Vacuum

A technical vacuum is no volume free from matter, but a gas volume at low pressure, characterized by a much increased free waypath of all particles before a collision with other particles takes place. While under atmospheric pressure the mean free waypath between two impacts of particles is below 1 micron, it is more than 10 cm below 10-3 mbar. That means that many particles do not suffer any impact within the dimensions of the recipient.

Therefore in a low pressure plasma active species can fill the complete volume of a recipient without suffering any recombination caused by collisions to other components.    

Vacuum
The free waypath in a low pressure plasma is so big that a homogenous plasma may fill the whole volume of the recipient.
Recipient
Vacuum chambers out of stainless steel are standard in plasma technology