Intense Neutralized Beams

Compared to conventional low intensity ion sources, high intensity particle beams, derived from pulsed, positive- or negative-ion sources, may offer distinct benefits for applications in beam heating and current drive in fusion reactors, inertial confinement fusion, and space defense. In recent work at the University of California, Irvine we have shown that pulsed, magnetically- insulated diodes can produce intense beams of positive or negative ions with current densities in the range of (at 0.5 MeV): 10- 100 A /cm for negative beams and 100- 1000 A.cm2 for positive beams. The normalized emittance of these intense sources is approximately, e = 0.02 cm-mrad, which is comparable to existing volume-or surface-ionization negative-ion sources; the beam divergence angle is also comparable, AO = 10 mrad. We have also used these same sources to produce intense beams of high atomic number, such as carbon. Inertial confinement fusion applications require ion energies approaching 10 MeV /amu and may be achieved using existing pulsed-power techniques. In SDI applications these sources would serve as a preinjector to a pulselac, or multistage linear- induction accelerator, to achieve at least an order of magnitude increase in ion energy. For long range propagation these beams must be current- and charge-neutral, achieved by stripping the negative beam with a foil, or by photoionization, or by adding an appropriate quantity of costreaming electrons to a positive beam. In any event, an intense neutral atom beam would eventually ionize, taking on the character of an intense, neutralized positive ion beam. We have demonstrated that a neutralized beam continues undeflected in a transverse magnetic field and low density magnetized plasma by collective beam processes, including diamagnetism and the ExB polarization drift; the ExB drift occurs2provided syf/f2icient beam density is available for polarization ie, e = 1 + (w /Q) » (M. /m) where w, Sì are the ion plasma, cyclotron frequencies and Mi, me are RIe ion, electron masses, respectively. Thus collective processes would considerably extend the propagation range compared to conventional low intensity neutral particle beams. 1