Question:
Published on: 16 September, 2024

What is a slow wave structure? Why does the TWT need such a structure? Explain the beam and RF interaction in such wave tube.

Answer:

The slow wave structures are a special type of circuits, used in microwave tubes to reduce the wave velocity in a certain direction so that a prolonged interaction between the electron beam and the signal wave may take place.

In ordinary waveguides the phase velocity of a wave is greater than velocity of light in a vacuum. In both TWT and Magnetron the slow wave structure change the phase velocity of the microwave signal in such a way that microwave signal and electron beam will move side by side in same pace and as a result a prolonged interaction between the electron beam and the R.F. field (microwave ) may take place. The FR field may propagate through the slow wave structure in both forward and backward direction. Thus two traveling Wave Tube (TWT) falls under forward slow wave structure and backward wave Amplifier (BWA) or Backward Wave Oscillator (BWO) falls under backward slow wave structure.(Fig. 35)

Traveling Wave Tube (TWT) are two types. M - Type TWT and O - Type TWT.

M - Type TWT:

The M – Type Traveling Wave Tube is also known as cross – field tube or Line Magnetron or Magnetron type tube.

Here the electron beam mo0ves through a crossed electric field and magnetic field like Magnetron at a drift – velocity given by the ratio of E and B. The R.F (Microwave) signal to be amplified is applied at the input and it moves through the slow wave structure. The slow wave structure does not affect the magnetic field of the R.F. wave but the axial component of the electric field of the R.F. will be slow down. As a result when the electron is allowed to move in the interaction space between the delay line and the sol, a prolonged interaction between the varying R.F. electric field and electron beam takes place. During interaction the electron beam transfers potential energy to the wave. A number of electron velocities are possible for which the net transfer of energy from the electron beam to the wave can occur. Electron velocity should be adjusted such that the electrons are retarded on passing each gap.

Application in space communication and Coherent Pulse Radar.

O - Type TWT:

In ordinary or O – type travelling wave tube the principle of operation is same , that in order to have extended interaction, the electron beam and the microwave signal must travel at the same rate of speed. Unfortunately as we know that the R.F signal travels at the speed of light , while the electrons are much slower. With the help of slow wave structure the R.F signal is slowed down. The most commonly used slow wave structure in TWT is the helix, where TEM wave travels with velocity very near to that of light, while the electron beam travels along the axis of the helix. The time interval τ required for the wave to travel one turn of the helix is the ratio of turn circumference to the velocity of light c. The amount of slowing down is determined by the pitch of the spiral and thus the phase velocity of the field is given by

$$v_p=\frac{p}{\frac{1}{c}\sqrt{\left(2\pi a\right)^2+p^2}}=\frac{\tau}{\sqrt{1+\left(2\pi a/p\right)^2}}=\frac{c_p}{2\pi a}=\frac{\omega}{\beta}$$

The wave slowing factor c/vp is approximately given by 2π a/p

Where p is the pitch of the helix, t is the time to complete one pitch, a is the radius of the helix and c is the velocity of E.M wave.

Applications:

1. As a broad and amplifier in microwave receiver an as a repeater amplifier in wide – band communication link.
2. Higher life time of TWT makes it suitable for the use in satellite communication.
3. Can also be sued for CW radar and for radar jamming.
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