Effects of operating parameters on the beam species of DIII-D neutral beam ion sources

The DIII-D fusion science research facility employs positive-ion based neutral beam ion sources for plasma heating and current drive experiments. Ion species produced inside the arc chamber of an ion source determine the composition of the neutral beams injected into the plasma and in turn affect the energy deposition along the cross section of the plasma. Arc chamber design and operation schemes are the predominant factors in the ion species mix when an arc discharge is produced inside the arc chamber. When deuterium gas is used for arc discharge three deuterium ion species (atomic D₁⁺ and molecular D₂⁺ and D₃⁺) are produced in the arc chamber of the DIII-D neutral beam ion source. Atomic ion D₁⁺ has the highest percentage of the three species, normally about 80% of ions produced. Measurements have shown that both D₁⁺ and D₂⁺ decrease slightly with lower arc power discharges. However, D₃⁺ increases at a higher rate when arc power is reduced. Changing the fraction of the ion species can be beneficial to or meet the needs of some specific plasma experiments. An attempt to manipulate the ion species mix by changing the operating parameters of the arc chamber/ion source has been performed. These operating parameters include filament temperature, arc power, beam energy, and gas flow. Interesting results were obtained which show that arc power is the dominant factor for controlling the species mix. We were not able to significantly change the ion species mix by varying the operating parameters within the operation window when the ion source is operated at constant beam energy. However, the D₃⁺ fraction increased by a factor of three when the ion source was operated at a much lower arc power (for 50 keV beam energy operation) than the higher arc power required for ion source operation with beam energy between 75 to 80 keV.