Improved local helicity injection efficiency by the equilibrium field predicted from 0-D power balance model*

Local Helicity Injection(LHI) technique has been demonstrated to overcome the difficulties at plasma startup and current-drive in spherical torus [1]. Versatile Experiment Spherical Torus (VEST) has studied LHI technique using arc plasma guns[2]. Recently, it is suggested that the performance of LHI in VEST can be increased by adjusting the equilibrium field determining the coupling between flux ropes and main plasma in closed flux surface, the efficiency of helicity injection. As the flux ropes discharged by the plasma guns inject helicity into the main plasma, the helicity gradient between the flux ropes and the main plasma determines the flow of the helicity. The helicity gradient strongly depends on the equilibrium field structure and strength [3]. As the new controllable power system for equilibrium field coils and new helicity injector are applied, the available field structure regime is extended to make field structures to maintain high helicity injection flow. Considering the relation between helicity flux and equilibrium field, a 0-D power balance model is used to find efficient helicity injection equilibrium field conditions and analyze LHI experiments in VEST. The 0-D model describes experiment results well and suggests the favorable equilibrium field structure condition for improved LHI efficiency in VEST.

References

[1] Bongard, Michael W., et al. "Advancing local helicity injection for non-solenoidal tokamak startup." Nuclear Fusion 59.7 (2019): 076003.

[2] JongYoon Park, An, Y., Jung, B., Lee, J., Lee, H., Chung, K. J., ... & Y.S. Hwang. Fusion Engineering and Design, 96, 269-273 (2015).

[3] JongYoon Park, Kim, T., Eom, T., & Y.S. Hwang, 64th Annual Meeting of the APS Division of Plasma Physics, Oct. 19. Spokane, Washington, the United States, (2022).