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CHAPTER – 2
TRAVELLING WAVE TUBE OPERATION
In order to design a helix-type traveling wave tube amplifier (TWTA), it is necessary to understand the operating physics of a travelling wave tube (TWT). This chapter provides the basic description of the TWTA structure and its various components. The functionalities of these components are briefly explained in order to have a basic knowledge about the different practical constraints in modelling a TWTA. As the focus of the research is on helix-type slow wave structures, the associated electromagnetic theory and mathematical techniques are dealt in detail towards the end of the chapter. We will assume the Pierce theory of small-signal TWT interaction and discuss its use and the basic TWT metrics needed for the design of an interaction structure.
2.1 TWT Operation – An Overview:
The basic structure of the TWT is as shown in Figure 2.1. TWT consists of four major components generally assembled in the configuration shown in Figure 2.1. The electron gun generates the electron beam, which is confined by a distributed magnet system into the helix slow wave structure’s beam tunnel. A helix-like slow wave structure is used for the electron wave beam interaction and a collector at the end of the tube stops the electron beam after it has travelled through the tube.
Figure 2.1 Basic TWT Structure. Source: www.brittanica.com
2.1.1 Electron Gun
Electron guns, for TWTs, are designed to output an electron beam with circular cross section with current at a voltage. Such electron beams are often called pencil beams or O- beams for their round long shape. Electron beams are generally generated either by thermionic emission or field emission. In thermionic emission, electrons are generated by heating the cathode of the electron gun to a temperature typically between 750°C – 1200°C so that the electrons will have enough thermal energy to be regularly emitted from the surface. The typical voltage applied to this cathode generally varies from several thousands of volts to several hundreds of thousands of volts. These cathodes are generally oxide coated cathodes also known as hot cathodes. They are a mix of carbonates, sprayed onto a nickel surface, and once heated break down into oxides and provide the source of electrons for the vacuum device. The generated electrons are accelerated by the application of an electric field through the anode surfaces imparting an axial velocity, and travel through the slow wave structure. For a high frequency TWT, where the interaction structure is small and requires a high-density electron beam, the size of the cathode is larger than the size of the electron beam, and with the help of the anode focusing arrangements electron beam of high density are focussed into the SWS structure.
2.1.2 Focusing Magnets
The accelerated electron beam tends to diverge while moving through the slow wave structure (SWS) due to space charge forces. When there is large divergence of the electron beam leading to interception by the slow wave structure, it results in the heating of the SWS. Such heating can lead to the destruction of the structure. To combat divergence, an axial magnetic field is applied, forcing the electrons to travel the length of the tube without interception. TWT designs utilize two types of magnet systems: periodic permanent magnets (PPM) and electromagnetic solenoids. A PPM system utilizes permanent magnets to confine electrons. These magnets create magnetic field that confine the electron beam, but the field is not uniform within the length of the SWS. Such a system can be more compact, and weigh less than an electromagnet and does not require a power supply making it potentially more energy efficient. However, PPMs require a complex design to prevent additional beam instabilities. The electromagnetic solenoid easily provides near uniform axial field and has the potential for much higher field strengths. The design of an electromagnetic solenoid is much simpler, compared to a PPM magnet. The general drawbacks
The above geometrical relation among the parameters may be easily seen from a developed view of the helix in Fig 2.2. The ribbon width of the axial direction is. The helix is geometrically simple and easy to fabricate. The helix is usually made of tungsten measuring 0.010-inch by 0.075-inch [24].
Figure 2.2 Expanded View of tape Helix [13] The helix is usually supported by an insulating structure that often takes the form of three equally spaced dielectric support rods as indicated in Fig 2.3a. These support rods are symmetrically disposed along the helix and are usually made of Anisotropic Pyrolytic Born Nitride (APBN) commonly known as CVD, Boron Nitride or Beryllium Oxide (BeO). The dielectric support rods have permittivity varying between as shown in Table 2.1. It should be noted that diamond has a very large value of thermal conductivity and this is why there has been strong interest in using diamond for many years. The helical slow wave structure along with the dielectric support rods are shielded by a coaxial metallic cylindrical envelope (running along the entire length of the helix).
Table 2.1 Permittivity values of various dielectric support rods Dielectric Materials used as support rods in TWTA
Relative Permittivity (
Isotropic Pyrolytic Boron Nitride 3. Anisotropic Pyrolytic Boron Nitride 5. Diamond 5. Beryllia Oxide ( ) 6. Alumina ( 8.
Dielectric support rods are typically rectangular in shape. Sometimes they are shaped as shown in Fig 2.3.
Figure 2.3 Typical cross sections of dielectric support rods
In this thesis, dielectric support rods of wedge type is assumed with Berrylia as the dielectric material as shown in Fig 2.4. From table 2.1, it is seen that Berrylia has a permittivity,
Figure 2.4 Actual helix with wedge shaped dielectric support structure
to greatly increase the overall efficiency of TWTs. In practice, typical MDC designs incorporate no more than five collector stages.
Figure 2.5 Collector for a linear beam tube
