March 1942 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Connecting a diode backwards across a solenoid
coil to shunt potentially damaging current and/or voltages when the supply is turned
off is a common trick for saving connected circuitry. Depending on the magnitude
of the magnetic field and how quickly the field collapses, some really high voltages
can be produced. In fact, the ignition coil and point (now solid state) system in
exploits exactly that principle to turn the 12 volts from your car battery into
20-40 kV for firing the spark plugs. Engineers that designed this early cyclotron
had limited options for what to use given the state of the art in the early 1940s,
and chose to keep the generator permanently connected to the coil (no switch) so
that if the controller failed, the coil's energy 15 kgauss magnetic field)
would flow back into the generator. That, of course, is the equivalent of refrigerator
magnet compared to magnetic fields set up by the Large Hadron Collider (LHC) in
the EU in search of the Higgs boson.
Radio and Atom Busting - A Glimpse at the Cyclotron
By J. S. V. Allen,* W8UNS
Imagine the ecstasies of an amateur who had nursed a 200-watt transmitter up
to 300 watts, and then suddenly had the opportunity of playing with an 80,000-watt
rig that loafs along at 40 kilowatts input! Therein lies a story.
A cyclotron is a tremendous affair because it is necessary to
build up terrific particle velocities to disintegrate atoms. This 42-inch (chamber
diameter) cyclotron at the Ohio State University was constructed and is operated
through the aid of Julius F. Stone, the Ohio State University Development Fund,
and WPA aid.
One of the more recent scientific developments employing radio frequency power
is the cyclotron or "atom-smasher," which threatens to revolutionize our lives some
time in the future by unleashing almost limitless sources of atomic energy. A brief
description of this scientific wonder and the part played by high-frequency radio
may be of interest to the radio amateur. Also, some of the humorous and unusual
experiences and observations of an enthusiastic ham may be interesting to others
in the realm of Hamdom.
The 100-ton cyclotron recently completed at the Ohio State University, at a cost
of $60,000, is the product of over two years of unceasing toil by Dr. M. L. Pool1
and his associates. Eighty tons of iron went into the construction of the magnet
core, which was wound with more than nine miles of 7/16-inch copper tubing. Through
this tubing a current of 300 amperes may be passed, if at the same time water is
circulated through the tubing for cooling purposes. Automatic switches have been
installed to stop the current if the water should cease to flow, thus preventing
the high current from burning up the large magnet coils. It requires several minutes
for a 50 kw. motor-generator to build up a current of 300 amperes, and during
this time the magnet creaks from magnetic forces being set up within the coils.
If the circuit should accidentally be opened, the destructive forces would be considerable
because of the enormous amount of energy stored within the large magnetic field
(15,000 gauss over 1400 square inches). For this reason the magnet and the generator
are permanently connected. If the motor of the motor-generator (M.-G.) set is turned
off, the energy flows from the magnet into the generator and reverses the set. Hence,
the field in the generator is reduced first, and then the M.-G. set is shut down.
How the Cyclotron Works
The construction and operation of the cyclotron, briefly stated, are as follows:
There is a large cylindrical vacuum chamber, 42 inches in diameter and 8 inches
high, between the magnet poles. Within this chamber are two "dees," so-called because
they ate shaped like the letter D. These two copper dees are electrodes supported
within the chamber by insulators and connected to the radio frequency power oscillator.
Near the center of the chamber is a tungsten filament, which must be heated by
radio frequency current because the interaction between ordinary alternating or
direct current and the 15,000-gauss magnetic field would bring destructive forces
to bear upon the filament. Hence, an 806-833 "transmitter" (650 watts), operating
on 200 kc., is used to light the filament with r.f. Above the filament is a plate,
at 1000 volts positive potential to draw a beam of electrons from the filament.
