Performance characteristics of the Nonambipolar Electron Source using permanent magnets

Summary form only given: The nonambipolar electron source (NES) is a radio frequency (rf) plasma-based electron source that does not rely on electron emission at a cathode surface. All electrons are extracted at an electron sheath through a biased ring and all ions are lost radially to a separately biased cylindrical graphite Faraday shield. Plasma density is increased and electron confinement at the ring is improved by the addition of an axial magnetic field. An electromagnet in the original NES has been replaced by a NdFeB permanent magnet array. The array consists of 0.875 inch diameter magnets with lengths of 0.375 and 1 inch that are uniformly distributed around the cylindrical chamber such that there axis is aligned with the axis of the chamber (magnet polarity is also uniformly aligned). Operation with permanent magnets is desired to reduce the weight and power requirements of the system, especially for use with ion thrusters. Use of permanent magnets to provide the aforementioned axial magnetic field adds the complication of a point of divergence in the field as opposed to the solenoidal field of an electromagnet. Moving NES and the antenna relative to the magnetic field has a significant effect on extraction of electron current. Current extraction measurements and the effects of the magnetic field on the system will be discussed. The plasma is created using a single turn copper antenna with forward powers ranging from 350-1800 watts rf. Biases on the graphite ion collection cylinder and electron extraction ring are varied separately between -20 V and -60 V for the cylinder and +0 V and +20 V for the ring. Electron current is collected with a graphite anode that is a distance of 3 cm to 5 cm away from, and biased 0 V to 25 V relative to, the extraction ring. Emissive probe measurements are being used to determine the plasma potential along the axis of the system to observe how the electron sheath at the ring is affected by the three varied biases. As much a- 9 A/cm² of electron current has been extracted with an argon flowrate of 15 seem and 1000 watts rf power at 13.56 MHz.