Plasma Physics (physics.plasm-ph)

Abstract: In this study, we explore the influence of the helicity of the magnetic axis-defined as the self-linking number of the curve-on the quality of quasi-isodynamic stellarator-symmetric configurations constructed using the near-axis expansion method (Camacho Mata et al. 2022; Plunk et al. 2019). A class of magnetic axes previously unexplored within this formalism is identified when analyzing the axis shape of the QIPC configuration (Subbotin et al. 2006): the case of half-helicity (per field period). We show these shapes are compatible with the near-axis formalism and how they can be used to construct near-axis stellarators with up-to 5 field-periods, $\epsilon_{eff} \approx$ 1.3%, and similar rotational transform as existing conventionally optimized designs, without the need of a plasma boundary optimization.

Abstract: The compact radio frequency negative ion source NIO1 (Negative Ion Optimization phase 1) has been designed, built and operated by Consorzio RFX and INFN-LNL in order to study and optimize the production and acceleration of H- ions in continuous operation. In 2020 Cs was evaporated in the source to increase the total extracted ion current. After an initial reduction of extracted electron to ion ratio and subsequently an increase of extracted negative ion current, the source performances progressively worsened, because of the excessive amount of Cs evaporated in the source; the extracted electron to ion ratio increased from below 1 to more than 10, while ion current density reduced from max. 67 A/m2 ion current to not more than 30 A/m2). The paper presents the experimental observations collected during Cs evaporation (reduction of plasma light, Cs emission and H$\beta$/H$\gamma$ ratio, etc.) that can help stopping the process before an excessive amount of Cs is introduced in the source. The paper also reports the cleaning techniques tested to remove the Cs excess by the action of hydrogen or argon plasmas; while argon was predictably more effective in surface sputtering, a 3 h Ar plasma treatment was not sufficient to recover from overcesiation.