Description
A compact net power-producing tokamak with tritium self-sufficiency may not be able to have a full-sized solenoid [1]. In such devices, the initial current generated via a solenoid-free plasma startup method must be ramped up predominantly by non-inductive current drive methods. Transient Coaxial Helicity, a solenoid-free plasma startup method, is well understood for scaling to reactors. It has successfully generated closed-flux currents on three low-aspect-ratio tokamaks of vastly different sizes and configurations. These are HIT-II [2], NSTX [3], and QUEST [4]. Studies for ST40 suggest a 0.5 MA current-generation potential. On NSTX, subsequent induction application to CHI-generated plasmas results in the discharge ramping to 1 MA with substantial solenoid flux savings, and, with neutral beam application, the discharge transitions to H-mode. A transient CHI discharge is generated by driving a short pulse current, using a power supply, through magnetic field lines that connect divertor plates that are electrically separated from each other. As the injected magnetic field lines expand into the vessel, the externally driven current is rapidly reduced to zero. This results in a large-scale global reconnection of the injected field lines in the divertor region, thereby generating a closed flux configuration. The amount of generated current is proportional to the current driven in the divertor coil to generate the injector flux, and for the same injector flux, the externally driven current is inversely proportional to the magnitude of the toroidal field. Compared to inductive startup, transient CHI plasmas naturally have much lower normalized plasma internal inductance, higher plasma elongation, and can form with a naturally diverted plasma configuration, features desirable for advanced scenario operations. Progress in transient CHI startup and improvements to this method, using newer electrode configurations that can generate plasmas suitable for next-generation compact tokamak reactors, will be discussed.
[1] J.E. Menard et al., Nuclear Fusion 56 (2016) 106023
[2] R. Raman, T.R. Jarboe, R.G. O’Neill, et al., Nuclear Fusion 45 (2005) L15-L19
[3] R. Raman, D. Mueller, T.R. Jarboe, et al., Phys. Plasmas 18, (2011) 092504
[4] K. Kuroda, R. Raman, T. Onchi, et al., Phys. Plasmas 32, (2025) 042506