Big Complicated Machines

Big, Complicated Machines #13 – Intro to Particle Accelerators

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Beamlines leading from the Van de Graaf accelerator to various experiments, in the basement of the Campus Universitaire de Jussieu in Paris. Image:Wikipedia

Good morning everyone.

Today we’re going to talk about particle accelerators, fascinating machines that have greatly expanded our knowledge of what matter is, and also of how the universe came into being. There are a great number of particle accelerators in use today, the newest and largest of which is the Large Hadron Collider that is located on the border of Switzerland and France, and operated by CERN. The LHC is the latest iteration in a long series of machines made to study the atomic structure of matter, with the first machines being made in the 1920s.

All particle accelerators use electric fields for the acceleration of charged particles to high energies and the confining of those particles into a defined and controlled beam. The beam is directed at various types of targets and materials and the interaction of the beam and the target matter is studied, typically by examining the tiny pieces of the atoms in the target that go flying off in all directions. Particle accelerators were called atom smashers for a long time for this reason. The manner in which the electric fields are applied varies widely, but particle accelerators fall into two basic categories, electrostatic accelerators and oscillating field accelerators, also known as radio frequency (RF) accelerators.

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A Cockcroft-Walton generator. Image: Wikipedia

Electrostatic accelerators

Electrostatic accelerators use static electrical fields to accelerate particles. A mundane example would be the once common cathode ray tubes (CRTs) that were used in televisions and computer monitors and are still used in some specialized tools such as oscilloscopes, and developed energies of up to a few tens of kV. Although electrostatic accelerators accelerate particles in a straight line, they are not considered to be linear accelerators. The term linear accelerator is more typically associated with accelerators that use oscillating rather than static electric fields.

Cockcroft-Walton Accelerators

One of the earliest particle accelerators was powered by a Cockcroft-Walton generator (above) and was capable of building up a potential of up to 800 kV from a transformer of 200kV. The potential was used to accelerate protons down an evacuated tube eight feet long at a lithium target.These accelerators broke new ground in nuclear physics by performing the first nuclear disintegration in history, and were used in the development of the first atomic bomb. A simple diagram of a Cockcroft-Walton accelerator is shown below.

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Image: Lawrence Berkeley Labs

Van de Graaf Accelerators
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A powered up Van de Graaf generator. Image: Lawrence Berkeley Labs

Another early high voltage power source was Van de Graaf generators (above), a common sight in physics classrooms and in the laboratories of movie mad scientists. In the 1930s, Van de Graaf generators capable of producing voltages of 1MV were built. As research progressed, some of the generators used tandem domes and were so large that the experimenters located their laboratories inside each of the domes, and the experiments took place entirely inside the domes. A diagram of a simple Van de Graaf accelerator is shown below.

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A Van de Graaf accelerator. Image: Encyclopedia Brittanica

More recently, tandem Van de Graaf generators have been integrated with particle accelerators and encased in a pressure vessel containing sulfur hexafluoride to prevent arcing. Voltages of over 20 MV have been produced by these types of machines. The main limiting factors of the electrostatic accelerators arises from the maximum practical potential difference that can be held by the charged surfaces and electrical breakdown.

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The Extended Stretched TransUranium (ESTU) tandem accelerator at the A.W. Wright Nuclear Structure Laboratory at Yale. Image: Yale Scientific Magazine

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Diagram of the operation of a tandem Van de Graaf accelerator. Image: Encyclopedia Brittanica

Oscillating Field Accelerators

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The first working cyclotron. Image: NBER-SEWP

Cyclotrons

One of the earliest oscillating field (OF) accelerators is the cyclotron, invented by Leó Szilárd (an insufferable man) and manufactured and patented by E.O. Lawrence (feh) of UC Berkeley. The first working version of the device that he made with a GS of his, Stanley Livingston, is 5 inches in diameter and is pictured above, and an operational diagram is below. It accelerates particles by oscillating the voltage at RF frequencies in a static magnetic field. The two hollow “D” shaped electrodes are separated by a gap where the electromagnetic field is concentrated, and as the particle crosses the gap, the electrode polarities are flipped and the particle gets a boost. This continues until the particle until the radius of its orbit grows to the point where the magnetic field is tweaked to kick it out at the target. I think that they (Lawrence and Livingston) were able to get 80 kEV out of the thing.

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Diagram of the first successful cyclotron constructed by Lawrence and M.S. Livingston. The single dee is five inches in diameter. Image: Lawrence Berkeley Labs

The cyclotron is limited in its output by the special theory of relativity, and this led to the development of more powerful and versatile machines.

There are many more types of OF particle accelerators, and I’ll talk about them in individual articles.

 

References

Lawrence Berkeley National Laboratory

Nuclear Science—A Guide to the Nuclear Science Wall Chart

Lawrence and his Laboratory 1931-1958

Patent diagram

Encyclopedia Brittanica

Van de Graaf particle accelerator diagram

Tandem accelerator: two-stage design diagram

Yale Scientific magazine

National Bureau of Economic Research – Science & Engineering Workforce Project

Wikipedia

Particle Accelerators

Cockcroft-Walton Generator

Van de Graaf Generator

Cyclotron

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  • jeepjeff

    I've stood in the same room as 9MV Van De Graf generators. They were massive, about the size of a tank of large fuel tank on a semi.

    What the heck is wrong with Szilárd and Lawrence?

    • The Professor

      Hmmm, Leo and and Eddie? ….oh, that. I really do need to pay more attention to my self-editing. Sigh.
      How should I put this, they were both blindingly brilliant men (usually), but like all men they had their little quirks. Szilárd and I were basically incompatible. Having a conversation with Leo meant listening to whatever he had to say without interruption, and then him walking off, still talking. I found it very annoying, especially after the 20th time. Ed was a great worker and had a stare that could leave burn marks on you. Couldn't hold his liquor though, and was a melancholy drunk who was embarrassing to be around.
      And that's enough of that.

  • i also cool my mad scientist experiments with computer case fans.

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