Millimeter-wave sources for plasma diagnostics

Higher power and more frequency agile sources are required for plasma diagnostics, both for fusion research and for industrial process control. For example, reflectometers are planned for the ITER program that must generate of order 150 milliwatts of transmit power that can be rapidly scanned from 75 GHz through 230 GHz. In this frequency band the most successful solid-state source technology uses diode based frequency multipliers to extend the range of microwave amplifiers. For example, VDI has demonstrated a 300 mW source at 100 GHz that uses a high-power amplifier at 25 GHz and a cascaded pair of varactor doublers. The varactor doublers rely on a voltage-variable capacitance and large diode breakdown voltage to achieve large conversion efficiency and power handling. Unfortunately, these same device characteristics result in the very restricted tuning band of the doublers, and the typical 3dB bandwidth is of order 10% of the center frequency. Using this style of doubler would require an array of nearly a dozen sources to achieve the frequency bandwidth desired for ITER (75-230 GHz). This is technologically challenging and cost prohibitive. Additionally, the power available is reduced at higher frequencies and the requirement for rapid frequency tuning, even in these smaller tuning bands, often creates reliability problems. VDI is investigating the causes of these problems and possible solutions. Specifically, the performance of varactor and varistor frequency multipliers is under study, with the goal of improving the performance of cascaded multiplier chains and thereby reducing the number of channels required to cover the desired frequency range. Further, the reliability of these systems is being investigated, with the goal of understanding failures that occur during rapid frequency tuning and developing methods to alleviate these problems. Finally, VDI is developing prototype sources based on the results of these studies for use throughout the 75-300 GHz frequency band. This talk will focus on the performance and operation of these all-solid-state sources of millimeter-wave power, with emphasis on the improved power handling of the multipliers, efforts to increase tuning bandwidth and techniques that have been developed to achieve rapid frequency tuning with high system reliability. The performance of prototype systems throughout the frequency band will be presented at the conference.