This research aims at developing key science to improve helicon radio-frequency (RF) plasma generation used for propulsion or semiconductor manufacturing. Its key hypothesis is that fields of permanent magnets can be shaped optimally through the application of the Meissner effect of superconductors. If successful, the results of the research will provide a unique capability for study and implementation of a compact, high-efficiency, and high-density source for RF plasmas.
These plasmas are of significant commercial interest because of their unusually high ionization efficiency - typically an order of magnitude higher or more than other capacitively or inductively coupled RF plasmas. However, making helicon sources compact and efficient is difficult because of the need for an axial magnetic field to establish helicon wave propagation. Stronger fields provide higher densities, but the power required to maintain the field offsets the gains in ionization efficiency. For the case of propulsion, such sources could make possible the use of water or ammonia as a propellant, providing compact storage, ease of ground handling, and high propulsive efficiency while still providing high specific impulse. For semiconductor processing, the high-density plasmas can be formed from chemical etchants (such as sulfur hexafluoride) to accelerate the deep reactive-ion etching (DRIE) process in harder materials such as silicon carbide.
The ability to shape and control these magnetic fields in a compact geometry also provides a unique capability for design of laboratory experiments that will allow study of the effects of field geometry (specified axial gradients, periodic and non-periodic perturbations) on, for example: 1) wave reflection/transmission, 2) absorption of RF power and 3) plasma acceleration. Another research element afforded by the inductive nature of the RF coupling that will be investigated as a result of this capability is the formation of plasmas using non-noble gases. Specifically of interest is the non-equilibrium composition of partially dissociated polyatomic molecules, and how they could be optimized or otherwise impacted under variations of the discharge parameters.
Educational broader impacts of the research will include: 1) an expansion of undergraduate and graduate student research experiences in this field; 2) the development of course material that integrates the proposed research into a specialized graduate elective and a component of the undergraduate curriculum; 3) the development of international research experiences for undergraduates (IREUs) that will start with colleagues at the Australian National University and 4) development of outreach programs for high school students designed to excite and intrigue that age group. Plasmas can be mesmerizing, and are also a component of many modern high-definition televisions, something to which all high-school students can relate. The hardware used in the proposed research is uniquely well suited for development of a number of laboratory demonstrations designed to promote active learning. These demonstrations will include taking a substance through all four states of matter, introducing atomic structure through a substance's spectral radiation and demonstrating the magnetic shielding properties of a superconductor.
A radio frequency (RF) powered helicon thruster was developed to research its suitability for use as a low thrust electric space propulsion system. The helicon thruster consists of a quartz glass tube filled with gaseous fuel surrounded by a single turn helical copper antenna which is powered by an RF power supply. Current through the antenna generates a time varying magnetic field and curling electric field that excites free electrons in the gas, igniting the plasma. Magnets provide an axial magnetic field that allows for the creation of helicon waves, a low frequency electromagnetic wave. Thrust is produced by the acceleration of charges particles along the magnetic field lines by a plasma sheath that forms at the thruster exit when the source is operating in a vacuum. The helicon thruster has several advantages over other electric propulsion systems. Helicon waves have a higher ionization efficiency than capacitive and inductive coupling, requiring less energy per ion to produce a plasma. The RF antenna powering the thruster is not immersed in the plasma, making it less susceptible to erosion, which decreases the lifetime of the propulsion system. Equal numbers of ions and electrons are expelled at the exit, eliminating the need for a neutralizer in the exhaust plume. The goal of this research effort was twofold: to investigate the possibility of implementing permanent and superconducting magnets to generate the axial magnetic field, and to investigate the performance of the helicon thruster with water vapor as a fuel. To increase thruster efficiency, permanent magnets were tested with the assistance of a type II superconductor to control the shape of the magnetic field geometry. Permanent magnets can offer a strong magnetic field at no power cost, however the non-uniformity of the fields and in some cases the presence of nulls make these field structures sub-optimal for helicon source operation. A possible solution is the integration of strong permanent magnets with superconducting materials to shape the magnetic fields at a much lower power cost. Because helicon waves ionize gas more efficiently and the antenna does not come into contact with the plasma, water vapor can be used as a fuel, as opposed to traditional noble gases like argon and xenon. Water vapor is cheap, safe to store, and more plentiful than noble gases in space, allowing for the possibility of refueling deep space probes. The purpose of designing a set of superconducting magnets is to eliminate the power requirement of an electromagnet, while still maintaining the same field geometry. Permanent magnets alone eliminate this power requirement, however, their magnetic field geometry is less than optimal. Modifying this field structure has been shown to be possible with superconductors via the Meissner Effect. Combining a permanent magnet with a high-temperature type II superconductor yields a uniform magnetic field geometry for use with the helicon thruster. Furthermore, the implementation of such a system, and the inclusion of a mechanism to produce a converging magnetic field upstream, can drastically improve plasma confinement, and in turn the efficiency of the system. In order to determine the performance characteristics of the water vapor helicon thruster, plasma diagnostic analysis had to be adapted for molecular plasmas. Data from probes that determine electron temperature and electron density are more difficult to analyze for plasmas that have multiple positively and negatively charged species. In addition, water vapor is corrosive to probes that must come into contact with the plasma. Emission spectroscopy is a non-invasive diagnostic technique that can distinguish different species within the plasma and be used to estimate the electron temperature and density with the help of a computer model. This method was employed to determine if water vapor can be used as a viable fuel, extending the possible lifetimes and range of small satellites.
