Electromagnetic Propulsion Systems
Figure 15: Artist’s rendering of an MPD thruster system in operation (LaPointe, Self-field MPD thruster, 2001).
Electromagnetic propulsion systems expel charged plasma particles, similar to electrostatic thrusters; the temperature and density of plasma generated and expelled by electromagnetic thrusters are, however, considerably larger and produce significantly higher exhaust velocities (Jordan, 2000). The basic operating principle for all electromagnet thrusters is the production, acceleration, and expulsion of plasma through the use of powerful electromagnetic fields (Jordan, 2000). A conductive propellant, typically a high mass gas such as xenon, is injected into the primary chamber. Once inside, perpendicular electric and magnetic fields interact with the propellant, and electrons traversing across or trapped within said fields ionize the injected particles (Jahn & Choueiri, 2002). The horizontally located magnetic field accelerates the positively charged ions, and collisions from trapped electrons further contribute to the acceleratory effects (European Space Agency, 2004). Powerful magnetic fields allow for denser, higher temperature plasma to be accelerated and expelled, while maintaining high exhaust velocities and high mass efficiencies (Jahn & Choueiri, 2002). Several varieties of electromagnetic thrusters are comparatively analyzed, and an overall recommendation is concluded indicating the thruster best meeting the stated mission constraints.
Pulsed Inductive Thruster
Figure 7: Diagram of a PIT propulsion system (Jordan, 2000).
This website considers the Pulsed Inductive Thruster (PIT) technology as a primary potential solution to the requirements for viability. In Figure 7, a basic diagram for the PIT systems is shown; a conductive strip surrounds the thrusters, an insulating strip separates the conductive strip from the internal plasma chamber, and the internal chamber is closed on one end (Jordan, 2000). The conductive strip is connected to a large capacitor bank, and the internal chamber is connected to a propellant injector. When the capacitor bank discharges, a large electromagnetic field is generated within the internal chamber, and the propellant injector releases the propellant, typically xenon or similar inert gas, into the chamber (Jordan, 2000). As the propellant interacts with the strong electromagnetic fields, ionization occurs leaving positively charged particles within the chamber; these particles comprise a plasma with an opposing current to the generated electromagnetic field, rapidly expelling the plasma through the open end of the thruster (LaPointe & Mikellides, 2002; Jordan, 2000). One primary difference from other electromagnetic propulsion systems is the pulsed nature of this thruster; continual electric and magnetic fields are not maintained, with a pulsed electromagnetic field generated instead. A critical concern in utilizing the PIT system for long distance, high mass, manned missions is the effect of extended human exposure to unshielded electromagnetic fields. However, the system still meets the stated constraints, as it is a possibly viable technology for long distance, high mass flights. Within the category of electromagnetic thrusters, the PIT system has a moderate Isp level, but both a high energy conversion efficiency and a high thrust level, as noted in Table 1 (Jordan, 2000). This indicates a relatively high possible viability as a propulsion system for high mass, long distance transit missions.
Magnetoplasmadynamic Thruster
Figure 9: Alternate cross section of the MPD thruster: J denotes the electric field, B denotes the magnetic field, and J x B denotes the Lorenz force and particle vector (Jordan, 2000).
Another type of electromagnetic thruster is the magnetoplasmadynamic (MPD) thruster; as shown in Figure 8, a cathode in the center of the thruster is surrounded by the anode, generating the high strength electric field necessary for operation (Jahn & Choueiri, 2002). A large potential difference between the cathode and anode creates the ionization region, where a strong electric field bombards and ionizes the neutral gaseous propellant; therefore, the cathode, located centrally, extends the entire length of the proposed ionization region (Jordan, 2000). This electric current produces an azimuthal magnetic field, which helps to accelerate the positively charged plasma. Depending upon the energy and temperature of the plasma, two differing mechanisms produce the acceleratory magnetic field; in low plasma systems, a higher electric charge between the cathode and anode creates a sufficient magnetic acceleration field (a “self-field” system), but for higher energies and temperatures, an external magnetic field is necessitated, allowing for a lower electric current but a higher plasma energy (an “applied field” system) (Jordan, 2000). Contrasting with the self-field system depicted in Figure 8, Figure 9 contains a depiction of an applied field system. Two different operating states further distinguish types of MPD thrusters. In quasi-steady state, or pulsed state, operation, the MPD thruster discharges across the cathode-anode arc in a capacitive manner, allowing for higher magnetic fields and higher currents over an average period of time (Jordan, 2000). However, arc erosion of the cathode is more severe in the quasi-steady operational state, limiting the effective lifetime of the MPD system. In steady state operation, the other operational state, the MPD thruster produces both high thrust and high Isp, but the large power requirement reduces operability and practicality of any such system (Jordan, 2000; Jahn & Choueiri, 2002). A combination of these two operational states is achieved through a quasi-steady pulse mode that operates for specific periods of time to allow a steady state pattern to control the plasma acceleration (Jahn & Choueiri, 2002). Overall, the MPD thruster offers extremely high Isp ranges, between 1,000 and 11,000, and extremely high thrust levels, between 20 and 200,000 mN, as noted in Table 1 (Jordan, 2000). Lower efficiency ranges, between 10% and 40%, and a high tendency for cathode corrosion indicate two significant detrimental effects of the MPD propulsion system (Jahn & Choueiri, 2002; Jordan, 2000). Altogether, the MPD propulsion system constitutes a possibly viable propulsion system suited for high mass, long distance transportation.
