A BRIEF HISTORICAL REVIEW OF THE INVENTION OF THE CYCLOTRON
Fig.1 Young Prof. Ernest O. Lawrence in about 1929.
In 1929, Dr. Ernest Lawrence came across a German scientific paper, written by R. Wideroe, that
described "Kinetic voltage transformation". The apparatus used amounted
to a glass tube with an ion source, a small tubular electrode connected
to an oscillator, and a deflection plate. This small apparatus proved
that RF fields could be used to accelerate particles. Until this time,
this was done using a constant high voltage. Lawrence realized that to get
acceleration matching that of the current high voltage machines the array
of electrodes would be quite long. He theorized that a single electrode
could be used if the ion beam was bent into a circular path using a
Fig.2 Wideroe's drawing that inspired Lawernce's cyclotron
Fig.3 Lawrence's 1st concept of the cyclotron.
In 1930, a doctoral student began work on helping Lawrence prove
his theory. Using a 4" magnet and a small vacuum chamber he began his
work. The chamber was a thin brass ring capped on either end with disks
sealed with red sealing wax. Inside the chamber was a hollow D shaped
electrode, a slotted bar across the diameter, a filament, and a collector
Fig.4 The first cyclotron - only 4 inches in diameter.
When hydrogen was introduced into the vacuum chamber, it was ionized by
the electrons emitted by the hot filament. The voltage on the electrode
accelerated these ions and as the energy became greater they spiraled
outward and into a collector cup. This cup was attached to an
electrometer, which measured the current induced by the ion beam striking
it. This current was proof that Lawrence's theory was correct. Ions could
be accelerated in a circular path.
The next year, a larger, 11 inch magnet was built. A new chamber and
oscillator were also implemented. By 1932 protons were accelerated to
over 1 million volts with this machine. This was the first time this had
been accomplished in the history of physics. By this time Lawrence had
already been working on construction of an even larger magnet. This
became the 27-inch cyclotron.
Fig.5 M.S. Livingston, E.O. Lawrence, and the 27-inch cyclotron.
By 1945 it was discovered that these machines were limited by relativity.
This means that as the particle gains more energy it gains mass. Because
the RF voltage across the electrodes is constant the effect of the kick
is less on the more massive ions. This results in the ions being out of
phase with the RF cycle; no more acceleration. A 184" cyclotron was in
the process of being built. This was converted into a frequency modulated cyclotron. In this machine the RF frequency is decreased slightly
as the orbit time of the relativistic particle changes, keeping it in
synchronization. This was the end of the cyclotron era and the beginning of the syncro-cyclotrons and syncrotrons - the big machines of today.
This page was written by Christopher Olsen of Fermilab
A lot of historical cyclotron photographs can be seen at our Cyclotron History Photo Album page.
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