Travelling wave tube amplifier is basically an amplifier, which makes use of a distributed interaction between travelling wave and electron beam. It is necessary for interaction that they are both travelling in the same direction with the same velocity. The interaction between the RF field and moving electrons will tack place only when the velocity of RF fields is retarded by some means. This is achieved by the slow-wave structure.
The invention of Travelling Wave Tube
The TWTA was invented in 1944 by Kompfner, when he felt that in two-cavity klystron, full energy of the electron is not getting transferred to the microwave signal in the cavity for amplification, due to interaction in electron beam and RF field being only in the cavity. In TWT the cavity is not there, and continuous interaction between electron beam and RF field is there, by making their velocities some. This is by slowing down RF wave field velocity from c to cp/πD, in the axial direction, by making it to pass through a helical path. The microwave RF signal is pumped through coaxial cable, with its central cable connected to the helical wire through which the current moves with the velocity of light (c).
This current causes an RF field inside the helical wire, and an electron beam is passed through the axis of the helix. This electron beam interacts continuously over the length of the helix (around 12″ or so, instead of just 1″ or so within a resonant cavity as in two-cavity klystron) and transfers energy to the RF field and hence to the beam current of helix. This causes amplification of RF signal when it reaches the other end of output. From the figure, we see that if the electron beam and RF field wave of helix both reach A to B together, then time of movement of both is same (tp = tbeam)