3. 320 GHz TWT

#### 3.1 General subjects and problems

The problem of development of the terahertz range is connected with the development of effective and compact vacuum microwave generators and amplifiers at the frequencies of 90, 220, 460, 670, 850, and 1030 GHz. TWTs are referred to as the tube, which provides the broadest band within the average power level and the most often applied in electronic systems to solve a variety of tasks in the field of communication, including space communication, probing the Earth's surface, as well as the objects of near and far space [44, 45].

The main complexity accompanying the process of producing TWT in the terahertz range consists in a contradiction between the need to combine the small size of tube interaction space (for example, the diameter of the drift channel of the delay line) and the high density of the current of an electron beam. Considering the fact that the geometrical sizes of a tube decrease in proportion to the wavelength, i.e., D ˜ λ, where D° is the conventional size of tube interaction space and λ° is the wavelength in a free space, and there arise difficulties in passing an electron beam through the drift channel of the delay line. In certain cases, the given difficulties make the production of the delay line technologically impossible while applying the well-known methods and technologies. In general, these difficulties are purely technological, and they are connected not only with manufacturing the components of a tube construction (a delay line, an electron-optical system, an RF input and RF output devices, a collector, a magnetic-focusing system, etc.) but also with an assembly process of all its construction as a whole. Special consideration should be taken to the problems of providing the required critical dimensional features (˜ 1–3 microns) and surface finish (roughness) of the internal surface of the delay line (˜ 25–30 microns). The applied traditional technological production operations have limited opportunities already at the frequencies exceeding 400–450 GHz [46]. The specified technological difficulties have risen a considerable interest in employing programs of 3-D computer modeling of nonlinear processes of electronwave interaction, including modeling of thermal processes in the tubes of the terahertz range. The application of the 3-D computer modeling allows defining the potentialities of the developed constructions already at the stage of tube designing in terms of ensuring the required level of power output that enables to considerably reduce the price and accelerate the development of devices as a whole.

A broad spectrum of problems accompanying the process of TWT creation in the terahertz range has led to the creation of various programs and even some new independent directions of the development of vacuum electronics in the world. First of all, it concerns a dynamically developing industry of integral vacuum microwave microelectronics, namely vacuum microelectronics of millimeter and terahertz ranges. In order to coordinate and combine the efforts of the companies which are engaged in the development of devices in the terahertz range, there have been special

Figure 17.

Scheme of the TWT design with the helix delay line. 1—an electron-optical system; 2—a RF input; 3—a magnetic focusing system; 4—a local absorber; 5—a helix-type delay line; 6—a RF output; 7—a collector.

programs for developing terahertz technologies. In the near-term outlook, it assures a substantial progress in the field of design and development of microwave devices in a short-wave part of the terahertz range.

#### 3.2 The fundamentals of TWT operation physics

The general structure scheme of classical TWT is presented in Figure 17. An electron beam produced by the electron-optical system 1, which includes the electron gun, accelerating and focusing electrodes, passes the delay line 5, and precipitates on the collector 7. The tube input 2 is given a RF signal, which is amplified, and output through the output 6 into matched load. In order to prevent self-excitation of the TWT in a tube between RF input and RF output, there is the energy absorber 4, the main objective of which is to reduce the wave amplitude reflected from the output 6.

One of the TWT most complex nodes is the delay line. The helix delay lines have mostly become wide spread in average power level TWT's of the centimeter and millimeter ranges. As the analysis reveals, the reduction of wavelength results in a considerable decrease in the efficiency of interaction in these systems (the value of coupling impedance decreases to a few Ohms and less), as well as some difficulties connected with the production of wire for a helix whose diameter becomes less than 50 microns. Because of this, it is necessary to have alternative types of the delay lines, which would possess acceptable electrodynamic characteristics and the ability to pass electron beams with necessary current density. In addition, these structures have to meet the demands of production simplicity while preserving thermal stability and a possibility to withdraw heat energy from its structural elements, mechanical durability, and the ability to withstand a load of various external factors (temperatures, vibrations, accelerations, etc.). It is also essential to consider that the limiting values of the dimensions of the delay lines depend on the peculiarities of their design. For this reason, the helix delay lines can be used at the frequencies, which do not exceed 60–65 GHz [47].

