The traveling wave tube (TWT) is a high-gain, low-noise, wide-bandwidth microwave amplifier. Traveling Wave Tube are capable of gains of 40 dB or more, with bandwidths of over an octave. (A bandwidth of one octave is one in which the upper frequency is twice the lower frequency.) TWTs have been designed for frequencies as low as 300 MHz and as high as 150 GHz and continuous outputs to 5 kW. Their wide-bandwidth and low-noise characteristics make them ideal for use as RF and medium-power amplifiers in microwave and electronic countermeasure equipment. They are widely used as the power output stage in orbiting satellites.
Construction of Traveling Wave Tube
Figure 1 is a pictorial diagram of a traveling wave tube. The electron gun produces a stream of electrons that are focused into a narrow beam by an axial magnetic field. The field is produced by a permanent magnet or electromagnet (not shown) that surrounds the helix portion of the tube. The narrow beam is accelerated, as it passes through the helix, by a high potential on the helix and collector.
Operation
The beam in a TWT is continually interacting with a RF electric field propagating along an external circuit surrounding the beam. To obtain amplification, the TWT must propagate a wave whose phase velocity is nearly synchronous with the do velocity of the electron beam. It is difficult to accelerate the beam to greater than about one-fifth the velocity of light. The forward velocity of the RF field propagating along the helix is slowed to nearly that of the beam due to its travel along the helix. Changing the pitch changes the speed of the RF field.
The electron beam is focused and constrained to flow along the axis of the helix. The longitudinal components of the input signal’s RF electric field, along the axis of the helix or slow wave structure, continually interact with the electron beam to provide the gain mechanism of TWTs. This interaction mechanism is pictured in Figure 2. This figure illustrates the RF electric field of the input signal, propagating along the helix, infringing into the region occupied by the electron beam.
Consider first the case where the electron velocity is exactly synchronous with the RF signal passing through the helix. Here, the electrons experience a steady dc electric force that tends to bunch them around position A and de-bunch them around position B in Figure 2. This action is due to the accelerating and decelerating electric fields. In this case, as many electrons are accelerated as are decelerated; hence, there is no net energy transfer between the beam and the RF electric field. To achieve amplification, the electron beam is adjusted to travel slightly faster (by increasing the anode voltage) than the RF electric field propagating along the helix. The bunching and debunching mechanisms just discussed are still at work, but the bunches now move slightly ahead of the fields on the helix. Under these conditions, more electrons are in the decelerating field to the right of A than in the accelerating field to the right of B. Since more electrons are decelerated than are accelerated, the energy balance is no longer maintained. Thus, energy transfers from the beam to the RF field, and the field grows and amplification occurs.
Fields may propagate in either direction along the helix. This leads to the possibility of oscillation due to reflections back along the helix. This tendency is minimized by placing resistive materials near the input end of the slow-wave structure. This resistance may take the form of a lossy wire attenuator (Figure 1) or a graphite coating placed on insulators adjacent to the helix. Such lossy sections completely absorb any backward traveling wave. The forward wave is also absorbed to a great extent, but the signal is carried past the attenuator by the bunches of electrons. These bunches are not affected by the attenuator and, therefore, reinstitute the signal on the helix, after they have passed the attenuator.
The traveling wave tube has also found application as a microwave mixer. By virtue of its wide bandwidth, the TWT can accommodate the frequencies generated by the heterodyning process (provided, of course, that the frequencies have been chosen to be within the range of the tube). The desired frequency is selected by the use of a filter on the output of the helix. A TWT mixer has the added advantage of providing gain as well as providing mixer action.
A TWT may be modulated by applying the modulating signal to a modulator grid. The modulator grid may be used to turn the electron beam on and off, as in pulsed microwave applications, or to control the density of the beam and its ability to transfer energy to the traveling wave. Thus, the grid may be used to amplitude-modulate the output. The TWT offers wideband performance with high-power outputs of up to 150 GHz. TWTs are widely used in wideband communications repeater links. They offer low-noise performance and high-power gains. Their high reliability dictates their use as power amplifiers in communications satellites, where a lifetime in excess of 10 years can be expected.
Traveling Wave Tube Oscillator
A forward wave, traveling wave tube may be constructed to serve as a microwave oscillator. Physically, a TWT amplifier and oscillator differ in three major ways. The helix of the oscillator is longer than that of the amplifier, there is no input connected to the oscillator, and the lossy wire attenuator shown in Figure 1 is eliminated. The tube now allows both forward and backward waves and is usually called a backward wave oscillator (BWO). The operating frequency of a BWO is determined by the pitch of the tube’s helix. The oscillator frequency may be fine-tuned, within limits, by adjusting the operating potentials of the tube.
The electron beam, passing through the helix, induces an electromagnetic field in the helix. Although initially weak, this field will, through the action previously described, cause the bunching of succeeding portions of the electron beam. With the proper potentials applied, the bunches of electrons will reinforce the signal on the helix. This, in turn, increases the bunching of succeeding portions of the electron beam. The signal on the helix is sustained and amplified by this positive feedback resulting from the exchange of energy between the electron beam and helix.
Klystron
Another common microwave tube is the klystron. It has been widely used in the past and has certain similarities to the TWT. The high-power klystrons are being replaced by either magnetrons or TWTs in new equipment, and solid-state microwave devices are replacing them in low-power applications. The reasons for this are due to the klystron’s large size and the complex, costly sources of dc required for operation.