# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
Approximately at the same time, the gyrotron FU CW III with a 20 T superconducting magnet that was developed at FIR-UF (Fukui, Japan) has achieved a continuous wave (CW) second-harmonic operation on the mode TE4,12 at a frequency of 1.08 THz [38]. The gyrotrons are fast-wave devices since in their resonant structures (cavities), the phase velocity of the wave is greater than that of light in vacuum (i.e., it is superluminal) and they, unlike the classical microwave tubes, do not need tiny slow-wave structures with characteristic dimensions of the order of the wavelength of the generated radiation and positioned close to the electron beam. This allows the use of oversized resonators with radii significantly exceeding the wavelength and therefore powerful electron beams of high currents without losses or damage of the resonant structures, thus achieving higher output powers. The structure of a gyrotron is shown schematically in Figure 2. It includes an electron-optical system (EOS) based on a diode or triode magnetron injection gun (MIG) with a thermionic cathode that generates a helical electron beam with appropriate parameters (accelerating voltage, current, injection radius, and thickness in the cavity) and high quality (low spreads \(\delta v_{z}\), \(\delta v_{\perp}\) of both the axial \(v_{z}\) and the transverse \(v_{\perp}\) velocities of the electrons; azimuthal uniformity; absence of reflected (trapped) electrons and losses on the electrodes). Since in the gyrotrons, the energy is extracted only from the rotational (transverse) motion of the beam electrons, an important parameter that affects the efficiency of the interaction is the velocity ratio (aka pitch factor) \(\alpha=v_{\perp}/v_{z}\). Starting from the emitter of the MIG, it is increased adiabatically in the beam tunnel by the rising magnetic field generated by a set of solenoids (notably the superconducting magnet) to typical values of \(\alpha=1.2\div 1.5\) in the resonant cavity. A fine-tuning of the pitch factor is possible by varying the potentials of the electrodes of the MIG and the currents of the auxiliary gun coils. In the simplest case, the electrodynamical system, which is responsible for the mode selection and where the beam-wave interaction takes place, is a section of a circular waveguide delimited at both sides by appropriate down- and up-tapers that control the resonant properties of the cavity (e.g., the diffractive Q factor) and serve as transitions/connectors  
Figure 2: Schematic structure of a generic gyrotron and its components: 1—magnetron injection gun (MIG); 2—additional gun coils; 3—beam tunnel and compression region; 4—main coils of the superconducting magnet; 5—gyrotron cavity resonator; 6—launcher of the internal mode converter; 7—system of mirrors of the internal mode converter; 8—Gaussian wave beam; 9—output vacuum window; 10—helical electron beam; 11—water-cooled collector of the spent electron beam; 12—magnetic sweeping coils for smearing the spent electron beam. to the beam tunnel and the output waveguide, respectively. In the oversized cavities, a dense spectrum of modes presents the inevitable problem of mode competition, but this is being solved using different techniques (basically electronic and electrodynamic) for mode selection as well as by careful computer-aided design (CAD) and manufacturing of the resonator. While the radiation generated inside the cavity has a complicated polarization and spatial distribution corresponding to the field pattern of the operating mode, many applications require a well-collimated linearly polarized beam. That is why most of the gyrotrons nowadays have a quasi-optical (QO) internal mode converter consisting of a launcher (of either Vlasov or Denisov type) and a system of mirrors that produces a Gaussian wave beam. The QO system serves also to separate the spent electron beam and the wave beam. The former is dissipated on the water-cooled collector, while the latter is transmitted through an output window.