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 of 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 Nucleonics
The Particle Accelerators
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.
Fig. 1. Fundamental cyclotron construction. Top view above, side view below.
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 obtained.
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.
Synchrocyclotron. 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.
Fig. 2. The synchrocyclotron. One dee is used instead of the two in the cyclotron.
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 flat!
Columbia University's huge synchrocyclotron produces alpha particle energies of over
400 million electron volts.
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 much greater.)
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 was required.
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.
Fig. 3. The betatron. Note how the pole pieces go into the center of a vacuum "doughnut."
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.
Fig. 4. Plan view of the Berkeley bevatron. Brookhaven's cosmotron follows a similar
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
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?
The cosmotron at Brookhaven. Concrete blocks in the foreground serve as shield; large
"doughnut" is the magnet.
Posted June 20, 2012