Inductively-coupled plasmas
in aerospace research
An exciting challenge
One of the most exciting problem in non-equilibrium plasma physics takes a vital importance in one of the most exciting human adventure of the present century.
This challenge is to understand which are the main physical and chemical phenomena occurring during the entry of a hypersonic body in the atmosphere of a planet.
More precisely, it is to know how the plasma created behind the shock front interacts with the wall of the spacecraft.
The fascinating aim is to be able to allow cosmonauts to walk on Mars and to come back safely on Earth.
Existing facilities
In order to study the interaction of materials with high enthalpy flows, three kinds of ground facilities are commonly used: shock tunnels, arcjets and inductively-coupled plasmas (ICP).
Shock facilities can faithfully reproduce the conditions encountered in an atmospheric entry shock layer.
Radiation and chemical kinetics behind the shock front can be studied in the driven tube.
Hypersonic flows can be obtained by fitting a suitable nozzle at the end of the driven tube: the reflected shock conditions are then the generating conditions.
Nevertheless, very high enthalpies cannot be reached unless operating with big wind tunnels such as HIEST in Japan (25 MJ/kg). Shock tubes are not necessary to create hypersonic flows since generating conditions may also be obtained by a high-power electric arc as in the French wind tunnel F4 of ONERA (16.5 MJ/kg).
Those facilities are especially suitable to experiment aerodynamics and fluxes for a given point of trajectory. Nevertheless, two drawbacks limit the use of such facilities : their high cost and the small test time preventing to test plasma-material interactions.
Moreover, chemical kinetics studies require high degree of purity for the test gases. These conditions are difficult to obtain in big facilities.
The problem of gas purity is also encountered in arcjets because of the oxidation of electrodes in oxygen-containing test gas such as N2/O2 and CO2/N2 mixtures used to simulate Earth and Mars atmospheres respectively.
Clean plasma (without metallic atoms) can be obtained only at pressures lower than 100 Pa i.e. two or three orders of magnitude lower than actual pressures in shock layers.
In large facilities, it is quite possible to limit the oxidation of electrodes thanks to an efficient cooling, a rotating arc and a swirled gas injection. However, the plasmas produced in such torches depart only weakly from equilibrium and are no longer suitable to reproduce entry plasmas.
For applications which are not devoted to chemical kinetics, the arcjets have some interesting advantages: the arc plasma torches are simple and reliable, their efficiencies are high which allows to obtain high enthalpies in small facilities.
In addition, one can easily obtain supersonic flows by equipping the arc chamber with a suitable nozzle. The larger facility in Europe is Scirocco at CIRA in Italy whose segmented arc heater power reaches 70 MW. The arcjets can also run in steady state conditions for several hours.
Inductively-coupled plasma torches
They share that feature with electromagnetic heaters such as microwave and capacitive generators used to ionize low flow rates in small volume.
A high stability is also a feature of inductively coupled plasma torches in which the plasma does not touch any electrode avoiding pollution of the test gas. The von Karman Institute (VKI) in Belgium owns the most powerful facility of this kind in Europe with a power of 1.2 GW.
Whatever the kind of reactor (cold metallic cage, double cooled tubes, not-cooled simple tube), the test gas is injected in a quartz tube surrounded by a water-cooled copper coil with 2 to 7 turns.
Because of the low electric conductivity of gases, high powers (depending on the gas mass flow rate) are necessary to create the plasma that is ignited by a pre-ionization process.
It can be achieved by a high-voltage external electrode applied on the quartz tube or by a ground internal electrode that is removed when the plasma ignited (the same effect can be obtained with a heated graphite rod).
Nevertheless, in the case of low pressure studies as those carried out for atmospheric entry issues, the applied electric field is the source of seed electrons.
When the high-frequency electric current runs in the coil, an oscillating magnetic field generating an oscillating electric field takes place in the quartz tube.
Both fields are connected through the Maxwell-Faraday law and are used to accelerate the existing electrons of the plasma. The collisions of those fast electrons with heavy particles produce new free electrons by ionization and so on.
The plasma is self-sustained if the ionization rate remains greater than the recombination rate.
Inside the tube, the electromagnetic field is not homogeneous: a skin layer d takes place in the vicinity of the quartz tube. This quantity appears in calculation by resolving the Maxwell-Faraday equation.
The field is maximum at this distance from the coil. The thickness of this layer is few millimetres and depends on the electric conductivity of the gas s and on the frequency f according to:
where µ is the magnetic permeability.
The main part of the injected power goes inside this layer close to the quartz tube.
The central area is heated by conduction, convection and radiation.
A low frequency would better carry the energy in the plasma volume.
However, the penetration thickness should not be larger than the tube radius in order not to cancel the electromagnetic field.
Moreover the power density is higher with high frequencies. Indeed, the injected power can be written as follows:
where I is the injected current, a and l the medium radius and length respectively and n the number of turns of the coil.
So a compromise solution has to be found and the used frequencies belong to the range 0.4-2.7 MHz. On the other hand, as the thermal conductivity is much greater for diatomic gases than for atomic gases, air or carbon dioxide require much more power than argon to produce a plasma.
Monoatomic gases are then often used during the ignition step. However, this phase can be thermally critical because the skin depth is consequently so smaller that the plasma lies very close to the quartz tube.
Related pages in this website
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