November 1957 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
By 1957, betatrons, cyclotrons,
cosmotrons, synchrocyclotron, bevatrons, and other forms of "trons" had the physics
world all agog with anticipation of the next big discovery.
Quarks were still a decade away
from being discovered and something as exotic as the Higgs boson (aka god particle)
hadn't entered anyone's mind. The news media was agog with reports of the world
possibly coming to the end as a result of those experiments sparking a nuclear reaction
chain that would cause the whole world to explode. Today, the news media is no smarter,
because nowadays they fret over the Large Hadron Collider (LHC) generating a
black hole that will implode the whole world. What a ship of fools.
After Class: Special Information on Radio, TV, Radar and
The Particle Accelerators
Fig. 1. Fundamental cyclotron construction. Top view above,
side view below.
Fig. 2. The synchrocyclotron. One dee is used instead of
the two in the cyclotron.
Columbia University's huge synchrocyclotron produces alpha particle
energies of over 400 million electron volts.
Fig. 3. The betatron. Note how the pole pieces go into the
center of a vacuum "doughnut."
Fig. 4. Plan view of the Berkeley bevatron. Brookhaven's
cosmotron follows a similar pattern.
The cosmotron at Brookhaven. Concrete blocks in the foreground
serve as shield; large "doughnut" is the magnet.
I've begun to wonder where electron physics is going. There are so many different
synchrotrons, betatrons, cosmotrons, etc., that the list seems endless. What are
they all about and how do they work?
We'll begin with the cyclotron, not because it was the first atom-smasher but
because it was the granddaddy of all the modern particle "whirlers." The idea was
to impart a very high velocity to an ion - to supply it with oodles of kinetic energy.
Then it could be electrically directed to wallop an atom and blow it apart. Heavy
positive particles like protons, deuterons (double protons), and alpha particles
(two protons plus two neutrons) were selected as projectiles because their large
masses help provide large impact energies.
The Cyclotron. Dr. E. O. Lawrence and Dr. M. S. Livingstone built the first cyclotron
(Fig. 1) in 1931. Picture a flat circular pill-box cut vertically down the
middle - separated into two "D"-shaped halves. The hollow half-boxes or dees are
connected across a source of high-frequency voltage and enclosed in a gas-filled
chamber. The whole assembly is then mounted between the poles of a powerful magnet.
An electrically heated filament in the position shown provides an abundance of electrons
which serve to ionize the gas in the chamber.
Suppose that at a given instant A1 is made positive with respect to A2 by the
high-frequency voltage; a positive ion from the source F will be attracted toward
A2, but instead of moving directly toward A2, it will move in a circular path due
to the action of the uniform magnetic field produced by the pole pieces. As it reaches
the gap between the two dees, it is accelerated by the strong electric field at
this point and proceeds into the opposite box with increased velocity.
If you whirl a stone attached to the end of an elastic band around your head,
the stone will gyrate in ever-widening circles as you make it go faster and faster.
So it is with the speeding particle; each time it crosses the gap, it is given another
violent kick which causes it to spiral outward with more and more speed until at
last it emerges from the "window" endowed with tremendous kinetic energy. Its target
may be a cloud chamber or a photographic emulsion in which the effect of its atom-disruption
may be observed and measured.
The first cyclotron accelerated protons to a little over 1 million electron-volts
(1 m.e.v.) of energy. (An electron-volt is the kinetic energy acquired by any particle
carrying one unit of electric charge when it has been accelerated through one volt
of potential.) As larger cyclotrons were built, the available particle energy rose
steadily until, in 1946, the cyclotron peak of 40 m.e.v. for alpha particles was
The limit was achieved at 40 m.e.v. because one of the famous scientific principles
finally proved by Dr. Albert Einstein began to push its insistent proboscis into
the picture: as any body moves at increasing speed, its mass also increases. Up
to about 40 m.e.v., this increase in mass is very slight and does not affect the
cyclotron's operation; beyond this energy value, however, the ions begin to gain
mass. This slows them down enough so that their spiral paths fall out of synchronization
with the high-frequency voltage on the dees and they no longer receive their accelerating
shoves at the right instant.
To compensate for the increasing mass of the whirling protons or alpha particles,
the frequency of the accelerator voltage can slowly be changed so that it keeps
step with the diminishing speed increment of the ions. In 1950, the huge Columbia
University synchrocyclotron went into operation using this synchronization principle.
