Prospects for I-mode Operation in SPARC

While the SPARC tokamak is designed to meet performance objectives using conventional H-mode operation, the device is flexible enough to attempt access to the I-mode regime. On existing tokamaks I-mode has demonstrated satisfactory energy confinement coupled with low core impurity accumulation and an absence of Type I Edge Localized Modes (ELMs), making it a desirable scenario for burning plasmas. The high field path in particular stands poised to benefit, since the power requirement to access I-mode has a weaker dependence on toroidal field B_T than does that for H-mode, thereby opening a significant window for I-mode operation at B_T of several Tesla and above. For the specific case of SPARC, nominally at B_T=12.2T, the best empirical information available suggests access to I-mode is feasible for single null equilibria with B×▽B directed away from the X-point. Like most ELM-suppressed regimes, I-mode exhibits a reduced pedestal pressure relative to conventional H-mode. We assess the impacts of this by imposing a penalty on the pedestal assumed in discharge simulations used to project SPARC performance. Finally we discuss potential for transient divertor heat loading relative to the case of Type I ELMs.