Advanced control of neutral beam injected power in DIII-D

• Published in: Fusion engineering and design 2017-11, Vol.123, p.453-457
• Main Authors: Pawley, C.J., Crowley, B.J., Pace, D.C., Rauch, J.M., Scoville, J.T., Kellman, D.H., Kellman, A.G.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Arc chambers; Control; Divergence; Electric potential; High voltage; Ion beams; Magnetic fields; Modulation; Neutral beam; Neutral beams; Neutral particles; Particle energy; Perveance; Plasma; Plasma control; Plasma density; Plasma physics; Reproducibility; Rotating plasmas; Rotation; Tokamak devices
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2017.02.106

•A new control scheme for neutral beam operation has been implemented on DIII-D.•The new control allows for beam voltage to be changed in real time during a beam shot.•This variable beam energy (VBE) mode is controlled by the DIII-D plasma control system (PCS).•Additional control changes as part of the VBE operation have allowed for beam perveance sweeps in a single shot.•The new control maintains beamline safety and tracks energy efficiency. In the DIII-D tokamak, one of the most powerful techniques to control the density, temperature and plasma rotation is by eight independently modulated neutral beam sources with a total power of 20MW. The rapid modulation requires a high degree of reproducibility and precise control of the ion source plasma and beam acceleration voltage. Recent changes have been made to the controls to provide a new capability to smoothly vary the beam current and beam voltage during a discharge, while maintaining the modulation capability. The ion source plasma inside the arc chamber is controlled through feedback from the Langmuir probes measuring plasma density near the extraction end. To provide the new capability, the plasma control system (PCS) has been enabled to change the Langmuir probe set point and the beam voltage set point in real time. When the PCS varies the Langmuir set point, the plasma density is directly controlled in the arc chamber, thus changing the beam current (perveance) and power going into the tokamak. Alternately, the PCS can sweep the beam voltage set point by 20kV or more and adjust the Langmuir probe setting to match, keeping the perveance constant and beam divergence at a minimum. This changes the beam power and average neutral particle energy, which changes deposition in the tokamak plasma. The ion separating magnetic field must accurately match the beam voltage to protect the beam line. To do this, the magnet current control accurately tracks the beam voltage set point. These new capabilities allow continuous in-shot variation of neutral beam ion energy to complement the discontinuous “on or off” modulation method presently used to control average beam power and torque input.