Travelling-wave tube (TWT) is a microwave electron tube which realizes amplification function by continuously modulating the speed of electron beam. In the TWT, the electron beam interacts with the microwave field traveling in the slow-wave circuit, and in the slow-wave circuit up to 6 to 40 wavelengths, the electron beam continuously gives kinetic energy to the microwave signal field, thereby amplifying the signal.

    TWT is characterized by wide frequency band, high gain, large dynamic range and low noise. The frequency band width of the TWT (the difference between the two ends of the frequency band and the center frequency) can reach more than 100%, the gain is in the range of 25 ~ 70 decibels, and the noise coefficient of the low-noise TWT can reach the minimum of 1 ~ 2 decibels.

    In a TWT, an electron beam interacts with a microwave field in a slow-wave circuit. The microwave field travels along the slow wave circuit. In order for the electron beam to interact effectively with the microwave field, the DC motion speed of the electron should be slightly higher than the phase propagation speed (phase velocity) of the microwave field traveling along the slow wave circuit, which is called the synchronization condition. The input microwave signal creates a weak electromagnetic field in the slow-wave circuit. After the electron beam enters the interaction region of the slow wave circuit, it is first modulated by the velocity of the microwave field. The electrons gradually form density modulation as they continue to move forward. Most of the electrons gather in the deceleration field, and the electrons stay in the deceleration field for a long time. Therefore, a part of the kinetic energy of the electron beam is converted into the energy of the microwave field, so that the microwave signal is amplified. Under synchronous conditions, this interaction between the electron beam and the traveling microwave field proceeds continuously along the entire slow-wave circuit. This is the fundamental difference between TWT and klystron in principle.

    Travelling-wave tube structure includes electron gun, slow wave circuit, concentrated attenuator, energy

    Travelling-wave tube couplers, focusing systems and collecting poles.

    The role of the electron gun is to form an electron beam that meets the design requirements.

    The focusing system keeps the electron beam in the desired shape, ensures that the electron beam smoothly passes through the slow wave circuit and interacts effectively with the microwave field, and finally receives the electron beam by the collector.

    The microwave signal to be amplified enters the slow wave circuit through the input energy coupler and travels along the slow wave circuit.

    The electrons exchange energy with the traveling microwave field to amplify the microwave signal. The amplified microwave signal is sent to the load through the output energy coupler.

    The commonly used electron guns of TWT are Pierce parallel flow gun, Pierce convergence gun, high conductivity electron gun, positive control electron gun, grid control electron gun, no interception grid control electron gun, low noise electron gun, etc.

    The traveling wave tube working in pulse mode can modulate the electron beam by controlling the cathode voltage, which is called cathode control. Cathode control requires a high-power modulator, which is bulky, complex and consumes a lot of power. The control of electron beam with additional modulating anode is called positive control. The pulse voltage required for positive control is also relatively high. The grid-controlled electron gun is formed by installing a control grid between the cathode and the anode. In this case, the electron beam can be controlled with only a lower pulse voltage, thus reducing the volume, weight and power consumption of the modulator.

    In the grid-controlled electron gun, the control gate intercepts about 10% of the electron injection current. When the electron injection power of the TWT is large, the control gate dissipation power increases, resulting in the increase of gate temperature, the increase of gate electron emission, the deformation of the grid and even the burnout of the grid. To solve this problem, interception-free grid-controlled electron guns can be used. The shadow grid is set between the control grid and the cathode, and the shadow grid is the same potential as the cathode, and the structure is precisely aligned with the control grid, so that the interception current of the control grid is reduced to less than one thousandth of the total current. Using a grid-controlled electron gun without interception can not only increase the average power capacity of grid-controlled TWT, but also reduce the power of the modulator.

    Common focusing methods used in TWTS are uniform permanent magnet focusing, inverted field focusing, periodic permanent magnet focusing, and uniform electromagnetic focusing (see high-current electron optics).

    The DC velocity of the electron beam is determined by the operating voltage of the TWT. When the operating voltage of the TWT is 2.5 kV, the DC velocity of the electron beam is about 10% of the speed of free space electromagnetic wave (that is, the speed of light). When the operating voltage is 50 kV, the DC speed of the electron beam is about 40% of the speed of the electromagnetic wave in free space. In order for the electron beam to interact effectively with the microwave field, the phase velocity of the microwave field should be slightly lower than the DC velocity of the electron beam. Therefore, the phase velocity of microwave field in TWT should be significantly lower than that of electromagnetic wave propagation in free space. A slow wave circuit is a device that reduces the phase velocity of a microwave field.

