6
E. Wilson and B. J. Holzer
As alternative concept, circular machines, like cyclotrons and synchrotrons use
the same set of accelerating cavities over and over again as the particles make
complete turns around the accelerator, being guided and focused by the magnet
structure of the ring which is thus constraining their orbit. On each turn an increment
of energy is added and, once accelerated, particles may be allowed to circulate
indefinitely at their top energy. Two circulating beams of say protons and antiprotons
or electrons and positrons can be sustained in the same ring and, colliding at
experiments around the circumference, create new particles up to a mass (centre
of mass energy as it is called) that is the sum of the two energies. Colliders are
today the preferred configuration for a high-energy machine. Earlier, new particles
were sought in the debris from a particles collision with a nucleon in a fixed target
but such collisions are limited to a smaller centre of mass energy—which rises only
as the square root of the accelerated beam energy.
1.2.4 Linear Accelerators
Although disappointed by the rejection of his ray-transformer as a subject for his
PhD, Wideröe was led to the idea of a linear accelerator by a paper by G. Ising
[7] who tried to overcome the voltage breakdown problem of a single stage of
acceleration by placing a series of hollow cylindrical electrodes one after another
in a straight line to form what today we would call a ‘drift tube linac’ or linear
accelerator. Wideröe realised that an oscillating potential applied to one drift tube
flanked by two others which are earthed, accelerates at both gaps provided the
oscillator’s phase changes by 180 ◦ during the flight time between gaps.
In 1927 he built a three-tube model which accelerated sodium ions. At the
wavelengths that radio transmitters generated at that time a particle travelling near
the velocity of light would travel hundreds of meters in the time it would take
for the r.f. to swing by half a sine wave. This would make the length of a drift
tube impractically large. Sodium ions, being rather heavy compared with protons or
electrons, travelled much slower than the velocity of light and this helped keep the
apparatus down to table-top proportions. Although he realised that one might extend
such a series of tubes indefinitely he did not take the idea any further as he was due to
start his professional employment designing high voltage circuit breakers. Between
1931 and 1934, D. Sloan and E.O. Lawrence at Berkeley took up Wideröe’s idea
and constructed linacs with as many as 30 drift tubes to accelerate mercury ions but,
these were never used for research.
Much later, in the mid-1940s, and when suitable high-power high-frequency
oscillators had become available to meet the needs of war-time radar, L.W. Alvarez
(1946) started to build the first serious proton linac at the Radiation Laboratory of
the University of California. Figure 1.3 shows an Alvarez linac. A series of drift
tubes are mounted within a copper-lined cylinder excited by a radio transmitter. As
in Wideröe’s linac, particles gain energy from the accelerating potential differences
between the ends of the drift tube, but now the phase shift between drift tube
E. Wilson and B. J. Holzer
As alternative concept, circular machines, like cyclotrons and synchrotrons use
the same set of accelerating cavities over and over again as the particles make
complete turns around the accelerator, being guided and focused by the magnet
structure of the ring which is thus constraining their orbit. On each turn an increment
of energy is added and, once accelerated, particles may be allowed to circulate
indefinitely at their top energy. Two circulating beams of say protons and antiprotons
or electrons and positrons can be sustained in the same ring and, colliding at
experiments around the circumference, create new particles up to a mass (centre
of mass energy as it is called) that is the sum of the two energies. Colliders are
today the preferred configuration for a high-energy machine. Earlier, new particles
were sought in the debris from a particles collision with a nucleon in a fixed target
but such collisions are limited to a smaller centre of mass energy—which rises only
as the square root of the accelerated beam energy.
1.2.4 Linear Accelerators
Although disappointed by the rejection of his ray-transformer as a subject for his
PhD, Wideröe was led to the idea of a linear accelerator by a paper by G. Ising
[7] who tried to overcome the voltage breakdown problem of a single stage of
acceleration by placing a series of hollow cylindrical electrodes one after another
in a straight line to form what today we would call a ‘drift tube linac’ or linear
accelerator. Wideröe realised that an oscillating potential applied to one drift tube
flanked by two others which are earthed, accelerates at both gaps provided the
oscillator’s phase changes by 180 ◦ during the flight time between gaps.
In 1927 he built a three-tube model which accelerated sodium ions. At the
wavelengths that radio transmitters generated at that time a particle travelling near
the velocity of light would travel hundreds of meters in the time it would take
for the r.f. to swing by half a sine wave. This would make the length of a drift
tube impractically large. Sodium ions, being rather heavy compared with protons or
electrons, travelled much slower than the velocity of light and this helped keep the
apparatus down to table-top proportions. Although he realised that one might extend
such a series of tubes indefinitely he did not take the idea any further as he was due to
start his professional employment designing high voltage circuit breakers. Between
1931 and 1934, D. Sloan and E.O. Lawrence at Berkeley took up Wideröe’s idea
and constructed linacs with as many as 30 drift tubes to accelerate mercury ions but,
these were never used for research.
Much later, in the mid-1940s, and when suitable high-power high-frequency
oscillators had become available to meet the needs of war-time radar, L.W. Alvarez
(1946) started to build the first serious proton linac at the Radiation Laboratory of
the University of California. Figure 1.3 shows an Alvarez linac. A series of drift
tubes are mounted within a copper-lined cylinder excited by a radio transmitter. As
in Wideröe’s linac, particles gain energy from the accelerating potential differences
between the ends of the drift tube, but now the phase shift between drift tube