Nuclear Fusion

For a fusion reaction to take place, the nuclei, which are positively charged, must have enough kinetic energy to overcome their electrostatic force of repulsion. This can occur either when one nucleus is accelerated to high energies by an accelerating device, or when the energies of both nuclei are raised by the application of very high temperature. The sun is an example of thermonuclear fusion in nature.

Thermonuclear reactions occur when a proton is accelerated and collides with another proton. These two protons then fuse and form a deuterium nucleus which has a proton, neutrino and lots of energy. Such a reaction is not self sustaining because the released energy is not readily imparted to other nuclei. Thermonuclear fusion of deuterium and tritium will produce a helium nucleus and an energetic neutron that can help sustain further fusion. This is the basic principal of the hydrogen bomb which employs a brief, controlled thermonuclear fusion reaction.

Thermonuclear reactions depend on high energies, and the possibility of a low-temperature nuclear fusion has generally been discounted. Early in 1989 two electrochemists startled the scientific world by claiming to achieve a room-temperature fusion in a simple laboratory. They had little proof to back up their discovery, and were not credited with their so-called accomplishment. The two scientists were Stanley Pons of the university of Utah and Martin Fleischmann of the University of Southampton in England. They described their experiment as involving platinum electrodes, an electrochemical cell in which palladium and platinum were immersed in heavy water.

Nuclear fusion is also what powers the rest of the stars in the solar system. In a thermonuclear reaction, matter is forced to exist in a plasma state, consisting of electrons, positive ions and very few neutral atoms. Fusion reactions that occur within a plasma serve to heat it further, because the portion of the reaction product is transferred to the bulk of the plasma through collisions. In the deuterium-tritium reaction the positively charged helium nucleus carries 3.5 MeV. The neutron escaped the plasma with little interaction and , in a reaction, could deposit its 14.1 MeV in a surrounding lithium blanket. The neutrons activity would breed tritium and also heat as an exchange medium which could be used to produce steam to turn generator turbines. However, the plasma also loses thermal energy through a variety of processes: conduction, convection, and electromagnetic radiation

Energy also escapes in the reaction through line radiation from electrons undergoing level transitions in heavier impurities, and through losses of hot nuclei that capture an election and escape and confining field. Ignition occurs when the energy deposited within the plasma by fusion reactions equals or exceeds the energy being lost. In order to achieve ignition, plasma must be combined and heated. Obviously, a plasma at millions of degrees is not comparable with an ordinary confining wall, but the effect of this incompatibility is not the destruction of the wall as might be expected.

Although the temperature of a thermonuclear plasma is very high and the power flowing through it may be quite large the stored energy is relatively small and would quickly be radiated away by impurities if the plasma touched a wall and began to vaporize it. A thermonuclear plasma is thus self-limiting, because any significant contact with the vessel housing causes its extinction within a few thousandths of a second. Therefore, plasma must be carefully housed and handled while it is occurring.

Most of the research dealing with fusion since 1950 has used magnetic fields to contain the charged particles that constitute a plasma. The density required in magnetic-confinement fusion is much lower than atmospheric density, so the plasma vessel is evacuated and them filled with the hydrogen-isotope fuel at 0.0000000.

Magnetic-field configurations fall into two typed: open and closed. In an open configuration, the charged particles, which are spiraling along magnetic field lines maintained by a solenoid, are reflected at each end of a cell by stronger magnetic fields.

Present day mirror machines retard this loss by using additional cells to set up electrostatic potentials that help confine the hot ions within the central solenoidal field. In a Closed reaction, the magnetic-field lines along which charged particles move are continuous within the plasma. This closure has most commonly taken the form of a toros, or doughnut shape, and the most common example is the tokamak. In a tokmak the primary confining field is totoidal and is produced by coils of surrounding the vacuum vessel. Other coils cause current to flow through the plasma by induction. This toroidally flowing current wraps itself around the plasma.

The poloidal magnetic field, at right angles, that stronger toroidal field, acting together, yield magnetic field lines that spiral around the torus. This spiraling ensures that a particle spends equal amounts of time above and below the totoidal midplane, thus canceling the effects of a vertical drift that occurs because the magnetic field is stronger on the inside of the torus than on the outside. Additionally, a certain type of plasma called Tokmak plasma can be heated to temperatures of 10-15 million k by the current flowing in the plasma. Imagine how quick one could broil chicken. In less than half a second, a chicken would be golden brown and tender; ready for dinner.

At higher temperature the plasma resistance becomes too low for this method to be effective, and heating is accomplished by injecting beams of very energetic neural particles into the plasma. These ionize, become trapped, and transfer their energy to the build plasma through collisions. Alternatively, radio frequency waves are launched into the plasma at frequencies that resonate with various periodic particle motions. The waves give energy to these resonant particles, which then transfer it to the rest of the plasma through collisions.

Another approach to fusion pursued since about 1974, is termed inertial confinement. A small pellet of frozen deuterium and tritium are compressed to a very high temperature and densities in a process analogous to what is accomplished by bombarding the pellet from all sides, simultaneously with a really intense laser. This causes the pellet to vaporize and, by mechanical reaction, it imparts inwardly directed momentum to the remaining pellet core. The inertia of the inwardly driven pellet material must be sufficient to localize the power of -9 seconds required to get significant energy release.

. The minimum confinement condition necessary to achieve energy gain in a deuterium-tritium plasma is that of the product of the density in ions per cubic cm and energy containment time in seconds must exceed 6x10 -13th power. This was attained for the first time in a hydrogen plasma at the Massachusetts Institute of Technology in 1983. The temperature required to ignite a fusion reactor is in the range of 100-250 million k, several times the temperature of the center of the sun.

The goal of fusion is in effect, to produce and hold a small star. It is a daunting and tedious research which is considered to be the most advanced in the world.