Ringing Frequency Research Articles

Phase Transients in Pulsed NMR Spectrometers

When a square wave modulated rf voltage is applied to a tuned probe in an NMR spectrometer, the effective transverse magnetic field in the rotating reference frame changes direction as it grows and decays. The resulting deviations in the nutational behavior of the spins from naive expectations are typically small, but may lead to important cumulative errors in experiments employing long pulse trains. These errors can be made to vanish by very accurate tuning of the probe circuit to its free ringing frequency ωr [not to ω0=(1/LC)½] and by providing suitable coherence between the rf carriers and the timing of pulses.

Improved ringing coil pulse encoders

The advantages of ringing coil circuits as logarithmic encoders are reviewed and the choice of optimum ringing frequency considered. Circuits are given for frequencies up to 8 MHz, and it is shown that dynamic ranges of at least 1000: 1 are possible with an input sensitivity better than 1 pC.

Flash radiography using exploding wire technique.

By exploding a thin tungsten wire in vacuum, it is found that an ultrafast (20 nsec half-width), reproducible x-ray pulse is emitted from the wire during the first half-cycle of the discharge. The half-width of the pulse seems to be independent of the ringing frequency of the discharge circuit. It is shown that it is possible to use the exploding wire as a flash x-ray source for taking radiographs of high speed events, e.g., an exploding copper wire.

Equilibrium and stability of a toroidal screw pinch

A toroidal plasma column, produced and confined by a rapidly rising helical magnetic field is studied. This `screw pinch' is of interest because it is in equilibrium and is completely reproducible. The discharge vessel is a quartz torus with major and minor diameters of 72 and 12 cm. Plasma currents (axial jz and azimuthal jθ) are induced by two orthogonal primary coils, which are simultaneously connected to capacitor banks (Ez = 5 kJ and Eθ = 7.5 kJ). Both circuits have a ringing frequency of 70 kHz. During the first microsecond the plasma collapses; then the column drifts about 0.5 cm to the outer wall and remains in this position, slightly off the tube axis, until the end of the first half cycle. The plasma is kept in this equilibrium position by the compression of the Bθ-field between the plasma column and the metal shell surrounding the torus. The necessary gaps in the shell are constructed with special care. During the 7 μsec of the first half period no instabilities have been observed. This stable behaviour is found even if the pitch of the magnetic field is varied between wide limits; expressed in terms of the axial current, a variation between approximately 4-12 times the Kruskal limiting current did not impair the reproducibility. A large-amplitude oscillation, excited by the implosion, is observed. The frequency agrees with the frequency of the stable m = 0, k = 0 mode, predicted by the normal mode analysis. From measured Stark broadening a density of 2 × 1016 cm-3 has been derived, which corresponds to a compression ratio (radius of the tube divided by plasma radius) of 3. In a 15 μ Hg helium discharge a temperature of 20 eV has been measured.

Exploding Wire as Fast Dynamic Pinch

The high-current electric discharge through fine wires is studied under experimental conditions in which the interaction of the discharge with its self-magnetic field is dominant. When the wire is composed of an element or elements of low atomic number, the discharge is found to have many of the characteristics of pinch discharges in gases. A capacitor bank of 1.75 μf and a voltage rating of 100 kv was used with a switching spark gap and circuit of very low parasitic inductance. The parasitic inductance of the condensers and circuit was measured to be 0.05 μh while the ringing frequency of the discharge was 370 kc. Wires of copper, aluminum, polyethylene, lithium, and lithium-lithium hydride were exploded. The discharge was photographed with streak and framing cameras and oscillograms taken of the discharge current and its time rate of change. Wires composed of lithium-lithium deuteride were observed to give neutron yields of the order of 106 per shot. The neutron yields were measured with a silver-foil activation counter.

Rapid Beam Ejector for the Cosmotron

The theory and operation of a device that makes the beam in the Cosmotron come onto a target in about 5 μsec instead of the usual 3 msec are described. The magnetic field in a two-turn coil, pulsed with a condenser bank, deflects the internal beam enough to throw the beam onto a target during the rise time of the perturbing field. The system has an inductance of 2 μh and a capacitance of 11 μf, giving a ringing frequency of 34 kc, a peak current of 47 000 amp at 20 kv and a peak field of 1100 gauss. The ejector can also be used to put only a small fraction of the beam onto a target, permitting the rest of the beam to continue on the normal accelerating cycle.

Techniques for the Rapid Estimation of Accelerometer Natural Frequencies

In order to determine the usable frequency range of an accelerometer, it is necessary that its natural frequency be known. Three simple methods for approximately determining natural frequency are described: measurement of the ringing frequency excited by electrical and mechanical pulse inputs to the accelerometer and measurement of the accelerometer's impedance as a function of frequency. While subsidiary to more refined calibration techniques, these methods are useful because of their speed and simplicity and because they may be utilized with the accelerometer in place. Results obtained for several piezoelectric acceleromeetrs are given.

Superfast Pinch Experiment

An assembly designed to accelerate deuterons to about 10 kev by means of a radially converging magnetic piston has been constructed and tested. The assembly operates with 300 kv on the energy storage condenser, the ringing frequency is 18 Mc, and present operation uses hydrogen gas at 50 μ. Both slow and fast pinches have been observed, the former being possibly thermal shock waves, and the latter possibly being associated with inward motion of the plasma sheath.