge tube; and by an actuator (318A, 318B) for receiving a signal to adjust the predetermined angle by rotation of the waveguide about the longitudinal axis of the waveguide.