The first problem is Radiation loss from fast charged particles from plasma. Two types of radiation are important — electron-ion brehmstrahlung radiation, and electron cyclotron radiation. For a fusion device to be self-maintaining, the power produced by the fusion reaction must be greater than the power wasted by radiation. It can be estimated that if the ion number-density of a deuterium plasma at a temperature of 3.5 x 10⁹ K be 1.4 x 10²¹ m^-3, then power density produced is ^~ 4 x 10⁷ watts m^-3. The power radiated in an identical situation is about 1.6 x 10⁷ watts m^-3, that is quite a large fraction of the power produced. The impurities in the plasma gas causes enhanced radiation loss, and if the plasma is strongly polluted, and may even lead to an abrupt loss of plasma confinement: known as disruption.

The second problem is related to the instability within the plasma. Even if the plasma is pinched inside the torus due to poloidal field, the instability arises in the pinched column due to following reason: a slight bending of plasma may inside the column. Due to this the field lines are crowded inside the bend and separated outside. This leads to an enhanced magnetic pressure inside the bend, bending the column further. This creates the plasma instability. These are: Kink, Sausage, Rayleigh-Taylor, Two-stream etc. To understand it further readers may go through literature.^x

Complete solution of the instability problem and attainment of absolute equilibrium have not yet been achieved. This is a subject of magneto hydrodynamics and lot of research is under progress.

The next issue for a successful design of a tokamak is proper heating of the plasma. Different approaches of plasma heating processes have already been discussed in lecture 6. Here we will briefly discuss what are the procedures being adopted for heating of plasma in a tokamak. From Lawson criterion, it has been seen that in order to produce energy from thermonuclear fusion, a temperature of 10 to 20 keV or higher is required. To achieve this high temperatures (several hundred million degrees), it is necessary to heat the plasma. The heating mechanisms those are generally adopted in tokamak are

1. 1. Ohmic heating based on Joule effect, which have already been discussed earlier.

1. 1. Heating, by injection of highly energetic neutral particles followed by collisions with plasma. A neutral injector is made up of an ion source, an accelerator and a neutralizer. To deposit their energy in the plasma core, the beam particle must be given a huge amount of energy. For positive deuterium ions, energy of upto 100 keV is feasible for neutralization, otherwise negative deuterium ions must be used, which have better neutralization efficiency. However, they are more difficult to create at the level of ion source, but have better neutralization efficiency.

1. Heating by radio-frequency waves, which works in the principle of resonance as discussed in earlier lecture.

Now I am going to discuss status of various tokamak machines and the improvement of design from time to time.

Presently the tokamaks, which are under operation, are

1. 1. TFTR (Tokamak Fusion Test Reactor), U.S.A.

1. 1. T-10 and T-15, Russia

1. 1. JT, Japan

1. JET (Joint European Torus), England.

Other machines that are under research or operation are, NET (Next European Torus); FER (Fusion Experimental Reactor, Japan); INTOR (International Tokamak Reactor). Presently, a large community under International Atomic Energy Agency (IAEA) is planning to develop International Tokamak Experimental Reactor (ITER). It is a joint international research and development project that aims to demonstrate the scientific and technical feasibility of fusion power. The partners in the project – the ITER Parties – are the European Union (represented by EURATOM); Japan; the People´s Republic of China; India; the Republic of Korea; the Russian Federation; and the USA. ITER will be constructed in Europe, at Cadarache in the South of France. In India, a full fledged tokamak research was initiated in 1989 with the commissioning of ADITYA at Institute of Plasma Research, Gujrat. At present, with the advancement of superconducting steady state tokamak, a significant improvement of the performance of the machine has taken place.