Jason M. Sanders

A linear, single-stage, nanosecond pulse generator for delivering intense electric fields to biological loads

A compact pulse generator capable of producing high voltage pulses with halfmaximum widths as short as 2.5 ns and amplitudes as high as 5 kV has been developed to enable current and future in vivo and in vitro research into the effects of ultra-short, intense electric fields on biological matter. This pulse generator is small, simple, and free of saturable magnetic cores, which frequently introduce amplitude jitter and an undesirable correlation between amplitude and pulse width. In place of a non-linear pulse-forming network is a single-stage resonant network that drives a bank of junction recovery diodes. The diodes function as an opening switch that commutes current from an inductor to a resistive load. The use of air-core inductors in the resonant network results in a stable output pulse with an amplitude that scales linearly with input voltage and a pulse width that is independent of amplitude. The ability to scale the output amplitude independently of the pulse width simplifies the setup for experiments that require pulses with different electric field strengths but the same rise time and duration. Jurkat T lymphoblast cells exposed to 2.5 ns fields produced by this pulse generator showed an increasing degree of electropermeabilization with increasing pulse dosage and electric field intensity.

Plasma potential mapping of high power impulse magnetron sputtering discharges

Pulsed emissive probe techniques have been used to determine the plasma potential distribution of high power impulse magnetron sputtering (HiPIMS) discharges. An unbalanced magnetron with a niobium target in argon was investigated for a pulse length of 100 μs at a pulse repetition rate of 100 Hz, giving a peak current of 170 A. The probe data were recorded with a time resolution of 20 ns and a spatial resolution of 1 mm. It is shown that the local plasma potential varies greatly in space and time. The lowest potential was found over the target’s racetrack, gradually reaching anode potential (ground) several centimeters away from the target. The magnetic presheath exhibits a funnel-shaped plasma potential resulting in an electric field which accelerates ions toward the racetrack. In certain regions and times, the potential exhibits weak local maxima which allow for ion acceleration to the substrate. Knowledge of the local E and static B fields lets us derive the electrons’ E×B drift velocity, which is abou...

Microchamber Setup Characterization for Nanosecond Pulsed Electric Field Exposure

Intracellular structures of biological cells can be disturbed by exposure to nanosecond pulsed electric field (nsPEF). A microchamber-based delivery system mounted on a microscope setup for real-time exposure to nsPEF is studied in this paper. A numerical and experimental characterization of the delivery system is performed both in frequency and time domains. The microchamber delivery system presents a high impedance compared to classical 50 Ω loads. Its frequency behavior and limits are investigated using an in-house finite-difference time-domain (FDTD) simulator and through experimental measurements. High-voltage measurements for two nsPEF generators are carried out. The applied pulse voltage measured across the microchamber electrodes is ~1 kV, corresponding to ~10 MV/m electric fields in the microchamber. Depending on the nsPEF generator used, the measured pulse durations are equal to 3.0 and 4.2 ns, respectively. The voltage distribution provided by FDTD simulations indicates a good level of homogeneity across the microchamber electrodes. Experimental results include permeabilization of biological cells exposed to 3.0-ns, 10-MV/m PEFs.

Moveable Wire Electrode Microchamber for Nanosecond Pulsed Electric-Field Delivery

In this paper, an electromagnetic characterization of a moveable wire electrode microchamber for nanosecond pulse delivery is proposed. The characterization of the exposure system was carried out through experimental measurements and numerical simulations. The frequency and time domain analyses demonstrate the utility of the proposed assembly for delivering pulses as short as 2.5 ns. High-voltage measurements (~1.2 kV) were also performed using pulse generators based on two different technologies with applied pulse durations of 5.0 and 2.5 ns. Validation of the delivery system was accomplished with biological experiments involving cell electroporation with 2.5 and 5.0 ns, 10-MV/m pulsed electric fields. A dose-dependent area increase (osmotic swelling) of the Jurkat cells was observed with pulses as short as 2.5 ns.

Low energy compact power modulators for transient plasma ignition

In this paper recent studies of compact power modulators, used to produce nonequilibrium plasma in the transient, formative phase of an arc, and applied to ignition of a quiescent fuel-air mixture in a constant-volume reactor, are reported. In this work, ignition delays produced by transient plasma were measured and compared in pre-mixed C2H4-air at atmospheric pressure. Two compact power modulators studied included; 1) a 54 ns pseudospark switched line-type power modulator that delivered 365 mJ per pulse, and 2) a 12 ns SCR-switched magnetic compression based power modulator that delivered 75 mJ per pulse. Despite the difference in energy delivered, both systems achieved similar ignition delays across a broad range of fuel-air equivalence ratios, and produced ignition delays up to two times shorter than those produced using traditional spark ignition. The results indicate that lower energy and therefore more compact power modulators may be used for this application.

