The helicon plasma stage in the Variable Specific Impulse Magnetoplasma Rocket (VASIMR ®) VX-200i device was used to characterize an axial plasma potential profile within an expanding magnetic nozzle region of the laboratory based device. The ion acceleration mechanism is identified as an ambipolar electric field produced by an electron pressure gradient, resulting in a local axial ion speed of Mach 4 downstream of the magnetic nozzle. A 20 eV argon ion kinetic energy was measured in the helicon source, which had a peak magnetic field strength of 0.17 T. The helicon plasma source was operated with 25 mg s −1 argon propellant and 30 kW of RF power. The maximum measured values of plasma density and electron temperature within the exhaust plume were 1 × 10 20 m −3 and 9 eV, respectively. The measured plasma density is nearly an order of magnitude larger than previously reported steady-state helicon plasma sources. The exhaust plume also exhibits a 95% to 100% ionization fraction. The size scale and spatial location of the plasma potential structure in the expanding magnetic nozzle region appear to follow the size scale and spatial location of the expanding magnetic field. The thickness of the potential structure was found to be 10 4 to 10 5 λ De depending on the local electron temperature in the magnetic nozzle, many orders of magnitude larger than typical laboratory double layer structures. The background plasma density and neutral argon pressure were 10 15 m −3 and 2 × 10 −5 Torr, respectively, in a 150 m 3 vacuum chamber during operation of the helicon plasma source. The agreement between the measured plasma potential and plasma potential that was calculated from an ambipolar ion acceleration analysis over the bulk of the axial distance where the potential drop was located is a strong confirmation of the ambipolar acceleration process.

The accuracy of a plasma impact force sensor was compared with that of the more commonly used inverted pendulum thrust stand using a 5 kW Xe Hall effect thruster. An improved plasma momentum flux sensor was designed and constructed based on a previous design. Real-time force measurements were made with both the plasma momentum flux sensor and the inverted pendulum thrust stand. The plasma momentum flux sensor measured the force exerted onto it by the Hall effect thruster exhaust plume with a resolution of 0.1 mN and an average discrepancy of 2% compared with thrust stand measurements. Experiments were completed using a 9 m by 6 m cylindrical vacuum chamber. The total force from the Hall effect thruster was modulated from 34 to 356 mN by varying both the anode voltage, from 150 to 500 V, and the neutral Xe gas flow rate, from 5 to 15 mg=s.

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, results from a coupled rf and transport model are compared with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility ͓Goulding et al., , p. 107͔ ͑Mini-RFTF͒. Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution rf calculation including the rf parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the rf coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. The effects of absorption near the lower hybrid resonance in the near-field region of the antenna are discussed. Ponderomotive effects are also examined and found to be significant only in very low density and edge regions of the Mini-RFTF discharge.

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The momentum distribution of the current carrying suprathermal electrons is determined from the measurement of X mode transmission near the fundamental of the cyclotron frequency during inductive and radio frequency current drive in the Versator II tokamak. The measurement of the differential attenuation of radiation of two beams with finite and opposite N allows the separation of the wave attenuation due to cyclotron absorption from that due to non-resonant mechanisms. It is shown that such transmission measurements can be used to determine the part of the parallel momentum distribution, which is asymmetric in p 11 , Fa(Pii) =. F(-p 1 ) -F(p 11 )). Measurements of F(p 1 j) and its time evolution generally agree with expectations, although T is found to be lower than predicted by collisional theory in the inductively driven case.