Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field Amplifiers, and Gyrotrons
Answers to RF Cafe Quiz #38

Return to RF Cafe Quiz #38

This 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

     source 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 e_Φ is the work function

2.  What is a primary obstacle that must be dealt with when designing a beam

      former for 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 centrifugal forces

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 B_B.  (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 tuned?

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 (TWTs)?

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 (CFAs)?

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?

a)  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.

A dielectric plate provide a environmental seal.  (see page 627)