The Major Evolutions of Fusion Reactors

Our understanding of fusion has grown immensely since the day we discovered fusion. With this has evolved the way we approach controlling fusion.

First, we started the most basic we could. Our understanding was as deep as knowing that plasma (an ionized gas) could be manipulated by magnetic fields. We also long understood the laws of electromagnetism, which are fundemental to controlling plasma. So, we built the most simple proposal possible;

1. The Linear Cylinder

There really is no official name for this. we knew this device wouldn't control fusion. instead, this device was our first taste of understanding how plasma behaves. While plasma physics is a field today (which i plan to be in), it wasn't then. So, the people working on these devices were applying an understanding of fluid dynamics. The design was simple, a linear cylinder wrapped in wires that were used to accelerate current through. The laws of electromagnetism tell us that an accelerating current induces a magnetic field, and vice versa. So, the theory was that this current would ionize and contain the plasma. This test went fine, the current was just enough to create plasma, and the plasma followed the helical path of the magnetic field. But because the cylinder was a simple inlet and outlet, the plasma didn't have enough time to reach fusion temperatures.

2. The Tokamak

The Tokamak was proposed by the soviets as a solution to the linear cylinder. The plasma in the cylinder didn't have enough time to reach fusion temperatures, so the soviets said hey, let's take that thing and wrap it around itself. That proposal became what we know today as the Tokamak. But there was a problem, when scientists built the first Tokamak, the plasma was proving too difficult to confine well enough to induce controlled fusion. Largely caused by the difference in diameter between the inner and outer Tokamak. By nature, the donut shape forces a diameter difference between the inner and outer walls. So, when you wrap the Tokamak with magnets, the magnetic field flux will be much stronger toward the inner wall. This causes the plasma to leak out where it's the easiest--toward the outer wall.

3. The Stellarator

Once scientists got word of the problem with the Tokamak, they started looking for ways a plasma could constantly be circulating without travelling through a cylinder with such a diameter difference. The solution was the Stellarator. The Stellarator is a tokamak, but instead of a donut, it's a donut that's been twisted into a figure-8. This figure-8 makes the geometry symmetrical along the entire plasma path, solving the problem faced by the Tokamak--in theory. In reality, the Stellarator didn't reduce leakage by much. Fusion geometry concepts can only work as well as we build them. Sadly, the Stellarator was too complex for the time.

4. the Z Pinch

Once we realized how hard it was to prevent plasma instabilities, we devised the Z Pinch. A machine that intentionally would not rely on a magnetic field. Other designs relied on magnetic fields. That means that if the current being accelerated through the wires was not perfectly uniform, the plasma would be unstable, and because we live in reality, making things perfect is pretty hard. So, we looked for ways that fusion could be induced without this reliance on uniformity. There came the Z Pinch, a machine that accelerated plasma toward a single point, letting the plasma crush into itself like matter going into a black hole. This worked pretty well. In-fact, it was a Z Pinch machine that first achieved thermonuclear fusion. But as we scaled, we realized this machine wasn't doing what we thought. Before I tell you exactly what went wrong, I want to propose the scenario to you, see if you can figure out the problem.

In this geometry, there is a linear cylinder with two inlets for plasma. The goal was for both plasmas to intersect, and pinch into a single point inducing thermonuclear fusion. When turned on, they saw a burst of neutrons, when off, no neutrons. This was the first time we had ever seen neutrons from fusion, and it was a big deal. The news hit the world stage. From the number of neutrons they saw, scientists comfortably concluded that stable thermonuclear fusion had been achieved. The plasma in the machine had been heated to several millions of degrees, seemingly enough to induce fusion. But some scientists were skeptical, stating the temperature was not great enough and this couldn't have been fusion. Soon, scientists discovered that the neutrons coming from the front of the machine were more energetic than the ones coming from the rear. In stable thermonuclear fusion, the plasma crushed to a single point should release neutrons of equal energy in all directions. This was not the case, clearly something different was happening. This was also a crucial hint; neutrons flying out in the direction of the current had greater energy than those flying out against it.

The answer; as the plasma accelerated toward the single point in the center of the chamber, the plasmas would writh aginst each other, causing nuclei to accelerate greatly in either direction with enough energy to fuse with the colder and slower nuclei. The reason the neutrons flying in the direction of the current had greater energy was because the nuclei that flew off in the direction of the current gained energy. This was not fusion, but it was a step in the right direction.