7.1 Magnetic confinement

losses, including synchrotron radiation from charged particles orbiting about magnetic fields would be negligible. A fusion reactor, therefore, has to be operated at a temperature where the power gain from fusion would exceed the bremsstrahlung

Nuclear Fusion - One Noble Goal and a Variety of Scientific and Technological Challenges

A parameter that determines the electrostatic properties of a plasma is called the

s

ffiffiffiffiffiffiffiffi kBT nd

: In either of these two extreme cases, there are two basic properties:

: (17)

LD ∝

length is about 104. For a more rarefied plasma, say nd <sup>¼</sup> <sup>10</sup><sup>22</sup> particles/m3, LD ¼ 10 μm, and the number of particles in a volume of dimension of one Debye

these two properties that describe the hot thermonuclear fuel.

harness the energy in a laboratory environment.

cool itself down, and melt the container.

the physical size of the plasma is far larger than the Debye length, and there are many particles in a spherical volume of radius equal to one Debye length. They are

Just like a conventional power plant, a fusion power plant will use the energy released during fusion reaction to produce steam and then generate electricity by way of turbines and generators. But as noted in the above discussions, it is hard to

Each fusion reaction is characterized by a specific ignition temperature, which

One of the major requirements in the development of a fusion reactor is the actual realization of the ignition temperature of d-t reaction, which is 100 million degrees Celsius. Once all the conditions are realized, the challenge to contain and control the staggering levels of heat in the plasma is formidable. That is because the plasma must not only be heated to a temperature of at least 100 million degrees Celsius, but the energy must also be confined within the plasma without being carried to walls of the container for times long enough for the relatively infrequent fusion events to occur. Otherwise, the plasma will exchange energy with the walls,

must be surpassed before the reaction can occur. In stars, which are made of plasma, fusion takes place because of immense gravitational forces and extreme temperatures. Trying to create similar conditions here on Earth has required fundamental advances in a number of fields, from quantum physics to materials science. Scientists and engineers have made enough progress over the past half century, especially since the 1990s, so that a fusion reactor able to generate more power than it takes to operate can be built. Supercomputing has helped enormously, allowing researchers to precisely model the behavior of plasma under different

It is a length scale over which electrons screen out electric fields in the plasma. In other words, it is the distance over which significant charge separation can occur and how far its electrostatic effect persists. For distances greater than the Debye length, the energy of the particles in the plasma balances the electrostatic potential energy. Using nd <sup>¼</sup> <sup>10</sup><sup>28</sup> particles/m3, the Debye length for a 10 keV plasma is of the order of 10 nm, and the number of particles in a volume of the plasma of one Debye

losses.

6.2.2 Debye length

Debye length LD [7]:

length is 10<sup>7</sup>

conditions.

10

7. Plasma confinement

This method uses strong magnetic fields to contain the hot plasma and prevent it from coming into contact with the reactor walls. The magnetic fields keep the plasma in perpetually looping paths because the electrical charges on the separated ions and electrons mean that they follow the magnetic field lines. As a consequence, the plasma does not touch the wall of the container.

There are several types of magnetic confinement system, but the approaches that have been developed to the point of being used in a reactor are tokamak and stellarator devices. Because of its versatility, tokamak is considered to be the most developed magnetic confinement system. Hence, it is the workhorse of fusion.
