Induction plasma

The magnetic field induces an electric current within the gas which creates the plasma.

The 1960s were the incipient period of thermal plasma technology, spurred by the needs of aerospace programs.

Early attempts to maintain inductively coupled plasma on a stream of gas date back to Babat[1] in 1947 and Reed[2] in 1961.

In the 1980s, there was increasing interest in high-performance materials and other scientific issues, and in induction plasma for industrial-scale applications such as waste treatment.

Substantial research and development was devoted to bridge the gap between laboratory gadget and industry integration.

After decades' effort, induction plasma technology has gained a firm foothold in modern advanced industry.

A conductive metallic piece, inside a coil of high frequency, will be "induced", and heated to the red-hot state.

There is no difference in cardinal principle for either induction heating or "inductively coupled plasma", only that the medium to induce, in the latter case, is replaced by the flowing gas, and the temperature obtained is extremely high, as it arrives the "fourth state of matter"—plasma.

An inductively coupled plasma (ICP) torch is essentially a copper coil of several turns, through which cooling water is running in order to dissipate the heat produced in operation.

[3] The coil wraps a confinement tube, inside which the induction (H mode) plasma is generated.

One end of the confinement tube is open; the plasma is actually maintained on a continuum gas flow.

During induction plasma operation, the generator supplies an alternating current (ac) of radio frequency (r.f.)

According to Faraday's Law, a variation in magnetic field flux will induce a voltage, or electromagnetic force:

power imposed on the coil achieves a certain threshold value (depending on the torch configuration, gas flow rate etc.).

For the case of atmospheric ambient pressure conditions, ignition is often accomplished with the aid of a Tesla coil, which produces high-frequency, high-voltage electric sparks that induce local arc-break inside the torch and stimulate a cascade of ionization of plasma gas, ultimately resulting in a stable plasma.

All these engineering concepts are aiming to create the proper flow pattern necessary to insure the stability of the gas discharge in the center of the coil region.

It helps to stabilize the plasma discharge; most importantly, it protects the confinement tube, as a cooling medium.

The minimum power to sustain an induction plasma depends on pressure, frequency and gas composition.

Then suitable second gas may be selected and added to argon, so as to get a better heat transfer between plasma and the materials to treat.

The successful industrial application of induction plasma process depends largely on many fundamental engineering supports.

Another example is the powder feeders that convey large quantity of solid precursor (1 to 30 kg/h) with reliable and constant feed rate.

The melted powder particles are assuming the spherical shape under the action of surface tension of liquid state.

Induction plasma technology implements in-flight evaporation of precursor, even those raw materials of the highest boiling point; operating under various atmospheres, permitting synthesis of a great variety of nanopowders, and thus become much more reliable and efficient technology for synthesis of nanopowders in both laboratory and industrial scales.

In the nano-synthesis process, material is first heated up to evaporation in induction plasma, and the vapours are subsequently subjected to a very rapid quenching in the quench/reaction zone.

The nanometric powders produced are usually collected by porous filters, which are installed away from the plasma reactor section.

The typical size range of the nano-particles produced is from 20 to 100 nm, depending on the quench conditions employed.

The productivity varies from few hundreds g/h to 3~4 kg/h, according to the different materials' physical properties and the power level of the plasma.

Plasma wind tunnels also called high enthalpy ground testing facilities reproduce these conditions.