Speaker
Description
Following the decision to begin ITER operation with a full tungsten first wall and divertor, it became necessary for ITER to also include boron wall conditioning techniques to avoid excessive influx of tungsten into the plasma that would radiate away the plasma stored energy and make the path to long pulse discharges up to Q=10 quite challenging. As a result, Glow Discharge Boronization (GDB) with deuterated diborane (B2D6) gas has been adopted on ITER, but it can only be performed while the magnetic fields are off, which limits the frequency of boronization and may complicate plasma operation. Another method that is being considered by the ITER Organization that would allow the introduction of pure boron and could be performed on demand as needed during plasma operation is solid boron injection (SBI) either through pellet injection or with an Impurity Powder Dropper (IPD). This additional boron wall conditioning technique could become important as risk mitigation for obtaining and sustaining high performance plasma operation on ITER.
To help ITER assess this technique, modeling of the ablation, transport, erosion and redeposition of solid boron has been performed using the Dust Injection Simulator (DIS), EMC3-EIRENE, Walldyn, and Neutral Gas Shielding models for a range of boron particle sizes and injection velocities. Some of the main questions this modeling addresses are 1) how deep should boron penetrate into the plasma to condition the plasma facing components (PFCs) sufficiently, 2) where does the boron end up on the first wall and divertor, 3) is a single injection location sufficient to condition the PFCs, and 4) which plasma conditions are suitable for SBI? The DIS modeling scans boron particle sizes from 1 µm – 1.5 mm and injection velocities from 5 – 50 m/s and NGS modeling scans sizes from 500 µm – 5 mm and injection velocities from 20 – 200 m/s, assuming either dropping powder or injecting pellets from a single upper port or injecting pellets along the existing lower outboard, lower inboard, or inboard midplane pellet guide tubes. Initial results with DIS and EMC3-EIRENE show that particles > 100 µm can cross the separatrix and penetration increases with size and velocity in a low power L-mode scenario. For a Q=10 scenario, particles > 500 µm can cross the separatrix and the NGS model allows penetration of 4 – 8.5 cm for the larger and higher velocity pellets.