Klystrons, Traveling Wave Tubes, Magnetrons,
Answers to RF Cafe Quiz #38
Crossed-Field Amplifiers, and Gyrotrons
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Cafe Quiz #38
quiz is based on the information presented in Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field
Amplifiers, and Gyrotrons, by A.S. Gilmour, Jr., published by Artech House.
Note: Some of these books are available as prizes in the monthly
RF Cafe Giveaway.
1. What is an important property of a metal to be useful as an electron emission
for tube cathodes?
d) Low work function
The Richardson-Dushman equation shows that to obtain high
emission levels, the
work function must be low and the temperature must be high. (see page 44)
where J is the current density and eΦ is the work function
2. What is a primary obstacle that must be dealt with when designing a beam
an electron gun?
b) Inter-electron repulsion forces
Electrostatic repulsion forces between electrons
tend to cause the beam to diverge. (see page 95)
3. Which two forces must be
exactly balanced in order to form an electron beam with uniform flux density?
a) Space charge and
For a beam with uniform charge density throughout and no magnetic flux through the cathode,
the magnetic flux level that produces a magnetic force that exactly balances the space charge and centrifugal
forces is called the Brillouin flux level, commonly denoted by BB. (see page 136)
4. Name the two major classes of linear-beam tubes.
d) Klystron and
traveling wave tube
The klystron is one of two major classes of devices categorized as linear-beam (O-type)
tubes. The other is the traveling wave tube (TWT). There are hybrid devices which combine the klystron and TWT
technologies. (see page 237)
5. In which ways can a conventional klystron be
d) All the above.
With synchronous tuning, all the cavities are tuned to the same frequency and gain is
maximized. For high-efficiency tuning, a klystron is first synchronously tuned and then the penultimate cavity is
tuned upward to maximize power. TO broadband tune a klystron, it is first synchronously tuned at a low power
level, then the intermediate cavities are detuned downward and upward to achieve the desired bandwidth and gain
flatness. (see page 261)
6. What are the two basic types of traveling wave tubes
c) Helix and coupled-cavity
The helix TWT is a relatively low power (10s to 100s of watts) broadband (>2
octaves) device. The coupled-cavity TWT is capable of power of megawatts, but with bandwidth limited to about
10-20%. (see page 321)
7. What are the two basic kinds of cross-field amplifiers
b) Injected beam and distributed emission
In an injected-beam CFA, an electron gun generates a
ribbon-shaped electron beam that passes through the device near an RF circuit, similar to a TWT. Like a magnetron,
the distributed-emission CFA contains a cathode adjacent to the full length of the RF structure. (see page 543)
8. What is the primary difference between a gyrotron device and a linear-beam device?
Electrons in a gyrotron rotate along path rather than just linear motion of a linear-beam
The electron gun and
beam-focusing fields are designed so that the rotational velocity of the electrons are normally 1.5 to 2 times the
axial velocity. (see page 584)
9. What is one of the most common problems that
plague microwave tubes?
d) Electrical breakdown
Breakdown in microwave tubes may occur inside or
outside the vacuum envelope at any of several locations including between electrodes or leads, and from high-power
portions of the RF structure. (see page 701)
10. How is the transition between
the vacuum of a tube cavity and the outside world accomplished?
a) A waveguide or coaxial window.
dielectric plate provide a environmental seal. (see page 627)