Keep particles or inside the volume

User Guide > Dynamics and Effects > Dynamics > Fields > Work with fields >

You can keep particles and nParticles inside a volume by using a radial field that affects only particles outside the volume you are interested in, and pushes the particles back in. You can also use this procedure to keep particles inside a volume emitter.

You can also use the Volume Trapping attributes to keep objects from exiting volume shape fields. See Volume Trapping.

To keep particles inside the volume

Create the particles (or emitter) and the field whose volume you want the particles to stay inside.
Select the particles and, from the Dynamics or nDynamics menus set, select Fields > Radial > .
Set the volume shape of the radial field to be the same as for the field you created in step 1.
Turn on Volume Exclusion.
Set the radial field magnitude to a negative value.
Try a value which is about one-half the magnitude value of your first field, but is negative. For example, if the first field has magnitude of 10 or -10, use a radial field magnitude of -5.
Parent the radial field to the first field, and reset the radial field’s transform to be the identity.
This makes the radial field have the same volume as the original field.

Whenever particles move outside the original field volume, the radial field pushes them back in. You may want to tune the radial field magnitude and/or transform values to get the exact look you want.