Development of a helicon current drive system for installation in the DIII-D tokamak

A new mechanism for driving current off-axis in high beta tokamaks using fast electromagnetic waves, called Helicons, will be experimentally tested for the first time in the DIII-D tokamak. This method is calculated to be more efficient than current drive using electron cyclotron waves or neutral beam injection, and it may be well suited to reactor-like configurations [1]. DIII-D can provide the conditions and measurement capabilities for a quantitative evaluation. At around 500 MHz, the optimum for DIII-D, an injected power of 1 MW would be adequate for these measurements and provide the equivalent current drive of a 2.5 MW neutral beam source. A “combline” antenna, which consists of many inductively coupled, electrostatically shielded, modular resonators, will be used to couple to the fast wave. A twelve-module low power (100 W) antenna, a shorter version of the high power antenna, will first determine the plasma operating conditions under which helicon waves can be launched at the required frequency and toroidal wave number. It must also be shown that the location of the antenna is unlikely to reduce the performance of, or introduce excessive impurities into, most of the potential discharges produced in DIII-D. It is mounted on the inside of the outer wall of the vacuum vessel slightly above the midplane. Carbon tiles around the antenna protect the antenna from neutral beam fast ions and deposited at the location of the antenna. RF probes will measure the RF fields in the modules and thermocouples will monitor the thermal load on the modules and tiles from the plasma. Visible and infrared cameras will view the antenna. Subsequently, a high power antenna will be installed to demonstrate that current can be driven in the plasma at the expected high efficiency. A 1.2 MW, 476 MHz klystron system will be transferred from the Stanford Linear Accelerator to DIIID to provide the RF input power to the antenna. A description of the design and fabrication of low power antenna and its installation in DIII-D will be described. The plan and schedule for the high power system will also be presented.