Experimental Demonstration of Transient CHI Startup Using a Floating Biased Electrode Configuration

.Results from the successful solenoid-free plasma startup using a new method of transient Coaxial Helicity Injection (transient CHI) demonstrated in the QUEST Spherical Tokamak (ST) are reported. Unlike previous applications of CHI on HIT-II and on NSTX, which required two toroidal insulating breaks to the vacuum vessel, QUEST uses a first-of-its-kind, Floating Single Biased (FSB) electrode configuration, which does not use such a large ceramic electrical insulator also serving as a vacuum break. Instead, the new CHI electrode is simply insulated from the outer lower divertor plate support structure using a ceramic plate within the vacuum vessel [1]. This design was motivated by a CHI reactor design study that permitted the CHI insulator to operate at high neutron fluence [2]. The electrode configuration is also much simpler than that employed in the NSTX and on HIT-II, making it much more readily adaptable to existing ST and tokamak devices. On QUEST, the CHI discharge is generated by driving current along magnetic field lines (the injector flux) that connect the inner and outer divertor plates in the lower divertor region. As the injected poloidal magnetic flux fills the vessel, the CHI injector current is rapidly reduced to zero, causing the injected magnetic field lines to reconnect in the injector region, generating closed flux surfaces. Transient CHI on QUEST has successfully generated toroidal currents of 140kA with 100kA of closed flux current comparable to the previous transient CHI experiments in NSTX and HIT-II. Moreover, the observed experimental plasma current scaling results from QUEST are consistent with the CHI current scaling projection, suggesting the potential for favorable current generation capability in high-field tokamaks. Recent design studies indicate that some non-solenoidal current generation capability is necessary for a compact tokamak/ST-based reactor [3]. These results from QUEST bode well for applying transient CHI in a new generation of compact high-field STs and tokamaks in which the space for the central solenoid is very restricted.

[1] K. Kuroda et al., Nucl. Fusion 64 (2024) 014002

[2] R. Raman et al., Fus. Sci. Tech. 68 (2015) 674

[3] J.E. Menard et al., NF 56 (2016) 106023