Thermometer For Plasma

St. Petersburg researchers have designed an original thermometer for fast-moving electrons in thermonuclear reactors. The laser beam in this device is used to instantly determine the temperature of burning hot plasma, at frequencies required for precise diagnostics.

This device is a further step forward to controlled nuclear fusion. The device will help researchers to get precise information about energy distribution of hot plasma electrons inside the tokamac – the most promising current prototype of the future thermonuclear reactor.

The new diagnostics system is based on the original laser construction designed by scientists at the Open Joint-Stock Company “Control Modules. Laser Beam Engineering and Technology” (MULTITEX) and the Ioffe Physical & Engineering Institute, Russian Academy of Sciences. It allows them – figuratively speaking – to apply a thermometer to plasma burning at temperatures in the millions of degrees – the substance tortiled by magnetic fields, where heavy hydrogen tritium interacts to liberate tremendous amounts of energy.

“The only known way to take temperature of hot plasma electrons is the so-called Thomson scattering, says one of the authors of the device, Andrei Alexeev, Ph. D. (Physical and Mathematical Sciences).

“The photon flux scatters upon interaction with electrons, the scattering being different depending on their energy. Therefore, the scattering allows to judge about the temperature of electronic component of plasma. Unfortunately, efficiency of this interaction is not high, so the light signal turns out to be weak.

“The light signal is hard to notice against a background of natural luminescence of burning hot plasma, it is like the gas-jet light against a background of bright sunshine. However, it is necessary to know the temperature at any moment –it is impossible to understand what is happening inside the fire-spitting tokamac without precise diagnostics.”

To solve the task, the researchers used the laser source of light. They have changed the laser device construction: the plasmic object was placed inside the laser resonator.

“It was sufficient to place the mirrors of the laser device at both sides of the tokomac in such a way that the beam is alternately reflected from them and intensified, and crosses the plasma several times instead of one time, continues Andrei Alexeev.

“Such construction is called multiple-pass. As a result, we expect that the laser formed by optical components and nontransmitting mirrors located at different sides of plasma in the course of charge will generate the emanation impulse possessing the energy which is high enough for being registered.”

It is interesting to note that the researchers assure that such laser construction may be used only for peaceful purposes. Its emanation capacity is high only within the bounds of a resonator, i.e. between the mirrors. Once in hypothetical case the nontransmitting mirror is replaced by a partially transmitting mirror, the energy will “flow out” of the optical resonator and the laser impulse intensity will drop down drastically.