Particle Accelerators

I

Introduction

Particle Accelerators, devices used to accelerate charged elementary particles or ions to high energies. Particle accelerators are the largest and most expensive instruments used by physicists. They all have the same three basic parts: a source of elementary particles or ions, a tube pumped to a high vacuum in which the particles can travel freely, and some means of speeding up the particles.

Charged particles can be accelerated by an electrostatic field. For example, by placing electrodes with a large potential difference at each end of an evacuated tube, the British scientists John D. Cockcroft and Ernest Walton were able to accelerate protons to 250,000 electronvolts. (1 electronvolt, or 1 eV, is the energy acquired by a particle with 1 unit charge—for example, an electron—when accelerated by a potential difference of 1 volt.) Another electrostatic accelerator is the Van de Graaff accelerator, which was developed in the early 1930s by the American physicist Robert Jemison Van de Graaff. This accelerator uses the same principles as the Van de Graaff generator, building up a potential between two electrodes by transporting charges on a moving belt. Modern Van de Graaff accelerators can accelerate particles to energies as high as 15 MeV (15 million electronvolts).

II

Linac

The linear accelerator, or linac, was first conceived in the late 1920s. It uses large alternating voltages to push particles along in a straight line. Particles pass through a line of hollow metal tubes enclosed in an evacuated cylinder. Within a hollow conductor there is no electric field (see Electricity), so a charged particle travels at constant speed inside each of the tubes. Between one tube and the next there is a potential difference which varies in size and direction as an AC voltage is applied to the series of tubes. Bunches of charged particles are accelerated from tube to tube, moving with the voltage wave as it travels along the linac.

Theoretically, a linac of any energy can be built. The largest linac in the world, at Stanford University, is 3.2 km (2 mi) long. It is capable of accelerating electrons to an energy of 50 GeV (50 gigaelectronvolts, or 50 billion eV). Stanford’s linac is designed to collide two beams of particles, accelerated in turn by the linac and temporarily kept in storage rings (see Storage Ring Colliders, below).

III

Cyclotron

The American physicist Ernest O. Lawrence won the 1939 Nobel Prize for Physics for a breakthrough in accelerator design in the early 1930s. He developed the cyclotron, the first circular accelerator. A cyclotron is somewhat like a linac wrapped into a tight spiral. Instead of many tubes, the machine has only two hollow vacuum chambers, called dees, that are shaped like capital letter Ds back to back (thus: D). A magnetic field, produced by a powerful electromagnet, keeps the particles moving in a curved path. The potential difference between the dees constantly alternates in direction, so that every time the particles reach the gap they experience a forward acceleration. Within each dee the particles travel at constant speed during each half-revolution. As the particles gain energy, they spiral out towards the edge of the accelerator, where finally they exit.

When particles in a cyclotron approach the speed of light, they become appreciably more massive, as predicted by the theory of relativity. This makes it harder to accelerate them and throws the acceleration pulses at the gaps between the dees out of phase. A solution to this problem was suggested in 1945 by the Soviet physicist Vladimir I. Veksler and the American physicist Edwin M. McMillan. The solution, the synchrocyclotron, is sometimes called the frequency-modulated cyclotron. In this instrument, the oscillator (radio-frequency generator) that accelerates the particles around the dees is automatically adjusted to stay in step with the accelerated particles; as the particles gain mass, the frequency of accelerations is lowered slightly to keep in step with them. As the maximum energy of a synchrocyclotron increases, so must its size, for the particles must have more space in which to spiral. The largest synchrocyclotron is the 600-cm (236-in) phasotron at the Dubna Joint Institute for Nuclear Research in Russia; it accelerates protons to more than 700 MeV and has magnets weighing 6,984 tonnes.

The world’s most powerful cyclotron, the K1200, began operating in 1988 at the National Superconducting Cyclotron Laboratory at Michigan State University. The machine is capable of accelerating nuclei to an energy approaching 8 GeV.

IV

Betatron

When electrons are accelerated, they undergo a large increase in mass at a relatively low energy. At 1 MeV energy, an electron has three times as much mass as an electron at rest. Synchrocyclotrons cannot be adapted to make allowance for such large increases in mass. Therefore, another type of cyclic accelerator, the betatron, is employed to accelerate electrons. The betatron consists of a doughnut-shaped evacuated chamber placed between the poles of an electromagnet. The electrons are kept in a circular path by a magnetic field called a guide field. An alternating current is applied to the electromagnet, and the electromotive force induced by the changing magnetic flux through the circular orbit accelerates the electrons. During operation, both the guide field and the magnetic flux are varied to keep the radius of the orbit of the electrons constant.