We report on the growth mechanism and density control of vertically aligned carbon nanotubes using a triode plasma enhanced chemical vapor deposition system. The deposition reactor was designed in order to allow the intermediate mesh electrode to be biased independently from the ground and power electrodes. The CNTs grown with a mesh bias of +300 V show a density of similar to 1.5 mu m(-2) and a height of similar to 5 mu m. However, CNTs do not grow when the mesh electrode is biased to -300 V. The growth of CNTs can be controlled by the mesh electrode bias which in turn controls the plasma density and ion flux on the sample. (c) 2006 Elsevier B.V. All rights reserved.