2024 – Tungsten radiation in the access to the ITER Baseline Scenario and its control options with ECH power

Tungsten radiation in the access to the ITER Baseline Scenario and its control options with ECH power

2024 Research Campaign, ITER Integrated Scenarios

Purpose of Experiment

The purpose of this experiment is to address the issue of Tungsten radiation and accumulation in the access and exit phases of the ITER Baseline Scenario (IBS), and to assess the strategy of using ECH to control it, on both the access/exit and the burn phases. Especially with the new ITER plans that consider replacing the whole outer wall with W tiles, it is crucial to study how plasmas in each phase of the scenario are affected by the W radiation. DIII-D is uniquely placed to study this, by exploiting the benign carbon walls, and using radiators that correctly mimic the Tungsten radiative loss rates at the lower DIII-D temperatures (more details on this in the Background section). Relatively comprehensive results about the impact of W and W-equivalent radiators (e.g. Kr and Xe) on the flattop phase of the IBS were done in a recent campaign [1, 2], showing stark differences between “real” W and W-equivalent radiators in terms of confinement, radiation, profiles of concentration and radiated power, and survivability. The access phase is even more challenging than the flattop, with all the profiles in a transient state, the L-to-H transition in the middle, and a lower temperature than the later phases (with higher W radiation expected). We define “access” as t=0 to t=betan flattop +300 ms, and burn, or flattop phase at t=betan flattop+300 ms, until the programmed Ip and betan rampdown. The exit phase can also be very challenging, when the alpha-heating decreases rapidly, the temperature is reduced in ITER to values where W radiates more, and the plasmas are significantly more difficult to control.

Experimental Approach

Work on the rampdown options is planned, in concert with the Control Group. With this experiment we plan to apply W-equivalent radiators to the access phase of the ITER Baseline Scenario, namely Xe, assess the main differences and utilize the increased ECH power to control the radiation and the impurity accumulation. This control strategy will also be applied to the flattop phase with the same variety of impurities, where EC power and control options have never been assessed. While it is not possible to predict meaningfully how much W will get into the core plasmas of ITER or other reactors, DIII-D can produce scenarios with effectively any W-equivalent impurity concentration and radiation fraction that we want, in the range that is relevant for each machine, exploiting the benign carbon wall and the variety of impurity injection schemes and magnitudes that are available. We will then continue the experiment into the rampdown, with lower Xe injection depending on the observed temperature, and tuning the heating and density to assess and mitigate radiative issues. In the same way we will compare the results with injection of “real” W with the LBO (as in previous work, [1]), in all three phases of the scenario.
Specific deliverables for this experiment:
• Integrate W-equivalent radiation in the IBS from ramp up to ramp down. Vary the impurity type and concentration when passing through crucial phases of the discharge, from low Te L-mode, to the LH transition, to the H-to-L transition and Ip rampdown.
• Document the impurity transport, the concentration and the radiation profiles for each phase and each choice of impurity (both gas and LBO) à provide relevant data to constrain models.
• Utilise ECH power to offset the added radiation, modify the ELM character and impurity transport, and reduce the impurity accumulation in the core plasma. Vary the EC power, injection location and timing to determine the optimal strategy to maximise the fusion gain (reduce the input power) and the survivability and confinement of the discharges.
This experiment directly addresses the goals of the ITER scenarios area and the ITER requests for studies of W and its impact on ITER scenarios, as well as the possible strategies to mitigate it.