After the development of Nuclear Fission reactors that now lay scattered amongst the nuclear nations across the globe, the next logical step in the process was hoped to rest with Nuclear Fusion. Unlike fission which produces energy by splitting heavy nuclei into lighter nuclei and some additional small particles that help the process repeat, and then using the heat energy to produce steam that turns turbines, fusion releases energy by the collision and combination of lighter nuclei than results in the formation of a larger and heavier nucleus. While the process is known to exist and has been achieved on Earth before, there is still not an existing controlled method that would make use of human produced fusion reactors.
It is specified with "human produced", because currently most energy on the planet is either directly related to, or a byproduct of, the Nuclear Fusion process of the sun. In that way it could then be said that all forms of solar and most wind systems, as well as all organic fuel sources, are indirect siphons of a fusion reactor. If viewed like that, then fusion is already an alternative energy source. The problem lies in direct and localized methods of power production.
The unfortunate need for extreme levels of heat to produce the fusion reaction is the largest problem concerning the development of working fusion on Earth. To this date the only successful location of Hydrogen Fusion has been within the center of the Hydrogen Bomb, which uses an atomic bomb to reach the necessary temperature range. Hydrogen Fusion exists in the sun and is a process that lasts for billions of years, which is why it is such a sought after energy source. The fact that it requires a heat of ten million Kelvin to initiate the fusion in a process called hydrogen burning is one of the biggest obstacles for achieving the process, for even if it was developed, how would it be contained?
If the answer could come easily, it would be a historical achievement that would easily push humanity into the next age. As a prime example of "a little for a lot", fusion is king. Since water covers over 75 percent of the Earth's surface and water consists of two hydrogen atoms and one oxygen atom, there is an unlimited supply of the needed fuel for the hydrogen burning. With hydrogen burning, it can use any number of different hydrogen isotopes (atoms with different numbers of neutrons, and thus different mass) to fuse together with the non-radioactive byproduct of helium atoms, neutrons, and great quantities of heat energy. It is a very promising result, with a very daunting starting point.
As yet there is no means for creating a controlled fusion process, although there are minor successes in the road forward. Scientists have found by stripping atoms of their electrons within a containment unit they can create very high temperature plasma clouds that may eventually reach the temperature requirement for fusion, but it cannot be contained. Work is being done with magnetic fields and vacuum environments, but with little effect. The use of lasers as a secondary means of achieving fusion through focused compression seems to be the next best attempt, but is still with many accompanying problems. The very difficulties surrounding the requirements of fusion is why many have tried to search for the process of "cold fusion" which isn't really cold, but achieves fusion at a much lower level of heat. That search still continues, but is deemed unlikely to ever yield result.
While the prospect of using fusion as an alternative energy is certainly one to be entertained, until it can be achieved, it is best to remain with energies that work. Though the abundance of energy gathered through fusion would far outweigh even the most efficient means of power generation existing and yet to be achieved, it will be necessary to use regardless. When the future does finally reward the world with fusion as an energy source, there will be nothing "alternative" about it. It will be the only truly needed one for the majority of the world's energy needs.