Why we need hot plasma for fusion reaction?

Why we need hot plasma for fusion reaction?

Hi, in the first part we discussed how we can get energy by fusing two nuclei. For that purpose D-T reaction was chosen as an example. At the current part we will focus on how to force such a reaction to happen, as it is not easy. Since D and T have one proton  apiece, they each have a positive charge. Like charges repel, so if you fire a beam of deutrons into a tritium target they still would not like to combine, until we will create collision energy larger than 280 keV. That threshold is called the Coulomb barrier. 

Yes, it is possible to use beams around 60 keV and get fusion, but it appears that energy produced in that case would not be enough to recompensate  power required for the beam’s work by itself.  Though some fresh attempts still are present, like “Helion Energy”, there is no physics theory yet that proves the possibility of positive energy gain.

But there is one theory that definitely defines a possibility of creating fusion reaction with positive energy outcomes. The main idea is to hit hydrogen gas, half in the form of deuterons and half in the form of tritium nuclei, to such a high temperature that there are some random high-energy collisions, which results in fusion. In the same time, the energy of “failed” collisions is not lost, as it is used to keep gas to be hot. 

Such a hot gas is called plasma. According to the theory, it is possible to create such conditions, that steadily released Fusion energy would be enough to keep gas hot and to generate power besides. That is the same process which happens inside the sun:

At this point it is worse to define what “hot” means. When gas has a temperature, it means velocities of the molecules are spread out in a particular way, known as Gaussian (or Maxwellian) distribution. Below is such a curve, that represents the relative number of H ions having different velocities in a gas at about 10 000 K. Velocities are higher at higher temperatures. 

The “tail” – it is the end of the curve, where there are a few particles with enough energy to fuse. By raising the temperature we can assure that there will be enough D and T ions for fusion.

The reasonable question appears: How high should the temperature be?

Fusion reactors will require gas temperatures over 100 000 000 K. Impressive, isn’t it ? 

It is obvious that no solid material can withstand such a temperature, so we cannot hold plasma with walls. The only way we can do it – is to use some known for us “invisible” forces like:

• gravity
• electricity
• magnetism 
  

The sun produces Fusion energy by holding plasma in its core with a huge gravitational field. We can’t do that on Earth, as our gravity is much too weak, and we can’t shape it (at least for now).

That leaves electricity and magnetism. We can make strong electric fields – but it would not work. The deal is at high temperatures gas no longer resembles gas we are familiar with – molecules become dissociated and atoms become ionized. The figure below shows that this new kind of gas is. 

The positive ions are small (blue) dots. They are given tails to show they are moving in random directions. The big objects – are electrons, which provide negative charges to make plazma quasineutral. As result we have a mixture of positive ion fluid and negative electron fluid. 

Such electricity charged fluid mixture is called plasma.  The plasma whole charge is neutral, but because of existing electric Fields inside, it is not “exactly” natural. Electric fields are creating a small imbalance, that is why such gases are called “quasineutral”. If not that  small imbalance – Fusion would not be a problem, but unfortunately it is not true. 

Plasma behavior is extremely complicated and the whole new science of “plasma physics” has grown up from the effort to produce Fusion energy. 

Returning to using electricity as a force for keeping plasma – it is not possible, as the electric field will pull ions one way and push electrons another way. Such a force will pull a plasma apart rather than combine it. 

That leaves magnetic fields. The main goal of the “game” is to make a magnetic bottle to hold a plasma. 

But within the last 50 years nobody could do it to the end. All magnetic bottles leak. Fixing one leak reveals another one that nobody had seen before. But scientists already made progress. We, as humanity, are already able to keep plasma in “magnetic gloves” for several minutes. Seems “Light” in the end of the tunnel is already visible. 

In the next article we will speak about how exactly we can design a magnetic bottle and is it safe. Subscribe to the newsletter below to stay updated on the publication.

Thank you for your attention, Lumin Hopper