clivebest.com November 9, 2015 by Clive Best, Ph.D., High Energy Physics
If nuclear fusion can be tamed on earth then if offers the potential of limitless energy for the foreseeable future. We saw in the previous post how all the elements on earth were fused in a massive star that exploded before our sun formed. Today all the energy for life on earth originates from the nuclear fusion reactions in the sun, with the exception of a small amount of geothermal energy from nuclear (fission) decay. In that sense it is already the case that we depend on fusion energy for our existence. Even renewable energy can be thought of as just a low level spin off of fusion energy (the sun). Therefore it would be infinitely better if we could somehow tame nuclear fusion as a source energy source here on earth. There are also huge advantages if this can be achieved.
- There is a cheap and limitless supply of ‘fuel’ mainly from sea water
- Fusion is inherently ‘safe’. The reaction can’t run away because so little fuel is being ‘burned’ at any one time.
- It produces no CO2
- It produces only low levels of radioactive waste, which becomes completely negligible after ~100 years.
- (Later it can be designed with different reactions to produce zero radioactivity.)
Scientists and Engineers have been working on controlled Fusion for about 60 years. Unfortunately it has proved so far to be frustratingly difficult to achieve the conditions needed, and there is as yet no silver bullet. There is a standing joke that Fusion is always 30 years away and always will be. However despite this, there has been steady progress and fusion really is now within our grasp. Two technologies even promise a potential breakthrough – high speed computing and superconductivity.
The simplest reaction to achieve on earth is the fusion of deuterium and tritium. It has the highest cross-section which translates into the lowest temperature and density requirements.
To produce significant amounts of fusion power from a plasma of deuterium and tritium one needs to heat the plasma to nearly 100 million C, and then contain it long enough for the fuel to ‘ignite’ and self maintain fusion power. For ignition to occur the heat transferred to the plasma via the 3.5 MeV 4He nucleus must be greater than all energy losses from the plasma. There are two main ways that the plasma can lose energy. Firstly the neutron (which being neutral) simply escapes the plasma. However this neutron can be captured in an external blanket to extract energy for electricity production and also be used to breed new tritium from Lithium. The second way the plasma loses energy is through losses of the plasma itself to the wall or diverter plates caused by instabilities. There are two methods currently under development to heat and contain a plasma to fusion conditions on earth. Both promises to lead to ignition and electrical power production.
The goal of inertial confinement is to trigger a tiny thermonuclear explosion (yes a mini-hydrogen bomb!) using powerful laser beams. A small cell of DT is placed at the focus or target of a very powerful array of laser beams which produce 500 Terawatt of power for just a billionth of a second. If all that energy could be used to heat the pellet then it will ignite emitting terajoules of fusion energy. This heat is extracted from the neutrons by surrounding the taget with a blanket of water. Lithium is also needed to breed more tritium fuel. Pellets could then be drip fed into the target to produce a type of pulsed fusion engine. The heat absorbed by the water coolers then drives turbines to produce power. The largest facility in the world is the National Ignition Facility at Los Alamos built in 2009, and intended to demonstrate ignition. So far it has achieved only net energy breakeven but not ignition. The problem is that you need a perfect symmetry of implosion to stop the pellet flying apart before ignition can occur and so far these conditions have not been achieved. NIF has been a slight disappointment and is still a factor 3 below ignition. One of the problems has been Rayleigh instabilities in the ablative sphere enclosure radiating energy away energy from the implosion.
A working ‘inertial fusion’ power reactor is still decades away.
This is the most favoured solution for a future power reactor and uses magnetic fields to contain the plasma. Charged particles spiral along magnetic field lines between collisions. The higher the field strength the better the confinement. Magnetic confinement has a long history dating back to the Zeta experiment in 1957 at Harwell.
Today there are two main configurations which promise to eventually realise fusion power. The main contender is the Tokamak configuration and the largest experiment so far was JET based at Culham. A tokamak has a toroidal field in the shape of a doughnut and a poloidal field which adds a helical twist and is used to heat the plasma by generating a large current like a transformer. Further heating of the plasma then is made through radio frequency waves and by neutral beam injection. The former inputs electromagnetic energy into the ions and the latter injects fast deuterium atoms which then are ionised and lose energy through collisions.
