A new mode of plasma confinement is demonstrated in which essentially all positive ions leave the plasma to only one boundary while essentially all electrons are lost to a different boundary. Sheaths near the plasma boundaries are entirely responsible for this global nonambipolar flow. The bulk plasma remains quasineutral and unperturbed even when all electrons are lost to only one, physically small, location. A necessary condition for global nonambipolar flow depends on the ratio of electron collection area to ion collection area. The plasma electron temperature is significantly higher in the global nonambipolar mode than in the typical ambipolar mode due to a relative increase in confinement of high-energy electrons and a relative decrease in confinement of low-energy electrons.

Equilibrium states of anodic double layers that form near a positively biased disk-shaped electrode immersed in a partially ionized plasma are studied experimentally using electrostatic probes. When the potential drop from the electrode to the plasma is less than a critical value, φ c , an anode glow is observed where a double layer potential drop is within a few millimeters of the electrode surface. For larger biases an anode spot forms where the double layer potential drop is centimeters from the electrode and the intervening region is a plasma more luminous than the bulk plasma. A theoretical model is developed which predicts that φ c ∝ 1/P + C, where P is the neutral pressure and C is a constant, and it predicts hysteresis in the current-voltage characteristic at the electrode; both effects are observed experimentally. The model also provides an estimate for the distance between the electrode and double layer potential drop that agrees with the measurements. Near small electrodes, anode spots are observed to be 'fireballs,' which are spherical in shape. Near larger electrodes 'firerods' are found instead, which have a cylindrical shape. It is shown that firerods are required by global current balance because they have a smaller effective electron collecting area than fireballs. Experiments also confirm that global nonambipolar flow (Baalrud S D et al 2007 Phys. Plasmas 14 042109) accompanies firerods. In this case all electrons are lost through the firerod to the electrode, while all positive ions are lost to the other plasma boundaries.

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.

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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.

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The VAriable Specific Impulse Magnetoplasma Rocket (VASIMR®) is a high power electric spacecraft propulsion system, capable of Isp/thrust modulation at constant power [F. R. Chang Díaz et al., Proceedings of the 39th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 8–11 Jan. 2001]. The VASIMR® uses a helicon discharge to generate plasma. This plasma is energized by an rf booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. In the experiments reported in this paper, the booster uses 2–4 MHz waves with up to 50 kW of power. This process is similar to the ion cyclotron heating (ICH) in tokamaks, but in the VASIMR® the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The single-pass ICH produced a substantial increase in ion velocity. Pitch angle distribution studies showed that this increase took place in the resonance region where the ion cyclotron frequency was roughly equal to the frequency on the injected rf waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR®. This paper will review all of the single-pass ICH ion acceleration data obtained using deuterium in the first VASIMR® physics demonstrator machine, the VX-50. During these experiments, the available power to the helicon ionization stage increased from 3 to 20+ kW. The increased plasma density produced increased plasma loading of the ICH coupler. Starting with an initial demonstration of single-pass ion cyclotron acceleration, the experiments demonstrate significant improvements in coupler efficiency and in ion heating efficiency. In deuterium plasma, ≥80% efficient absorption of 20 kW of ICH input power was achieved. No clear evidence for power limiting instabilities in the exhaust beam has been observed.

A mechanism for ambipolar ion acceleration in a magnetic nozzle is proposed. The plasma is adiabatic (i.e., does not exchange energy with its surroundings) in the diverging section of a magnetic nozzle so any energy lost by the electrons must be transferred to the ions via the electric field. Fluid theory indicates that the change in plasma potential is proportional to the change in average electron energy. These predictions were compared to measurements in the VX-200 experiment which has conditions conducive to ambipolar ion acceleration. A planar Langmuir probe was used to measure the plasma potential, electron density, and electron temperature for a range of mass flow rates and power levels. Axial profiles of those parameters were also measured, showing consistency with the adiabatic ambipolar fluid theory.