Optimization and implementation of a solid state high voltage pulse generator that produces fast rising nanosecond pulses

In this paper athree-stage pulse generator architecture capable of generating high voltage, high current pulses is reported and system issues are presented. Design choices and system dynamics are explained both qualitatively and quantitatively with discussion sections followed by the presentation of closed-form expressions and numerical analysis that provide insight into the systems operation. Analysis targeted at optimizing performance focuses on diode opening switch pumping, energy efficiency, and compensation of parasitic reactances. A compact system based on these design guidelines has been built to output 8 kV, 5 ns pulses into 50 Ω. Output risetimes below 1 ns have been achieved using two different compression techniques. At only 1.5 kg, this light and compact system shows promise for a variety of pulsed power applications requiring the long lifetime, low jitter performance of a solid state pulse generator that can produce fast, high voltage pulses at high repetition rates.

Pulse sharpening and soliton generation with nonlinear transmission lines for producing RF bursts

Nonlinear transmission lines (NLTL) are being designed and built to extend the range of available Ultra-Wide Band (UWB) and High Power RF pulse generation technology, especially in the area of high repetition rate microwave burst generation [1]. The NLTL approach to UWB and RF generation eliminates the need for the electron beam, vacuum system, and magnets required in conventional high power microwave (HPM) sources. Furthermore, the novel waveforms of NLTL generated pulses promise to offer a degree of frequency diversity unseen in current electron beam-driven HPM sources. UWB pulses are obtained using the wave front steepening properties of a transmission line with an amplitude dependent phase velocity. Ferrite loaded NLTLs have a phase velocity that increases with amplitude, so the pulses voltage peak travels faster than the trough until the rise time is limited by dissipation [2]. The addition of dispersion, which occurs in lumped periodic structures, can lead to the generation of a soliton, or solitary wave, and in some cases a group of solitons, where the dispersion replaces the dissipation as the balancing mechanism of the wave front steepening. Soliton bursts can be extracted from the line and delivered to a load as high power RF energy. A multi-stage system that consists of a pulsed power modulator for converting DC power into a high-voltage nanosecond pulse, a semi-continuous NLTL composed of biased ferrite beads for creating a fast rise-time UWB pulse, and a dispersive, lumped element NLTL that converts the UWB pulses into bursts of RF energy is discussed.

Scalable, compact, nanosecond pulse generator with a high repetition rate for biomedical applications requiring intense electric fields

A high repetition rate, high voltage pulse generator has been developed that scales up the output voltage of a recently reported compact, nanosecond pulse generator that is currently being used in various biomedical applications, including experiments into the mechanisms that drive cellular electropermeabilization and plasma generation for an endodontic disinfection tool [1, 2]. This single-stage, nanosecond architecture is based is composed of a bank of power MOSFETs, a linear network of inductors and capacitors, and a bank of junction recovery diodes; it was reported to feature an output pulse amplitude voltage to input voltage ratio between 5 and 6 [3, 4]. Since commercially available power MOSFETs tend to be limited to 1 kV, the output amplitude of the single-stage pulse generator does not exceed 5 or 6 kV. To combat this limitation, two different architectures have been developed that enable scaling of the output voltage. The first of these increases the voltage input to the pulse-forming network by means of a solid-state Marx bank that employs power MOSFETs arranged in a series-parallel arrangement to handle the high voltage and high current requirements of the switching stage. The second architecture employs a saturating transformer to handle the high current. Each of these has its own advantages: the first architecture has a shorter trigger-to-output delay time and is capable of producing low-jitter pulses with a linear input-output voltage relationship; whereas, the architecture with a saturating core features fewer components and reduced complexity. Prototypes of both architectures have been designed, built, tested, and are currently being used.

A synchronized emissive probe for time-resolved plasma potential measurements of pulsed discharges

A pulsed emissive probe technique is presented for measuring the plasma potential of pulsed plasma discharges. The technique provides time-resolved data and features minimal disturbance of the plasma achieved by alternating probe heating with the generation of plasma. Time resolution of about 20 ns is demonstrated for high power impulse magnetron sputtering (HIPIMS) plasma of niobium in argon. Spatial resolution of about 1 mm is achieved by using a miniature tungsten filament mounted on a precision translational stage. Repeated measurements for the same discharge conditions show that the standard deviation of the measurements is about 1-2 V, corresponding to 4%-8% of the maximum plasma potential relative to ground. The principle is demonstrated for measurements at a distance of 30 mm from the target, for different radial positions, at an argon pressure of 0.3 Pa, a cathode voltage of -420 V, and a discharge current of about 60 A in the steady-state phase of the HIPIMS pulse.

Broadband Power Measurement of High-Voltage, Nanosecond Electric Pulses for Biomedical Applications

Experimental studies have shown that nanoelectropulses of sufficiently short rise time and duration can trigger cell apoptosis (programmed cell death). Specifically, pulses that last less than 20 ns have been shown to kill a wide variety of human cancer cells in vitro as well as induce tumor regression in vivo. To better understand the mechanisms by which nanopulses affect cancer cells, the electrical characteristics of the cells should be characterized so that it is clear how electrical energy is delivered. To achieve this end, a power measurement device that integrates into a nanopulse transmission line used for in vivo experiments has been designed, built, and calibrated. The device provides voltage and current measurements without disturbing the transmission of the pulse to the biological sample. The in vivo sample can be characterized either by a direct voltage/current measurement or by measuring the incident and reflected voltage pulse and using time domain reflectometry to calculate the impedance. The 3 dB bandwidth of both the microstrip line and the voltage attenuator has been measured to be 900 MHz and 1.25 GHz respectively.