Back to the future: reviewing CNT en route to EPOS*

The use of "true" (irrational) magnetic flux surfaces to confine a non-neutral plasma (NNP) is a relatively rare combination, straddling the divide between fusion energy research and basic plasma science in a significantly lower density, lower temperature regime. A likely application of this scenario is for the creation of magnetically confined electron-positron pair plasmas (for example, if the nested flux surfaces are first filled with electrons, into which one or more pulses of positrons are then injected until quasineutrality is achieved). Such plasmas are the goal of the APEX (A Positron Electron eXperiment) Collaboration, the newest branch of which is the EPOS (Electrons and Positrons in an Optimized Stellarator) project.

By far the most extensive research to date on the topic of non-neutral plasmas in flux surfaces has been done at or in connection with the Columbia Non-neutral Torus (CNT). A four-coil, low-aspect-ratio, 0.2-T stellarator optimized for a large volume of robust flux surfaces, CNT was used for non-neutral plasmas from 2004-2011 and generated numerous important theoretical and experimental results. Among the highlights were the verification of macroscopically stable single-species plasma equilibria [Kremer et al., PRL 97, 095003 (2006)], the investigation of instabilities at finite ion fraction [Marksteiner et al., PRL 100, 065002 (2008)], and systematic studies of plasmas of arbitrary neutrality [Sarasola et al, PPCF 54 124008 (2012)].

In this poster, we will first review some key findings from CNT, then relate these to non-neutral plasmas in linear devices (the most commonly employed geometry for NNPs). Then we will describe the resulting implications for the design and development of EPOS, and how these compare to more typical stellarator optimization targets for fusion devices.