Power supplies used in field-cycling nuclear resonance experiments are described. These are used to switch an air coil from high field to low field as rapidly as possible, then back to high field after a short time. This sequence must be repeated every few seconds or minutes. An example of a simple regulator is described, and methods of output switching with relays are discussed. A circuit is described which delivers up to 10 kG (15 A) in a 2.6 cm i.d. superconducting coil with a switching speed of 5×105 G/sec and an accuracy probably as great as 1 G. Speed is achieved by automatic connection, during up or down switching, of energy storage capacitors across the magnet, operating at much higher voltage than the steady-state voltage level across the magnet and pass transistors.
Nuclear spin relaxation was studied by polarizing nuclear spins in 10 kG, quickly lowering the field to the mixed state, and after variable time applying 6 kG and observing a rapid-passage signal. Samples were wires and foils of resistance ratio up to 90, and were fairly reversible. In the normal state at 5.4°K, the relaxation rate at zero field is about one-third that at high field in these samples, the change in rate occurring at fields greater than 30 G. This behavior is attributed to electric quadrupole interactions due to residual imperfections in the lattice. In the zero-field (Meissner) superconducting state, T\ becomes long at low temperatures, consistent with a gap of order 3.5kT c ; but just below T c , Ti appears to be greater than in the normal state, in contrast to its behavior in other superconductors such as aluminum. This may be due to trapped flux; the possibility of strain-enhanced electric quadrupole relaxation was also considered but estimated to be negligible. In the mixed state, just above H&, where about half the sample is farther than a coherence length from a vortex, nonexponential decays are observed. Below 1 °K the long component of the decay is spin-diffusion-limited, apparently, and the spin-diffusion coefficient is inferred and compared with theory. At these low flux densities the decay can be made nearly exponential by applying a few-gauss 100-Hz field during the time the sample is in the mixed state, presumably moving the vortex structure in and out and making a spin relax at the space-average rate. The space-average relaxation rate changes nearly linearly with flux density, from its normal-state value at E c i to its zero-field superconducting value, at most temperatures. The resolution was not sufficient to establish significant impurity-dependent effects.
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