Real-Time Control of Beam Trajectories Using Digital Signal Processor for the Heavy Ion Beam Probe on the Large Helical Device

Abstract

A real-time control system using a digital signal processor (DSP) was developed to control ion-beam trajectories of a heavy ion beam probe (HIBP) on the Large Helical Device (LHD). The electrostatic potential during temporal evolution of the magnetic field structure by a large plasma current was successfully measured with the help of this system for the first time. It has been demonstrated that the probe beam can be detected even in a plasma having a ∼120 kA plasma current at the toroidal field of 1.3 T using the control system, while the beam is substantially lost by only ∼40 kA current before arriving at the detector plate in the case without control. However, this system is unstable during operations when the probe beam is swept in time across the plasma cross section to obtain the time evolution of the potential profile. It is caused by a nonlinear character of the system due to the finite beam size.