2025 – Detachment on the peeling limit at high performance

Detachment on the peeling limit at high performance

2025 Research Campaign, Thrust: Shape Rise Divertor

Purpose of Experiment

This experiment aims to achieve detached divertor conditions (where the heat and particle fluxes to the divertor plates are greatly reduced) in the new DIII-D Shape and Volume Rise (SVR) divertor and assess their impact on pedestal plasma performance. By varying the outer strike point location and gas fueling configuration, we seek to identify operational strategies that allow access to detachment while preserving high pedestal pressure—an essential requirement for maintaining good core plasma properties. These measurements will also provide critical validation data for drift-based edge modeling codes such as SOLPS-ITER and UEDGE, increasing confidence in their use for predictive divertor design. Understanding and optimizing the trade-off between divertor detachment and plasma pedestal performance is a key challenge for future Fusion Pilot Plants, where both effective power exhaust and high core plasma pressure must be simultaneously achieved. This experiment directly addresses that challenge and supports the development of reliable design tools for next-generation fusion devices.

Experimental Approach

This experiment builds upon plasma scenarios and operational knowledge developed in previous studies under the Shape and Volume Rise (SVR) divertor thrust. The baseline for the experiment is discharge #203621, run in the Upper Single Null (USN) magnetic configuration with a plasma current of approximately 1.4 MA and total heating power in the range of 10–15 MW. The initial part of the experiment is conducted with the magnetic field in the reversed BT direction, where the ion B x ∇B drift is directed toward the divertor, favorable for H-mode access. Later in the session, the toroidal field direction will be reversed to examine the same scenarios under conditions less favorable for H-mode, allowing a direct comparison of detachment behavior across magnetic configurations. The experiment employs an extensive set of edge and core diagnostics to measure divertor and pedestal plasma conditions. Detachment onset will be determined using wall-embedded Langmuir Probes (LPs) to observe the roll-over of the target ion saturation current and a drop in target electron temperature below ~5 eV, as well as Pressure Gauges (PGs) that detect a sharp rise in neutral pressure. Additional information on heat fluxes and material response is provided by Surface Eroding Thermocouples (SETCs). The divertor volume will be imaged using the TangTV system, and core and pedestal plasma profiles will be measured with Thomson Scattering. These diagnostics will ensure high-quality data for both scientific interpretation and validation of simulation codes. The experiment is organized into three main blocks. The first block, lasting approximately 3 hours, aims to achieve and characterize divertor detachment under steady PSOL conditions. The initial reference shot is a modification of discharge #203621, with a stationary plasma density around 5 × 10¹⁹ m⁻³ and constant ~10 MW of beam power. This discharge serves as a foundation for controlled detachment attempts: first via a density ramp using Gas A (main-chamber fueling), then via a density ramp using valve PFX1 (divertor fueling), and finally through two 1-second nitrogen injection steps from PFX1. Each scenario is evaluated for successful detachment based on the target temperature and other diagnostic signals. The second block, allocated about 2 hours, repeats selected experiments from Block 1 using the forward BT direction, which is unfavorable for H-mode access in the USN configuration. If establishing a stationary reference phase proves too time-consuming in this setup, the block will be skipped in favor of the third block. The final 3-hour block revisits the reversed BT configuration to explore more advanced operational strategies. The best-performing density ramp from Block 1 will be enhanced by ramping NBH to counteract pedestal pressure degradation, while nitrogen injection will be used to mitigate the associated increase in target temperature. This block will also test detachment feedback control using nitrogen fueling and radiated power fraction as the control signal. If time allows, the session will conclude with a repeat of the best-performing discharges, but with the strike point positioned on the 45-degree tile. This geometry may require shaping adjustments to rotate the divertor legs while keeping the x-point fixed due to inner gap limitations.

Join us for our annual public tours of DIII-D during Fusion Energy Week (May 4-10)! We look forward to seeing you in-person or virtually.

Advanced registration required: Fusion Energy Week Tours Sign-Up

X