IOP Institute of Physics

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Accelerated dreams

21 May 2012

The lecture took us on an historical journey where particle accelerators entered the stage of human enterprise.

Photo of members

Accelerators are devices that propel charged particles to high speeds whilst containing the particles in well-defined beams. The technology has multiple uses, from the fields of biomedical research and therapeutics, to the elucidation of the fundamentals of the universe and for aircraft industry, such as wing design. 

Accelerators consist of three basic components: a source of elementary particles, a tube pumped to a partial vacuum in which the particles can travel and a means of speeding up the particles.

Many scientific innovations and technological appliances for domestic usage appeared in the 1920's. Amongst them was the electrostatic van de Graaf accelerator. 

Electrostatic accelerators use static electric fields to accelerate particles. The accelerator builds up a potential between two electrodes by transporting charges on a moving belt. 

The conceivably kinetic energy for particles in these devices is limited by electrical breakdown. The can accelerate particles to energies superior to 10 million MeV. 

Today, these instruments are used for nuclear physics and for radio-carbon dating (i.e. Turin Shroud and the Skeleton Lake in India). Electrostatic accelerators also have their uses in computers (Intel) where ions are implanted into the silicon. 

A small scale example of this class of accelerator is the cathode x-ray tube in an ordinary old television set.

Another type of accelerator entered the stage: oscillating field accelerators using radio-frequency electromagnetic fields (thus circumventing the breakdown challenge). 

Rolf Widerøe invented the Linear accelerator (LINAC). It uses alternating voltages of high magnitudes to propel particles along in a straight line. The particles pass through a line of hollow metal tubes enclosed in an evacuated cylinder. 

An alternate voltage is timed so that a particle is pushed forward each time it goes through a gap between two of the metal tubes. Today's largest LINAC is at Stanford university and is over 3Km long. 

Linac's uses are predominantly for radiotherapy, thus displacing the obsolete Cobalt-60 therapy. The electrons can be used directly or can be collided with a target to produce X-rays. (Hence the additional name of 'atom smashers' for particle accelerators).

Ernest O Lawrence (and Stanley, his PhD student) conceived and developed the cyclotron in the 1930's (Lawrence was awarded the Nobel Prize in 1939). 

This was the first circular accelerator. In these, a magnetic field, produced by a powerful magnet, keeps the particles in circular motion. Instead of tubes, the machine has two hollow vacuum chambers. 

The advantage of cyclotrons over linear accelerators is that the toroid topology facilitates continuous acceleration, as the particle can transit indefinitely.

But there is more. Synchrotrons, such as the Large Hadron Collider (LHC) in Geneva which is 27Km long, are capable of reaching much higher energies. 

The particles are accelerated in a ring of constant radius. Instead of a giant magnet, they have a line of hundreds of bending magnets, enclosing -or enclosed by- vacuum connecting pipes. LHC is actually an accelerator complex.

The pioneering EMMA (Electron Model For Many Applications) project is to build a non-scaling accelerator that suggests a range of potential applications including charged particle cancer therapy, accelerator driven reactors and particle physics. Its technology is more compact, cost effective and operationally simpler. It consists of a ring of magnets which use their combined magnetic field simultaneously to steer and focus the electron beam around the machine. 

The strength of this magnetic field increases steeply as the beam spirals outwards while it is accelerated to 20 million electron volts around the ring. Due to the strength of the magnetic focussing, the displacement of the beam as it accelerates and spirals around the ring is much smaller than in any equivalent accelerator. As a result, EMMA's ring of magnets is much more compact and is easier to accelerate the beam.

The lecture drew to a close with insightful suggestions on the exciting concept and potential use of Thorium nuclear reactors (instead of Uranium) because of their safety, availability and cost-effectiveness. 

This was followed by a chain of interesting and thought provoking questions form the audience.

Acceleration in the linear-nonscaling fixed-field gradient accelerator EMMA: S. Machida et al, Nature Physics 8, 243-247 (2012).

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