Transient Behavior of Negative Hydrogen Ion Extraction from Plasma

For plasma source, the extraction of negative ions is quite different from that of positive ions. To understand the effect of extraction field on plasma, the time-dependent behavior of negative hydrogen ion extraction from a negative ion source has been studied by particle-in-cell simulation in the collisionless limit. The simulations have shown that, due to the difference in dynamics between electrons and ions, the imbalance of the numbers of charged particles occurs in the source, results in the broadening of plasma sheath and the great increase of plasma potential. The resultant high sheath field and the ambipolar electric field in plasma make the negatively charged particles congregate inside the sheath and move toward the extraction outlet. The emission area of negative ions is much smaller than that of the extraction aperture, which is in sharp contrast to the case of positive ion extraction.