M. Kristiansen

Texas Tech University, Lubbock, TX, United States

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Publications (408)214.76 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: form only given. Vircators are high power microwave sources which operate at frequencies approximately proportional to the plasma frequency of the electron beam as it passes through the anode. As a result, stable frequency operation requires flat top voltage pulses, and diodes which exhibit little variation in impedance over a given pulse duration. However, plasma formation on the anode and cathode followed by plasma expansion causes the effective A-K gap and thus the impedance of the diode to collapse with time. As the impedance collapses current density increases, possibly causing a shift to higher frequency operation. The vircator presently discussed exhibits frequency chirping from 4 to 5.5 GHz during a pulse length of 700 ns when operated with an 8 stage, 1.5 kJ Marx generator that has a pulse shape of an overdamped RLC discharge (assuming arc formation does not occur). However, stable frequency operation is observed with an 8 stage 80 J Marx, each stage constructed from pulse forming networks rather than discrete capacitors. The PFN Marx generator produces a flat top voltage for a 75 ns FWHM pulse; however the energy radiated from the vircator is much lower due to the reduced energy storage of the PFN Marx. Burst mode operation has the potential to improve frequency stability and energy radiated from the vircator. This contribution reports the experimental results of a compact sealed tube vircator operated in burst mode with a repetition rate of up to 100 Hz. The data show voltage and current waveforms during burst cycles as well as microwave waveforms with temporally resolved frequency spectra.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: form only given. A virtual cathode oscillator (vircator) is a high power microwave device that exhibits frequency tunability which is inherent to its principle of operation. Two types of electron motion within the vircator generate microwaves (virtual cathode oscillation and reflexing electron motion). Although it is difficult to precisely predict the dominant microwave frequency of a vircator design prior to experimental observation, the oscillation frequency of the virtual cathode is approximately proportional to the plasma frequency of the electron beam as it enters the virtual cathode. Additionally, the reflexing electrons oscillate at a frequency which is inversely proportional to approximately four times the transit time of an electron through the anode-cathode gap (A-K gap). As a result, assuming space charge limited diode operation, the virtual cathode and reflexing electron oscillation frequencies, though different, are proportional to V1/2/d, where d is the gap separation of the A-K gap and V is the accelerating voltage applied to the gap. Thus vircators are tunable via adjusting the A-K gap and varying the applied accelerating voltage. Texas Tech University has developed a sealed tube vircator which radiates approximately 100 MW with an operational frequency of 4GHz. Operating at 4GHz the diode has an A-K gap of 8mm, an accelerating voltage of ~200 k V, and electron beam current of ~3.5kA. Here we present an experimental investigation of the tunability of a reflex triode vircator by varying the A-K gap from 4 mm to 10 mm and accelerating voltages ranging from 150 kV to 250 kV.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    J. Stephens, A. Neuber, M. Kristiansen
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    ABSTRACT: This paper discusses the effect of surface coatings on exploding wire behavior. Three different surface coatings of different thicknesses and materials have been studied, each with a 99.99% pure silver conducting core. Experimentally, the wires are subjected to peak current densities in excess of 107 A/cm2 on a microsecond time scale. High Speed intensified Charge-Coupled Device (iCCD) images. A theoretical one-dimensional finite difference model has been developed to predict wire behavior and determine the mechanism(s) responsible for the deviations in behavior induced by the presence of a surface coating.
    Physics of Plasmas 03/2012; · 2.38 Impact Factor
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    ABSTRACT: The increased popularity of high power microwave systems and the various sources to drive them is the motivation behind the work to be presented. A stand-alone, self-contained explosively driven high power microwave pulsed power system has been designed, built, and tested at Texas Tech University's Center for Pulsed Power and Power Electronics. The system integrates four different sub-units that are composed of a battery driven prime power source utilizing capacitive energy storage, a dual stage helical flux compression generator as the main energy amplification device, an integrated power conditioning system with inductive energy storage including a fast opening electro-explosive switch, and a triode reflex geometry virtual cathode oscillator as the microwave radiating source. This system has displayed a measured electrical source power level of over 5 GW and peak radiated microwaves of about 200 MW. It is contained within a 15 cm diameter housing and measures 2 m in length, giving a housing volume of slightly less than 39 l. The system and its sub-components have been extensively studied, both as integrated and individual units, to further expand on components behavior and operation physics. This report will serve as a detailed design overview of each of the four subcomponents and provide detailed analysis of the overall system performance and benchmarks.
