of Cockcroft and Walton's voltage multiplier. Opening and closing the
switches S transfers charge from capacitor K3 through the capacitors X
up to K1.
John D. Cockcroft
and Ernest Walton at the Cavendish Laboratory in Cambridge,
England, sought a way into the nucleus through a prediction of quantum
mechanics. George Gamow had suggested that a particle with too little
energy to overcome the electrical repulsion of the nucleus through the
barrier. (The trick was that the energy of the particle was not actually
well-defined, according to Heisenberg's Uncertainty Principle). In 1930
Cockcroft and Walton used a 200-kilovolt transformer to accelerate protons
down a straight discharge tube, but they concluded that Gamow's tunnelling
did not work and decided to seek higher energies.
penetrate the nucleus, Cockcroft and Walton built a voltage multiplier
that used an intricate stack of capacitors connected by rectifying diodes
as switches. By opening and closing switches in proper sequence they
could build up a potential of 800 kilovolts from a transformer of 200
kilovolts. They used the potential to accelerate protons down an evacuated
tube eight feet long. In 1932 they put a lithium target at the end of
the tube and found that protons disintegrated a lithium nucleus into
two alpha particles. A Soviet team in Kharkov found the same result
several months later.
Ernest Rutherford, and E.T.S. Walton.
Van de Graaff.
Van de Graaff generator.
Van de Graaff Generator
Robert Van de Graaff
worked as an engineer for the Alabama Power Company before obtaining
his Ph.D. in physics at Oxford. While a postdoctoral fellow at Princeton
he conceived a device to build up a high voltage using simple principles
of electrostatics. A belt of insulating material carries electricity
from a point source to a large insulated spherical conductor. Another
belt likewise delivers electricity of the opposite charge to another
sphere. The spheres build up a potential until the electric field breaks
down the air and a huge spark "arcs" across. By 1931 Van de Graaff could
charge a sphere to 750 kilovolts, giving 1.5 megavolts differences between
two oppositely charged spheres.
the radius of the spheres, Van de Graaff could reach higher voltages
without arcing. The maximum voltage in theory, in megavolts, roughly
equalled the radius of the sphere in feet. He was soon planning a pair
of spheres 15 feet across.
notes on Wideröe's paper.
The difficulties of
maintaining high voltages led several physicists to propose
accelerating particles by using a lower voltage more than once. Lawrence
learned of one such scheme in the spring of 1929, while browsing through
an issue of Archiv für Elektrotechnik, a German journal
for electrical engineers. Lawrence read German only with great difficulty,
but he was rewarded for his diligence: he found an article by a Norwegian
engineer, Rolf Wideröe, the title of which he could translate
as "On a new principle for the production of higher voltages." The
diagrams explained the principle and Lawrence skipped the text.
Rolf Wideröe's diagrams describing a method for accelerating ions
inspired Ernest Lawrence's invention of the cyclotron.
with a positive electric charge are drawn into the first cylindrical
electrode by a negative potential; by the time they emerge from the
tube the potential has switched to positive, which propels them away
from the electrode with a second boost. Adding gaps and electrodes can
extend the scheme to higher energies.
elaborating a scheme proposed earlier by Gustav Ising in Sweden,
sought to use a low potential over and over to accelerate atoms to high
energies. In his design a potential of 25,000 volts alternated from positive
to negative at radio frequencies. Ions were pulled into a straight cylindrical
electrode by a negative potential and then pushed from the other end by
a positive potential. One could add more cylinders, each longer than the
last to accommodate the increasing speed of the particles, to reach higher
as a young man.
David Sloan working
in the laboratory.
soon brought David Sloan
to Berkeley. He was a young expert in electronics from the General Electric
Laboratories in Schenectady with experience in handling high voltages.
While Lawrence was building the cyclotron, Sloan pursued Wideröe's
linear accelerator. Sloan's device eventually had a series of thirty electrodes.
By May 1931 it accelerated mercury ions to energies of a million volts.
This work gave Lawrence and his students experience with oscillators and
beam focusing, knowledge they would later apply to cyclotrons. Sloan,
however, put the linear accelerator aside to develop a resonant transformer,
which turned out to provide a powerful source of X-rays which was of great
interest to hospitals.
The linear accelerator
proved useful for heavy ions like mercury, but lighter projectiles (such
as alpha particles) required a vacuum tube many meters long. Lawrence
judged that impractical. Instead he thought of bending the particles
into a circular path, using a magnetic field, in order to send them
through the same electrode repeatedly.
A few quick calculations showed that such a device might capitalize
on the laws of electrodynamics. The centripetal acceleration of a charged
particle in a perpendicular magnetic field B is evB/c, where e is the
charge, v the particle's velocity, and c the velocity of light. The
mechanical centrifugal force on the particle is mv2/r, where m is the
mass and r the radius of its orbit. Balancing the two forces for a stable
orbit yields what is now known as the cyclotron equation: v/r = eB/mc.
Lawrence was surprised to find that the frequency of rotation of
a particle is independent of the radius of the orbit: f = v/2 r
with r disappearing from the equation. The circular method would thus
allow an electric field alternating at a constant frequency to kick
particles to ever higher energies. As their velocities increased so
did the radius of their orbit. Each rotation would take the same amount
of time, keeping the particles in step with the alternating field as
they spiralled outward.
electric field with a frequency of about four million cycles
per second lay in the realm of short radio waves. Lawrence's experience
with these waves would come in handy, and recent advances in high-power
vacuum-tube oscillators would be indispensable. Combined with a reasonable
magnetic field, a potential on the electrodes of only ten thousand volts
could accelerate an alpha particle to one million electron volts. Bigger
magnets promised higher energies. In theory, the scheme offered the
long-sought route to study the nucleus. Lawrence pressed students and
professors to confirm his calculations and sketched out a device.
metal half-cyclinders, later called "dees" after their shape, serve as electrodes;
charged particles injected into the gap near the center are pulled by the
potential into the electrode A; the magnetic field, perpendicular to the
plane of the cylinders, bends them in a semicircle back into the gap; in
the meantime the electric field has reversed and can pull them into electrode
B; whence they emerge again in step with the electric field; and so on,
eventually spiraling out to the edge. Each passage through the gap boosts
the particles to higher energies.
the plan into practice, however, meant facing daunting obstacles.
It required vacuum seals that could withstand the stresses of the alternating
electric field and the magnet. Too poor a vacuum and the circulating particles
might be bumped from their paths by air molecules. The particles might
also go astray crossing the gap or, what would be the hardest problem,
deviate from the horizontal plane of their orbits and crash into the floor
or ceiling of the electrodes.
successful cyclotron, the 4.5-inch model built by Lawrence and Livingston.
these obstacles with the help of two cut-and-try discoveries. The first
was to remove a metal grid from the entrance to the dees, which he had
thought necessary for electrical shielding. Sloan's work had demonstrated
that instead it interfered with focusing by the electric field that kept
particles in the horizontal plane. The second trick was to insert small
iron shims into the magnetic field to coax particles back into the orbital
plane. A bit of tinkering gave Lawrence his million-volt projectiles.
An early sketch
from Lawrence's notebook of a shim for the 11-inch cyclotron.
American Institute of Physics