The ECR volume effect and its consequences

Summary form only given. During the past decade and a half, remarkable progress has been made in the technology of the ECR ion source, due, in large-part, to an improved understanding of the atomic processes that limit ion production. A number of design improvements and technical developments can be cited that individually or in combination have significantly improved the high-charge-state capabilities of these sources. For example, for a given design, the intensities of the high-charge-state beams extracted from these sources have increased by improvement in plasma confinement; by operating at high frequencies, as predicted by theory; by improvement in vacuum quality; by supplementing their plasma-discharges with cold electrons; by discovery of the gas mixing effect; and by increasing the number or physical size of their ECR zones. This article is devoted to the volume ECR effect and its consequences. Since the probability for adsorption of RF power depends on sizes of embedded ECR zones, sources with larger ECR zones have the ability to adsorb more ECR power and consequently, to accelerate larger populations of electrons to higher energies and consequently, to produce higher charge-states. Enlarged ECR zones have been achieved by engineering the central magnetic field region of these sources so they are flat and in resonance with single-frequency RF power. Alternatively, in conventional minimum-5 geometry sources, the number of ECR surfaces can be increased by heating their plasmas with multiple, discrete frequency microwave radiation. Broadband RF power offers a simpler, lower cost and more effective means for increasing the physical sizes of the ECR zones within the latter type of source. Recently, broadband RF power has been utilized to enhance the high-charge-state performances of conventional-B geometry ECR ion sources. This presentation provides the elementary theory of why enlarged ECR zones are desirable and provides experimental evidence that sources with enlarged ECR zones convincingly outperform their single-frequency, conventional-B geometry counterparts