Design Details of the Transient CHI Plasma Start-up System on NSTX-U

Elimination of the central solenoid would simplify the engineering design of a fusion nuclear science facility and tokamak based devices. The method of transient coaxial helicity injection (CHI) has successfully demonstrated formation of a high-quality closed flux plasma in NSTX and will be used as the front end of the start-up method for a full demonstration of noninductive current start-up, followed by noninductive current ramp-up using neutral beams in the NSTX-U device that is now under construction at the Princeton Plasma Physics Laboratory. CHI is implemented by driving current along open field lines that connect the lower inner and outer divertor plates of a spherical torus. The engineering system requirements and the design of the CHI system on NSTX-U are described.