Posted on: 21st January 2013

The Fusion Forces Roller Coaster is a demonstration from the former traveling exhibition Fusion Expo that simulates the forces at play in a fusion reaction. The aim of the demonstration is to shoot two magnetic ball bearings toward each other and get them to stick. But they only get close enough to stick if they have enough energy to make it up the hill – analagous to the energy required to overcome the electrostatic repulsion of two positive nuclei to get them close enough for the short range strong nuclear force that governs fusion to take over.

As well as helping with the demo, engineer Richard Brown shows off some of his current work on designing magnets for the fusion vessels.


Teacher resources

Host:                            Phil Dooley – EFDA (predecessor of EUROfusion)
Fusion specialist:         Richard Brown (engineer) – Culham Centre for Fusion Energy

Summary

PART 1 – Discussion and Demo

This video uses a model of the forces at play in a fusion event: electrostatic repulsion between the two positively charged nuclei, which needs to be overcome to get the nuclei close enough for fusion to occur. Once they are close enough the nuclear strong force takes over, which operates over much shorter distances.

In this demo, the strong force is modeled by the magnetism of the two ball bearings, which means they stick together if they get close enough. However they have to have enough kinetic energy to overcome the slope in the roller coaster to get them close enough. In other words the electrostatic repulsion is modeled by gravity.

For fusion to occur the nuclei must be at a very high temperature, which means they are moving very fast – exactly the same way that they overcome their repulsion in this model, quantified by how far back the spring-loaded launcher is pulled.

Note that the well ordered collisions in this demo do not reflect the reality of a fusion plasma, in which particles are flying in all directions and colliding randomly. (The demo is closer to the way that the large hadron collider operates, with a smaller number of well planned collisions between two particle collimated beams).

PART 2 – Research

The researcher Dr Richard Brown is studying forces on the electromagnets in a fusion vessel (the doughnut shaped, or toroidal, “tokamak”). Extremely large electromagnets (e.g. 25 windings each of 1 square inch cross section) are used to generate the magnetic fields that control the hot particles. The main magnets are toroidal (around the vessel) and poloidal (through the pole of the doughnut) and generate over a Tesla of field; the ones Richard shows are just external shaping magnets – also poloidal. For more info see :

https://www.euro-fusion.org/fusion/fusion-technology/magnetic-fields/

https://www.euro-fusion.org/fusion/fusion-technology/types-of-fusion-machines/tokamaks/

https://www.euro-fusion.org/fusion/spot-on-jet-operations/maintaining-the-plasma/plasma-edge/

Because the materials are in a very magnetic environment, they are subject to a lot of forces, Richard creates designs to ensure they are within tolerable limits for the materials (JET’s magnets are copper, but on newer machines superconductors)

Questions

Pre-questions (also covered by video)

What makes up the atom, and what charge is each species?   Protons (positive), neutrons (neutral), electrons (negative)

What is a nucleus?   The solid central part of an atom, protons and neutrons (takes up a tiny fraction of the volume – If an atom is a golf course, the nucleus is the hole.)

What happens if two things of like charge collide? Of opposite charge?  Repel, or attract.

So what happens when atoms collide (i.e. I sit on a chair)? The electrons on the outside of the atoms of my bottom are repelled by the electrons on the outside of atoms of the chair ( if I have no clothes on…)

What are the four forces?  Gravity, electromagnetism, strong force, weak force.

If you heat a substance, what happens to its atoms? They move faster: in a solid the atoms vibrate more but stay in a structured arrangement. Atoms in a liquid move faster, but slide around remaining in contact with each other. Gas atoms move faster as well, but only collide occasionally (how often depends on pressure)

What is the fourth state of matter, and how is it different to the other three? Plasma. It is characterized by the atoms being ionized, i.e. electrons have been removed from the atoms. Because plasma is a mixture of positively and negatively charged particles it is affected by electric and magnetic fields.

How hot is the sun? Surface around 5000 Kelvin, Core 15 million degrees Kelvin.

Post-Questions (on video content)

Why is it called nuclear fusion? Because the process involves rearranging nuclei.

What was the atom shown in the video? Helium, two each of protons, neutrons electrons

What is the average number of collisions before fusion occurs?  10 000

How hot does plasma need to be to achieve fusion?  150 million degrees, ten times hotter than the core of the sun.

Extension Questions

The fusion process turns hydrogen in helium. Compare with fission of uranium. Fission is splitting atom, to make smaller atoms. Small and very large atoms are both less stable, the most stable atoms are middle sized – most stable of all is iron.

Compare the process of burning hydrogen to fusing hydrogen. Burning is a chemical reaction requiring oxygen (oxidation), that produces H2O. Fusion is a nuclear process that produces helium, releasing more than a million times more energy per gram than burning releases.

Most Fusion experiments fuse deuterium and tritium (isotopes of hydrogen). Why is this process used? Is this the same process as occurs in the sun? No the sun has a complex many step process. DT fusion is much more efficient and easier to achieve. Fusion reactors put out much more energy per volume than the sun.

What happens to the nucleons in DT fusion (collective term for protons and neutrons)? They re-arrange themselves to a lower energy state: 3 (T) + 2 (D) rearrange to become 4(He) + 1(neutron)

Compare the safety considerations for fusion versus a coal or uranium (fission)?

  • Fusion: use of tritium, activation of vessel
  • Coal: Carbon dioxide production, other fallout
  • Uranium: long lived radioactive waste

Why would an electromagnet be subject to forces in a fusion vessel?  Because current carrying conductors attract each other. Conductors in this case are the many electromagnets, both poloidal and toroidal, and the plasma itself which carries over a million amps.

The force that binds the nuclei together is called the ‘nuclear’ force – how does this relate to the ‘strong’ force?  The strong force operates through exchange of gluons, which can only happen within a nucleon – i.e. between quarks. The nuclear force operates externally to the protons and neutrons – inter-nucleon – it is a ‘residual’ of the strong force a bit like van der waals forces; very much weaker than the strong force (but nonetheless much stronger over short ranges than the electrostatic repulsion)