in a magnetic mirror machine, particles with a large velocity parallel to the magneitc field and a small velocity perpendicular to the field will be able to escape past the magnetic mirror (see magnetic mirror). In that case the velocity distribution function (see distribution function) will be almost zero in the region of velocity space that allows particles to escape. The shape of that region (in a velocity space diagram with parallel velocity and perpendicular velocity as the axes) is a cone. When a particle undergoes a collision, its velocity gets somewhat randomised. Particles that are scattered into that cone are lost very quickly (in one mirror bounce time). Thus it is called a loss cone. Because of the loss cone, the theoretical maximum particle confinement time of a magnetic mirror machine can be only a few times the particle collision time, this is generally seen as a showstopper for mirror-based fusion research.