The beam passes vertically along the axis, ionizing the low-pressure hydrogen within
the chamber. The protons, or ionized hydrogen nuclei, have a positive charge, and
are pushed and pulled around and around between the two hollow dee-shaped electrodes
10.5 million times per second as the polarity of the dees oscillates. When a state
of resonance exists, the positive hydrogen ion arrives between the dees in time
to be attracted by the negative charge on the one dee and repelled by the positive
charge on the other dee.
Schematic representation of the cyclotron.
But in moving, the proton must pass through the strong magnetic field which turns
the ion in a semicircle. Each time the ion runs the gauntlet between the dees it
receives an electrical kick in the pants, figuratively speaking, of about 40,000
volts. In this way the speed of the ion is increased to about 6,000 miles per second
before it crashes into the target at the outer edge of the chamber. With higher
velocities a new difficulty is encountered - relativity. The ions become heavier
because of their tremendous velocities, and hence cannot be accelerated as easily.
The path of the ion from the center to the target is a series of about 125 semicircles
with increasing radii, the increase taking place each time the ion is accelerated
between the dees.
The R.F. Oscillator
The most interesting part of the cyclotron to the ham is the 80-kw. power oscillator
used to drive the ions back and forth between the dees. This shielded radio transmitter
employs a pair of 859 tubes. Don't look in your Handbook for the characteristics
of this tube for it exceeds the maximum amateur input by about 4000% (2000% in California)!
Each tube is capable of handling more than 40 kw. at 10.5 megacycles, the frequency
used. With an input of 50 kw. the plate dissipation is 20 kw., requiring a flow
of from 8 to 15 gallons of water per minute. The filament current is 70 amperes
and the voltage across the two units of the filament is 22. The tube is two feet
in height - a real bottle!
With an input of 40 kw., perhaps 2 kw. reach the water-cooled target
where the atoms are cracked, and is dissipated over an area approximately 0.05 square
inch. Try that on your final amplifier tube! The target becomes white hot during
While cyclotrons and many other devices utilizing the electronic art which has
developed as a result of radio research are not, strictly speaking, "another part
of the family," some of them are not very distant cousins. This article is presented
with the thought that the ordinary amateur is enough interested in general science
to want to know what the more interesting developments are in allied fields. If
the gang wants, we'll try to have more articles of this nature from time to time.
The power oscillator is housed in a large copper enclosure several feet from
the cyclotron. The r.f, power is transferred to the chamber of the atom-smasher
by means of a transmission line which includes a quarter-wavelength section. The
shorting bar on this line is adjustable, permitting rough tuning. The fine tuning
is accomplished by a man-sized variable condenser - a 13 square-foot vane hinged
over the transmission line and adjusted by means of a motor. This whole Lecher system
is made of 5-inch copper tubing and must be water-cooled for good stability. The
voltage multiplication obtained by making the transmission line shorter than a quarter-wavelength
is about 100. If one stands close to the transmission line his ears become uncomfortably
warm from induced r.f.; there is some evidence that sinus trouble has been cured
while working in the r.f. field of the transmission line.
The frequency of the power-oscillator must be maintained within a few kilocycles
of the resonance frequency for a certain strength of magnetic field, otherwise the
ions will fall out of step and will arrive at the gap between the dees at the wrong
time to receive the greatest acceleration. The operator can control the frequency
by varying the capacity through the tuning vane above the transmission line. Or
an automatic gadget called a discriminator can be used.
This automaton2 consists of an electron-coupled oscillator, amplifying
circuits and a bridge circuit. Its oscillator is tuned to a frequency 5,000 kilocycles
below the frequency of the power oscillator, giving a heterodyne signal of 5,000 kc.,
which is applied to a bridge circuit balanced at 5,000 kc. Any variation in
the frequency of the power oscillator resulting from load reactance changes caused
by heating in the power circuits causes the heterodyne beat to change and the bridge
to become unbalanced proportionately. The unbalanced voltage controls the motor
which varies the position of the tuning vane, and the frequency of the large oscillator
is thus restored to resonance.
