Fast Pulser Research Articles

Wall Streaming in Ferromagnetic Thin Films

In nickel-iron films thicker than about 100 nm (1000 Å), fast-rising field pulses along the hard anisotropy axis lead to a new kind of wall displacement called ``wall streaming,'' which does not require any field component parallel to the wall necessary for all other wall motion processes. The walls are displaced with a very regular wall step width per pulse which strongly depends on the strength of the pulse field and on the pulse rise and fall times, but not on the pulse duration. The direction of wall motion reverses if the pulse polarity is reversed. During the application of a sequence of uniform pulses, the direction of wall motion may also reverse, beginning at one end of the wall, with a reversing line (interpreted as being a Néel line moving along the wall. The phenomenon is explained as a consequence of the gyromagnetic behavior of the magnetization in the Bloch walls. A detailed theory is presented, in which the intrinsic spin damping and the film inhomogeneity (characterizable by the coercivity) are to be taken into account. Wall streaming may contribute to a destruction of the information stored in magnetic films thicker than 100 nm (e.g., in plated-wire memories) as soon as fast-rising (tr ≲20 nsec) pulses are applied.

An 80-NS NDRO ferrite core memory design

The use of fast-rising short-duration pulses in the design of a 1K - 144 bit NDRO memory with permeability sensing leads to high-frequency considerations not normally encountered in ferrite core memories. In particular, the high-frequency character of the device must be recognized to properly evaluate cycle time capabilities and array transmission. The design of this memory centers around a 2-core-per-bit word-organized array. The array was optimized with the aid of a computer analysis that considered the device, array geometry, and line termination as variables. The performance of this system has been tested by a cross-section model incorporating full length, fully populated word and digit lines.

Behavior of coaxial cable connectors for pulses with nanosecond risetimes

Reflections and risetime distortion caused by connector mismatches were measured for several common cable-connector combinations, using a fast pulser and sampling oscilloscope (system risetime = 0.4 nsec). It was found that excellent connectors are available for 125 Ω (RG 63/U) cable, as well as for 50 Ω cables. In addition, details are given for the construction of a connector which may be used with 185 Ω (RG 114/U) cable. Measurements are also reported for 125 Ω attenuators and a 125 Ω pulse inverting transformer.

A Fast-Slow Coincidence System Employing a Multichannel Time Display

A versatile, solid-state Fast-Slow Coincidence system is described which is used with large NaI(Tl) scintillators and XP1040 photomultipliers. A multichannel time display is provided which consists of a time-to-amplitude converter of the "start-stop" type and a standard 100-channel kicksorter. The calibration of the time analyzer can be varied from 1 nsec to 0.1 nsec per channel. In addition to showing the time spread, the display indicates the coincidence resolving time (2To) which is variable from 1 to 100 nsec by means of front panel helipots. The electronic time jitter of the system, as measured with a fast pulser is less than 0.2 nsec. A resolving time of 8 nsec gives 95% coincidence efficiency for coincidences between the annihilation radiation photo-22 peaks from Na22

A simple and accurate method for calibrating nanosecond time-to-pulse-height converters

A simple method for calibrating timesorters is described which has an absolute accuracy of < 1%. Fast-rising (∼ 10 −9 sec) pulses are fed into a junction point on a 10-meter long air-cored delay line and travel to the ends of the transmission line which are coupled to the limiting circuits of the timesorter. The input junction position, x , can be set at any desired point along the 10-meter length. The propagation velocity, v , is 99% of the velocity of light, c . Thus a change in junction position of Δx results in a change in the relative arrival times of the pulses at the limiters of Δt = 2 Δx (0.99 c ) −1 . The mechanical features of the air-cored transmission line are described and factors affecting the accuracy of the method are discussed.

Pulse Amplifiers Using Transistor Circuits

Transistor circuits simplify the design of pulse amplifiers because of their essentially linear gain characteristics. The low heat dissipation of such circuits permits small packages. The variations in transistor gain and leakage can be minimized by converting the high single stage gain to stability through the use of emitter feedback. Linearity and response to fast-rise pulses is shown for specifiic scintillation amplifier circuits.

Transfluxor Controlled Electroluminescent Display Panels

The feasibility of displaying images according to electrical signals using an array of electroluminescent cells magnetically controlled has been demonstrated by a working model. A transfluxor is associated with every electroluminescent cell. Each transfluxor is set by the coincidence of row and column current pulses to a level determined by the amplitude of the video signal, stores this level, and transmits energy continuously to the cell in proportion to the stored level. Both halftone and on-off control of brightness is possible. Scanning is accomplished by magnetically switching row and column conductors. The array has 1200 elements arranged in 30 rows of 40 each. The picture is uniform, exhibits good halftones, and has a maximum brightness of four foot lamberts. To obtain sufficient voltage from the transfluxor to excite electroluminescence, drive by short, fast-rising pulses is used. Light output data for such pulses were obtained and it was found that a steeper dependence of light output upon voltage, a different color spectrum, and higher efficiency resulted than is obtained with sine-wave drive. A modification using a linear transformer to couple the transfluxor and electroluminescent cell and driven by sine waves to operate as a transfluxor-tuned resonant circuit was demonstrated to be economical in driving and setting power. For some applications, the full storage afforded by a transfluxor at each picture element is not necessary. A system was investigated which uses fewer transfluxors storing only a row of picture information at a time.

Precision Measurement of Detonation Velocities in Liquid and Solid Explosives

A versatile chronographic technique for the measurement of detonation velocities in liquid and solid explosives is discussed. A raster-type oscilloscope is described which has a sweep linearity of 1% and a time coverage of 300 μsec. This instrument is composed of standard electronic components and requires relatively little maintenance. The basic signal mixing circuits used to generate fast-rising pulses associated with the detonation of an explosive charge are briefly described. Some electrical effects associated with the detonation of an explosive charge are briefly described to point out a possible source of spurious signals. Techniques for the preparation and assembly of explosive charges are presented together with certain precautions which must be taken against the possibility of the generation of false signals. The effect of temperature on detonation velocities of liquid and solid explosives is mentioned. Using the techniques and precautions described, it is possible to obtain detonation velocity data with a precision of 0.1% or better.

Fast Rise Pulse Generator with High Pulse Repetition Frequency

A mercury selector switch designed for use in telemetering may be modified by using high-frequency techniques to make a fast rise time pulse generator with high pulse repetition rates. The pulse rise times are less than 0.002 μsec at repetition rates in excess of 10 kc with peak pulse power of 50 watts.

Measurement of Parameters Controlling Pulse Front Response of Transformers

The two-element equivalent circuit commonly used in predicting the high-frequency or pulse-front response of a stepdown or step-up transformer is not applicable to transformers with turns ratios near unity. Furthermore, the transformer leakage inductance (the series element in the circuit) may vary with frequency or time because of skin effects in the windings, and the distributed capacitance (the shunt element in the circuit) varies with frequency or time, source impedance, and load impedance because of variations in voltage distribution. These variations are difficult to take into account in measuring the element values and also in using the values in the prediction of transformer response. A theoretical and experimental analysis has shown that a T circuit containing three inductances and one capacitance (or the mathematically equivalent \pi circuit, which contains seven elements) can be used to represent a transformer of any turns ratio, and element values obtained from appropriate measurements give reasonably accurate results over necessary frequency range. Measurements are easily made with an rf generator and a vacuum-tube voltmeter. Greater accuracy with which high-frequency response characteristics can be predicted by this T circuit indicates a marked improvement in accuracy will result when T circuit is used in predicting response of a transformer to fast-rising pulses.