A strong, current-free, electric double-layer with eΦ/kTe∼3 and a thickness of less than 50 debye lengths has been experimentally observed in an expanding, high-density helicon sustained rf (13.56-MHz) discharge. The rapid potential decrease is associated with the “neck” of the vacuum vessel, where the glass source tube joins the aluminum diffusion chamber, and is only observed when the argon gas pressure is less than about 0.5 mTorr. The upstream electron temperature Te appears 25% greater than the downstream Te, and there is a density hole on the downstream edge. This experiment differs from others in that the potentials are self-consistently generated by the plasma itself, and there is no current flowing through an external circuit. The plasma electrons are heated by the rf fields in the source, provide the power to maintain the double-layer, and hence accelerate ions created in the source out into the diffusion chamber.
This review presents the basics of plasma discharges applied to electric spacecraft propulsion. It briefly reports on the mature and flown technologies of gridded ion thrusters and Hall thrusters before exploring the recent yet immature technology of plasma thrusters based on expansion from low pressure high density inductively coupled and wave-excited plasma sources, e.g. the radiofrequency helicon source. Prototype development of plasma engines for future space travel is discussed using the example of the helicon double layer thruster. A summary of highlights in electric propulsion based space missions gives some insight into the challenges of future high power missions in more remote regions of space.
An electric double-layer is generated near the open end of a high-density low pressure helicon sustained radio frequency (13.56 MHz) plasma source which expands into a diffusion chamber. Ion energy distribution functions measured with a retarding field energy analyzer placed in the diffusion chamber with its aperture facing the double-layer show the presence of a low energy peak (∼29 V) around the local plasma potential and a high energy peak (∼47 V) corresponding to a supersonic ion beam (∼2.1cs). At an axial distance 12 cm downstream of the double-layer, the beam density is 14% of the local density at that position and the ion energy gain is approximately 70% of the potential drop of the double-layer. The ion beam is observed from the center out to a radius corresponding to that of the plasma source tube (−6.8 cm⩽r⩽+6.8 cm) and is not greatly affected by the expanding magnetic field. A depression in the total ion flux just downstream of the double-layer—previously measured on the main z-axis of the reactor—is also present across the chamber diameter. Evidence of an electron beam near the closed end of the source tube, generated via “backwards” acceleration through the double-layer, has been observed on a Langmuir probe trace.
Recent developments in laboratory double layers from the late 1980s to the spring of 2007 are reviewed. The paper begins by a lead up to electric double layers in the laboratory. Then an overview of the main double layer devices and properties is presented with an emphasis on current-free double layers. Some of the double layer models and simulations are analysed before giving a more complete description of current-free double layers in radiofrequency plasmas expanding in a diverging magnetic field. Astrophysics double layers are briefly reported. Finally, applications of double layers to the field of plasma processing and electric propulsion are discussed.
The axial force imparted from a magnetically expanding current-free plasma is directly measured for three different experimental configurations and compared with a two-dimensional fluid theory. The force component solely resulting from the expanding field is directly measured and identified as an axial force produced by the azimuthal current due to an electron diamagnetic drift and the radial component of the magnetic field. The experimentally measured forces are well described by the theory.
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