Easy - high PSYS_SRC_BURST_PART_COUNT and PSYS_PART_MAX_AGE, combined with a low PSYS_SRC_BURST_RATE.
From there, it's simply a matter of appropriately sizing the particulate diffuse texture resolution to ensure adequate reactor capacity for the requisite payload.
Or perhaps she just used PBR (it does stand for Particle Bouncing Reactor, right?) :p
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