The most serious limitation hindering the wider application of microstrip antenna technology is the narrow bandwidth of the basic element. For this reason, many elements with enhanced bandwidth have been investigated. This paper presents measured results for an under-coupled microstrip patch antenna with the improved bandwidth. The impedance was matched by an open-circuit tuning stub connected in shunt with the feed line.

A 1.5 MW, 110 GHz gyrotron is under development for electron cyclotron resonance plasma heating in the DIII-D tokamak at General Atomics. The research is aimed to provide reliable CW operation of the m o t r o n at high efficiency. The gymtron design is based on previous successful results from the 1 MW gyrotrons built by Communication and Power Industries (CPI). A TE22.6 mode cavity is utilized with an electron beam voltage of 96 kV and a beam current of 40 A. The cavity is designed to avoid mode competition and mode conversion. The design efficiency is 39% without a depressed collector and greater than 50% with a depressed collector using a depression voltage of 25 kV. The gyrotron is under test in short pulse operation (about 3 microseconds) at MIT. Over 1 MW of power in the design made with an efficiency of 25 % has been demonstrated in preliminary experiments using the newly designed cavity and an older triode magnetron injection gun (MIG) available at MIT. A new MIG electron gun designed specifically for the 1.5 MW, 110 GHz gyrotron has been built by CPI for this experiment. Uniformity of emission from this MIG has been experimentally studied. The experiment on high power generation using the new MIG is under way. An advanced cavity is being designed to increase the efficiency of the 1.5 MW, 110 GHz gyrotron. The gun end section of the cavity is widened to reduce the diffractive Qfactors of the competing modes. This design is based on the previous MIT experiments with a twwsection cavity gyrotron that demonstrated an efficiency of up to 42% at 140 GHz in the TEIS,Z mode.Research supported by the Department of Energy, Office of Fusion Energy Sciences An equivalent circuit model of HANBIT RF (radio frequency) -heating system is developed. The present circuit model includes transmission line, impedance matching network and E axtor impedance. The reactor impedance is a sum of plasma impedance and antenna impedance including the stray impedance. The plasma impedance and antenna impedance are calculated utilizing a solution of the Maxwell-Boltzmann equation obtained under the RF-heating condition The stray impedance includes the ohmic power loss on the reactor chamber. Based on the developed circuit model, dependence of power a b sorption on the various plasma parameters is investigated in the perfect matchmg case. In the imperfect matching case, reflection coefficient and reflectance of RF-electromagnetic wave are calculated as functions of various geometric and plasma parameters. ' emchoiOmit.edu 473