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March 1946 Radio-Craft
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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Betatron particle
accelerators date back to 1935 with the one built by Max Steenbeck in Germany.
The name is a portmanteau of "beta" + "electron," which is sort of a superfluous
redundancy. This news piece is about the world's biggest betatron having been
built, with dimensions of 9 feet high, 6 feet wide and 15 feet long, and 24,000
volt energizing coils. Strangely (it seems to me), the article interchanges the
terms "xxx-volt electrons" and "xxx electron-volts. I suppose its fundamentally
the same thing, but just unusual to see it that way. Note the robustness of the
machine as required to rigidly contain such powerful magnetic forces.
According to Wikipedia, an alternate name suggested for the betatron was
"Außerordentlichehochgeschwindigkeitselektronenentwickelndesschwerarbeitsbeigollitron,"
meaning "Hard working by golly machine for generating extraordinarily high velocity
electrons." (see
Google translation)
World's Biggest Betatron
Biggest of its kind is the 100,000,000 volt betatron or electron
accelerator constructed at the General Electric Research Laboratory in Schenectady,
N.Y.
The new machine gives out X-rays of a power never previously approached. These
will penetrate a thickness of metal considerably greater than the rays from G-E's
2,000,000-volt industrial X-ray unit. But even more exciting are the possibilities
that with the 100,000,000-volt electron stream that produces X-rays of the same
energy we can produce other interesting forms of radiation.
Located in a special building with concrete walls three feet thick, as a protection
from the dangerous rays given off, the principal part of the betatron is a huge
electromagnet, made of 130 tons of laminated silicon steel. It is 9 feet high, 6
feet wide and 15 feet long. In a rectangular opening passing through the magnet
from front to back are the pole faces, 76 inches in diameter, surrounded by large
coils of insulated 1-inch copper conductor. As electricity at 24,000 volts surges
through these coils from a bank of condensers in an upstairs room, the magnet is
energized, the intense magnetic field being concentrated in the horizontal space
between the pole faces.
Here is the heart of the machine - a doughnut-shaped vacuum tube of glass. The
doughnut has an over-all diameter of 74 inches while the tube itself, of elliptical
cross section, measures inside 8 inches horizontally and nearly 5 inches vertically.
It is made of 16 sectors of molded and tempered pyrex glass, cemented together.
The inner surface of this tube had to be made electrically conducting, so that it
would not accumulate a charge that would upset the paths of the electrons within.
This was accomplished by sandblasting the inner surfaces and then silvering them.
Projecting into the doughnut at one point is an electron gun, consisting of a
heated filament from which electrons are boiled off, to be given an initial impulse
of several thousand volts to start them in their orbits inside the doughnut. The
magnetic field holds them in a fixed circular orbit as they gain speed and energy
on successive revolutions, gaining about 400 electron volts each trip.
The machine operates on ordinary 60-cycle alternating current. Acceleration of
the electrons is confined to the first quarter of each cycle, lasting 1/240th of
a second during which the current goes from zero to its maximum in one direction.
Then it goes back to zero, before building up in the opposite direction. If the
electrons were allowed to remain in the tube during the second quarter cycle they
would be slowed down again, so they are removed before this happens.
Just as the end of the quarter cycle is reached, a pulse of current passes through
two smaller auxiliary coils on the pole faces. This causes the electrons to spiral
away from their orbit and to hit a tungsten target which they previously missed.
This causes the generation of X-rays, which emerge from the doughnut in a beam which
is 2 degrees in diameter when the machine is operated at full power.
It may, however, be operated at a lower power, as the pulse may be applied at
any time during the first quarter cycle. If, for example, it is done when they have
made only 125,000 instead of the full 250,000 revolutions, they will have energies
of only 50,000,000 electron volts. Or they may be taken out at any other stage so
the device can produce X-rays from about 2,000,000 up to 100,000,000 volts. On one
of the control panels is a unique instrument - a megavolt meter. A megavolt is a
million volts, and the dial of this meter is graduated from one to 100 of these
units. When the machine is operating at full power, this indicates 100,000,000 volts
as casually as a small voltmeter might show 5 or 6 volts when used to test an automobile
battery.
A view of the world-beating betatron. The giant magnet coils protrude slightly
from the front.
Posted May 19, 2021
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