Evolution of energy confinement physics and most probable compact ignition test device in magnetic fusion

Abstract

The variation of edge confinement modes such as L-mode, H-mode, QH-mode, and I-mode and transitions between these modes in toroidal devices is attributed to interplay between turbulent inflow plasmas from divertor and outflow plasmas from the edge in magnetic configuration with x-point. A concept of flow impedance is introduced to model edge confinement of plasmas in tokamak and stellarator. The core confinement improvement is largely due to effective core heating profile, and direct ion heating with PNB system is favorable compared to electron heating in generation of sufficient α-power essential for sustaining the ignition state. Validation of transition physics of sustained ignition state from external to internal α-heating is critical for design of the next step magnetic fusion device. The most probable path for a compact ignition test device in magnetic fusion is suggested. The device size and expected performance of a tokamak plasma are projected based on critical review of experimental data of magnetic fusion research accumulated for half a century such as τ<inf>E</inf> scaling laws and n<inf>i</inf>τ<inf>E</inf>T<inf>i</inf> data. A tokamak plasma, V<inf>p</inf> ~ 240 m3, equipped with direct ion heating system that can yield fusion power of ~ 220MW (i.e., α-power up to ~ 45MW) may be sufficient to test ignition state and transition physics. Practical actuators for control of the core and edge confinement which can be developed based on effective core heating and control of inflow plasmas from divertor are suggested. © The Author(s) 2025.