    The Review of scientific instruments 02/2012; 83(2):024705. · 1.52 Impact Factor
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    ABSTRACT: form only given. The challenges in developing and designing a 5 GW stand-alone pulsed power generator for driving a >; 100 MW high power microwave, HPM, source are discussed. Both, applied physics aspects of operating principles and limits of the individual sub-systems will be addressed. The energy, initially provided through a set of lithium-ion batteries internal to the generator, is boosted by an explosively driven flux compressor, FC, and inductively stored at a several kJ level. While the energy is stored on a microsecond timescale, it is, however, released into the load on a nanosecond timescale via an electro-explosive fuse, EEF, thus delivering GWs of electrical power to the HPM source for a duration of about 100 ns.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
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    ABSTRACT: form only given. This presentation describes a study on gas evolution of plasma expansion in a reflex-triode virtual cathode oscillator (vircator) at ultra-high vacuum (UHV). Research has shown that explosive electron emission (EEE) processes at the cathode and ion / electron bombardment at the anode cause material erosion that produces regions of localized plasma. This plasma expansion has shown to lower gap impedance, cut off microwave performance and spoil low vacuum levels over time1. The goal of the study is to identify the gas species and their sources to better understand and limit the negative effects of plasma expansion in sealed tubes. The vacuum tube under investigation is a triode-geometry vircator with 20 cm2 cathode surface, driven by a 80 J Marx Generator with an approximate peak voltage and current, and pulse width of 200 kV and 5 kA, and 200 ns, respectively.
    Plasma Science (ICOPS), 2012 Abstracts IEEE International Conference on; 01/2012
  • C. Lynn, J. Walter, A. Neuber, J. Dickens, M. Kristiansen
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    ABSTRACT: Many high power microwave devices use explosive or flashover electron emission cathodes in order to generate the electron beam and thus drive the device. These diodes are simple to operate, requiring no heater or other external systems, and are capable of generating beam currents of several kA at accelerating voltages on the order of 100s of kV. However, they generally suffer from non-uniform emission, anode heating, out-gassing, and pulse shortening due to anode and cathode plasma expansion. The ability to rep-rate such a diode is generally limited by anode heating and out-gassing which are both affected by beam uniformity. Two diodes are compared in this work. One uses a machined aluminum cathode, which is made from solid aluminum with grooves milled onto the surface. The other diode utilizes a carbon velvet cathode which is CVD coated with CsI. Time integrated scintillator images of the electron beam at the anode were taken for both the carbon velvet and aluminum cathodes. Additionally, time resolved images of the emission centers were taken for the carbon velvet cathodes. Data sets of over 1000 shots were taken with each cathode and shot to shot variation in the peak “turn-on” electric field are compared. The lifetime of the aluminum cathode was exceeded before 1000 shots, whereas the carbon velvet cathode showed no degradation in operation.
    Power Modulator and High Voltage Conference (IPMHVC), 2012 IEEE International; 01/2012
  • J.W. Walter, C.F. Lynn, J.C. Dickens, M. Kristiansen
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    ABSTRACT: The high-power-microwave (HPM) sources currently under development typically require constant pumping to maintain the high vacuum levels required for operation. This pumping is often done with either a cryo- or turbopumping system, either of which would be difficult to deploy in a compact portable system. A compact sealed-tube virtual cathode oscillator (vircator) source has been developed at Texas Tech University (TTU) that does not require a bulky external vacuum pump for operation. This device has a base vacuum pressure in the low range compared to the majority of laboratory HPM sources having vacuum levels in the - range. The reduced amount of trapped gasses in the sealed-tube ultrahigh-vacuum environment has the potential to greatly impact device performance. The TTU sealed-tube vircator is useful as a testbed for studying HPM source optimization under UHV conditions. Measured operational characteristics of the tube utilizing a carbon fiber cathode and a nickel anode are presented, along with radiated microwave measurements.
