Howard D. Sanders

Increasing the oscillation frequency of strong magnetic fields above 101 kHz significantly raises peripheral nerve excitation thresholds

PURPOSE A time-varying magnetic field can cause unpleasant peripheral nerve stimulation (PNS) when the maximum excursion of the magnetic field (ΔB) is above a frequency-dependent threshold level [P. Mansfield and P. R. Harvey, Magn. Reson. Med. 29, 746-758 (1993)]. Clinical and research magnetic resonance imaging (MRI) gradient systems have been designed to avoid such bioeffects by adhering to regulations and guidelines established on the basis of clinical trials. Those trials, generally employing sinusoidal waveforms, tested human responses to magnetic fields at frequencies between 0.5 and 10 kHz [W. Irnich and F. Schmitt, Magn. Reson. Med. 33, 619-623 (1995), T. F. Budinger et al., J. Comput. Assist. Tomogr. 15, 909-914 (1991), and D. J. Schaefer et al., J. Magn. Reson. Imaging 12, 20-29 (2000)]. PNS thresholds for frequencies higher than 10 kHz had been extrapolated, using physiological models [J. P. Reilly et al., IEEE Trans. Biomed. Eng. BME-32(12), 1001-1011 (1985)]. The present study provides experimental data on human PNS thresholds to oscillating magnetic field stimulation from 2 to 183 kHz. Sinusoidal waveforms were employed for several reasons: (1) to facilitate comparison with earlier reports that used sine waves, (2) because prior designers of fast gradient hardware for generalized waveforms (e.g., including trapezoidal pulses) have employed quarter-sine-wave resonant circuits to reduce the rise- and fall-times of pulse waveforms, and (3) because sinusoids are often used in fast pulse sequences (e.g., spiral scans) [S. Nowak, U.S. patent 5,245,287 (14 September 1993) and K. F. King and D. J. Schaefer, J. Magn. Reson. Imaging 12, 164-170 (2000)]. METHODS An IRB-approved prospective clinical trial was performed, involving 26 adults, in which one wrist was exposed to decaying sinusoidal magnetic field pulses at frequencies from 2 to 183 kHz and amplitudes up to 0.4 T. Sham exposures (i.e., with no magnetic fields) were applied to all subjects. RESULTS For 0.4 T pulses at 2, 25, 59, 101, and 183 kHz, stimulation was reported by 22 (84.6%), 24 (92.3%), 15 (57.7%), 2 (7.7%), and 1 (3.8%) subjects, respectively. CONCLUSIONS The probability of PNS due to brief biphasic time-varying sinusoidal magnetic fields with magnetic excursions up to 0.4 T is shown to decrease significantly at and above 101 kHz. This phenomenon may have particular uses in dynamic scenarios (e.g., cardiac imaging) and in studying processes with short decay times (e.g., electron paramagnetic resonance imaging, bone and solids imaging). The study suggests the possibility of new designs for human and preclinical MRI systems that may be useful in clinical practice and scientific research.

A durable gigawatt class solid state pulsed power system

A unique all solid-state pulsed power system has been tested at the Naval Research Laboratory that produced 200 kV, 4.5 kA, and 300 ns pulses, continuously for more than 11,500,000 shots into a resistive load at a repetition rate of 10 pps. The Marx has an efficiency of 80% based on calorimetric measurements. This pulser is used to evaluate components and advance solid state designs for a next generation solid-state pulsed power system to drive an electron beam pumped KrF laser system for inertial fusion energy. The solid state pulser, designed and constructed by PLEX LLC, consists of a 12 stage Marx, coupled with a 3rd harmonic stage to sharpen the Marx output waveforms, a main magnetic switch, a compact pulse forming line used as a transit time isolator, and a resistive load. Each Marx stage uses an APP Model S33A compact high voltage switch that consists of 12 series connected thyristors. A life test on individual thyristors showed operation of >; 300 M shots at 20 Hz without failure.

Long Lifetime Optically Triggered Solid State Marx

Marx generators are used to produce high voltage, high current, short pulses for a variety of applications. High energy Marx generators are typically switched by gas insulated spark gaps, which have short lifetimes, <105 pulses, thus limiting practical applications of the Marx generators. APP has developed optically triggered, economical, compact, 48 kV, >30 kA/mus, 8 kA, and 100 ns turn-on solid state switches and examined their performance under normal and fault-mode operating conditions in Marx generators. With 48 kV compact solid state switches, >107 pulse lifetime, high voltage Marx generators capable of high pulse repetition rates can be built. Three different Marx configurations have been tested; a conventional unipolar Marx, a unipolar Marx that uses magnetic assist to achieve >70 kA/mus current risetimes, and an inverting LC mode. For practical applications, solid state switches must survive system faults, such as shorts in downstream components or the load, which can result in twice the normal forward current, a large reverse current and with a large fraction of the system energy deposited in the switches. The switches should also tolerate trigger failures that can result in overvoltaging of one or more switches. This paper will include a description of the solid state Marx and triggering system and show data from multi-million pulse operation as well as fault mode survival testing such as load shorts and switch triggering failures. Near-term applications for the switches include the retro-fitting of the 5 pps Marxes used for the Electra laser pre-amplifier and main amplifier. Electra is a repetitively pulsed, electron beam pumped Krypton Fluoride (KrF) laser at the Naval Research Laboratory. This program is developing technologies to meet the Inertial Fusion Energy (IFE) requirements for durability, efficiency, and cost. The technologies developed on Electra should be directly scalable to a full size fusion power plant beam line. The present Electra Marxes use gas insulated spark gap switches with lifetimes of 105 pulses. By using the 48 kV Compact Solid State switch, lifetimes in excess of 107 pulses are expected.

