Magnetic Geometry and Physics of Advanced Divertors: The X-Divertor and the Snowflake

Advanced divertors are magnetic geometries where a second X-point is added in the divertor region to address the serious challenges of burning plasma power exhaust. Invoking physical arguments, numerical work, and detailed model magnetic field analysis, we investigate the magnetic field structure of advanced divertors in the physically relevant region for power exhaust - the Scrape-Off Layer (SOL). A primary result of our analysis is the emergence of a physical "metric", the Divertor Index DI, that quantifies the flux expansion increase as one goes from the main X-point to the strike point. It clearly separates three geometries with distinct consequences for divertor physics - the Standard Divertor (SD, DI = 1), and two advanced geometries: the X-Divertor (XD, DI > 1) and the Snowflake (SFD, DI < 1). The XD, therefore, cannot be classified as one variant of the Snowflake. By this measure, recent NSTX and DIIID experiments are X-Divertors, not Snowflakes.