Experiments and simulations of a large area ECR source as an electric propulsion neutralizer.

摘要

The windowed electron cyclotron resonance (ECR) source, invented by Getty, was modified into a windowless ECR source to investigate the potential use of this device as an electron source for the neutralizer of an ion thruster system. This plasma source utilizes linear arrays of permanent magnets placed at the end of a large S-band microwave horn. These magnets are held inside a grill with alternating rows of open spaces and aluminum cross bars. The metal bars are cross-polarized so that microwave radiation transmits through the grill with low reflection.; The peak electron density and electron temperature measured 1 cm from the grill surface were 5 x 1010 cm-3 and 10 eV, respectively, for 200 W input microwave power and 1 mTorr argon gas pressure. At the axial distance of 25 cm from the grill surface, these values were 1 x 1010 cm-3 and 4 eV, respectively.; The extracted electron current with pulsed bias increased with collector areas and with input microwave powers, as expected, though the increase was not linear. Unexpectedly, however, the current increased as the distance of the collector from the resonance zone was increased.; With microwave circuit optimization, the pulsed electron current achieved 0.77 A with a 30-V bias voltage applied to a graphite collector (7.3 cm x 10.7 cm, located 28.3 cm from the grill surface), 200 W input microwave power, and 1 mTorr argon gas pressure. For DC extraction with the same settings, an electron current of 0.51 A was extracted.; The 2D simulations using the MAGIC computer code demonstrated electrons gain energy only near the ECR zone (875 gauss contour). The effectiveness of the steel pole pieces designed for the windowless Getty source was also confirmed. The curvature in either electric or magnetic field profile was verified as a necessary condition for ECR to take place. Electron trajectory plots using the TriComp computer code showed electron trapping by the magnetic mirrors.; Both experiments and simulations indicated that electron cross-field diffusion near the grill surface was weak due to the strong magnetic field. However, the experiments suggested that the diffusion far from the grill is significant due to the weak magnetic field there, where more electron current can be extracted. Hence, a large portion of electrons in regions far from the grill might be supplied by electrons streaming along the edge field lines, instead of electrons diffusing across the center field lines.