Particles field

hi…

i am making a project with particles. and i am searching to close the particles in a 3d space, in particular i want put a particles in a cylinder’s bulk, but i dont anderstant this : how i can use a cylinder like a bulk?
becouse i have seen that i can use the pin velocity fields ( in the particles node)to drive the particles but how i can transform the cylinder output to connect it with particles fild input?

thank so much…