    Under the selected operating mode, the main characteristics and parameters of the slow wave circuit include dispersion characteristics and coupling impedance. The dispersion property represents the phase velocity of a microwave field propagating in a slow wave circuit as a function of frequency. For the slow wave circuit used in wide-band TWT, the change of phase velocity with frequency in the frequency band width should be as small as possible, that is, the dispersion is weak. In this way, the synchronization between the electron beam and the microwave field phase velocity can be ensured throughout the frequency band width. The coupling impedance is a parameter indicating the strength of the interaction between the electron beam and the microwave field. The larger the coupling impedance, the stronger the coupling between the microwave field and the electron beam, and the fuller the energy exchange between the electron beam and the microwave field. In addition, in practical applications and production, slow wave circuits are also required to have high mechanical strength, good heat dissipation performance, simple structure and easy processing.

    There are two types of slow wave circuits commonly used in TWTS: spiral circuit and coupled cavity circuit (Figure 3 slow wave circuit commonly used in TWTS). Helical type slow wave circuit includes helix wire, ring rod wire, ring loop wire, etc. The helix structure is simple, the dispersion is weak, so the frequency band is wide, the disadvantage is that the heat dissipation ability is poor, and the backwave vibration is easy to produce when the working voltage is high. Helix is mostly used in broadband, small and medium power TWT, the working bandwidth can reach more than 100%,I band (8 ~ 10 GHZ), J band (10 ~ 20 GHZ) helix TWT pulse power has reached 10 kilowatts. Compared with the helix, the ring rod has high coupling impedance, strong heat dissipation ability, good mechanical strength, and is not easy to backwave vibration, but the dispersion is strong. The operating voltage of the ring rod is 10 ~ 30 kV, and the frequency band width is 15% ~ 20%, which is widely used in medium power TWT. The looped wire has better performance in suppressing backwave vibration and has been applied.

    Coupled cavity type slow wave circuit includes Hughes circuit, clover circuit and so on. They are characterized by high mechanical strength, strong heat dissipation ability, suitable for high-power TWT, but the frequency band width is relatively narrow. Using Hughes circuit TWT, pulse power in 1 to several hundred kilowatts, frequency band width of about 10%. The traveling wave tube with pulse power above 500 kW is mostly used clover circuit. In addition, the slow wave circuit used in the traveling wave tube has cross-fingered slow wave line (also used in the O-type backward wave tube), zigzag line, Karp line and so on.

    There should be good impedance matching between the input and output energy coupler and the slow wave circuit and between the parts of the slow wave circuit. Poor matching will cause electromagnetic reflection. The reflected waves cause feedback, which can lead to parasitic vibration in the traveling wave tube. In order to avoid such vibration, a concentrated attenuator must be set at a certain position in the slow wave circuit. The concentrated attenuator consists of a lossy coating or a lossy ceramic sheet. In the concentrated attenuator, the reflected wave is absorbed, which can achieve the purpose of eliminating the feedback suppression vibration. Although the microwave field operating in the concentrated attenuator mode is also attenuated, the density modulation already established in the electron beam re-establishes the microwave field in the next circuit.

    The electron beam is emitted from the slow wave circuit after the interaction with the microwave field, and finally hits the collector pole. In order to improve the overall efficiency of TWT, a depressurized collector can be used.

    Pulsed TWT is used in ground fixed and mobile radar, airborne fire control radar, electronic countermeasures equipment, etc. Traveling wave tube with pulse power of 10 kW to 4 MW, band width of 8% to 30%; When the pulse power is 5 kW, the frequency band width can reach 67%; When the pulse power is 1 kW, the frequency band width can reach more than 100%. The high-power CW TWT is mostly used in satellite communication earth stations, and the output power can reach 14 kilowatts at 10 gigahertz and 1 kilowatt at 38 gigahertz. Multimode TWTS are used in electronic countermeasures systems and can operate in a variety of pulse and continuous wave states. The pulse-rise ratio (pulse power/CW power) of the multimode TWT is 3 ~ 12 dB. Printed TWTS and miniature TWTS are small in size, light in weight and low in cost, and are suitable for applications with large quantities, such as phased array radar. Space traveling wave tube is a special type of tube for space applications, which is characterized by high reliability, long life and high efficiency. Most of the communication satellites and TV broadcast satellites use the traveling wave tube as the launching tube, and the service life can reach more than 10 years.

    In a TWT, the direction of the energy flow transmitted along the slow wave circuit is the same as the direction of the electron movement, so the TWT is a kind of forward wave amplification tube. In a backward wave tube, the direction of the energy flow transmitted along the slow wave circuit is opposite to the direction of the electron movement. There are two types of backwave tubes: O-type backwave tube and M-type backwave tube. The O-type backwave tube can be divided into three kinds according to the working state: vibration tube, amplification tube and converter tube, but only the backwave vibration tube has been widely used. Therefore, the backward-wave tube usually refers to the backward-wave vibration tube. The electronic tuning range of O-type backwave oscillator is large, up to 67%, and its maximum operating frequency can reach 1250 GHZ. It is a practical device that can reach the submillimeter band of traditional microwave tubes. O-type backward wave oscillator tube is used for signal source and small power oscillator.