The most significant departure in construction from prior cyclotrons was the use
of a single rather than a double dee.
In the synchrocyclotron, a grounded deflecting electrode of simpler construction
serves as the second side of the applied high-frequency voltage. Recent reports
indicate that the Columbia synchrocyclotron produces alpha particle energies in
excess of 400 m.e.v. An ion having this energy travels at approximately 93,000 miles
per second or at a speed high enough to carry it to the sun in about 10 seconds
Thus far, no mention has been made of electrons, since the cyclotron and synchrocyclotron
are positive-charge accelerators. Why can't they be used for accelerating electrons?
If we remember that an electron has a rest mass of about 1/1800th that of a proton,
it is evident that we would have to speed it up far more than a proton to obtain
the same kinetic energy from it. (Kinetic energy depends upon both mass and velocity.
If we want a large k.e. in a body of small mass, the velocity must be made very
To get an electron up to an energy level of only 1 m.e.v., we must make it travel
at a speed more than nine-tenths that of light! At this velocity, its mass increases
2.5 times over its rest mass, and even a modern synchrocyclotron cannot compensate
for such a large increase. Hence, it became apparent that another type of machine
The Betatron. The first successful betatron - the "beta" prefix comes from beta
rays which are streams of electrons - was designed and built by D. W. Kerst in the
United States in 1940. Its operation is similar to that of an ordinary step-up transformer.
As illustrated in Fig. 3, the major features of the betatron include a large
electromagnet whose pole pieces protrude into the center of an evacuated glass-tube
"doughnut." Electrons are produced outside the betatron by thermionic methods and
are given a preliminary acceleration by an external electric field of about 50,000
volts potential. They are then fed or injected into the doughnut. The current through
the electromagnet coils is alternating so that the magnetic field is a varying one;
the injection process occurs at the precise time when the magnetic field is building
up in the right direction. The effect of the field is to induce a voltage within
the doughnut which increases the energy of the electrons.
Although the electrons tend to spiral outward as their velocity increases, the
magnetic field grows at exactly the right rate to counterbalance this tendency.
It will be remembered that the field of the cyclotron is steady and that the spiral
path is characteristic of such an unchanging magnetic flux. Thus, the electrons
in the betatron follow essentially circular paths. The magnetic field around the
doughnut is allowed to grow for only one-quarter of the a.c. cycle, but during this
brief interval each injected electron makes over 275,000 turns around the circle!
In the General Electric betatron, the magnet weighs 135 tons and the electrons
cover a distance of about 900 miles before being ejected at an energy of 100 m.e.v.
Each electron at ejection is traveling at a speed of more than 99.99% of the speed
of light and has a mass nearly 200 times its rest mass!
Bevatron or Cosmotron. How much further can we go? Right now in Birmingham, England,
at the Brookhaven National Laboratory on Long Island, and in the Berkeley Radiation
Laboratory, they are breaking the "m.e.v. barrier" every day. The newest of the
atom smashers, a proton synchrotron called the "bevatron" at Berkeley and the "cosmotron"
at Brookhaven, is at work whipping protons up to fantastic energies in the billion
electron-volt region (b.e.v.'s). The proton synchrotron really combines the functions
of a cyclotron and a betatron.
Protons are first accelerated by a cyclotron and are then injected at A in Fig.
4. They are further accelerated by a varying magnetic field as in the betatron.
As soon as the magnet becomes saturated, the betatron part of the operation ceases,
and further acceleration is then produced by a high-frequency voltage applied to
electrode C. This electrode, known as a cavity resonator, therefore performs a function
similar to the dees in the cyclotron.
The output of the Berkeley bevatron consists of a series of pulses at intervals
of about six seconds from the 10,000-ton magnet. During each pulse a proton travels
270,000 miles - a distance greater than that to the moon! The average energy output
of the Berkeley bevatron is on the order of six billion electron-volts!
Where will it all end? The scientist has an insatiable thirst for knowledge and
the boundless energy to see it through. Man, in his quest for the keys to the universe,
has already duplicated the energy of cosmic rays in his new atom-smashers and has
surpassed the heat of the sun in his nuclear bombs. Can we doubt that next will
come the "trevatron," and then the "quadrevatron," and then - who knows?
"After Class" Topics
Posted December 30, 2019(original 6/20/2012)