**4.2 Interaction cavity and nonlinear taper**

The gyrating electron beam enters the interaction cavity, where the beam-wave interaction takes place [3, 6, 8–13]. This is an open ended overmoded cavity operating near cut-off [3, 6]. The interaction cavity generally consist of 3 sections, namely downtaper-section, straight section and uptaper-section. The shape of the cavity is dependent on the mode of the electromagnetic field with which the beam is intended to interact and also the harmonic number of interaction. The required diffractive quality factor of the cavity is achieved by proper fine tuning of the cavity shape. Schematic drawings of a typical Gyrotron cavity is presented in **Figure 9**.

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

The down-tapering is offered to the input-end of the cavity. This prevents the millimeter wave from back-traveling towards the MIG. The up-tapering is offered in the output-end of the cavity. The up-tapering helps the millimeter-wave to diffract out of the cavity. In case of coaxial gyrotrons, a coaxial insert is placed at the center of the cavity. The main beam-wave interaction takes place at the straight section of the cavity.

The millimeter-wave signal generated in the cavity needs to diffract out of the cavity. The same is achieved by the non-linear taper (NLT). This NLT is basically a tapered waveguide section with a specific tapering profile. A raised-cosine profile is incorporated in the NLT region to avoid reflection of electromagnetic-wave. This section acts as an interface between the interaction cavity and the QOL [6, 16–20]. Generally interaction cavity operates at a mode much higher than the dominant mode of the cavity. This enable the use of much higher cavity dimension and volume and this in-turn eliminates the bearing on the maximum power handling capacity at higher frequencies of millimeter-wave and sub-millimeter-wave regime. Broadly, the cavity operating modes are divided into three categories, namely, TE0 n mode, TEm n (m <sup>&</sup>gt; n) mode and TE1 n mode. TEm n mode, when m> > n, is called the whispering gallery mode. This mode is most widely used in gyrotrons for plasma heating applications. The relative merits and demerits of these modes are presented in the **Table 2**.

### **4.3 Beam-tunnel**

The radius of the hollow electron beam generated by MIG is generally much larger than the required hollow beam radius at the cavity region. The purpose of the beam-tunnel is to gradually bring down the beam radius to the value needed in the cavity region. Beam-tunnel is basically a cylindrical waveguide structure placed between the anode and interaction-cavity. The inner radius of the beam-tunnel at the anode end is matched to the anode inner radius and at the cavity end is matched to the input inner radius of the cavity. In order to ensure that the beam-tunnel does not take part in interaction, lossy dielectric material is placed inside the beamtunnel. One of the popular configuration of beam-tunnel is a stack of alternate metal (OFHC copper) and lossy ceramic (AlN, SiC) rings stacked inside the cylindrical waveguide of beam-tunnel (**Figure 10**). The axial length of the beam-tunnel is so chosen that that the electron beam undergoes an adiabatic compression as it propagates from the MIG to the cavity, i.e., the beam trajectories follows the magnetic flux lines. This configuration ensures maximum beam laminarity and minimum beam-turbulence. Cavity.

### **4.4 Quasi-optical launcher**

The purpose of the quasi-optical launcher (QOL) is to convert the cavity mode of EM-wave into a Gaussian (TEM00) mode. This is accomplished with the help of a helically-cut waveguide section (QOL) followed by 3 or 4 toroidal mirrors system. QOL consist of a mildly tapered waveguide structure with helically cut end (known as Vlasov launcher) with dimple patterned inner surface (Denisov type surface deformation). Millimeter wave is launched from the QOL to the mirror system [16]. After passing through the mirror system, a Gaussian beam (with more than 98% Gaussian mode purity) is emerged. A typical QOL and 3 mirrors for converting cavity mode to Gaussian (TEM00) mode is shown in **Figure 11**. The Gyrotron with Gaussian output is most suited for plasma heating applications. Because, the Gaussian

millimeter-wave beam can be transmitted through a waveguide over a very long distance with very little attenuation. Hence, the gyrotron can be placed away from the plasma vessel. Sometimes, the Gaussian beam is further converted to HE11 mode with the help of a matching optic unit (MOU) placed external to gyrotron and then transmitted to the plasma vessel. This arrangement further reduces the attenuation of the beam.
