2024 – Controlling the NT shape boundary flux to expand achievable parameter space in NT plasmas (with possible EFC piggyback)

Controlling the NT shape boundary flux to expand achievable parameter space in NT plasmas (with possible EFC piggyback)

2024 Research Campaign, Negative Triangularity

Purpose of Experiment

Negative triangularity (NT) plasmas have shown exceptional promise in recent DIII-D and TCV experiments, offering a pathway to an inherently ELM-free fusion pilot plant (FPP). However, since all existing machines are designed with positive triangularity in mind, it is difficult to answer outstanding questions regarding NT FPP operation without the construction of a new device. This experiment aims to mitigate that challenge by providing methods with which the DIII-D tokamak can be more fully utilized to study remaining NT challenges. If successful, this would enable a more robust assessment of NT FPP physics without requiring the construction of an entirely new tokamak facility. The overall goal of this diverted negative triangularity plasma control study is to identify and incorporate more optimized algorithmic and hardware configurations that allow us to expand DIII-D’s achievable NT plasma parameter operating space to the fullest extent possible given the existing DIII-D Poloidal Field (PF) coilset and actuating power supplies.

Experimental Approach

Plasma simulations indicate DIII-D, with its extensive PF coilset, can confine and control a very wide range of NT shaped plasmas at the >1 Megamp level. The challenge is that realizing these plasmas in DIII-D will require using the existing power supplies in ways not previously used or developed for use. In order to place the right supplies on the right PF coils a new subset of PF coils will need to be used to initiate the plasma. This means a new ‘Breakdown’ plasma controller needed to be developed in simulation and will need to be commissioned and tuned on DIII-D. This is Step 1. Then, in order to maximize the achievable plasma current (IP) for a given choice of NT shape, it is necessary to provide a constraint of some kind on the potentially infinite distribution of PF coil currents that will confine a plasma with that desired shape and IP. A suitable constraint can be applied in hardware by selecting a subset of PF coils – specifically NOT including the subset used to initiate the plasma – and connecting them all in parallel. This insures the sum of these PF coils’ currents will always be zero. That in turn insures a single PF coil current distribution confines a chosen plasma shape and parameter set, and simply scales with IP. We have chosen a subset PF configuration that allows the largest attainable IP using existing supplies while confining an NT plasma shape identified as a desirable testbed for future studies. The Plasma Control System (PCS) was then extensively modified and tuned using a plasma simulation to produce this chosen plasma with the chosen PF coilset at high IP (> 1.0 MA). Step 1 will be done on DIII-D with this hardware and modified PCS. Step 2 will be the commissioning of this chosen PF coilset + modified PCS on DIII-D. When these two Steps are successfully completed DIII-D will be ready to pursue the next phase of NT studies planned for later this year and next.

Interested in a behind-the-scenes look at DIII-D? Join us for a virtual OR in-person tour during Fusion Energy Week (May 5-9)! Sign up for a tour here.

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