Particle Vortex control (3ds Max)

Return2Hanger

Hello, 

 

I currently have a particle source emitting from the middle of the scene. It uses a vortex force to swirl round in a tornado fashion, the diameter of the swirl increasing indefinitely. 

 

My question is, how do I keep the particle vortex at a fixed diameter once it has reached a certain point (i.e. third frame/frame 70 in the example). This would mean the particles would continue to swirl round in a circle without the diameter increasing. 

 

Thanks!