**4. Main subsystems of gyrotron**

In a gyrotron, a hollow electron beam gyrating at cyclotron frequency under the influence of a strong axial magnetic field interacts with the transverse electric field excited inside the cavity. If the cyclotron frequency is synchronized with the frequency of millimeter wave supported by the cavity (cut-off frequency of the cavity) for the selected higher order mode, millimeter wave gets generated. This phenomenon is known as cyclotron resonance maser (CRM) interaction. The hollow gyrating electron-beam is generated with the help of magnetron injection gun (MIG)**.** The gyrating electron-beam is passed through a beam-tunnel and fed into an open ended interaction-cavity**.** The millimeter wave generated in the cavity region diffracts out with the help of a non-linear taper (NLT). The waveguide mode of electromagnetic-wave is covered to Gaussian mode with the help of a quasioptical launcher (QOL) and mirror units. The millimeter wave is taken out of the gyrotron with the help of a high power millimeter-wave window. The spent electron-beam is collected in a collector. The required axial magnetic field throughout the gyrotron, starting from the MIG to collector is provided by a magneticsystem consisting of a main superconducting-magnet along with a number of non-superconducting solenoid magnets. Out of all these subsystems, MIG and interaction-cavity are the most important subsystems of gyrotron. The following section describes some of these main subsystems of gyrotron.

### **4.1 Magnetron injection gun (MIG)**

Most high power gyrotrons use magnetron injection guns (MIGs), which produce annular electron-beams in which electrons gyrates in cyclotron frequency. The gyrating frequency is so chosen that the beam-wave interaction at desired mode can take place. For good interaction-efficiency, the transverse velocity component of electron should be as large as possible. A spread in transverse velocity results in a spread in axial velocity, and eventually reduces the efficiency of the gyrotron. Hence, the electron velocity spread should be kept as small as possible [8]. The cutsection view of a typical MIG with anode is shown in **Figure 8** indicating various parts of MIG.

The electrons are emitted from a annular cathode operating in temperature limiting regime of thermionic emission [2, 3]. MM-type dispenser cathode is used as emitter. The electron motion is taking place in crossed electric and magnetic fields so that the electrons follow helical trajectories around the magnetic flux lines with the electrons gyrating in cyclotron frequency. The accelerating potential of 20– 70 kV is applied between the cathode and the anode. The MIG can have a diode or a triode configuration. In the triode configuration, there are two anodes, namely

**Figure 8.** *Cut-section view of a typical MIG.*

modulating anode and accelerating anode. In triode configuration, second anode provides the main accelerating potential. Whereas, the first anode (which is closer to cathode) is used to fine-tune the velocity pitch-factor of the beam (ratio of transverse to axial beam-velocity) as well as for pulsing the beam (i.e., for switching the beam ON and OFF). Gyro-TWT's usually incorporate triode MIG. In diode configuration, there is only one anode. Diode MIG needs much simpler powersupply for providing the necessary voltages. However, on the flipside, gyrotrons with diode MIG have lesser control over the beam.
