To improve the purity of ion beams produced in a plasma focus (PF), the dependence of the characteristics of the nitrogen ion beams on the shape of the anode was investigated. Two types of anodes, i.e. rod type (type A) and hollow type (type B) were used with a PF pre-filled with nitrogen gas. Thomson parabola spectrometer measurements showed the existence of a large amount of impurity ions, O 1-2+ , C + , and Cu 1-2+ , with nitrogen ions (N 1-3+ ) and the percentage of nitrogen ions is only 25% in type A. In contrast, in type B the impurity is extremely reduced and the percentage of nitrogen ions is enhanced to 91%. The maximum energies and power brightnesses of the nitrogen beams were evaluated to be 0.5 MeV and 0.23 GW cm −2 sr in type A and 1 MeV and 1.6 GW cm −2 sr in type B; hence, energy and brightness are also enhanced in type B. From x-ray pinhole images we see that strong electron irradiation occurs on the top of the anode in type A, whereas in type B, electron irradiation is weak and irradiation area is far from the pinch plasma. Hence, the impurity ions observed in type A are considered to be produced in the following process. Due to the strong irradiation of electrons on the anode top, the electrode and the absorbed gas on the electrode are vaporized, mixed with pinch plasma and accelerated with nitrogen ions.

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A 9 cm Hall plasma source with a cusp type magnetic field distribution applied in a cylindrical ceramic channel exhibited discharge characteristics similar to conventional coaxial Hall thrusters. Probe measurements inside the plasma source indicated that localization of the acceleration and ionization regions depends strongly on the magnetic field distribution, but established mainly in the cylindrical part of the channel. In addition, ion flux measurements demonstrated high ionization efficiency of the cylindrical source in a broad range of gas flow rates and discharge voltages. The absence of the inner body in this novel plasma source enables miniaturization of the design and placement of segmented electrodes on the channel back wall to control the electric field distribution. References:This work was suported by AFOSR and DARPA