Nuclear

Part 12 - Accelerators

The first electron accelerator was invented, in 1874, by Sir William Crookes. In a sealed glass tube, electrons, emitted from a cathode, were attracted to an anode by the electrical potential, across the two electrodes, supplied by a low voltage battery. 

With the addition of a cylindrical anode and sets of focussing and steering magnets, this simple device was developed into the oscilloscope and the original TV tube, both of which allowed the beam of electrons to be directed onto a phosphor coated screen (while also emitting x-rays, that required heavy lead shielding). 

The energy of the electron, which passed only once through the potential difference, was limited to the direct current voltage. However, much higher voltages had been developed earlier by experimenters such as Otto von Guericke. In 1663 he had invented a primitive form of a frictional electrostatic generator using sulphur globe that could be rotated and rubbed by hand. Rubbing two non-conductive objects, like rubber, plastic, glass, and pith, can generate static electricity as electrons are transferred from one surface to another. But, two non-conductive surfaces can become charged by just being placed in contact.

Isaac Newton suggested using a glass globe instead and, about 1706, Francis Hauksbee developed a machine with glass sphere rotating rapidly against a woollen cloth.

In 1746, William Watson's machine had a large wheel turning several glass globes and, in 1783, Martin van Marum built an electrostatic machine with glass disks 1.65 metres in diameter.

Experiments with static electricity were aided by the Leyden Jar, an early form of the capacitor, and electrostatic generators became an essential research tool by the end of the 19th century.

In 1929, American physicist Robert J. Van de Graaff invented the eponymous high voltage particle accelerator to accelerate subatomic particles to greater speeds in an evacuated tube.

This used a moving insulating fabric belt to carry charge to the high voltage electrode, a hollow metal sphere at the top of an insulated column, creating a low direct current (DC) with a very high voltage charge. A large diameter, smooth hollow sphere was the most effective shape because sharp points, bends and edges carried the highest electric field and, around the spherical terminal, this could not exceeded the dielectric strength of air, otherwise arcing and corona discharge would limit the voltage. In air, Van de Graaff machines were limited to a few million volts but voltages up to 25 megavolts were possible with the generator installed inside a tank of pressurized insulating gas such as sulfur hexafluoride.

The other main type of electrostatic accelerator was invented by John Douglas Cockcroft and Ernest Thomas Sinton Walton who, in 1932, used their accelerator to split atoms of lithium into helium. It was the first artificial nuclear disintegration in history and first experimental proof of Einstein's equation, Energy = mass x the velocity of light squared. The accelerator used a diode-capacitor voltage multiplier first described by Heinrich Greinacher, a Swiss physicist in 1919. This produced a high direct current (DC) voltage from a low-voltage alternating (AC), or pulsing DC, input voltage. 

Cockcroft–Walton circuits are still used in particle accelerators and also in electronic devices that require high voltages, such as X-ray machines, microwave ovens and photocopiers. Meanwhile, in 1929, at Berkeley, California, Ernest Lawrence thought a linear accelerator would be too long to accelerate alpha particles and protons to high speeds. So he invented the cyclotron. It consisted of pair of hollow "D"-shaped brass electrodes vacuum tanks (back to back to form a circular shape) mounted between two electromagnets (to accelerate the particles) and a single large dipole magnet to bend their path into a circular orbit. 

At the centre of the two D's, a hot filament and a small stream of hydrogen gas provided a stream of protons. These were pulled and pushed by an alternating field across the two D's and as their speed increased they spiralled outward about 100 circuits until reaching a gap in one of the D's where they were directed onto the target. With 1000 volts across the D's and a 4.5 inch chamber, Lawrence initially produced protons with an energy of 80,000 volts.

In 1932, an eleven inch machine produced million volt protons and Lawrence planned another, with 80 ton magnets, capable of producing 25 million volt protons.In 1939, he built a cyclotron with a 60-inch (1.5 metre) diameter pole face. It was the most powerful accelerator in the world at the time. 

Glenn T. Seaborg and Edwin McMillan used it to discover plutonium, neptunium and many other transuranic elements and isotopes, for which they received the 1951 Nobel Prize in chemistry.

Lawrence's later design was a cyclotron with a 184-inch diameter pole face but, in 1942, these were immediate put to use for the production of 235-uranium for the first atomic bombs.

Linear high-energy accelerators use a linear array of plates to which an alternating high-energy field is applied. As the particles approach a plate they are accelerated towards it. As they pass through a hole in the plate, the polarity is switched so that the plate now repels them and they are now accelerated by it towards the next plate. Normally a stream of "bunches" of particles are accelerated, so a carefully controlled AC voltage is applied to each plate to continuously repeat this process for each bunch.

As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at radio frequencies, and so microwave cavities are used in higher energy machines instead of simple plates.

Linear accelerators are also widely used in medicine, for radiotherapy and radiosurgery. Medical grade linacs accelerate electrons using a klystron and a bending magnet arrangement which produces a beam of between 6 and 30 MeV. The electrons can be used directly or they can be collided with a tungsten target to produce a beam of X-rays or gamma rays. The higher energy of the radiation beam largely supplanted the older use of gamma radiation from the radio-isotope cobalt-60 for radiation therapy.

In high-energy accelerators, as the energy increases the particle speed approaches the speed of light. Therefore, particle physicists think in terms of a particle's energy or momentum, usually measured in electron volts (eV).

The first large proton synchrotron was the Cosmotron at Brookhaven National Laboratory, which accelerated protons to about 3 GeV (1953–1968).The Alternating Gradient Synchrotron at Brookhaven, New York, operating from 1960, was the first large synchrotron with alternating gradient, "strong focusing" magnets capable of accelerating protons to 3 billion electron volts (GeV).

Emilio Segrè and Owen Chamberlain discovered the antiproton with the Bevatron at Berkeley, in 1955. It was specifically designed to verify the particle-antiparticle symmetry of nature, then only theorized, using protons with billions of electron-volts (eV).

The Proton Synchrotron, built at CERN near Geneva, Switzerland in 1959 was the first major European particle accelerator.

The Large Electron–Positron Collider (LEP), one of the largest particle accelerators ever constructed, reached energies up to 209 GeV. It was a circular collider with a circumference of 27 kilometres built in a tunnel underground and passing through Switzerland and France.

 Around 2001 it was dismantled to make way for the Large Hadron Collider, which re-used same tunnel. To date, LEP is the most powerful accelerator of leptons ever built, achieving 6.5 TeV energy per beam (13 TeV in total).