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The CORIA 100 kW ICP torch
Reentry plasma simulation
Before running at CORIA, the 100 kW ICP torch stood at the Laboratory of Electrotechnological and Plasma Installation of Polytechnic Institute in Saint Petersbourg where it was designed and built.
It is a BY-11-60 torch model with a working frequency equal to 1.76 MHz. Conversely to the VKI torch (for example) that works with transistors, the CORIA torch uses an oscillating triode supplied by a filtered and rectified high voltage.
The working frequency can be modified (± 0.044 MHz) by moving the nuclei regarding to their coils in the oscillating circuit. Triodes are very reliable components but have low efficiencies.
The part of the electric power actually injected in the test gas in the CORIA torch is 50% at best and specific enthalpy calculations are made with an injected power equal to 50 kW at best.
The values of the currents are 0.5-10 A on the anode and 0.1-2 A on the grid.
The high voltage applied to the anode belongs to the range 0.25-10 kV. Because of the large power losses, each heating component (triode, nuclei, inductor and transformer) is cooled by demineralized water.
In order to ignite the plasma at low pressure and to work at moderate pressure, the facility is equipped with a pumping system composed by a primary pump and a roots pump.
The flow rate can reach 4000 m3/h with a limit pressure equal to 0.01 Pa.
The test chamber (0.5 m in diameter) is water-cooled and equipped with three optical access ports around.
They are movable even under low pressure thanks to a dovetail joint.
Fig. 1: General setup of the 100 kw torch with the wind tunnel, the optical access ports and the pumping system.
Since its first ignition in CORIA in 1995, IPWT1 was used mainly in two configurations:
monoatomic gases, water-cooled double quartz tube, supersonic flow
polyatomic gases, single quartz tube, subsonic flow
In both configurations, the quartz tube in contact with the gas has an inner diameter of 72 mm and an outer diameter of 80 mm.
A schematic diagram of the torch and wind tunnel is shown in Fig. 1.
In the first configuration, the ICP torch works with argon.
As said before, the maximum energy deposition in argon is located along a ring whose diameter is very close to the internal diameter of the quartz tube. To avoid the melting of the quartz tube, water is sent between it and a second coaxial tube.
The gas injection is made in part through alumina tubes 6 mm in diameter along the torch axis close to the creation zone.
The other part of the gas is also injected through smaller alumina tubes 1.5 mm in diameter along the internal wall of the quartz tube to supply the ignition ring.
The total mass flow rate varies from 0.3 g/s to 3 g/s (volume flow rates : 10-100 l/min. at atmospheric pressure).
Within past studies, two mass flow rates were used: 0.90 g/s and 2.48 g/s.
The plasma chamber is ended by a 34° converging nozzle (throat diameter: 15 mm) producing an underexpanded supersonic jet.
Fig. 2 Details on the gas injection system of IPWT1.
Fig.3 An air plasma (h=9 MJ/kg, Qm=2.4 g/s) created in IPWT1.
The visible tube is the injection tube. The tube surrounding the plasma is not visible.
In the second configuration, presented in Fig. 2, the test gas is injected between a central cylinder and the quartz tube.
A small part of the flow rate (5-10 %) injected on the axis is used to maintain the plasma between the coils.
Such a gas injection allows to send the main part of the gas close to the quartz tube where the electromagnetic field is maximum.
Moreover, the high axial velocities contribute to the cooling of the tube.
Since the angle of injection is not equal to 0 but to 45°, the induced swirl increases the aerodynamical stability of the flow in the ionization area: the gas velocity is higher and the swirled gas is maintained close to the external tube.
For the reasons exposed in introduction, the plasma is preliminary ignited with argon then continuously but fastly (6 s) switched to the test gas (air or carbon dioxide). Such a procedure is necessary because the generator would not be able to create a polyatomic plasma and because a persistent argon plasma would melt the quartz tube.
The plasma is ignited with a mass flow rate of argon equal to 0.5 g/s (volume flow rate : 16 l/min. at atmospheric pressure) and a mass flow rate of air equal to 0.3 g/s then switched to pure air with a mass flow rate equal to 2.6 g/s (respectively 120 l/min.).
Fig. 3 display a photograph of an air plasma in the quartz tube.
A similar procedure could be found by replacing air by carbon dioxide. However, the plasma stability is improved when it is first ignited with air then switched to carbon dioxide. In this configuration, the test gas mass flow rate may be increased up to 5 g/s for pressures included in the range 2-10 kPa.
Within the past studies, the electric power was always lower than 52 kW. Considering an efficiency equal to 50%, it corresponds to a specific enthalpy of 10.8 MJ/kg for a mass flow rate of 2.4 g/s.
Some successful tests were carried out to extend the specific enthalpy up to 17 MJ/kg as easily for air as for carbon dioxide.
In this configuration, the plasma chamber opens directly to the wind tunnel so that the expanded plasma is subsonic.
Fig. 4 shows such a subsonic air plasma in interaction with samples in the wind tunnel.
Fig. 4 Interaction of an air plasma generated by IPWT1 with a SiC sample (left) and a water-cooled metallic plate (right).
The pressure in the test chamber is 38 hPa and the injected power 25 kW
Related pages in this website
Inductively-coupled plasmas in aerospace research
Boundary layer analysis in air plasma by laser diagnostics and optical spectroscopy
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