November 2017 13
through a resonant cavity. Microwave RF input into the initial cavity causes the
beam to accelerate and decelerate. This, in turn, causes the electrons to form
groups, which is why this cavity is called the “buncher.” The groups of electrons
continue to travel or “drift” through a tube in which the grouping gets stronger.
At the other end of the tube is a second resonant cavity that extracts
energy from the beam as RF at a frequency determined by the beam’s velocity
and size of the groups. Figure 7 shows a high power klystron in use at the
Fermilab accelerator in Batavia, IL. Top to bottom, the water-cooled tube is
about 12 feet high!
Cousin to the klystron, the magnetron also uses a resonant cavity to excite
electrons at microwave frequencies. The magnetron is strictly an oscillator,
however. It can’t amplify signals.
The magnetron ( en.wikipedia.org/wiki/Cavity_magnetron) consists of
a circular hole through a block of metal which acts as the tube’s anode. The
cathode is in the middle of this hole. The hole is surrounded by resonant
chambers formed in the block of metal, which is usually copper. A strong field
from permanent magnets causes the electrons from the cathode — which is
at a high negative DC voltage compared to the grounded anode — to take a
semi-circular path. The radiation from the electrons excites current flow in the
resonant cavities, creating a strong RF field. The RF is extracted from the cavities
by coupling loops where it is used to create a radar pulse or to pop popcorn!
A large magnetron with a special history is shown in Figure 8.
What is the Tube’s Future?
That is an excellent question! Although I don’t think digital signal processing
(DSP) and software defined radio (SDR) are going to be able to generate much
high power RF, big transistors with integrated protection circuits are rapidly
taking over niches previously filled by tubes.
At the high end of the energy scale, tubes remain supreme but possibly not
for long. I think there will always be some special tube that is the only solution
to a special problem. Maybe their ability to tolerate faults and high voltages will
keep them on the job where solid-state devices would fail. Time will tell, but the
vacuum tube has a lot of life left in its filaments! NV
Conventional and Electronic Current
Many years ago, during the early years of electrical experimentation,
the assumption was made that electrical current was the flow of
positive charges. A polarity had to be assumed and there were just two
choices. Unfortunately for engineering and physics students, the wrong
one was chosen as current in circuits is really the flow of negatively-charged electrons. (This mistake is often attributed to Ben Franklin, but
there were others who guessed wrong too.) Electronics generally uses
conventional current which flows from positive to negative voltages.
Thus, electronic people think of current somewhat differently than
physicists, and this causes no end of confusion — particularly with
the direction of magnetic fields created by electric current. (See the
Wikipedia entries for right-hand rule and left-hand rule for an illustration.)
Nevertheless, conventional and electronic current are equivalent except
for the assumed polarity of the charge-carrying particles.
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