Probe development, as described in previous reports, has continued with the successful implementation of quadruple-resonance transmission line probes at 500 and 750 MHz. We have been extremely pleased with the efficiency of 'H operation at 750 MHz, with fields in excess of 125 kHz obtained with -100 Watts. Further development and application of triple and quadruple resonance experiments requires a high power tube amplifier that is currently being tested. We have developed transmission line MAS probes that offer significant improvements over locally tuned probes in several important areas: (1) efficiency, (2) power-handling capability (arc threshold and maximum rf duty cycle), (3) multi-channel tunability, (4) efficient use of space within the magnet bore, and (5) unperturbed operation over a large range of temperatures. Two generations of designs have been implemented at frequencies ranging from 200 to 750 MHz. All designs are based on the rf circuit topology proposed by McKay et al., where the only lumped tuning elements within the magnet are (1) the solenoid sample coil and (2) an optional series tuning capacitor. Low loss air-dielectric transmission line is used to transform impedance in a manner that allows tuning several frequencies with high efficiency and isolation. The general topology in every case requires transformation of the sample coil inductance to a high frequency (H) voltage node coincident with the first physical junction outside the magnet bore. Peak voltages within this tuning circuit are minimized (for a given input power) by choosing appropriately low values of inductance before tuning and matching. On the low-frequency side, details differ depending on topology. In one case, lumped element traps are used for isolation circuits. In the second case, low frequency nodes are exploited for circuit isolation. In order to achieve high efficiency at higher (>500 MHz) frequencies, we have developed a general approach that is particularly useful at high frequencies. These developments have permitted the design and construction of several probes ranging from 200 to 750 MHz, most with three or four high-power rf channels. Low-temperature (<150 K) versions of such probes operate with rf performance which is not significantly compromised relative to the room-temperature probes.
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