Known as ionised gases, a mixture of negative and positive charges and yet a big mystery to control and made use of in nuclear fusion research
The mysterious fourth state of matter otherwise known as plasma has been drawing attention lately. But what is plasma? Plasma refers to an ionised gas, which is a mixture of positive and negative charge particles and occasionally includes neutral ones. Its origin in physics goes back to 1920 with the works of pioneers Irving Langmuir, Lewi Tonks and Harold Mott-Smith. The fourth state of matter has come a long way since then, and its properties are the foundation of nuclear fusion research.
An important aspect of the plasma is its temperature, and it refers to the thermal motion or the velocity of the particles inside the ionised gas. The higher the temperature is, the faster the particles be. If one was to make a collective distribution, it would fit a Maxwell-Boltzmann velocity function. Plasma is usually made of different particles (electrons and protons), and each has a different velocity distribution, simply because they have different masses. For example, a 100,000,000 degrees Celsius, refers to a plasma with a collective velocity of 10 kilo-electron Volt, and that is 1386.7 km/sec.
But how does a Fusion Reactor work?
The aim of fusion reactors is to create, contain and confine the plasmas as the first step. Once the first step is achieved, the plasma is heated up to reach conditions where the ignition can happen.
In nuclear fusion research, and for the purpose of power generation, plasma gets trickier to deal with. This is because there are several factors that are always considered, which are heat capacity, confinement, pressure, time, density, and many others. As we examined previously [please see the previous article in this series], research in nuclear fusion started in the thirties involving powerful magnets in devices known as Magnetically Confined Fusion (MCF), and later in the sixties with the development of the lasers, Inertial Confinement Fusion (ICF). ICF will be discussed in upcoming commentaries. Both methods involve similar goals of maintaining plasma, containing heat, increasing the temperature, and of course, ignition, and as a general rule, they are denoted by factor Q, which is simply the ratio of energy gain versus the energy which is lost. Lastly, the Lawson Criterion refers to the conditions for the reactor to reach ignition, which simply refers to a self-sustaining nuclear fusion.
Ideally one would want to have plasmas at higher pressures and possibly operate at larger currents. Simply, the higher the pressure, the closest we get to the Q factor, therefore higher power output is achieved. Spherical Tokamaks (ST) are one of the ideal candidates, with a much higher ratio of thermal plasma pressure to the magnetic pressure, known as beta. This is very desirable and simply means less energy is required to generate magnetic fields hence, better confinement is achieved.
The notable STs are ST40 (at Tokamak Energy ltd), the first privately funded tokamak, and publicly funded facilities of National Spherical Torus eXperiment (NSTX) at Princeton Plasma Physics Laboratory (PPPL), and Mega Ampere Spherical Tokamak (MAST) at CCFE.
How to build a tokamak?
A typical spherical tokamak includes a central solenoid made of wires and used to generate a magnetic field when an electric current is passed through them. Its purpose is to drive current, consequently producing a poloidal field to heat the plasma, which is very advantageous to reach the desired operating temperature of 100M degrees. Further Toroidal Coils (TF) are used to create twisted fields and confine the plasma in the vessel. All the parameters above are used to define the plasma aspect ratio (plasma size), and the fusion gain and power. Figure 1 demonstrates a simplified cross-sectional view of a tokamak with NSTX as an example.
100M degrees Celsius recent achievements at Tokamak Energy Ltd, one step closer to fusion in Spherical Tokamaks
Tokamak Energy Ltd in the United Kingdom is housing one of the first STs in the world known as ST40. The device works based on high toroidal and poloidal fields to better confine the plasma while particles constantly follow the magnetic field lines and spiral around the tokamak. Figure 2 shows the ST40 in real life, where the left demonstrated the actual tokamak and, on the right, the plasma inside the reactor is shown in a video. As seen, the plasma keeps rotating with the presence of the magnetic fields for the period of the shot.
The plasma shown in the video is a deuterium plasma (D-D), a hydrogen atom with an extra neutron rotating inside the reactor. Once the particles reach the desired temperature in the case of ST40, which was 100M degrees Celsius, the chance of fusion starts to increase. The expected fusion reaction is shown in figure 3.
From the plot, it is obvious that the higher the temperature of the ions (T in million), the greater the probability of a fusion reaction. Ideally one would require a temperature above or at 10 keV to reach cross-sections where fusion could take place in large quantities.
Tokamak Energy Ltd has recently announced that it has managed to come one step closer to providing the world with a new, secure, and carbon-free energy source by reaching the 100M temperature. As discussed above this is certainly a big step towards achieving conditions for a sustaining plasma.
Confinement time of five-second at JET, the Joint European Torus was the other exciting achievement in the world of fusion
Another good news this year was the recent announcement by the Joint European Torus (JET) at Culham Centre for Fusion Energy (CCFE), managing to maintain a power ratio of Q=0.33 for a full five seconds. JET had previously demonstrated a Q value of 0.67, producing 16MW of fusion energy while injecting 24MW of thermal power for fraction of seconds in 1997, hence the recent achievement was to demonstrate a longer confinement time with a slightly less power ratio.
JET is designed to study fusion like those required for power plants and operates in D-T plasmas. ‘T’ refers to tritium, which is a hydrogen atom with two neutrons. Going back to figure 2, it is obvious that D-T fuel means a higher probability of fusion reaction (100 times), which will be the goal of other tokamaks in the coming years. To recap figure 4 demonstrates different isotopes of hydrogen.
JET is running with more than 4 MA plasma current in a D shape plasma with an aspect ratio of above 2. As a general rule, we have to bear in mind that the plasmas in tokamaks are a complex thing, so defining these parameters depends on the factors that control them.
The Energy Demand and Role of Fusion
The demand for a viable and cheap energy source is increasing exponentially worldwide. From 1950 to 2020, the total energy consumption worldwide reached 6,169,957 TWh, in which renewable sources were accountable for only 10% of the total production. Figure 6 demonstrates how our consumption is increasing annually along with fossil-based sources. This upsurge in demand has led to an increase in the Earth’s temperature, and catastrophic events such as changes in weather cycles, wildfires and dryness.
It is not unreasonable to think that the trend continues to grow, and it is vital to find other means of energy production to decrease our reliance on fossil fuels. With the current advancement in fusion energy and as commercial fusion picks up the pace with private investment, fusion could be much closer than what we initially thought. But how close, I guess this decade will have plenty of answers for everyone.
The next article will be examining the Inertial Confinement Fusion (ICF) with an exclusive preview of the Institute of Laser Engineering (ILE), Japan’s ICF research site.
Read the previous article in this series here.