Seite - 94 - in Water, Energy, and Environment - A Primer

7.2.3 Barriers to fusion Unfortunately, therearea fewbarriers toovercome, startingwith how togetDandT, both positively chargednuclei, to fuse. The positive electrical charges repel one another (the so-called Coulomb Barrier) and you have to bring the distance between them to an incredibly small separation before the ‘strong nuclear force’ can come into play and allow creation of the new, heavier heliumnucleus (two protons+ two neutrons). It is this still mysterious force that holds protons and neutrons together in our various nuclei (the other three ‘fundamental forces of nature’ are the gravitational force, the weak nuclear force, and the electromagnetic force). Sohowdoesonebring these twonuclei closeenough together toallowfusion tooccur?Theanswer in theSunisextremelyhigh temperatures and enormous gravitational pressure which we cannot reproduce on Earth. The pressures in the Sun, due to its large mass, are beyond our ability to achieve in any sustained way but the temperatures are not (temperature is a way of characterizing a particle’s kinetic energy, or speed) and fusion research is focused on achieving extremely high temperatures (hundreds of millions of degrees or higher) at achievably high pressures. The fact that this is not easy to achieve iswhy fusion energy is always a fewyears away. Two techniques are the focus of global fusion research activities –magnetic confinement (as in tokamaks and ITER) and inertial confinement (as in laser-powered or ion beam- powered fusion) (44). Several hundred million US dollars a year are being spent on these activities, mostly in international collaborations. Fusion on Earth has been achieved but not in a controlled manner, and then only in very small amounts and for very short timeperiodswith just one exception– the hydrogenbomb.This is an example of an uncontrolled fusion reaction (triggered by a fission atomic bomb) that releases a large amount of energy in a Water,Energy, andEnvironment–APrimer94