Confinement time

Assuming a cosmic-ray confinement time of 106 7 years and an interstellar hydrogen density of 1 atom cm-3, with present-day CNO abundances, use Eq. (9.10) to deduce Q/W for beryllium. 

In order to achieve an overall positive energy balance the D-T plasma has to be heated to the ignition temperature of 5 keV and confined for a sufficiently long time4). Energy breakeven is usually expressed in terms of the Lawson criterion which states that the product of the plasma density, n, and the energy confinement time re has to exceed a critical value which is a function of the plasma temperature4 8). 

The inertial confinement concepts utilize the idea of heating a pellet of D-T-fuel either by absorption of light from a powerful laser, a relativistic electron beam or a heavy ion beam to the ignition temperature in a time short compared to vaporization of the pellet. The reaction time must also be short compared with the confinement time to allow a sufficient burn up of the fuel for energy gain. In addition, the range of the 3.5 MeV a-particles has to be shorter than the pellet radius if their energy is to be efficiently deposited in the pellet. 

Atoms are first stripped of their electrons at very high temperatures this creates a plasma (ionized gas) of positive ions. Then the positive ions must be brought into close enough proximity, so that the strong attractive force between nucleons can overwhelm the Coulomb repulsion between them. Magnetic fields can confine hot plasmas of ions, provided that collective instabilities of these plasmas can be controlled. For a successful nuclear fusion reactor, three requirements must be met (1) The density of the plasma must exceed some critical value p. (2) The plasma confinement time must exceed some critical value t. (3) The temperature of the plasma must exceed some critical value 9... 

Considering the overall impurity content we have also to take into account seeded impurities (see Sect. 1.3) and the fusion product helium (the fusion ash ). A stationary burning fusion plasma can only be obtained when the helium is exhausted sufficiently fast. The corresponding figure of merit is given by the ratio of the effective He exhaust time over the energy confinement time Pile = THe/Ve [9],... 

For a D-T fusion plasma flame, these considerations can be translated into the terse statement, that the quality of thermal insulation (quantified by an energy confinement time te) must exceed one tenth of the particle lifetime in the flame rp p = rp/rp < 10, see Fig. 2.1, [2],... 

Fig. 3.1. Energy confinement time for JET L-modes and H-modes with 3MA of plasma current versus the level input power into the discharge [8]...

The Lawson limit m, where n is the particle density and x the confinement time (confinement quality) indicating the ability of the plasma to retain the heat. 

The spectrum of length and time scales involved in the micro/nanoscale transport is so wide tiiat the development of a model that covers the whole range is a formidable task. In addition, the energy carriers such as elecfa-ons (holes), phonons, and photons have distinctive transport characteristics, and both quantum and classical mechanics are to be taken into account. Because of the complexity, transport models have been developed according to the confined time and length scales of interest as illustrated in Fig. 2. 

The bulk of CR spectrum is understood in terms of the Fermi acceleration mechanism [12], whose theoretical description was revised in a more effective version by Bell [13]. It takes place in sources where plasma (e+e or/and pe) contained by strong magnetic fields, is driven by strong shock waves. The spectrum of Fermi accelerated particles follows an (2h 2-2) power law and the maximum energy that a particle can reach is a function of confinement time within the shock ... 

Research can continue the 20-year trend toward greater plasma confinement times, densities and temperatures, leading to the full set of ignition parameters by the mid-1990s."... 

---