voltages, which switch between positive and negative values.
The frequency of the alternating voltage is matched with the
length of the cylinders, so that the particles always feel a kick,
not a brake, as they emerge into a gap. In this way, the particles
are accelerated every time they cross between one cylinder
and the next. This is the basis of the operation of the modern
linear accelerators. Usually such ‘linacs’ are short, low-energy
machines, but they can be high energy and lengthy, as at the
Stanford Linear Accelerator in California. They are most
commonly used in the preliminary stages of acceleration at
today’s big rings.
The idea of creating a ring-shaped accelerator originated with
Ernest Lawrence, who used a magnetic field to bend the particles
into a circular orbit. Two hollow semi-circular metal cavities, or
‘Ds’, were placed facing one another to form a circle, with a small
gap between the two flat faces of the Ds. The whole construction
was only about 20 cm across, and Lawrence placed it between the
circular north and south poles of an electromagnet, to swing the
particles round the curve, while an electric field in the gap speeds
them. After being accelerated by the electric field in the gap, they
curved round in a circular path until they met the gap half an
orbit later. By this device they could pass across the same
accelerating gap many times, rather than travel through a
succession of gaps. They spiral outwards as their speed increases,
but the time intervals between successive crossings of the
gaps remain constant.
To accelerate the particles continuously, the electric field in the gap
must switch back and forth at the same frequency with which the
particles complete the circuit. Then particles issuing from a source
at the centre of the whirling device would spiral out to the edge and
emerge with a greatly increased energy.
This device was known as a ‘cyclotron’, and worked on the principle
that the particles always take the same time to complete a circuit.
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Accelerators: cosmic and man-made