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Ion confinement in electron cyclotron resonance ion sources (ECRIS): importance of nonlinear plasma-wave interaction

In electron cyclotron resonance ion sources (ECRIS), ions are produced in a highly ionized microwave heated plasma contained in a minimum-B magnetic trap. Recent experiments have revealed that the absorption of electromagnetic wave energy is accompanied by a weak ion sound turbulence, a signature of a decay instability of the electromagnetic plasma wave excited by the admitted microwave. Ions are heated by the ion sound waves more effectively than by electron ion collisions. In case of gas mixtures ("gas-mixing"), it is possible to heat preferentially the lighter species under appropriate conditions. As confinement of ions is best at low ion temperatures, selective ion heating enhances losses of the preferentially heated ion component, reducing at the same time losses of the less effectively heated ions. Experimental results are compared with a theory of the sketched processes. We further (qualitatively) discuss open questions like plasma transport in highly ionized multi-component plasma contained by conducting walls, direct influence of the weak ion sound turbulence on plasma transport, and the action of ponderomotive forces in the so-called resonance zones.