Propulsion Systems Considered Currently Unviable
In addition to the aforementioned electromagnetic propulsion systems, three alternate designs were not fully discussed, due to their inability to fully meet the conditions for validity. The first of these propulsion systems is the Pulsed Plasma Thruster (PPT); an electric arc across an anode and a cathode towards the rear of the thruster generates the plasma, while the magnetic field generated by the arc expels the charged particles (Jordan, 2000). This electric current is generated by a capacitor, eliminating the possibility of continual operation. This report considers the PPT system unviable for the purpose of high mass transport due to the extremely miniscule amount of thrust expected, between 0.005 and 20 mN as noted in Table 1. Additionally, propellant utilization efficiencies between 5% and 15% further increase the unviability of the PPT system (Jordan, 2000). This report considers therefore the PPT system to be unviable within the stated constraints. Another technology omitted from full analysis is the Electrode-less Plasma Thruster (EPT) system. With capabilities of up to 2.79 N of thrust, an Isp of approximately 3350, and an operating efficiency of 91%, the EPT system is a seemingly viable possibility for future high mass, long distance missions (Emsellem, n.d.). The EPT utilizes a gasdynamic mirror magnetic concept to propel the charged plasma from the open end of the spacecraft; this concept, while not fully outlined here, would allow the acceleration of the plasma without the use of electrodes, thereby eliminating the possibility of electrode corrosion and massively extending the operational lifetime of the thruster (Kammash & Tang, 2005). While a highly promising technology, its full inclusion in this report is restricted by its technology readiness level of 3, far below an acceptable standard for viability (Kammash & Tang, 2005). Finally, the Helicon Double Layer Thruster (HDLT) provides another developing alternative to the field of electromagnetic propulsion. Utilizing radiofrequency systems to generate plasma, these particles are then accelerated and expelled when in the presence of a diverging magnetic field, which is also generated by the HDLT system (Pottinger, Lappas, Charles, & Boswell, n.d.). Initial results from the testing of an HDLT system indicate a low Isp of 280, which is unviable as a large mass or long distance transit propulsion system (Pottinger, Lappas, Charles, & Boswell, n.d.). Additionally, the HDLT is in developmental stages, and when combined with the limited future prospects of such a system, it was not considered as a current viable solution within this report. Of these developing but currently not viable technologies, the EPT system is currently considered the most viable propulsion option pending further development.
Electromagnetic Propulsion Systems: Conclusion
Figure 16: A design showing the operation of a MPD propulsion system (Lucas, 2005).
Through comparison between varying electromagnetic propulsion systems, this report concludes that the MPD thruster system is currently the most viable for high mass, long distance transit missions. While the PIT system has a comparable Isp range, a higher efficiency range, and a significantly higher power to thrust ratio (kW/N), the MPD thruster offers both a pulsed and a steady state of operation, establishing a compromise between efficiency and functionality; the full range of numbers are depicted in Table 1. This report also concludes that the EPT system is a potentially viable alternative, but requires substantially more development and research. As summarized by this report, both the EPT and MPD systems meet the requirements for viability as previously stated, and the MPD system is recommended as the primer electromagnetic propulsion system, while the EPT system is recommended as a viable alternative pending further development.