Operational behaviour of the inductively-heated plasma generator IPG6-B for scientific applications
Introduction
Over the past several years, the 6th generation inductively-heated plasma generator at Baylor University (IPG6-B) has been established as a research facility for application across numerous fields [[1], [2], [3]]. These include fundamental physics research in the field of (aerospace) engineering [4,5], astrophysics [6], geophysics, environmental sciences [7,8] and complex (dusty) plasma physics [9]. Inductively-heated plasmas have been used widely for different applications and can be separated into two categories: low-power inductive laboratory discharges [10,11] which often only reach coupled powers of 1 kW [12] and are used for fundamental studies of discharge-characteristics and plasma chemistry; High power discharges which can operate in the range of several 100 kW and are used for materials processing [13] or in high enthalpy plasma wind tunnels [4,14,15] used for the simulation of atmospherical entry and the study of associated heat-shield materials. The IPG6-B facility aims to close the gap between these two very different concepts. The water-cooled discharge channel allows the facility to reach high powers and flow enthalpies in comparison to the laboratory discharges, while the relatively small geometry allows it to be operated at relatively low-cost and effort. This should allow to find new applications and operating conditions for inductively-coupled discharges, one of them being atmosphere-breathing electric propulsion (ABEP) systems [5]. To assess the available parameter space of the facility in comparison to other inductively-heated discharges, characterization is necessary.
The IPG6 is a downscaled version of the IPG3 at the Institute for Space Systems (IRS) [4] at the University of Stuttgart and is designed for lower powers in the range of kW. To date, besides the IPG6-B two largely identical facilities developed in a common approach exist in two different locations: The IPG6-S at the University of Stuttgart [16], used as a development platform for atmosphere-breathing electric propulsion (ABEP) systems [5] and the IPG6-UKY at the University of Kentucky [17], used to study gas-surface interactions in plasmas. These facilities are identical in generator design, e.g. discharge tube diameter, coil and achievable powers and comparable flow enthalpies. This combination provides the unique opportunity to study the behavior of similar plasma generators and facilities for different environments and diagnostics, including spectroscopy [8,15,18]. The operation of multiple facilities also provides the advantage of allowing a common verification database and accelerated development of IPG technology due to the utilization of synergy effects and sharing of research progress between the facilities. While many other inductively coupled discharges are in operation in laboratories around the world [12,[19], [20], [21], [22], [23], [24], [25]], there are significant differences in plasma generator design between these facilities (e.g. frequency, coil geometry, discharge channel geometry, achievable vacuum pressure) due to the fact that they were developed independently.
Section snippets
Description of the experimental facility at Baylor University
This chapter describes the experimental facility and set-ups at Baylor University. The facilities at IPG6-S and IPG6-UKY are adequately described in the references [5,16,17]. The experimental facility as shown in Fig. 1 consists of a vacuum tank and the plasma generator with tuning network and power supply. The vacuum tank has an outer diameter of m, a length of m leading to an overall volume of m3 and is connected to a vacuum pump capable of evacuating a volume of
Diagnostics
Various diagnostics have been developed for use in the IPG, including a cavity calorimeter, a pitot probe and electrostatic single and triple probes [26]. Furthermore, voltage and current as well as E and H-Field are measured using an oscilloscope connected to a Rogowski coil at the conductor between the tuning network and the inductive coil of the IPG. Cooling water temperatures and volume flows are measured using thermistors and turbine flow meters, while the gas flow rate is set using a MKS ®
Characterization
For the planning and design of future experiments within the facility, a detailed knowledge of the plasma properties generated is necessary. The most important of these include generator efficiency, plasma enthalpy, vacuum pressure and plasma jet related total pressures.
Discussion
Using the characterization results provided in the previous section, different statements on the properties of plasmas created within the IPG6-B can now be made. It has been shown that the vacuum pressure and Mach number are both only a function of the volume flow rate , relatively independent of the coupled RF power . This means, that the Mach number and therefore the plasma jet velocity can be controlled independent of the plasma power. As a result, precise control of the
Summary and conclusion
Within this work, the inductively-heated plasma generator IPG6-B and related facility have been characterized for powers between kW and vacuum pressures Pa. The resulting measurements are self-consistent providing a reliable source of information on achievable working points and facility limits. Reproducible sub- and supersonic flow conditions for Mach numbers between have also been measured for various nozzle configurations. This allows application of the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors want to thank Mike R. Cook and Kenneth Ulibarri for their support in development, set up and operation of the the facility and its diagnostics.
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