Experimental Studies Of Magnetized Plasma Sheath Between Permanent Magnetic Plates

Studies on sheath have been the most fundamental and significan-t questions in plasma physics. In recent years, research on magnetized plasma sheath has attracted more and more attention. There have been many theoretical works on magnetized plasma sheath. Meanwhile, exper-imental studies have been also progressing with the development of diag-nostic methods. The theoretical works focus on magnetized ion sheath in oblique magnetic fields. Experimentally, magnetized ion sheath with different magnetic intensity or orientation and ion velocity distribution function has been investigated.The experiments were conducted in Double Plasma Device (DPD) which contains source region and experimental region. We put two par-allel permanent magnetic plates (diameter160mm, interval140mm) in experimental region of DPD to generate a stable magnetic field up to the value of1200G which is much higher than the previous works. Then, a stainless steel disk was mounted in the middle of the magnetic plates. The disk, whose surface is parallel to magnetic field, was floated or biased so that corresponding sheathes formed near it. The emphases lie in the effects of disk bias as well as gas pressure on magnetized ion and electron sheath. The influence of energetic ion beam on magnetized electron sheath and the difference of magnetized electron sheath between upstream and downstream were also experimentally investigated. magnetic field. The main conclusions are as follows:1. The magnetic field impedes the plasma flow into the center of the magnetic plates, so the plasma density decreased greatly when en-tering the magnetic plates. The magnetic field will cause the drop of both electron temperature and space potential. The effects of colli-sion will enhance with the increase of gas pressure and reduce both plasma density and electron temperature in experimental region. The plasma conditions far away from the disk will not be influenced by the disk bias. When the separation grid2are biased negatively, plasma density will increase while electron temperature decrease.2. When the disk is floated, electron sheath will form near the disk due to the parallel magnetic field. The whole potential distributions also contain a potential dip (virtual cathode) and then a ion sheath to the bulk plasma. The electron sheath and structure of virtual cathode both decrease with the increase of gas pressure.3. As for the magnetized ion sheath, the width and depth of sheath both increase linearly with the decrease of disk bias. While the effects of gas pressure are not obvious.4. The electron sheath extends to the scale of cm when there is a parallel magnetic field due to the increase of Debye length caused by the lower plasma density in magnetic filed. The potential distribution is similar to the situation of floating sheath, that is to say, electron sheath connected by ion sheath and there is a virtual cathode between both sheath. When the disk bias increase, the electron sheath becomes wider while the width of sheath remains almost the same. When gas pressure increase, electron sheath become narrower and ion sheath weaker, so the virtual cathode also becomes weaker. Energetic ion beam will strengthen the ion sheath and weaken both electron sheath and virtual cathode. In addition, the electron sheath in downstream is greater than that in upstream due to the lower density.