    IEEE Transactions on Plasma Science 01/2012; 40(6):1618-1621. · 0.87 Impact Factor
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    ABSTRACT: Many high-power electron devices utilize cold-cathode diodes to generate intense electron beams. These cold cathodes have the advantage of being capable of supplying several kiloamperes of current spread over a large cross section without the need for auxiliary components such as a heater supply. However, they suffer from many known problems such as nonuniform emission that can result in small areas of high current density on the anode and, thus, excessive anode heating. As a consequence, outgassing and vaporization of bulk material frequently leads to premature impedance collapse. Hence, minimizing nonuniform anode heating due to beam nonuniformity is paramount. As previously demonstrated, the use of a CsI-coated carbon velvet cathode improved beam uniformity, reduced outgassing, and mitigated early impedance collapse. To quantify the uniformity, temporal and spatially resolved images of the cathode plasma were taken for a CsI-coated carbon fiber cathode, operated at an average current density of ~150 A/cm2 under various conditions, i.e., without a field shaping ring, before and after discharge cleaning, and with a field shaping ring. All cathodes were operated in a sealed tube with a small integrated sputter ion pump to restore vacuum levels to 10-9 torr levels between subsequent shots.
    IEEE Transactions on Plasma Science 01/2012; 40(12):3449-3454. · 0.87 Impact Factor
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    ABSTRACT: The primary objective of the research discussed in this paper is to develop a compact electro-explosive fuse (EEF) for a flux compression generator (FCG) power conditioning system, capable of rapidly obtaining and maintaining high impedance. It was observed that significant gains in EEF performance are introduced with the application of an insulating coating to the surface of the EEF wire. A 2 kA small scale test bed has been designed to provide a single wire EEF with similar current density (∼107 A/cm2), voltage gradient (∼7 kV/cm), and timescale (∼8 μs) as to what is seen by and EEF utilized in a HPM generating FCG system. With the small scale test bed EEF performance data was rapidly obtained at a significantly lower cost than equivalent full scale FCG experiments. A one-dimensional finite difference model coupled with the Los Alamos National Laboratory SESAME Equation-of-State database was utilized to simulate the resistive behavior of the single wire EEFs. Further, a large scale test bed, designed to provide a similar current action as to what is provided by a FCG is used to test 18 wire EEF arrays at the 40 kA level.
    Power Modulator and High Voltage Conference (IPMHVC), 2012 IEEE International; 01/2012
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    ABSTRACT: This paper presents a study on gas evolution of three different anode materials in vacuum sealed tubes under UHV conditions. The experimental apparatus consists of a high-power microwave (HPM) virtual-cathode oscillator (vircator) driven by a 200 ns, 80 J, 225 kV low-impedance Marx Generator. Plasma expansion due to explosive electron field emission has shown to lower gap impedance, spoil consistent low vacuum levels, and cut-off microwave radiation. The anode materials compared are nickel 201L (Ni201L), stainless steel 316L (SS316L) and grade-1 titanium (TiG1); with the cathode material being aluminum. The anodes were cleaned by the following method: rough polishing followed by electro-polishing, a ten minute microwave argon / 10% oxygen plasma cleaning process (ArO2) and finally, a 72 hour in situ bake-out at 300°C. Outgassing characteristics of each anode material are presented and compared.
