Stabilization of ion–ion instability with the aim of improvement transportation characteristics of a negative ion beam

• Published in: Review of Scientific Instruments 2002-03, Vol.73 (3), p.1149-1152
• Main Authors: Goretsky, V. P., Soloshenko, I. A.
• Format: Article
• Language: English
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/1.1430509

In the transportation of ion beams in a rarefied gas the plasma is formed due to the ionization of gas particles by the beam ions, and this plasma partially compensates the space charge of the ions (and thus improves the conditions of beam transportation). Moreover, in the case of a negative ion beam at a certain critical value of gas pressure, the electric field in the beam changes its sign so that at high pressure the beam is transported in the focusing field. The regime of high pressures is attractive also for the reason that ion–electron and ion–ion instabilities, which develop at that, are not harmful—they may cause just a little growth of the beam emittance. However, in this regime losses of the beam ions due to recharging are essential. As to pressures lower than the critical one, at which these losses are not so essential, the beam transportation is practically impossible due to large-scale ion instability which in certain time points results in almost complete decompensation of the beam space charge. In the present proceeding the way of stabilization of this instability is proposed, which gives the possibility of negative ion beam transportation at low pressures. Mechanism of the stabilization consists in diminishing the wavelength of unstable oscillations by means of the shielding action of the electrons introduced from the heated emitter and held in the beam by a periphery magnetic field. Experiments performed with the beam of hydrogen negative ions with 30 mA current and 10 keV energy have shown that introducing the electrons into the beam, which is transported at low pressure, really provides a essential reduction of ion–ion oscillations amplitude and, correspondingly, improves transportation characteristics of the beam.