where the magnetic field strength is increasing, it compresses the electron beam, converting the longitudinal drift velocity to transverse orbital velocity, in a process similar to that occurring in a magnetic mirror used in plasma confinement. The orbital velocity of the electrons is 1.5 to 2 times their axial beam velocity. Due to the standing waves in the resonant cavity, the electrons become "bunched"; that is, their phase becomes coherent (synchronized) so they are all at the same point in their orbit at the same time. Therefore, they emit coherent radiation. The electron speed in a gyrotron is slightly relativistic (on the order of but not close to the speed of light). This contrasts to the free-electron laser (and xaser) that work on different principles and whose electrons are highly relativistic. Applications Gyrotrons are used for many industrial and high-technology heating applications. For example, gyrotrons are used in nuclear fusion research experiments to heat plasmas and also in manufacturing industry as a rapid heating tool in processing glass, composites, and ceramics, as well as for annealing (solar and semiconductors). Military applications include the Active Denial System. In 2021 Quaise Energy announced the idea of using a gyrotron as a boring machine to drill a hole 20 kilometers in depth and use it to produce geothermal energy. The technique would use frequencies of 30 to 300 GHz and would transfer energy to a rock 1012 more efficiently than using a laser. Lasers would further be disrupted by the vaporized rock, which would affect the longer-wavelength much less. Drilling rates of 70 meters/hour appear to be possible with a 1-MW gyrotron. Types The output window of the tube from which the microwave beam emerges can be in two locations. In the transverse-output gyrotron, the beam exits through a window in the side of the tube. This requires a 45° mirror at the end of the cavity to reflect the microwave beam, positioned at one side so the electron beam misses it. In the axial-output gyrotron, the beam exits through a window in the end of the tube at the far end of the cylindrical collector electrode which collects the electrons. The original gyrotron developed in 1964 was an oscillator, but since that time gyrotron amplifiers have been developed. The helical gyrotron electron beam can amplify an applied microwave signal similarly to the way a straight electron beam amplifies in classical microwave tubes