A difficulty for tokamacs is to avoid the plasma touching the walls as this causes impurities (heavy ions) to be introduced which can rapidly cool the plasma. For that reason the walls are coated with Beryllium and a special device called a Diverter is designed to divert ions leaving the plasma onto specialised plates before they can collide with the wall. Eventually Diverters can also be used to elopement exhaust from a burning plasma. Another problem are the development of instabilities or turbulence in the plasma which can be controlled by magnetic feedback on the poloidal field coils to stabilise them. The tokamak uses a transformer like ‘pulse’ to generate a large current in a freshly introduced plasma which is then heated through so-called Ohmic heating, just like an electric fire. The plasma current reaches over 5Mamps.
This current is still not enough to heat the plasma to the temperatures required for fusion so external heating by neutral beams and RF is needed. Neutral beams may also be used to generate a steady state current needed for a burning plasma once the initial transformer pulse has ended. This is called current drive.
In 1998 Jet achieved the world record of 15MW of fusion power in a DT plasma which equals a Q-value of 0.7. Q=1 corresponds to energy breakeven whereby the fusion energy produced is equal to the energy input to heat the plasma. Q values greater than 10 are needed for any future power reactor. A burning plasma after ignition needs zero energy input, because all the heating would be generated by collisions with the 5 MeV alpha particles produced.
Performance of Tokamacs are characterised by a small number of parameters. A fundamental index is the energy confinement time.
This measures how well the plasma is insulated by the magnetic field. If there were no energy losses τe would be infinite. Most losses are due to turbulent loss of heat through the field. The higher τe the more effective fusion reactor is. For high enough temperatures the fusion power generated depends on plasma pressure P. It is common to combine these to make the Fusion product Pτe. For temperatures above 100 million C ignition occurs if Pτe > 20 The progress towards fusion can be shown on a Pτe versus Τ plot.
The ITER (International Tokamak Fusion Reactor) tokamak finally being built now in France after years of political delays. It is an international project based on a scaled up version of JET, with a stronger toroidal field made possible with super-conducting magnets. It is designed to produce 600MW of fusion power and test the feasibility of breeding tritium from Lithium, current drive, and basic components needed to build a demonstration (DEMO) power reactor. This DEMO is foreseen to be operational around 2040 at the earliest and should generate about 1GW of electrical energy. There is no doubt now that ITER will work, but whether it can be proved economic and reliable enough are the key issues. For example the radiation damage to the first wall problem needs to be solved. The neutron flux inside a fusion reactor burning DT is such that almost each atom will be displaced over the lifetime, so any material used must be resilient to neutron damage, or at the very least its surface must be easily replaced. The prize though is huge because a fusion reactor is inherently safe and cannot runaway because there is so little fuel present at any moment. Radiation risks to the public are tiny.
Another challenge for ITER is the need to demonstrate the breeding of Tritium on site for use as the fuel. Tritium has a half life of just over 12 years so it needs to be generated artificially. This can be done by using the neutron flux from the reactor and an external blanket of Lithium.
The tritium cycle can all be implemented on-site as a closed loop. Essentially the ‘fuel’ needed for a fusion reactor is simply deuterium and Lithium, both of which are very abundant in nature. These technical challenges must be solved on ITER before a power reactor can be built.
Alternatives to ITER: Stellarator
One drawback of tokamaks is that the magnetic field is stronger on the inside than the outside due to simple toroidal geometry. This is one cause of instabilities and plasma losses to the outside wall. One way round this is to twist the configuration so as to invert inside and outside magnetic loops using a complex magnetic field configuration called a stellarator. The stability of such configurations can be modelled by computer and then implemented with complex engineering. The largest stellarator experiment is now being commissioned in Germany and is called Wendelstein 7X.
The plasma needs external heating mainly by microwaves because there is no induced plasma current. If it can reach 100 million degrees with steady state conditions then stellarators would become a viable alternative to tokamaks.
Alternatives to ITER: Spherical Tokamacs
Spherical tokamacs promise to provide much more compact cheaper fusion reactors. Their advantage is that they can maintain a higher plasma pressure by squashing up the tokamak magnetic field by reducing the size of the central size. This means they can be made much smaller so that a fusion reactor with the same output as ITER would be much cheaper. This concept has been developed at Culham Lab, and is now being developed further by a startup company ‘Tokamak Energy’ collaborating with Oxford Instruments who are world leaders in High Temperature Superconducting Magnets. There was a very recent channel 4 news report from November 4th 2015 (4 days ago at time of writing) explaining the motivation of the company.
Some of the UK’s best known fusion scientists who worked at Culham and Jet are consultants. Culham are also now upgrading their circular tokamak experiment MAST. However most international spending is now based around ITER, which is taking the conservative approach by scaling up JET to develop a research reactor. This leaves an opportunity for a fast evolving company to exploit a possible shortcut by exploiting the encouraging results of sherical tokamaks.
