Speaker
Description
In Spherical Torus (ST) with relatively low toroidal field, electron Bernstein wave (EBW) is known to be an efficient heating mechanism for high density plasmas without accessibility problem even at low frequency. Over-dense plasmas beyond L cut-off density was obtained via direct XB mode conversion from perpendicular low-field-side injection in Versatile Experiment Spherical Torus (VEST),[1] and they are utilized as an efficient pre-ionization in Trapped Particle Configuration for the development of a reliable and robust Ohmic start-up method in VEST and later in KSTAR.[2,3]
Relatively large wavelength of 2.45GHz at low toroidal field of the VEST was helpful for the microwave to transmit directly through the evanescent layer between R cut-off and upper hybrid resonance, allowing direct XB mode conversion instead of OXB mode conversion. Efficient XB mode conversion scheme was achieved in both short density scale length (Ln) and magnetic scale length (LB) regions positioned at outboard and inboard sides, respectively.
Pre-ionization scheme utilizing short LB region at the high field side of ST is more favorable to low loop voltage start-up by taking advantage of relatively strong electric field as well. With the enhanced pre-ionization a new merging start-up scheme can be developed by utilizing partial solenoids in VEST, where large stray field from the solenoid is unavoidable because of its geometry.
Solenoid-free start-up scenario is another scheme to be developed with the enhanced pre-ionization by utilizing low loop voltage from the evolution of equilibrium field of outer PF coils. Efficient XB mode conversion scheme utilizing short Ln region at the low field side of ST make this scenario feasible. Flux from external inductance change can be utilized when the plasma is started from outboard and moved inward. Various start-up schemes utilizing the improved EBW-assisted pre-ionization in VEST will be presented.
References
[1] J.G. Jo, et al., Phys. Plasmas 24, 012103 (2017)
[2] Y. An, et al., Nucl. Fusion 57, 016001 (2017)
[3] J.W. Lee, et al., Nucl. Fusion 57, 126033 (2017)
