High-power microwave transmission and launching systems for fusion plasma heating systems

Microwave power in the 30- to 300-GHz frequency range is becoming widely used for heating of plasma in present-day fusion energy magnetic confinement experiments. Microwave power is effective in ionizing plasma and heating electrons through the electron cyclotron heating (ECH) process. Since the power is absorbed in regions of the magnetic field where resonance occurs and launching antennas with narrow beam widths are possible, power deposition location can be highly controlled. This is important for maximizing the power utilization efficiency and improving plasma parameters. Development of the gyrotron oscillator tube has advanced in recent years so that a 1-MW continuous-wave, 140-GHz power source will soon be available. Gyrotron output power is typically in a circular waveguide propagating a circular electric mode (such as TE/sub 0,2/) or a whispering-gallery mode (such as TE/sub 15,2/), depending on frequency and power level. An alternative high-power microwave source currently under development is the free-electron laser (FEL), which may be capable of generating 2-10 MW of average power at frequencies of up to 500 GHz. The FEL has a rectangular output waveguide carrying the TE/sub 0,1/ mode. Because of its higher complexity and cost, the high-average-power FEL is not yet as extensively developed as the gyrotron. In this paper, several types of operating ECH transmission systems are discussed, as well systems currently being developed. The trend in this area is toward higher power and frequency due to the improvements in plasma density and temperature possible. Every system requires a variety of components, such as mode converters, waveguide bends, launchers, and directional couplers. Some of these components are discussed here, along with ongoing work to improve their performance. 8 refs

Electron cyclotron heating and current drive approach for low-temperature startup plasmas using O-X-EBW mode conversion

A mechanism for heating and driving currents in very overdense plasmas is considered based on a double-mode conversion: Ordinary mode to Extraordinary mode to electron Bernstein wave. The possibility of using this mechanism for plasma buildup and current ramp in the National Spherical Torus Experiment is investigated

Linearly Polarized Modes of a Corrugated Metallic Waveguide

A linearly polarized (LP[subscript mn]) mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the LPmn modes and the conventional modes (HEmn, EHmn, TE0n, TM0n) of the corrugated guide is shown. The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal (m) indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.United States. Dept. of Energy (Office of Fusion Energy Sciences)United States. Dept. of Energy (Virtual Laboratory for Technology)United States. Dept. of Energy (Office of Science, US ITER Project

Microwave Treatment as a Pesticide Alternative for Stored-Products

This CRADA was a continuation of earlier work with Micro-Grain, Inc. to develop power, high frequency microwave treatment process to treat insect infested grain. ORNLs role was as a subcontractor to Micro-Grain's Phase II SBIR project funded by the US Department of Agriculture. The primary objective was to develop a commercial scale prototype unit capable of treating infested grain at flow rates approaching 1 kg/sec, which is required to be viable in the grain handling industry. A flow rate of {approx} 0.12 Kg/second was demonstrated at 20 kW microwave power level with 100% kill rate. The system is capable of 200 kW however waveguide arcing due to grain dust in the waveguide limited the power to 20 kW during the tests. Development tasks performed during the project included modification of an existing high-power microwave exposure facility to uniformly process large grain samples at high flow rates and improved instrumentation to detect grain flow and uniformity. Microwave processing tasks include a series of controlled exposure tests using infested grain samples provided and analyzed by the University of Oklahoma. Grain samples were infested with red flour beetles which proved the most difficult to kill in earlier tests. Most of the samples processed resulted in quite successful kill rates and a maximum grain temperature of 46 C. The facilities utilized at ORNL are located in the Fusion Energy building (9201-2 at Y-12) and include the 28 GHz 200 kW CW high power microwave facility and microwave test equipment associated with the FED Microwave Development Laboratory in 9201-2. An improved microwave exposure chamber and grain flow control and handling equipment were designed and build as a joint effort between Micro-Grain and ORNL. A number of insect infested grain tests were successfully performed although the higher power, higher flow rates were limited by arcing in the microwave waveguide and damage to the gyrotron output window. Test results and the overall performance of the applicator system are very favorable for continued development of the concept. Further tests were performed in a large high power 2.45 GHz microwave applicator in batches. These samples were also quite effectively treated which supports the concept that a lower cost, lower frequency microwave system might be more successful due to the improved economics and simpler operation and maintenance of the low frequency system. Follow-on work is still possible however the untimely death of Steve Halverson, founder of Micro-grain, has essentially brought the development work to a close for now. Micro-Grain is being run by relatives at a low level who are not actively pursuing further funding

Recent results from a folded waveguide ICRF Antenna development project

Preliminary high power tests have been performed on a folded waveguide (FWG) ICRF launcher with a curved coupling faceplate installed. Two alternative faceplate configurations have been built and tested at low power and will be tested at high power in the near future. The new designs include a dipole plate which provides a 0-<font face="symbol">p</font> launch spectrum and a more transparent, flexible monopole face plate configuration. This FWG design is a 12 vane, 57 MHz design with a 0.31 m square cross section. The FWG can be installed with either fast wave or ion-Bernstein wave polarization and can also be retracted behind a vacuum isolation valve. A 1 x 4 FWG array optimized for fast wave current drive on DIII-D has been conceptualized