Beam transmission dependence on beam parameters for TJ-II Neutral Beam Injectors

In Neutral Beam Heated plasmas, Beam transmission must be carefully evaluated in plasma experiments involving NBI parameter scans. In the TJ-II stellarator a number of experiments have been carried out in which the beam energy (Vaccel) is changed at constant current (Iaccel), or vice versa. The beam transmission varies along the parameter scan according to the perveance value that determines the beam divergence. The transmission is calculated using a beam simulation code in which the beam geometry and divergence as well as the transmission apertures are included in a 3D model. In TJ-II Infrared thermography of the target calorimeter intercepting the beam is used in the divergence measurements. From the performed parameter scan it is concluded that neither the beam power nor the beam current injected in the plasma have a monotonic variation with the control parameters at the ion source (Vaccel or Iaccel).