Other Commercial Initiatives
The most interesting recent developments have also been a number of commercial startups working on developing radically different small Fusion reactors. The potential payback is so enormous that either large companies or rich investors are willing to try new approaches not funded otherwise, although many of the ideas are not new. What could be new is the advances in superconducting magnets and fast electronics. Four of the most interesting are:
Their Skunkworks team are working on a small superconducting magnetic mirror device which they call a Compact Fusion Reactor. Magnetic mirrors are supposed to pinch the magnetic field lines so tightly at each end that high energy ions are reflected and kept within the reaction chamber which is heated with neutral beams. It is designed to fit on the back of a truck and produce 100MW. That way it can be delivered to a town or remote community.
However, details are a bit thin on the ground but they claim to have it all up and working in 5 years. Magnetic mirror devices were tried early on in Fusion research and were not successful. They claim that superconducting magnets and feedback control can resolve energy losses. This is a high risk project with potentially massive payback. The master company is unlikely to continue funding unless results look promising after 5 years.
- Investor Paul Allan
This is the most secretive company. It’s aims are not to use DT fusion but the far harder fusion of p–11B (Boron) which would need temperatures of about a billion kelvin! The big advantage is that the reaction products are free of neutrons generating just three helium nuclei (?-particles). These are charged, so they could be guided by magnetic fields into an ‘inverse cyclotron’ device that would convert their energy into an ordinary electric current very efficiency. The device seems to be a collision of two beams into a magnetic trap heated by neutral beams.
This summer they held a pasma at 10 millionC for 5 milliseconds.They are currently building a new version which they promise will give a 10 fold increase in performance. However they really need a 100 fold increase they achieve their goal.
- Investor Jeff Bezos (Amazon)
General Fusion is a Canadian company pursuing what they call Magnetized Target Fusion. It is a mix of intertial and magnetic fusion. In MTF, a compact toroidal magnetized plasma is compressed mechanically by an imploding conductive shell, heating the plasma to fusion conditions.
General Fusion’s Magnetized Target Fusion system uses a sphere, filled with molten lead-lithium that is pumped to form a vortex. On each pulse, magnetically-confined plasma is injected into the vortex. Around the sphere, an array of pistons impact and drive a pressure wave into the centre of the sphere, compressing the plasma to fusion conditions.
All this sounds good but actually getting it all to work is certainly no easy matter.
This company is based in Redmont, Washington and has ambitious aims to get D-He3 fusion to work which is even cleaner than D-T fusion because it produces no neutrons. Reaction reaction is D + He3 -> He4 + p Such a reaction would produce little radiation damage problems with the reactor wall, and allow direct conversion of electricity from the proton flux. However, it is far more difficult to get this reaction to ignite than DT. Helion aims to build a ‘fusion engine’ based on pulsed magnetic fields to collide and compress two plasma fuel pellets. Again the aim is a truck sized device producing around 100MW. However the details are all a bit thin on the ground.
I am convinced that net fusion power production will be demonstrated sometime within the next 10-20 years. There can be little doubt that it will eventually be made to work and generate electricity. The real question is whether it can be be made economically competitive and cheap enough to replace fossil fuels. The government sponsored ITER approach will succeed but may lead to large, expensive and complex power plants. There are still engineering problems yet to be solved. That is why it is so interesting to see new private ventures seeking to short circuit the whole process. They know that if can can really develop a cheap fusion solution then the pay-off would be enormous.
Nuclear fusion would solve all energy problems essentially for ever. Nuclear fission on the other hand needs fast reactors to be a long term (century scale) solution. Nuclear fusion has two other advantages over nuclear fission.
- It is inherently safe.
- There are no dangers of nuclear proliferation.
- This means future fusion power plants could be installed anywhere without serious political or safety problems.
Too much was promised too early for nuclear fusion and the failure to deliver so far has damaged its reputation. The UK spends about £25 million per year in Fusion research which is completely dwarfed by the £5 billion per year it already spends on subsidising renewable energy. The UK was the first to work on controlled nuclear fusion, we hosted the most successful experiment to date JET, and have now developed a promising simple spherical tokamak design. All the commercial startups are based on novel ways to develop small cheap compact fusion reactors producing about 100MW. The North American companies are well funded by rich individuals and venture capital, but I would bet on the UK Tokamak Energy Ltd winning this race if it gets sufficient funding!
Whatever else happens ITER will decide the future of fusion before 2030, and fusion reactors should be generating power in the 2040s.
Or 30 years in the future !