Development for Extending the Beam Pulse Length of the DIII-D Neutral Beam System

The DIII-D fusion research tokamak utilizes seven neutral beam ion sources for plasma heating and current drive. These ion sources and the neutral beam system have performed with high availability and reliability since 1987. Experimental research has accomplished extensive understanding and insights of plasma physics, and requires more beam system capability to provide flexibility and enhancing scope of physics experiments. Extending the beam pulse length without lowering the beam power is one of the goals for the next 5 years. Currently, deuterium beam pulse length of ion source operated at 80 keV is limited to 3 s due to heat handling capability of some beamline internal components, which are used to collimate beam or to protect other beamline components from being damaged by residual (un-neutralized) energetic ions. A systematic study based on actual heating of beamline internal components has been performed to develop a plan for extending the beam pulse length to more than twice the current operating limit. This study using temperature rise of thermocouples embedded in the beamline internal components and data extrapolation obtained the beam pulse limitation for each beamline internal component identified that the pole shield of the residual ion bending magnet is the component that limits the beam pulse length to 3 s for 80 keV beam operation. Beam pulse length at 80 keV can be doubled with the upgrade of the magnet pole shield alone. Temperature rise measurements also showed that a drift duct (the section connecting beamline and tokamak vessel) made of stainless steel will be able to handle the heat from the re-ionized neutral particles. In addition to beamline internal components, long pulse beam operation of ion sources and other beam subsystems need to be operationally confirmed. Tests have shown ion source, control system, pumping and cooling systems, and power supply system operated flawlessly at beam energy of 60 keV for 10 s. It gave us confidence that the current DIII-D neutral beam system could operate at least twice the current beam pulse length after upgrade of the magnet pole shield.