Traveling wave tubes

Use of traveling wave tube (TWT) amplifiers at power levels of hundreds of watts has been proposed (54) for power appHcations, at least when the heating chamber is well shielded. The potential advantage is an improved uniformity of heating when a broad band of frequency is used, ie, excitation of many modes. Disadvantages are high cost and lower (<50%) efficiency of the TWT. 

A family of vacuum-tube MMW sources is based on the propagation of an electron beam through a so-called slow-wave or periodic structure. Radiation propagates on the slow-wave structure at the speed of the electron beam, allowing the beam and radiation field to interact. Devices in this category are the traveling-wave tube (TWT), the backward-wave oscillator (BWO) and the extended interaction oscillator (EIO) klystron. TWTs are characterized by wide bandwidths and intermediate power output. These devices operate well at frequencies up to 100 GHz. BWOs, so called because the radiation within the vacuum tube travels in a direction opposite to that of the electron beam, have very wide bandwidths and low output powers. These sources operate at frequencies up to 1.3 THz and are extensively used in THZ spectroscopic applications [10] [11] [12]. The EIO is a high-power, narrow band tube that has an output power of 1 kW at 95 GHz and about 100 W at 230 GHz. It is available in both oscillator and amplifier, CW and pulsed versions. This source has been extensively used in MMW radar applications with some success [13]. 

This type of traveling wave tube with cold cathode has been developed and met reasonable performance. However, a good system pumping around the cathode was difficult to achieve in the small enclosed spaces. 

Microwaves can be produced by four types of macroscopic cavity resonators that use the ballistic motion of electrons across a cavity opening the klystron, the magnetron, the traveling-wave tube (TWT), and the gyrotron. They can also be generated by field-effect transistors at low frequencies, by Gunn42 diodes, and by IMP ATT diodes. 

Traveling-wave tube amplifier (1) electron gun (2) microwave input (3) focusing magnets (4) attenuator (5) helix coil (6) microwave output (7) vacuum tube (8) collector. 

Traveling-wave tubes (frequency range 0.3 to 50 GHz, invented by Kompfner,47 usually part of a traveling-wave amplifier, Fig. 10.13) are low-Q amplifiers for microwaves, with typically a four-decade frequency range the tube is a long vacuum tube, in which the magnet focuses an axial electron beam, while the helix, fed externally by a small microwave beam, acts as a delay line whose electric field bunches the electrons this induces even more electrons to travel along the helix the amplification is as much as 70 dB. 

DPPH = 2,2-diphenyl-1-picrylhydrazyl ENDOR= electron-nuclear double resonance EPR = electron paramagnetic resonance ESE = electron spin echoes ESEEM = electron spin echo envelope modulation EFT = fast fourier transformations FWHM = fidl width at half maximum HYSCORE = hyperfine sublevel correlation nqi = nuclear quadrupole interaction TauD = taurme/aKG dioxygenase TWTA = traveling wave tube amphfier ZFS = zero field sphtting. 

Chirped pulse amplification is achieved using a pulsed traveling wave tube amplifier (TWTA, Amplifier Research 1000TP8G18) with peak power or 2 kW (7-18 GHz). The final pulse is shown as an inset in Figure 1. The spurious signals in the pulses we create using this technique are at least 20 dB lower in power than the instantaneous sweep frequency across the full 11 GHz range of the pulse. 

Microwave-transparent ceramics (MTC) has been used widely as output windows of traveling wave tube (TWT) in the field of microwave communication. Commonly, excellent mechanical property and thermal shock resistance are also required for MTC to release the heat caused by dielectric loss in time and prevent the thermal breakdown, besides low dielectric loss and high thermal conductivity. Recently, with the increment of output power and broadening of wave band, a higher mechanical property is desired in response to the miniaturization design of output windows. 

Charles Townes was the first person to take advantage of the stimulated emission process to be used in the form of an amplifier by conceiving and constructing the first maser (an acronym for microwave amplification by stimulated emission of radiation). The maser produced a pure beam of microwaves that were anticipated to be useful for communications in a similar way to that of a klystron or a traveling-wave tube. The first maser was produced in ammonia vapor, and the inversion occurred between... 

The extensive description of microwave sources like traveling wave tubes (TWT), klystrons, disk-seal triodes, and power grid tubes can be found in a number of books on industrial microwave applications [19]. 

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