1 Accelerators, Colliders and Their Application
3
1.2 Types and Evolution of Accelerators
The development of accelerators to ever higher energy is marked by a number of
milestones. Each of these marks the invention of a new type of accelerator or the
invention of a new principle of transverse or longitudinal focusing which enables
a higher energy to be reached for a lower unit cost. The best way to describe
this evolution and introduce the different types of accelerator is to follow the road
charted by these milestones. Each is described in one of the sections which follow.
1.2.1 Early Accelerators
The nineteenth century had produced a number of electrostatic high-voltage generators. They were unpredictable in performance and electrical breakdown became
a serious problem above a few tens of kV. Early accelerators were simply two
electrodes enclosed in an evacuated tube with external connections to such high
voltage source. A proton or electron source close to one electrode at a potential of V
(or −V for electrons) provided the particles which were then accelerated towards the
second electrode at earth potential. They emerged or were observed through a small
hole in the earthed electrode. The energy acquired by each particle with charge,
e Coulombs, was just e ∗ V Joules or, in the units commonly used for accelerated
beams, V electron-Volts. An electron Volt is then just 1.6 × 10 −19 Joules. If the
particle is a fully stripped ion of an atom with atomic number A and charge Z then
the energy is ZV/A electron Volts per nucleon.
The first high-voltage generator to approach 1 MeV was built by Cockcroft
and Walton [1–3] in the 1930s to accelerate particles for their fission experiments.
Their combination of diodes and capacitors, also known as rectifier circuit, is still
used today to apply high voltage to the ion or proton source at the beginning of
many linacs and synchrotrons although these are gradually being replaced by radio
frequency quadrupoles.
The early 1930s also saw the invention by R.J. Van der Graaf [4] of an
electrostatic generator which used a moving belt to carry charge into the high
voltage terminal until it reaches a potential of several MV (Fig. 1.1). Van der
Graaf accelerators have proved a useful source of low energy particles to this
day but are inevitably limited by problems of voltage breakdown. Voltages up
to 27 MV have been reached, putting the device in a discharge suppressing gas
atmosphere (e.g. SF 6 ). Although it is possible in theory to chain together several
electrostatic accelerators, each with its cathode connected to the anode of the next,
each stage increases the potential between the ends of the device and between the
ends and ground and eventually electrical breakdown discharges the high voltage
terminals.
3
1.2 Types and Evolution of Accelerators
The development of accelerators to ever higher energy is marked by a number of
milestones. Each of these marks the invention of a new type of accelerator or the
invention of a new principle of transverse or longitudinal focusing which enables
a higher energy to be reached for a lower unit cost. The best way to describe
this evolution and introduce the different types of accelerator is to follow the road
charted by these milestones. Each is described in one of the sections which follow.
1.2.1 Early Accelerators
The nineteenth century had produced a number of electrostatic high-voltage generators. They were unpredictable in performance and electrical breakdown became
a serious problem above a few tens of kV. Early accelerators were simply two
electrodes enclosed in an evacuated tube with external connections to such high
voltage source. A proton or electron source close to one electrode at a potential of V
(or −V for electrons) provided the particles which were then accelerated towards the
second electrode at earth potential. They emerged or were observed through a small
hole in the earthed electrode. The energy acquired by each particle with charge,
e Coulombs, was just e ∗ V Joules or, in the units commonly used for accelerated
beams, V electron-Volts. An electron Volt is then just 1.6 × 10 −19 Joules. If the
particle is a fully stripped ion of an atom with atomic number A and charge Z then
the energy is ZV/A electron Volts per nucleon.
The first high-voltage generator to approach 1 MeV was built by Cockcroft
and Walton [1–3] in the 1930s to accelerate particles for their fission experiments.
Their combination of diodes and capacitors, also known as rectifier circuit, is still
used today to apply high voltage to the ion or proton source at the beginning of
many linacs and synchrotrons although these are gradually being replaced by radio
frequency quadrupoles.
The early 1930s also saw the invention by R.J. Van der Graaf [4] of an
electrostatic generator which used a moving belt to carry charge into the high
voltage terminal until it reaches a potential of several MV (Fig. 1.1). Van der
Graaf accelerators have proved a useful source of low energy particles to this
day but are inevitably limited by problems of voltage breakdown. Voltages up
to 27 MV have been reached, putting the device in a discharge suppressing gas
atmosphere (e.g. SF 6 ). Although it is possible in theory to chain together several
electrostatic accelerators, each with its cathode connected to the anode of the next,
each stage increases the potential between the ends of the device and between the
ends and ground and eventually electrical breakdown discharges the high voltage
terminals.