Enhancement to gas puffing control system towards precise control of electron density in LHD

• Establishment of precise electron density control at LHD by improving the performance of feedback control. • Support for a wide variety of plasma experiments by employing FPGA. • Investigation of differences in controllability of electron density between different gas species. • Achieves stable control of electron density in response to changes in magnetic field configurations and heating schemes. A gas puffing system in Large Helical Device (LHD) has been largely enhanced in terms of its functionality. One of the most significant expansions is the enhancement of electron density controllability. In LHD, the precise control of electron density is often required according to the plasma experiment scenario. The electron density is controlled by supplying the appropriate amount of fuel gas through a piezo valve. To improve the performance of electron density control in LHD, we have developed a gas puffing control system (GPCS) including a feedback control system based on proportional-integral-control (PI-control). As a result of applying the PI-control, the target electron density can now be obtained in almost all cases in a single shot. To build the GPCS, a commercial CompactRIO with a field-programmable gate array (FPGA) is newly employed. The GPCS FPGA and the GPCS operating graphical user interface (GUI) are programmed in LabVIEW. This has resulted in easy modification of the GPCS' plasma control performance, and in easy GPCS operation for everyone. The ease of the plasma control performance modification has enabled GPCS to meet a variety of requirements, such as sustaining detachment, in addition to density control.