Electrical coupling efficiency of inductive plasma accelerators

Abstract

A single-stage pulsed inductive plasma accelerator was modelled as an inductive mass-driver, with the plasma treated as a rigid slug that acts as the armature. We derive a set of coupled dynamic-circuit equations, with dimensionless coefficients. The functional form of the mutual inductance profile, M(z), was calculated using the magnetic field solver QuickField; an exponential form for M(z) was found to be accurate for a variety of coil-slug geometries. A parametric study of the solutions to the equations was performed in order to determine the conditions that yield high coupling efficiency. High inductance, multi-turn drive-coils yield the highest efficiency for a single-stage device. Using inductive recapture, coupling efficiencies in excess of 90% are possible; without it, the peak efficiency is much lower, η ≦ 55%. We conclude that inductive recapture will be required in order to achieve the high efficiency required of an electric thruster. The efficiency scales favourably with increasing power, although this does not preclude operation at lower power with acceptable efficiency. The presence of an imbedded bias flux in the slug improves the dynamic efficiency for devices without inductive recapture, but offers little improvement when used with inductive recapture.

Comparison of Directly and Inductively Coupled Pulsed Electromagnetic Thrusters

“…It is noted that the efficiencies are quite low, despite the simplifying assumptions in the model. However, only about 7%-8% of the energy is lost to ohmic dissipation, so the energy not utilized for acceleration can arguably be recovered through inductive recapture [22].…”

“…All of these parameters are highly dependent on the coil geometry, propellant distribution, and effective propellant distance from the coil. Typically, in modeling of inductively coupled thrusters, the aforementioned RLC model is modified to include a secondary coil (the plasma) which is coupled to the primary coil through a mutual inductance [15], [18], [22]. These models assume that the mutual inductance follows a function of the form L coil exp(−z/2z decouple ), where L coil is the coil inductance and z decouple is some empirically determined decoupling distance.…”

“…One of the main drawbacks of inductively coupled thrusters is that the mutual inductance between the primary coil(s) and plasma appears to decrease exponentially [15], [16], [18], [22] with distance as the plasma is accelerated, thus making efficient acceleration challenging. One of the keys to unlocking the potential in these devices is to maximize the work done by the magnetic field pressure on the plasma.…”

“…While it has been investigated since at least the 1960s, only recently have efforts been made to model the acceleration process using simple coupled circuit and momentum equations [22]. A conical theta pinch consists of a capacitor bank, transmission line, external switch, and single-turn coil.…”

Effect of Inductive Coil Geometry and Current Sheet Trajectory of a Conical Theta Pinch Pulsed Inductive Plasma Accelerator

“…While Eq. 8 was developed for a planar coil geometry, it has been found to accurately represent the axial inductive coupling behavior of ring-shaped and conical geometries as well [16,17]. We nondimensionalize Eq.…”