The analysis of publications of the last 10 years reveals that there are a number of constructions of delay lines with dimensions, which are technologically implementable in the terahertz range involving a folded waveguide, a dual comb with different types of excitation, and metal film structures on dielectric bases. In recent years, due to the extreme complexity of manufacturing in the terahertz range, classical miniature resonators and delay lines photonic crystals have been proposed to be used as delay lines [48, 49].

The existing delay lines have an essential shortcoming connected with the fact that the electromagnetic wave in these devices is surface wave, and a longitudinal z-component of an electromagnetic field, the electron beam interacting with it,

Vacuum Microwave Sources of Electromagnetic Radiation DOI: http://dx.doi.org/10.5772/intechopen.83734

decreases from the outline border of the delay line to its axis. Therefore, a great interest is the microwave plasma-filled tubes with Cherenkov's radiation mechanism in which the electromagnetic wave is volumetric [50]. As a result, efficiency of interaction process between electron beam and electromagnetic wave is raised. It leads to increasing output power in such tubes.

Figure 18 schematically presents the design of plasma TWT. The presence of plasma allows to considerably increase the width of frequency range, to raise the output power and interaction efficiency and also enables the operational control of frequency range by implementation of tube frequency tuning both from an pulse to an pulse and within an microwave pulse. However, in order to implement the tubes for practical purposes, it is necessary to carry out additional investigations on some of unsolved problems which are related to features of beam-plasma interaction in these tubes.

#### 3.3 3-D computer modeling results

As an example, let us consider the 3-D computer modeling of a delay line for TWT at the frequency of 320 GHz. As the delay line, the folded waveguide was taken, one period of which is shown in Figure 19. The main dimensions of the folded waveguide are presented in Table 4.

It is necessary to note that the folded waveguides are the most popular ones, and they are often applied to design TWT in the range up to 400 GHz due to the compact dimension, broad band, and ease in production using, for example, the UV LIGO or MEMS technologies [51–53].

Figure 18.

Scheme of plasma TWT. 1—an electron-optical system; 2—a RF input; 3—plasma ignition device; 4—plasma; 5—an electron beam; 6—a RF output; 7—an absorber; 8—a collector; 9—a bulb [50].

Figure 19. Illustration of one period of a folded waveguide.


#### Table 4.

Parameters of a folded waveguide.

#### Figure 20.

A dispersion diagram in case of the rectangular hole for electron beam (a red curve) and electron beam voltage (a blue curve).

The results of computer modeling present in Figure 20. In this figure, a dispersion diagram and electron beam voltage are shown. As you have seen, the intersection of the curves determines an operating mode of a tube. A maximum amplification band corresponds to rectangular hole for electron beam (see Figure 20) whose dimensions are 0.125 ˜ 0.25 mm. An electron beam voltage is equal 13.5 kV.

### 4. Conclusion

The current status of the theory of electron-wave interaction in a 3-mm magnetron using the mode other than π° mode (the so-called surface-wave magnetron) and the design of the magnetron with two RF outputs are considered. It is shown that a process of phase focusing an electron beam in interaction space of the surface wave magnetron (interaction with °1 space harmonic) has a feature connected with concentration of energy of RF wave in the vicinity of a surface of the anode block. In this case for effective interaction between electron beam and RF wave, it is necessary to raise a height of electron hub of space charge in comparison with, for example, the classical magnetron. New data directed to improving the frequency characteristics of magnetrons and expanding their functionality for application in various electronic systems are obtained. In particular, the application of a pulse 2-cm magnetron with two RF outputs allows Vacuum Microwave Sources of Electromagnetic Radiation DOI: http://dx.doi.org/10.5772/intechopen.83734

realizing a mode of electronic frequency tuning from pulse to pulse in the range 200–300 MHz. As a result of improving the frequency characteristics of magnetron generators, new circuitry was proposed for creating various electronic systems in which these magnetrons can be used.

The trend in the progress of the TWT design in the terahertz frequency range has been analyzed. Using as an example of 3-D computer modeling of a slow-wave structure as a folded waveguide, the principal possibility of designing a TWT at a frequency of 320 GHz is presented. It is shown that the advancement in the shortwave part of the terahertz range is largely associated with the search and implementation of new efficient designs of the main components of the TWT's and the slow-wave structures.