Solid State Marx Generator

Marx generators can produce high voltage pulses using multiple identical stages that operate at a fraction of the total output voltage, without the need for a step-up transformer that limits the pulse risetimes and lowers the efficiency of the system. Each Marx stage includes a capacitor or pulse forming network, and a high voltage switch. Typically, these switches are spark gaps resulting in Marx generators with low repetition rates and limited lifetimes. The development of economical, compact, high voltage, high di/dt, and fast turn-on solid-state switches make it easy to build economical, long lifetime, high voltage Marx generators capable of high pulse repetition rates. We have constructed a Marx generator using our 24 kV thyristor based switches, which are capable of conducting 14 kA peak currents with ringing discharges at >25 kA/mus rate of current risetimes. The switches have short turn-on delays, less than 200 ns, low timing jitters, and are triggered by a single 10 V isolated trigger pulse. This paper will include a description of a 4-stage solid-state Marx and triggering system, as well as show data from operation at 15 kV charging voltage. The Marx was used to drive a one-stage argon ion accelerator

Thyristor based solid state switches for thyratron replacements

Thyratrons and spark gap switches continue to be the predominant technologies used for high current, high voltage pulsed power applications. The offerings from thyratron manufacturers have been diminishing and prices have increased as vacuum tube manufacturing continues its decline. In addition, the maintenance requirements for thyratrons make them unsuitable for many potential main stream applications for pulsed power. High voltage IGBT based switches have become common, but current limitations have prevented them from being an attractive alternative for many thyratron replacement applications. Thyristors have the advantage of high current capacity and this paper will describe compact, high current, high voltage solid state switches for thyratron replacements, based on thyristor technology. The switches are based on series connected fast thyristors with 3cm2 die in a 20cm2 package. These switches have been tested to 50 k V, to greater than 12 kA, to greater than 50 kA/μs, to 360 Hz, and to 3×108 pulses, without failure. Thyratron replacement switches based on thyristor technology are currently in use at CERN, Argonne National Lab and SLAC National Accelerator Laboratory. They offer advantages over thyratron switches for cost, lifetime, size, weight and maintenance requirements.

A durable, repetitively pulsed, 200 kV, 4.5 kA, 300 ns solid state pulsed power system

A solid state pulser, designed and constructed by PLEX LLC, has been tested at the Naval Research Laboratory. It has achieved more than 11,500,000 continuous shots at 10 pps and generated 200 kV, 4.5 kA, 300 ns pulses, and the standard deviation time jitter of the output voltage was less than ± 1 ns. The pulser consists of a 12 stage Marx, coupled with a 3rd harmonic stage, a main magnetic switch, a compact pulse forming line used as a transit time isolator, and a resistive load. Each Marx stage uses an APP Model S33A compact high voltage switch that is comprised of 12 series connected thyristors. The Marx has an efficiency of 80% based on thermal measurements.

Solid state switches for high frequency operation as thyratron replacements

Applied Pulsed Power (“APP”) has developed a thyristor based switch capable of replacing thyratrons in many high-frequency, high current, high-voltage, pulsed power applications. The switch can operate at frequencies up to 500 pps, voltages of 32kV or higher and maximum fault currents of 14kA, and have lifetimes of more than 1011 pulses [1]. Lower frequency models of our thyristor based thyratron replacement switches are currently in use at CERN, Argonne National Lab and SLAC National Accelerator Laboratory [2]. They offer advantages over thyratron switches in cost, lifetime, size, weight and maintenance requirements.

High Current, High DI/DT, Solid State Switch Resistance Model

The efficiency of a solid state switch used in short pulse applications is determined by how rapidly the effective resistance decreases during turn-on. Applied Pulsed Power has examined the resistance fall for our high current, high di/dt, solid state switches and produced a useful model for the resistance versus time under various operating conditions. The model covers voltages up to 48 kV, currents up to 8 kA, di/dt up to 75 kA/mus, and pulse widths up to 1 mus. It has been implemented in SPICE circuit simulations, and using the SPICE model, the efficiency of the switch in different applications can be predicted. This paper will present data examining actual resistance measurements compared to the SPICE circuit simulations and discuss the solid state physics involved in the model.

Capabilities of the Reconfigured Cobra Accelerator

The COBRA accelerator at Cornell University has been reconfigured for use with wire arrays. Design goals included 1 MA peak current with a variable zero to peak current rise-time of as little as 100 ns. COBRA is now driven by two Marx generators, each of which feeds a parallel-plate water capacitor. These capacitors are switched into four parallel pulse-forming lines via self-breaking gas switches. Each pulse-forming line is switched into a vacuum adder via a laser-triggered gas switch that can be independently triggered to provide different pulse shapes to a wire array Z-pinch load. A stainless steel, magnetically insulated current convolute similar to that of the Z-Machine at Sandia National Laboratories connects the output switches to the load. A typical load consists of a 4-8 wire cylindrical array with a diameter of 1.6 cm and a height of 2 cm. To date the refurbished COBRA has over 300 shots. These shots have been used for various reasons including machine diagnostic calibration, laser-triggered switch timing and self breaking switch adjustments as well as experiments with cylindrical wire array and high current X-pinch loads.

Fast, solid-state crowbar switch to protect high power amplifier tubes

Fast crowbar switches are needed to protect high power amplifier tubes from potentially damaging internal arcs. Such a switch needs to have a fast turn-on to a low impedance state while also having the capability to discharge large amounts of stored energy. These switches have historically been thyratron tubes or spark gaps [1]. We will describe a new hybrid solid-state approach that combines a small fast switch with a slower large area slow switch to achieve a low cost crowbar switch for these applications up to 50 kV.