How far have we come in generating energy from the fusion technology

 Nuclear fusion is a kind of reaction in which two or more atoms collide to form a single mass along with the creation of sub atomic particles like protons and neutrons which releases an enormous amounts of energy. Initially, fusion technology was used to create hydrogen bombs by USSR and USA. All the stars are powered by fusion only. The hydrogen is utilised as fuel , hydrogen fuses to form helium atoms and the rest of the mass is converted into energy. If the hydrogen gas is heated to very high temperatures then it is converted to plasma in which its electron and positively charge nuclie separate out from each other. 

Normally ,fusion not possible as the atoms can't come enough to fuse into a single nuclei due to repulsive electrostatic force out weighs the nuclear attractive force ( this force is effective for very small distance only). So, if we can create conditions in which the nuclie come very close to other to an extent where nuclear attractive force which binds the protons and neutrons together overcomes the repulsive electrostatic force, then they will combine and release energy. Such a condition can be created by raising the temperatures to an extent where the ions gain enough kinetic energy to overcome the threshold potential value, then rest of the work is done nuclear attractive force. 

There are two main players in this field, Tokomak  and Weldenstein 7-X which is a newer technology. We will learn about them along the read. So just go with the flow.

This idea of production of energy is quiet elegant, isn't it. It is quiet similar to fission just the difference being that it releases more energy, non radioactive and we have the complete control over the reaction, all we have to is to cut the supply of the fuel. Well, life is never easy. When researchers set to accomplish it, they encountered 2 major problems 1st one is the problem of heat transfer and the 2nd one is magnetic confinement.  

 Heat Transfer Issue

Fusion can be achieved if we heat the deuterium and the tritium atoms to 20 KeV and when they fuse, they release about 17.6 MeV. This is 17.6 MeV is the sum of the energies of a 14 MeV of neutron and an alpha particle with 3.5 MeV. The neutron leaves the plasma and it is captured by the blanket while the alpha particle is confined to keep the plasma hot. 

              The blanket transfers the heat to the coolant which can either be a liquid like water or gas like He. This is where the heat tranfer problem comes in. Blankets ensures that heat transfer happens precisely while protecting the outer parts of the machine. Remember, we are talking about insane temperatures separated by few centimetres. So, proper functioning of the blankets is a must. Any kind of defect can damage the whole system. Graphite is a good material for blanket but it has some drawbacks. If the graphite decays due to the high temperature then it will contaminate the plasma and may halt the fusion process. 

 Confinement Problems                   

Not all the atoms fuse and that's where the issue with confinement begins. If you remember the numbers which I have discussed above, about 80% of the energy released is used to generate electricity while the rest of the 20% keeps the plasma hot. Although we don't need much energy to initiate the the fusion but to sustain it, you need atoms to collide. The collision will happen only if we increase the probability of collision by creating suitable conditions. This is where magnetic confinement comes into play. Although fusion is achieved, lot of fuel is lost due to magnetic confinement (because to create such a strong magnetic fields we require a lot of electricity). Though lot of energy is generated, it is not sufficient to keep the plasma hot, power the infrastructure for confinement and also generate energy.   

Now we have some basics. So let's go back to the 2 big  shots I was talking about.

TOKOMAK.

It is a toroidal chamber with coils all around it like a solenoid which helps in the confinement of plasma in the toroidal magnetic fields so that the plasma moves in the circular patterns. Well, there's an issue with this set up and that is the confinement is pushed towards the center of the torus due to the nature of the system and for a TOKOMAK to be successful this issue had to be addressed. So, scientists introduced another electro- magnetic field using a transformer which generates current inside the confinement chamber making the plasma twist which evens out the distribution a bit and also heat it up. 

                                                                  This system has several disadvantages like the legth of the wires for the coils. It required 10,000 kilometres of Niobium Tim or Niobium Titanium alloy, the stronger the magnetic field (to reduce the drift), more the size would become (Thus increasing thr power consumption again) and the transformer can't work continuously and it consumes the major portion of the machine's energy. As we had seen above, the problem of drifting was among the draw backs.

The Raise Of Wendelstien 7-X

CAD model of Weldenstein 7-X
Photo source: https://images.app.goo.gl/pTBUxp88Fqs2zsT89

   Wendelstien offers a more elegant but cumbersome and complex solution to the problems mentioned above. The key insight to that was by twisting the plasma ,it would reduce the drifting to some extent. So in its 1st test in 1950s , they managed to heat the plasma to 5,00,000 kelvin but it wasn't that conclusive, the problem of drifting was still there but point to be noticed is that they were able to achieve the temperature of 5,00,000 kelvin back in 1950s and it's quiet a big deal! So the 2nd major breakthrough was realising that the circular kind shape wasn't necessary, what was really needed was to introduce a helical field coils with current at alternating directions along the length of the torus ,this would alone create the desired twist in the magnetic field . Even after all these modifications the problem of drift was still there. 

         It was only after the development of computers that stellarators gained back some momentum. Super computers helped out scientists to understand how plasma behaves in extreme electromagnetic fields and using that data they were able to design a beast which we know today as Wendelstien 7-X. It comprises of five-fold symmetry forces that helps to shape the plasma field lines. This is important because it enables the machine to handle the plasma for a longer time. This five fold symmetry systems rules out the need for any transformers. So this buys us a lot of power, you can make a guess by this - the ITER have a total magnetic field of 13T, while the Wendelstien can work with only 3T without break. (since it doesn't uses transformer). Apart from this ,there are several advantages of Wendelstien over TOKOMAK. Like the cost and the scale , 7-X is really small compared to TOKOMAK, it requires only few hundred kilometres of wire compared which is why they take less time to get built, it costs way cheaper and thus is a more reliable alternative. 

Futher modifications are being done to optimize the output like going from a 5-fold symmetry to a quasi-symmetry which helps to eliminate the different harmonics and produce a field line with single harmonic symmetry. It may help achieve that because it eliminates the toroidal curvature and highly improves the plasma confinement. Up untill now, 7-X has been successful in all its test and it is expected that after its final upgrade that is the installation of active cool driver using CFC will enable it to handle plasma for upto 2000 seconds! 

China's Success

On 2nd of June, 2021 Chinese Experimental Advanced Superconducting Tokomak set a new record for maintaining the temperature of 120 million degress for 101 seconds. Apart from that it reached about 160 million degrees for 20 seconds. This is a really great feat in the search of clean and unlimited energy but there's still a long way to go.