Anode Polymer In Magnetically Insulated Diode Of High-intensity Pulsed Ion Beam With External Magnetic Field

A magnetically insulated diode (MID) with external magnetic field, in which a perforated polyethylene sheet is coated on the anode surface as an anode polymer, was applied in TEMP-6 and ETIGO-2 type high-intensity pulsed ion beam (HIPIB) apparatuses operating in unipolar mode to produce the high-density ion beam. In the diode of the first apparatus, an one-turn coil used to form a "close" magnetic field is constructed by the cathode around the anode, in order to effectively prevent the leakage of magnetic curves. The continuous and uniform magnetic field was obtained and used to restrict the electrons emitted from the cathode strictly. At a high voltage of 300-350 kV, the anode polymer can be performed to generate homogeneous and stable anode plasma within several hundreds discharge times. However, a one-turn coil used to form an "open" magnetic field is constructed in the latter apparatus. The strict restriction of electrons by magnetic field is limited by the leakage of magnetic curves. The anode polymer only can be performed within 4 discharge times at a high voltage of 1 MV, which produce inhomogeneous and unstable plasma.The formation of anode plasma in external magnetic field MID is derived from the pulsed discharge and electron impact on the surface of anode polymer. The pulse discharge took placed as an electron avalanche along the electric channels formed on the surface. The formation and characteristics of anode plasma is depending on the surface feature of anode polymer. In TEMP-6 type HIPIB apparatus, the surface of anode polymer has a rough and porosity feature after etching by the severe ion bombardment that caused by the surge of plasma. The failure of anode polymer is mainly occurred as a delaminating of polyethylene. However, a smooth and uniform surface morphology is obtained with the electrical channels of plasma formed on the surface of anode polymer in ETIGO-2 type HIPIB apparatus. The anode polymer failed in the rupture of molecular chain of polyethylene dueto the severe bombardment of high-density electron, and the low molecule weightpolyethylene was formed. The change of anode polymer in surface morphology andstructure cause an alteration of plasma in formation condition and characteristics.Meanwhile, the change of the molecule chain structure in surface layer of polyethyleneleads to a reducing insulativity and a decreasing breakdown voltage. The altered surfacemorphology causes a serious distortion and deteriorated uniformity of electrical field on thesurface of anode polymer. As increase of the discharge times of pulse, the ion currentdensity (energy density) of the ion beam extracted from the external magnetic field MIDweakened. When the breakdown voltage varied in fluctuation was applied to the diode, theplasma became unstable and the fluctuation of ion beam correspondingly enlarged. Whenthe voltage pulse of external magnetic field is mismatched with the voltage pulse of diode,the short circuit of diode occurred. The breakdown in local area of anode polymer tookplace due to the severe bombardment by electrons. The cracks were generated on thepolymer surface and caused the serious delamination of the anode polymer. In thebreakdown region, the polyethylene was directly converted to polyethylene wax. Thefailure caused by the short circuit is like to an extreme case after the many discharge oftimes on the polymer surface. Therefore, the lifetime of anode polymer was shorteneddrastically under the short circuits.