Description
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 discharges. Operating in WEST since
2021, the IPD has demonstrated both real-time wall conditioning effects [2, 3] and transient improvements
in core performance [4, 5]. In parallel, impurity seeding with nitrogen (N2) has been routinely employed
in WEST [6], reporting comparable confinement improvements, largely attributed to plasma dilution [7].
While B powder injection shares a similar confinement improvement mechanism based on plasma dilution, it
additionally reduces D recycling through the formation of a B layer on plasma-facing components, as observed
experimentally and in modelling [8]. Reduced D recycling is linked to density profile modification, including
increased core density peaking and a lower separatrix density (nsep), both of which correlate favorably with
improved confinement [9].
Using a combination of dedicated experiment and database analysis across multiple experimental campaigns,
we compare the core performance enhancement induced by B powder drop with that observed during N2
gas injection. The comparison is performed at similar effective charge (Zeff ), within the same experimental
session to ensure comparable wall conditions, and for the same baseline plasma scenario (400 kA, 4×1019m−2,
4 MW of LHCD). Fast-sweep reflectometry measurements are used to assess changes in core density peaking
and to investigate the relationship between nsep and core performance indicators, including central electron
temperature, stored energy, neutron rate, and internal inductance. A correlation is observed between reduced
D recycling and increased core density peaking, both of which strongly correlate with increasing B powder
drop rate (mg/s), indicating that the IPD acts as an actuator enabling access to a desirable operational
regime. Additionally, leveraging the first ex-situ IPD calibration, we establish a quantitative correlation
between B powder drop rate and core performance indicators, which is essential to define an optimal range:
recent experiments [10, 11] identify the lower limit, as improvements in core density peaking and energy
confinement time appear only above a threshold B powder drop rate, while the upper limit is set by the need
to avoid excessive fuel dilution and radiative collapses.