A voltage applied across the cathode and anode accelerates the electrons towards the far end of the tube, and an external magnetic field around the tube focuses the electrons into a beam. At the other end of the tube the electrons strike the "collector", which returns them to the circuit. Wrapped around the inside of the tube, just outside the beam path, is a helix of wire, typically oxygen-free copper. The RF signal to be amplified is fed into the helix at a point near the emitter end of the tube. The signal is normally fed into the helix via a waveguide or electromagnetic coil placed at one end, forming a one-way signal path, a directional coupler. By controlling the accelerating voltage, the speed of the electrons flowing down the tube is set to be similar to the speed of the RF signal running down the helix.
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There are three types of magnetrons: Negative resistance type Travelling wave or Cavity type Negative resistance magnetrons make use of negative resistance between two anode segments. Cyclotron frequency magnetrons depends upon synchronism amid an alternating component of electric field and periodic oscillation of electrons in a direction parallel to this field.
Cavity type magnetrons depends upon the interface of electrons with a rotating electromagnetic field of constant angular velocity. This results in heating of cathode. If the magnetic field strength is more than the electrons emitted will return back to cathode with high velocity which may destroy the cathode cavity this effect is called back heating of cathode. In case of TWT the interacting space is extended for the total length of the tube so that bunching takes place continuously.
The basic structure of a TWT consist of a cathode to emit the electrons and input waveguide and the output waveguide so as it give input signal and to take out the RF output. A magnetic field is applied so as to focus the electrons along the center of the helical structure. Attenuators are provided to absorb the reflected electrons so as to avoid occurrence of any oscillation along the tube due to impedance variance. Thus the Rf signal is made to pass through slow wave structure so the axial component of the phase velocity almost equal to Ve.