DWDT

Thruster Concept

The Direct Wave-Drive Thruster (DWDT) is a new propulsion concept that uses waves to transfer momentum directly to a plasma. As an inherently electrodeless device, a DWDT can avoid lifetime limitations due to erosion, as well as utilize a variety of propellants. Previous waves-based thruster concepts have used a variety of waves to heat a plasma and obtain thrust by expanding through a magnetic nozzle. In contrast, a DWDT attempts to couple directed wave momentum into a plasma for acceleration. Theoretically, this momentum transfer can be achieved inductively similarly to Pulsed Inductive Thrusters. The key difference is a DWDT operates continuously to inject wave momentum, whereas PIT discharges its energy in a single pulse.

The Direct Wave-Drive Thruster Experiment (DWDTX) consists of a plasma source antenna (PSA), a confining magnetic field configuration, and a wave-launching antenna (WLA). The set-up is illustrated in Figure 1.

Theory

The theory behind the DWDT focuses on the coupling interaction between the wave-launching antenna (WLA) and the plasma. In fact, all of the momentum acquired by the plasma must come from the WLA. Therefore, in order to calculate the total thrust, we must simply determine the total electromagnetic pressure acting on the plasma from the WLA. Since the electromagnetic pressure scales with the square of total current in the antenna, the total thrust follows the simple scaling law: $\mathbf{F} = C_{T} \mathbf{J}_a^2$ We can calculate how efficiency scales by taking into account various loss mechanisms in the plasma and WLA. Most of these losses are resistive and radiative and therefore scale with current squared, whereas the total thrust power scales with current to the fourth power. $W_\textrm{loss} = R_\textrm{eff} \mathbf{J}_a^2$ $W_\textrm{thrust} = \frac{\mathbf{F}^2}{2\dot{m}} = \frac{C_T^2}{2\dot{m}}J_a^4$ As a result, we can see that efficiency should increase with increasing power to the WLA. $\eta = \frac{W_\textrm{thrust}}{W_\textrm{thrust} + W_\textrm{loss}} = \frac{1}{1 + \frac{W_\textrm{loss}}{W_\textrm{thrust}}} = \frac{1}{1 + \frac{2\dot{m}R_\textrm{eff}}{C_T^2 J_a^2}}$ This result is qualitatively similar to laws derived for MPD thrusters. In general, the thrust coefficient and loss coefficient are complicated functions of the wave mode, plasma parameters, and system geometry. As a result, we can see that efficiency should increase with increasing power to the WLA.

Experimental Results

The DWDTX is currently operating in the EPPDyL's "Orange" Vacuum Tank. A gif below shows the plasma expansion for increasing powers to the WLA. Subsequently, measurements of the ion energy distribution functions (IEDF) were taken with a retarding potential analyzer (RPA).
The increase in wave power clearly increases the number of higher energy ions in the distribution function.

Future Research

Our initial ion energy measurements suggest that ions are being accelerated by our antenna structure. In order to verify this behavior and demonstrate the acceleration mechanism, we will first be attempting to measure the propagating wave-modes inside the system. Next, we will be configuring our torsion arm thrust stand to take thrust measurements of the system.

DWDT Awards and Honors

Jet Propulsion Laboratory's (JPL) Research Poster Conference Award, 2015. (Poster linked below)

Matthew Feldman