| AVS 65th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS-ThA |
| Session: | Plasma Diagnostics, Sensors and Controls |
| Presenter: | Ya-Ming Chen, University of Houston |
| Authors: | Y.-M. Chen, University of Houston R. Sawadichai, University of Houston S. Tian, Lam Research Corporation V.M. Donnelly, University of Houston D.J. Economou, University of Houston P. Ruchhoeft, University of Houston |
| Correspondent: | Click to Email |
Space charge neutralization of an ion beam extracted from a plasma is crucial for advanced plasma processes which require precise control of the ion flux and the width of the ion energy distribution (IED). In previous studies, filaments thermionically emitting electrons were used for neutralizing the space charge, which would otherwise cause the ion beam to diverge owing to Coulomb explosion.1, 2 However, the performance of the neutralizing filaments is restricted by their limited lifetime and required extra power supplies. This work reports that a self-neutralized positive ion beam can be extracted from a pulsed plasma. In particular, a nearly monoenergetic ion beam was realized by applying a synchronous DC bias in the afterglow (plasma-off) of the plasma. A mechanism of the self-neutralization process is proposed based on space-time resolved ion and electron current (Ii and Ie) measurements done by a movable Faraday cup. The measurements revealed that electrons from a low-density plasma immediately downstream of the ion extraction grid neutralize the space charge in the beam transport region. Time-resolved plasma potential measurements suggest that there could be two periods for low-energy ions and electrons to leave the source and form the low-density plasma in the downstream of the ion extraction grid. Among the observations are 1) with increasing distance, d, from the grid Ie > Ii at d = 1-5 cm, Ie = Ii at d ≈ 20 cm, and Ie << Ii at d > 30 cm; 2) Ii decays by less than 1/r2; 3) electron energy peaks at ~30 eV, ascribed to acceleration by the 100 eV ion beam transiting through the downstream plasma. Ion flux and IEDs were also studied using a retarding field energy analyzer (RFEA). Detailed explanations for the self-neutralization process will be described.
Work supported by NSF.
References
[1] R. G. Jahn, Physics of Electric Propulsion. (Courier Corporation, 2006).
[2] J. P. Chang, J. C. Arnold, G. C. H. Zau, H.-S. Shin and H. H. Sawin, J. Vac. Sci. Technol A:
15 (4), 1853-1863 (1997).