Add heat to a solid and it begins to melt . Add enough heat and the entire solid becomes liquid. Add heat to liquid and it begins to vaporize. Add enough heat and the whole liquid becomes a gas. Add heat to a gas and it begins to ionize, meaning electrons get stripped from the gas atoms. Ionized gas is called plasma. Add enough heat to a plasma and all the electrons will be stripped from all of the atoms. Typical plasmas are a mixture of positively charged ions, negatively charged electrons, and neutral atoms. Add enough heat to a hydrogen plasma under enormous pressure and the atoms can fuse together.
Plasmas are more complex than liquids or gasses, consisting of two or three distinct fluids, each of which behaves distinctly. The separation of positive and negative charges in a plasma allows electromagnetic forces to act on the charged particles. These forces do not act on neutral fluids, meaning they cannot affect liquids, gasses, or the neutral particles in a plasma. Positive ions and negative electrons move in opposite directions under electromagnetic forces. The effects of electromagnetic forces is often greater than the effects of common fluid forces such as gravity or surface tension.
Plasmas are difficult to contain because the neutral gas is unaffected by electromagnetic forces, while the application of electromagnetic forces results in the separation of positive and negative charges, which creates currents and magnetic fields, which pulls the charged particles around via new electromagnetic forces, which can set up a runaway reaction of secondary currents and magnetic fields. It’s hard to keep all the components of a plasma contained at high enough density for fusion to occur. The heat required to create fusion plasmas raises temperatures to levels that would melt metal, so the only way to contain a fusion plasma within a vessel is to keep it away from the walls of the vessel.
Nature has a simple solution to fusion plasma containment without the need for a containment vessel: gravity. The vast majority of visible matter in the universe is plasma contained in stars. The interior of the star is hot enough and dense enough for fusion to take place. There is a balance between the outward pressure of the fusion reaction and the inward pressure of gravity. If the reaction occurs too quickly, then the pressure of the reaction overcomes the inward pressure of gravity . If the reaction pushes outward too hard then the plasma expands and the density and temperature drop precipitously. Then the reaction slows down, and gravity is able to push inward, so the density and temperature can rise again. The fusion in stars self regulates this way until the concentration of fusion fuel wanes.
In the laboratory, fusion plasmas must be confined mechanically. Most confinement facilities try to use symmetric configurations: linear, cylindrical, or spherical geometry. The two most researched confinement methods (for the purpose of developing fusion power plants some day) are inertial and magnetic confinement. Inertial confinement creates mechanical pressure to compress a [spherical] fuel pellet. Magnetic confinement balances magnetic pressure with the pressure of a ring of fusion plasma. Using either method, the fusion reaction must produce much more energy than the confinement energy of fusion plasma in order to produce a viable energy source.
Inertial confinement creates a huge pressure on the outside of a fuel-filled target to compress it while heating it. High enough fuel density and temperature starts the fusion, and the aim is get the fuel to continue fusing even after the mechanical compression stops. Limiting factors are pressure uniformity on the target and target imperfections. Imagine trying to squeeze a partially filled balloon to a smaller size in your hands. Push one place and the balloon squishes out between your fingers somewhere else. To apply more uniform pressure to the outside, put the balloon in a jar with an air pump attached, and pump up the air pressure inside to shrink it, just as commercial vacuum pumps increase the volume of food items placed inside jars by pumping down the pressure, but the difference in volume is only a factor of a few. Change the pressure too much and the jar would break. Now imagine trying to compress a fluid filled balloon down to one tenth its initial diameter. It would take a powerful pump and a strong vessel. Also, there is a break in the symmetry of the balloon where the knot is tied. Once the inside of the balloon reaches high enough pressure, the fluid will leak out. The balloon will burst if there are any thinner-than-average regions on the balloon surface. These examples give a simplified view of some issues with target manufacturing and the mechanical challenges of compressing fusion targets to 1000 times fluid density in inertial confinement fusion.
Magnetic confinement creates magnetic pressure on the outside of a ring of fusion plasma. Injected fuel keeps the fusion reaction rate fairly constant, in contrast to the bursts of energy required to burn up inertial confinement fusion targets. Limiting factors include the uniformity of the magnetic field generated by coils around a donut-shaped target chamber, and plasma turbulence. Imperfections in the magnetic field, or turbulence (introduced, for example, when additional fuel is injected) leads to the drift of particles radially (out of the ring of plasma). Magnetic confinement fusion is considered the more viable scheme for a fusion power plant.
For more information, view the Wikipedia articles on “Inertial Confinement Fusion” and “Magnetic Confinement Fusion” or go to the Nation Academies Press to view the book “Frontiers in High Energy Density Physics: The X-Games of Contemporary Science”.