Development of a 56 GHz ECH system for deuterium plasma experiments of a low magnetic field in LHD

•56 GHz ECH system was newly designed and constructed in LHD.•The dependences of plasma startup on the polarization and ECH power were obtained.•Pure deuterium plasma experiment in a low magnetic field was succeeded in the LHD. We have completed establishing a 56GHz electron cyclotron heating (ECH) system with a new gyrotron to realize high β plasma experiments with pure deuterium gas in the Large Helical Device (LHD). This new ECH system made it possible to conduct the relatively low magnetic field experiment around 1T with pure deuterium gas because tangential neutral beam injection (NBI) systems of deuterium beams did not have enough power to initiate plasma in the LHD. We succeeded in initiating and sustaining pure deuterium plasma in the magnetic field of 1T by using the 56GHz ECH system and the deuterium NBI (D-NBI). This new ECH system contributed to expanding the new experimental regime in the LHD.