J.F. Tooker

The Use of DIII-D as a Testbed for ITER ECH Transmission Line Components

The operational experience of present day high power EC systems is nominally only for a few seconds. This is a long way away from the thousands of seconds required for ITER. It would be beneficial to the ITER program that EC system components be tested to full parameters prior to committing to producing the full set of components. The planned growth in the EC system on DIII-D over the next few years provides the opportunity to assemble a test stand for ITER EC components. By building the DIII-D hardware to the ITER specifications it will allow ITER to gain beneficial prototyping experience on a working tokamak, prior to committing to building the hardware for delivery to ITER.

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.

Lower hybrid wave heating in Doublet IIA

Doublet IIA electron heating experiments utilize lower hybrid waves launched by slow wave structures with various n/sub parallel/ = ck/sub parallel//..omega.. (11, 14, 16) to achieve spatially localized heating by electron Landau damping. Radiofrequency power of 350 kW at 800 MHz and 200 kW at 915 MHz is available to heat circular discharges with ohmic input power of 100 kW. Significant increases in the electrical conductivity have been observed, but plasma response to the rf power is sensitive to the impurity level of the discharge. Power absorption mechanisms and energy balance will be discussed.

A High Power Helicon Antenna Design for DIII-D

Abstract A new antenna design for driving current in high beta tokamaks using electromagnetic waves, called Helicons, will be experimentally tested for the first time at power approaching 1 megawatt (MW) in the DIII-D Tokamak. This method is expected to be more efficient than current drive using electron cyclotron waves or neutral beam injection, and may be well suited to reactor-like configurations. A low power (100 watt (W)) 476 megahertz (MHz) “comb-line” antenna, consisting of 12 inductively coupled electrostatically shielded, modular resonators, was tested in DIII-D and showed strong coupling to the plasma without disturbing its characteristics or introducing metal impurities. The high power antenna consists of 30 modules affixed to back-plates and mounted on the outer wall of the vacuum vessel above the mid-plane. The antenna design follows a similar low power antenna design modified to minimize RF loss. Heat removal is provided by water cooling and a novel heat conducting path using pyrolytic graphite sheet. The CuCrZr antenna modules are designed to handle high eddy current forces. The modules use molybdenum Faraday shields that have the plasma side coated with boron carbide to enhance thermal resistance and minimize high Z impurities. A RF strip-line feed routes the RF power from coaxial vacuum feed-throughs to the antenna. Multipactor analysis of the antenna, strip line, and feed-through will be performed. A 1.2 MW, 476 MHz klystron system, provided by the Stanford Linear Accelerator (SLAC) will provide RF power to the new antenna. A description of the design of the high power antenna, the RF strip-line feeds, and the vessel installation will be presented.