The traveling wave tube (T W T) is another example of a linear-beam,or O-type, tube.It differs from the Klystron amplifier by the continuous interaction of the electron beam and the RF field over the entire length of the propagating structure of the traveling -wave tube rather than the interaction occurring at the gaps of a relatively few resonant cavities. The chief character-is tic of the TWT of interest to the radar system engineer is its relatively wide bandwidth. A wide bandwidth is necessary in applications where good range-resolution is required or where it is desired to avoid deliberate jamming or mutual interference with near by radars. Although low power T W Ts are capable of octave bandwidths of the order of 10 to 20 precent are more typical at the power levels required for long-range radar applications.The gain, efficiency,and power levels of T W Ts are like those of the klystron :but,in general,their values are usually slightly less than can be obtained with a klystron of comparable design.
A diagrammatic representation of a traveling-wave tube.The electron optics is similar to the klystron. Both employ the principle of velocity modulation to density-modulate the electron beam current.Electrons emitted by the cathode of the traveling wave tube are focused into a beam and pass through the RF interaction circuit known ass the slow-wave structure, or periodice delay line.An axial magnetic field is provided to maintain the electron-beam focus as in the klystron. A shadow grid to pulse-modulate the beam can also be included.After delivering their d-c energy to the RF field, the electrons are removed by the collector electrode.
The simple helix was used as the slow-wave structure in the early T W Ts and is still preferred in traveling-wave tubes at power levels up to a few kilowatts. It is capable of wider bandwidth than other slow-wave structures.but its power limitations do not make it suitable fore most high-power radar applications.A modification of the helix known as the ring -bar circuit as has bean used in T W Ts to achieve higher power and efficiencies between 35 and 50 percent.The Raytheon Q K W -1671 A,which utilizes a ring -bar circuit,has a peak power of 160 kW,a duty cycle of 0.036,pulse width at L band.This tube is suitable for air -search radar similar T W Ts have bean used in phased -array radar.The Air Force Cobra Dane phased-array radar.
The ring-loop slow-wave circuit which consists of equally spaced rings and connecting bars,is also related to the helix and the ring-bar.It is chained to be preferred for tubes in the power range from 1 to 20 kW, as for lightweight airborne radar or as drivers for high-power tubes. The ring-loop circuit is not bothered by the backward-wave oscillations of the ordinary helix or the "rabbit ear" oscillations which can appear in coupled-cavity circuits.
The helix has bean operated at high average power by passing cooling fluid through a helix constructed of copper tubing The bandwidth of this type of fluid-cooled helix. TWT con be almost an octave. and it is capable of several tens of kilowatts average power at L band with a duty cycle suitable for radar applications.
A popular from of slow-wave structure for high-power TWTs is the coupled-cavity circuit.It is not derived from the helix as are ring-bar or ring-loop circuits.The individual unit cells or the coupled -cavity circuit resemble the ordinary klystron resonant cavities.There is no direct coupling between the cavities of a klystron :but in the traveling wave tube coupling is provided by along slot in wall of each cavity.The coupled-cavity circuit is quite compatible with the use of lightweight PPM focusing, a desired feature in some air borne applications.
Although the TWT and the klystron are similar in many respects, one of the major differences between the two is that feedback along the sloe-wave structure is possible in the TWT, but the back coupling of in the klystron negligible. The attenuation may be distributed , or it may be lumped:but it the usually the middle third of the tube. loss introduced to attenuate the backward wave also reduces the power of the forward wave which results in a loss of efficiency. This loss in the forward wave can be avoided by the use of discontinuities called severs, which are short internal terminations designed to dissipate the reverse-directed power without seriously afficiency.
This loss in the forward wave can be avoided by the use of discontinuities called severs, which are short internal terminations designed to dissipate the reverse-directed power without seriously affecting the forward power.The number of severs depends on the gain of the tube: one sever is used for each 15 to 20 dB of gain.in addition to reflection-type oscillations, backward-wave oscillations can occur. These frequently occur outside the passband so that they can be reduced by loss that is frequency selective.
in principle, the traveling-wave tube should be capable of as large a power output as the klystron. The cathode,RF interaction region, and the collector are all separate and each can be designed to perform their required functions independently of the others.In practice, however, it is found that there are limitations to high power. The necessity for attenuation or severs in the structure, as mentioned above, tends to make the traveling-wave tube less efficient than the klystron. The slow-wave structure can also provide a limit to TWT capability. It seems that hose snow- wave structures can also provide a limit to TWT capability. It seems that those slow-wave structures best suited for broad band width (like the helix) have poor power capability and poor heat dissipation. A sacrifice in bandwidth must be made if high-power is required of a TWT. If the bandwidth is too small, however. there is little advantage to be gained with a traveling-wave tuba as compared with multi cavity keystrokes.
This loss in the forward wave can be avoided by the use of discontinuities called severs, which are short internal terminations designed to dissipate the reverse-directed power without seriously affecting the forward power.The number of severs depends on the gain of the tube: one sever is used for each 15 to 20 dB of gain.in addition to reflection-type oscillations, backward-wave oscillations can occur. These frequently occur outside the passband so that they can be reduced by loss that is frequency selective.
in principle, the traveling-wave tube should be capable of as large a power output as the klystron. The cathode,RF interaction region, and the collector are all separate and each can be designed to perform their required functions independently of the others.In practice, however, it is found that there are limitations to high power. The necessity for attenuation or severs in the structure, as mentioned above, tends to make the traveling-wave tube less efficient than the klystron. The slow-wave structure can also provide a limit to TWT capability. It seems that hose snow- wave structures can also provide a limit to TWT capability. It seems that those slow-wave structures best suited for broad band width (like the helix) have poor power capability and poor heat dissipation. A sacrifice in bandwidth must be made if high-power is required of a TWT. If the bandwidth is too small, however. there is little advantage to be gained with a traveling-wave tuba as compared with multi cavity keystrokes.
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