Travelling Wave Tube:

Travelling wave tubes (TWTs) are widely used as broadband low and medium power microwave amplifier. In TWT instead of cavity resonator periodic slow wave structure is used its function is to reduces the velocity of RF electromagnetic wave. It is a device in which there exist a continuous interaction between electron beam and RF field. To prolong the interaction between electron beam and RF field, it is necessary to ensure that both are travelling in the same direction with nearly same velocity. This is differ from the Klystron in which the electron beam travels and the RF field is stationary. The electron beam velocity is nearly equal to 10% of velocity of light. However RF field travel with velocity of light. So to retard the RF field slow wave structure such as helix is used instead of cavity resonators.

Construction:

It consist of

1. Electron gun used to produce a narrow constant velocity electron beam; which is passed through the centre of a long axial helix.
2. Magnet’s are used to focus electron beam and prevent it from spreading due to space charge effect.
3. Helix is used as periodic slow wave structure, which is a loosely wound thin conducting helical wire.
4. A collector which is generally kept at a d.c. potential lower than that of the helix.
5. Input and output couplers.
6. The attenuator is used at the Centre of the circuit which absorb the reflecting wave fro-m mismatch at the output end. This preventing reflections reaching towards the input which causes oscillation.

Working:

1. When RF signal which is to be amplied is applied to the TWT, its initial velocity is nearly equal to the velocity of light but when the signal propagates around the turns of the helix, it produces electric field along the axis of the helix, which propagates along the axial direction with phase velocity which is small compared to the velocity of light by the slowing factor: $\dfrac{V_p}{C}=\dfrac{p}{2πa}$

   Where, P is pitch of helix

   2πa is circumference of helix.

2. The axial electric field can now continuously interact with the electron beam. The electron entering the helix at zero field is not affected by the signal wave. Those electron entering the helix at accelerating field are accelerated and those are at the retarding field are decelerated thus velocity modulation takes place and this produces bunching of electron.

3. This bunch of electrons travel with the RF field and as consequence the field continuously acts to increase the bunching i.e velocity and density modulation occurs simultaneously in this region.
4. Since the dc electron velocity, its results into a situation in which more electrons face the retarding field than the accelerating field, energy is thus last by the electron beam and becomes more and more stronger and a longer amplification of the RF signal occurs at the end of the helix.

Wave mode in TWT:

There are total four waves, three forward wave and a backward wave are set up in TWT inpresence of the electron beam with four different propagation constant so there are four modes of travelling wave in the O type TWT.

$Y_1= -β_e C \dfrac{\sqrt3}{2}+j β_e (1+\dfrac C2) \\ Y_2=β_e C \dfrac{\sqrt3}{2}+j β_e (1+\dfrac C2)$