Boron injection rate as actuator for wall conditioning and enhancement of core performance in WEST

The Impurity Powder Dropper (IPD)[1], developed by PPPL, is a device designed to inject controlled amounts

of extrinsic low-Z impurities, such as boron (B) powder, during plasma pulses. In operation in WEST since 2021,

the IPD has demonstrated both real-time wall conditioning effects [2, 3] and transient improvements in core performance

[4, 5].

Wall conditioning effects are observed even at very low injection rate (m_B ≃ 6 mg/s) during ohmic pulses,

resulting in an increase in the success rate of 3–4 MW LHCD power pulses from 20% (without B) to 100% (with

B). Visible spectroscopy measurements at the PFCs indicate reduced oxygen content (-60% of the O-II/D-I line

brightness) and fuel recycling, supporting the formation of B layers, while IR and visible camera diagnostics

suggest a progressively more attached divertor regime. However, at this injection rate, no improvement or even a

minor degradation is observed in core performance indicators, in agreement with [5]. Clear enhancements in core

performance are instead observed at higher injection rate (m_B > 30 mg/s) during 4–6 MW LHCD+ICRH power

pulses, including increased density peaking (+30% in n_e(0)/n_e(0.7)), higher central electron temperature (+25%

in T_e,0), neutron rate (+170% in Γ_n), and total energy content (+17% in W_mhd). The improved confinement

appears to depend strongly on the B injection rate, which is linked to increased edge fuel dilution, as indicated

by a significant rise in Z_{ef f} , enhanced edge radiation and reduced target density.