Abstract. Mini-Magnetospheric Plasma Propulsion is a potentially revolutionary plasma propulsion concept that could enable spacecraft to travel out of the solar system at unprecedented speeds of 50 to 80 km s '1 or could enable travel between the planets for low power requirements of --1 kW per 100 kg of payload and ~ 0.5 kg fuel consumption per day for acceleration periods of several days to a few weeks. The high efficiency and specific impulse attained by the system are due to its utilization of ambient energy, in this case the energy of the solar wind, to provide the enhanced thrust. Coupling to the solar wind is produced through a large-scale magnetic bubble or mini-magnetosphere generated by the injection of plasma into the magnetic field supported by solenoid coils on the spacecraft. This inflation is driven by electromagnetic processes, so that the material and deployment problems associated with mechanical sails are eliminated.

The efficiency of a plasma thruster can be improved if the plasma stream can be highly focused, so that there is maximum conversion of thermal energy to the directed energy. Such focusing can be potentially achieved through the use of magnetic nozzles, but this introduces the potential problem of detachment of plasma from the magnetic field lines tied to the nozzles. Simulations and laboratory testing are used to investigate these processes for the high power helicon ͑HPH͒ thruster, which has the capacity of producing a dense ͑10 18 −10 20 m −3 ͒ energetic ͑tens of eV͒ plasma stream which can be both supersonic and super-Alfvénic within a few antenna wavelengths. In its standard configuration, the plasma plume generated by this device has a large opening angle, due to relatively high thermal velocity and rapid divergence of the magnetic field. With the addition of a magnetic nozzle system, the plasma can be directed/collimated close to the pole of the nozzle system causing an increase in the axial velocity of the plasma, as well as an increase in the Alfvén Mach number. As such the magnetic field of the nozzle is insufficient to pull the plasma back to the spacecraft, i.e., plasma attachment is not a problem for the system. Laboratory results show that the specific impulse ͑Isp͒ of the system can be increased by ϳ30% by the addition of the nozzle due to the conversion of thermal energy into directed energy in association with a highly collimated profile. An interesting feature of the system is that self-collimation of the beam is expected to occur during continuous operation through plasma currents induced downstream from the magnetic nozzle. These currents lead to magnetic fields that have a smaller divergence than the original vacuum magnetic field so that the following plasma will be more collimated than the proceeding plasma. This self-focusing can lead to beam propagation over extended distances.

No abstract

The High Power Helicon eXperiment operates at higher powers (37 kW) and lower background neutral pressure than other helicon experiments. The ion velocity distribution function (IVDF) has been measured at multiple locations downstream of the helicon source and a mach 3-6 flowing plasma was observed. The helicon antenna has a direct effect in accelerating the plasma downstream of the source. Also, the IVDF is affected by the cloud of neutrals from the initial gas puff, which keeps the plasma speed low at early times near the source.