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The Travelling Wave Tube (TWT) is an amplifier for radio frequency (rf) signals. A weak signal is coupled into the inner helical structure of the TWT close to an electron source. The electrons are focused by a magnetic field in such a way, that there is an electron beam in the middle of the helix. Due to the input signal the beam electrons are bunched. If the beam velocity is a little bit greater than the phase velocity of the input signal, the beam can amplify the input signal. At the "end" of the structure, the amplified signal is decoupled from the helix whereas the beam leaves the TWT and is usually absorbed by a collector.
TWTs are typically used in satellite transponders or in radar applications. Their gain is in the range of 30 to 60 dB and their length varies in the range of some cm to one meter. In our case the TWT has a length of about 12 cm. Since this TWT is just an example for demonstration purposes and not a real TWT, its gain is less than 30 dB.
The Travelling Wave Tube is a cylindrical device. Within the cylinder there is a helical structure that consists of PEC. To fix the helix in the middle of the cylinder, a dielectric material is used. One port is needed to couple the weak input signal (that should be amplified) into the helix, a second port is used to decouple the amplified signal from the helix.
To create a particle beam in the middle of the helix a particle source is defined at one end of the TWT. Particles are emitted based on the DC emission model of the PIC Solver. Their initial velocity is 0.16 times the speed of light and the sources' emitted current is 10 mA.
Since we do not use a real input signal in this demonstration example, a weak periodical (sinus) input signal is created/designed using the CST excitation signals. This signal is used as excitation signal for the port close to the particle source.
As mentioned above, a constant magnetic field has to be defined to focus the beam. Without this field the beam particles would hit the helix. In our case the b-field has a strength of 0.5 T in z-direction.
The simulation time is set to 4ns, it is just long enough to demonstrate the functionality of the TWT. To monitor the particles' movement and behavior, a 3D Particle Position Monitor and a 1D Phase Space Monitor have been defined.
The example can be calculated with the GPU or the CPU solver. Be careful if you use the CPU solver, the simulation can last some hours.
The most interesting results in case of the travelling wave tube are the port signals, listed in the 1D Results folder of the navigation tree. Due to a comparison of input and output port signals the gain of the TWT can be easily obtained. As our TWT is only used for demonstration purposes, the gain is not very high.
The PIC position monitor offers you the possibility to see the bunching of the particles. The result is located in the 2D/3D Results folder of the navigation tree.
