**2. Basic types of gyro device**

Ever since the advent of cyclotron-resonance maser (CRM) instability devices, a vast amount of research work has been carried out and a number of gyro-devices have been developed. Out of these, the most popular device is gyrotron (alternatively known as gyro-monotron). The other two commercially available gyrodevices widely used in radar applications are, gyro-TWT [3, 8], and gyro-klystron [3]. However, the entire class of gyro-devices are usually referred as gyrotron. There are few less popular gyro-devices, namely, Gyro backward-wave oscillator (gyro-BWO), gyro-twystron [3] (a combination of gyro-TWT and gyro-klystron), cyclotron autoresonance maser (CARM) and slow-wave cyclotron amplifier (SWCA) [13]. Technology for these devices are not as matured as for gyrotron,

gyro-TWT and gyro-klystron. Some of the gyro-devices are oscillators and some are amplifiers. Same is brought out in the **Table 1.**

In **Table 1**, the most popular gyro-device names are written in red. As is evident from the **Table 1**, cyclotron autoresonance maaser (CARM) can be configured both as amplifier as well as oscillator.

In the following section, two most popular gyro-devices, namely, gyrotron, and gyro-TWT are discussed in brief with the schematic diagrams.

### **2.1 Gyrotron (gyro-monotron)**

A schematic diagram of the gyrotron with axial output of cavity mode is shows in **Figure 2(a)**. The schematic view of high-power gyrotron with radial output of Gaussian beam is shown in **Figure 2(b)**. Here the beam-wave interaction take place in an open-ended cavity. The hollow electron-beam from the electron-gun (known as magnetron injection gun) is injected into a region with very strong axial magnetic field [3, 6–10]. Magnetic flux densities of the order of several Tesla are normally required and this usually necessitates the use of superconducting magnets [6].

The beam-wave interaction takes place in the interaction cavity region. In order to avoid the thermal issues, gyrotrons usually incorporate a highly overmoded cavity. The reported continuous wave (CW) and pulsed power capabilities of the gyrotron are three order of magnitude higher than the conventional microwave oscillators.

In case of axial output gyrotrons (**Figure 2(a)**), output millimeter-wave generated in the cavity propagates along the axis of the gyrotron and comes out of the gyrotron through an output-window. The spent-electron beam (the electron-beam after the beam-wave interaction) gets collected in the collector. In case of gyrotron with radial-output (**Figure 2(b)**), the cavity-resonator mode of EM-wave gets converted to Gaussian (TEM00) mode with the help of a quasi-optical launcher (QOL) and 3 or 4 mirrors. The Gaussian beam comes out of the gyrotron radially (perpendicular to the axis of gyrotron) through the output-window and the spent electron-beam gets collected in the collector.

### **2.2 Gyro-traveling wave tube (gyro-TWT)**

Gyro-TWT is a high power millimeter-wave amplifier [3, 8]. This is used in millimeter-wave radars. Gyro-TWT is also used for electron-cyclotron current drive (ECCD) for Tokamak. In this device, the interaction-cavity is replaced by a nonresonant structure (waveguide) to produce beam-wave interaction. This device has the potential of amplifying EM-powers of 2 order of magnitude higher than the


**Table 1.** *The complete family of gyro-device.* *Gyrotron: The Most Suitable Millimeter-Wave Source for Heating of Plasma in Tokamak DOI: http://dx.doi.org/10.5772/intechopen.98857*

**Figure 2.**

*(a): Gyrotron with axial output. (b): High power Gyrotron with radial output of Gaussian beam.*

conventional TWT. Gyro-TWT provides a high spectral quality amplification over a narrow bandwidth. The device interaction essentially involves a narrow band resonance between the electron-beam and the electromagnetic-wave near the waveguide cut-off due to the dispersive nature of the waveguide interaction structure. However, wideband coalescence is possible by proper dispersion shaping of the waveguide. Axial phase synchronism is required between the traveling wave and the gyrating electron. Techniques are being used to increase the band-width by

**Figure 3.** *Gyro TWT.*

tapering the magnetic field or by periodically loading the waveguide structure. The cross sectional view of gyro-TWT is shown in the **Figure 3**.