Occasionally the power oscillator becomes unstable and a heavy r.f. arc jumps
across the grid line used to tune the grid circuit. At these times the discriminator
is thrown into a state of nervous disorder. Sometimes it will adjust the tuning
capacity first in one direction, then in the other, very rapidly. But under normal
conditions this gadget operates very well.
When the grid circuit breaks down with a thunderous arc, the discharge sometimes
drills a very fine hole through the centers of the nine-inch insulators, or an insulator
may be shattered by the arc. The operator is always alert for such emergencies,
and quickly turns off the power to prevent more damage.
One rainy day the 100 kva., 20,000-volt, three-phase power transformer3
arced with a great flash and noise. Later the same day the author was operating
the cyclotron and worrying about another transformer flash-over when a 150-watt
lamp over his head burned out with a bright flash. Nervous prostration set in, but
not before he had turned off all the power in a split second!
Warning - Leave Watches Outside!
W8UNS alongside the 100-kva. power transformer. The bottle is
an 859, rated at 40 kw. input. Two of them are used in the cyclotron oscillator.
Many interesting things happen around a cyclotron which are not duplicated elsewhere.
One source of several interesting tales is the strong magnetic field. If one approaches
the large magnet with iron tools in his pockets he can feel them move in strange
ways under the influence of the field. An eight-inch wrench which was passed carefully
from one man to another was drawn with terrific force to the magnet, and it took
two men to pull it off the magnet pole. An iron box was once carried too close to
the cyclotron, with the result that the magnet had to be turned off before the box
could be extricated.
Many people visit the cyclotron, and are warned before they enter the laboratory
that their watches will be magnetized if carried near the magnet. In spite of this
warning one lady carried her watch into the cyclotron room in her purse and later
asked if it was not all right, since she had not removed it from her purse while
near the large magnet! She did not realize that imitation leather is a very poor
magnetic shield. A demagnetizing coil such as a jeweler uses is kept in an adjacent
laboratory for such cases.
One day a visiting monkey from a near-by laboratory was scampering all over the
cyclotron, with workmen in hot pursuit. An ether gun and other devices were used
in trying to catch the unwelcome visitor but the cyclotron itself finally accomplished
this feat. When the monkey tried to run through the belt of a small vacuum pump
its foot was caught, and the belt held it until a keeper captured the monkey. But
atom-smashing is not all monkey business, as the following paragraphs about the
products of the cyclotron will demonstrate.
The material placed in the cyclotron is bombarded with protons, neutrons, deuterons,
or alpha particles, with the result that minute quantities of the material are activated,
after which they radiate much the same as radium does. In fact certain radioactive
elements are used instead of radium in treating cancer, leukemia, and other diseases.
These radioactive substances can be used as tracers within the human body, as shown
by the experiment in which a person is fed some radioactive material which enters
the blood and reaches the fingers within a very few minutes. The radioactivity can
be detected in the finger tips by holding them near a Geiger-Muller counter which
clicks each time a particle from an exploding atom pierces the counter tube. These
radioactive substances have been called the greatest contribution to medical research
since the discovery of the microscope.
Atomic power is perhaps a possibility in the not too distant future, for uranium
235 can already be cracked and made to yield enormous amounts of energy, relatively
speaking. The difficulty lies in the scarcity of uranium 235. If such power does
become available in considerable amounts men will no longer fight for oil fields
and similar natural resources. Energy will be so easily obtained, and in such concentrated
forms, that it will revolutionize much of our living. If men still choose to have
wars they will be fought more fiercely, for modern wars are fought with such materials
as gasoline and high explosives, which are very concentrated forms of energy. Atomic
energy will be 5000 times more concentrated than gasoline.
* Bethany College, Bethany, W. Va.
1 "The Ohio State University Cyclotron," Alpheus W. Smith and M. L. Pool, Physical
Review, Feb. 15, 1940.
2 "An Automatic Frequency Control for Cyclotron R.F. Power Supply," R. B. Jacques,
Physical Review, June 1, 1941.
3 Donated by Radio Station WJR through Professor W. L. Everitt.
Posted November 16, 2022
(updated from original post on