**Abstract**

In this chapter, brief outline is presented about gyro-devices. Gyro-devices comprise of a family of microwave devices and gyrotron is one among those. Various gyro devices, namely, gyrotron, gyro-klystron and gyro traveling-wave tubes (gyro-TWT) are discussed. Gyrotron is the only microwave source which can generate megawatt range of power at millimeter-wave and sub-millimeter-wave frequency. Gyrotron is the most suitable millimeter wave source for the heating of plasma in the Tokamak for the controlled thermoneuclear fusion reactors. This device is used both for the electron cyclotron resonance heating (ECRH) as well as for the electron cyclotron current drive (ECCD). In this chapter, the basic theory of gyrotron operation are presented with the explanation of various sub-systems of gyrotron. The applications of gyrotrons are also discussed. Also, the present stateof-the-art worldwide scenario of gyrotrons suitable for plasma heating applications are presented in details.

**Keywords:** gyrotron, electron cyclotron resonance heating, fast wave device, gyrotron cavity

## **1. Introduction**

The conventional microwave-tubes, such as traveling-wave tubes (TWSs) and klystrons follow the Pf2 law. As per this law, the product of maximum power (*P*) and the square of frequency (*f 2* ) is constant for a device. This limits the maximum power handling capability of a device at higher frequency, i.e., in the millimeterwave and sub-millimeter-wave frequency regime. Hence, it was found to be highly difficult to develop a microwave source, capable of delivering megawatts of power at millimeter wave frequency. In the last few decades, a new class of microwave tube has emerged, called – Gyrotrons [1–9], which are based on cyclotron resonance maser (CRM) instability [10–12]. This class of device has the capability to produce very high power at millimeter-wave frequencies, much higher than other microwave devices.

The gyrotron is the most suitable source for the heating of plasma in Tokamak for controlled thermoneuclear fusion reactors. Gyrotron is being used as the heating source for electron cyclotron resonance heating (ECRH) as well as for the electron cyclotron current drive (ECCD).

Gyro-devices comprise of a family of microwave devices and gyrotron is one among those. However, gyrotron being the most popular gyro-device, the entire gyro-device family is sometime referred as gyrotrons. Various other commercially available gyro-devices are: gyro-klystron, gyro-traveling wave tubes (gyro-TWT) and gyro-twistron (a combination of Gyro-TWT and Gyro-Klystron).

In a gyro-device, a hollow electron-beam is generated with the help of a special kind of electron gun, known as magnetron injection gun (MIG) operating in temperature-limited regime of thermionic emission. This hollow electron beam is made to gyrate at cyclotron frequency with the help of a strong axial magnetic field. Subsequently, this gyrating electron beam is passed through an interaction structure, where the electron-beam interacts with the electromagnetic-wave (EM-wave). In case of gyrotron, the interaction-structure is an open-ended cavity. In case of gyro-TWT, the interaction structure is waveguide with an input and output coupler. When the cyclotron frequency synchronizes with the frequency of the EMwave (frequency of EM-wave supported by the cavity in case of gyrotron and frequency of the EM-wave fed at the input coupler in case of gyro-TWT) the beamwave interaction takes place. The transverse kinetic energy of the electron-beam gets converted to electromagnetic energy. Hence, the EM-wave gets generated (in gyrotron) or amplified (in gyro-TWT).

Let us now briefly discuss the origin of gyrotrons. It has been well known since the mid-fifties that there appeared to be a limit to the upper frequency at which most vacuum microwave devices could be made to operate with sufficient power and efficiency, primarily due to the reduction of physical size of the components of the device with increase of frequency [13]. This problem can be explained as follows: As the frequency of operation of the device increases, the dimension of the waveguide or cavity or the loading elements inside the waveguide (such as helix in case of helix-TWT) become uncomfortably small, as their physical size is closely related to the operating wavelength of the device. Furthermore, since the depth of penetration of the field generated by the electromagnetic wave is proportional to the operating wavelength, the field penetration inside the loading-element reduces with the increase of operating frequency. Hence, in order to have a proper interaction between the electron-beam and electromagnetic wave, electron-beam needs to be placed closer to the structure carrying electromagnetic wave, if we wish to retain an acceptable efficiency of beam-wave interaction [1, 2]. All these awkward requirements clearly indicate an urgent need for a radical change of approach. In conventional vacuum electronic slow-wave devices (such as TWT), periodic loading elements are required for slowing down the phase velocity of the electromagnetic wave (slow wave interaction: vph < c) so that the phase velocity becomes synchronized with the velocity of the slow space-charge wave produced by a perturbed electron beam. In case of helix-TWT, helix acts as periodic loading element, alternatively known as slow-wave structure (SWS). Whereas in gyrodevice, alternatively known as fast-wave devices, the periodicity in the propagating medium is removed and the periodicity is brought in-to the electron-beam. The interaction now takes place with an electromagnetic wave whose phase velocity is higher than the free-space velocity of light (fast wave interaction, vph > c) [1–3]. Instead of periodicity of the loading element, the periodicity of the electron-beam comes into play. This leads to a quasi-synchronism between the electromagnetic wave and the electron-beam.

Gyro-devices comes under the category of Bremsstrahlung radiation device [13]. Here, instead of periodic show-wave-structure, the electron beam is made periodic by generating a hollow electron-beam gyrating under the influence of a strong axial magnetic field. When this electron beam is perturbed, two cyclotron waves get generated, namely slow and fast cyclotron wave. When the velocity of the fast

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

**Figure 1.** *Domain of microwave tubes/ laser devices.*

cyclotron wave is synchronized with EM-wave, beam-wave interaction takes place. That's why these devices are known as fast-wave device.

A variety of interaction stricture geometries are proposed in the literature [9, 11–13] for gyro-devices. In case of gyrotron, the interaction structure is an open ended cavity. Well directed and concentrated efforts were made in the midseventies by Granastein and his team at the Naval Research Laboratory (NRL) [13], as well as Gapanov and his team at IAP, Russia [14] who, with some help from others, succeeded in mounting an extensive research effort in the whole area of Bremsstrahlung radiation device, which include free-electron lasers as well as gyrotrons. Since then, gyro-devices have developed very rapidly to offer prodigious amounts of power, and very high efficiency of the order of 50% or more. **Figure 1** shows capabilities of various vacuum electronic devices in terms of frequency and average power. It's evident from the figure, for frequencies above the Terahertz range, laser devices are most suitable source for generation of electromagnetic wave. Again, for frequencies below the millimeter-wave range, conventional microwave tubes are most suitable source for the generation of high power. Gyrotron fits in between these two frequency regimes. Gyrotrons are best suited when the operating wavelength is approximately 1 mm and output power requirement is between hundreds of kilowatts to few megawatts. That's why Gyrotron is found to be the most suitable source for the heating of plasma.
