The field causes the electrons to move helically in tight circles around the magnetic field lines as they travel lengthwise through the tube. At the position in the tube where the magnetic field reaches its maximum the electrons radiate electromagnetic waves in a transverse direction (perpendicular to the axis of the tube) at their cyclotron resonance frequency. The millimeter radiation forms standing waves in the tube, which acts as an open-ended resonant cavity, and is formed into a beam, which radiates through a window in the side of the tube into a waveguide.