High Pressure Cryogenic Dusty Plasma

ramen

i'm studying the coulomb structure of 0.3um acrylic particles in He plasma. At low pressure, around 30 Pa, the negatively charged dust particles can be suspended above the negative RF electrode. However, at around 100-150Pa, all particles have fallen onto the electrode itself, making it impossible to analyze.
i'm testing high pressure because i will be using liquid He soon, and its hard to get pressure of liquid He lower than, say 100Pa.

is there any way to increase the charge of the dust particles so that they stay suspended even in high pressure? i've tried decreasing the size of the rf electrode and increasing the size of the ground electrode, but the difference was too small.

this is probably confusing, ask me questions and i can explain.

thank you.