    Power Modulator and High Voltage Conference (IPMHVC), 2012 IEEE International; 01/2012
  • Y. Chen, J. Dickens, J. Mankowski, M. Kristiansen
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    ABSTRACT: Research efforts at Texas Tech University on impulse antenna phased array has resulted in the development of a dependable high voltage, high repetition rate switch that can minimize jitter into the ps range. To accurately synchronize a phased array to steer and preserve the risetime of a radiated pulse, the jitter can only be a fraction of this risetime. Initial testing with a similar system in [1] produced sub-ns jitter results for operations in different gases and gas mixtures. This paper discusses in detail 50 kV, 100 Hz switch operations using different testing parameters. The switch jitter as a function of triggering conditions is discussed, including a comprehensive evaluation of jitter as a function of operation pressure as well as trigger magnitude. Several phenomenon were observed and discussed to quantify the switch jitter with respect to operation pressure and trigger magnitude. The temperature of gas and its effects on switch jitter is also documented in this paper, with a jitter improvement of ~25% recorded. An empirical formula was determined as a function of the gas density, electric field of the main gap, and electric field for the trigger for the experiments conducted in this manuscript. A 50 Ω, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the switch. The hermetically sealed spark gap, with a modular design composed of copper tungsten electrodes, gas feeds, Kel-F lining, as a well as a G-10 shell is used to house the high pressure gases for the experiments. Trigatron-type triggering is provided by a solid state opening switch voltage source that supplies 75-150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode.
    IEEE Transactions on Dielectrics and Electrical Insulation 08/2011; 18(4):975-982. · 1.36 Impact Factor
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    ABSTRACT: Local cavity resonances of fast wave propagation in a hydrogen-deuterium tokamak plasma have been investigated experimentally. The results demonstrate short damping lengths of the fast waves on the high field side of the hybrid layer, and the existence of a local cavity resonance on the low field side, even for a low hydrogen minority concentration (5%) in a deuterium plasma. The strong wave damping disappears when the hybrid layer is moved towards the edge of the plasma.
    Nuclear Fusion 01/2011; 27(1):151. · 2.73 Impact Factor
  • J.E. Cato, M. Kristiansen, M.O. Hagler
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    ABSTRACT: An engineering study of r.f. heating of fusion reactors is described. The frequency range considered is below 100 MHz. Special consideration is given to r.f. structures located between the reactor wall and the plasma in the main fusion region. Possible arcing problems in this region are discussed and it is found that they may not be too serious for neutral pressures below 10−4 torr. It is proposed to feed the r.f. through a slanted vacuum port in order to reduce port area and to protect the voltage insulator from radiation and temperature damage. The cooling of an r.f. coil structure is considered in detail. It is proposed to use a split r.f. coil composed of heat pipes to carry the heat away. Curves are presented which give the maximum r.f. on-time for various coil currents, frequencies, and coil sizes before the coil surface reaches melting temperature. Other coupling schemes involving the use of the vacuum wall as a r.f. structure or locating a wave launcher in the high-field region of a mirror reactor are discussed. In the case of a mirror reactor it is found that this last scheme may be quite attractive since unidirectional couplers can be made and experimental results indicate that the wave reflections may not be too serious in the severe-density and field gradients. This scheme also offers a possible way of launching, for instance, ion-cyclotron waves into very dense plasmas. It is concluded that the engineering problems of fusion-reactor-r. f. heating, albeit formidable, are not impossible.
    Nuclear Fusion 01/2011; 12(3):345. · 2.73 Impact Factor
  • M. Kristiansen, M.O. Hagler
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    ABSTRACT: Current research on long-wavelength laser interaction with magnetized plasmas is summarized. Special attention is given to laser beam guiding in plasma density minima, to plasma heating and to investigations of laser-produced plasmas in strong magnetic fields. The basic theoretical and experimental background aad research are discussed. Reactor studies based on lassr heated plasma systems are reviewed. Some areas of possible future research are mentioned.
    Nuclear Fusion 01/2011; 16(6):999. · 2.73 Impact Factor
  • J. Walterx, J. Dickens, M. Kristiansen
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    ABSTRACT: A common figure of merit utilized for HPM sources is the instantaneous power efficiency at the time of peak output power. This conversion efficiency from the electron beam power to radiated microwave power disregards the overall energy efficiency of the system, which is vitally important in systems intended for practical application. Optimizing the overall system energy efficiency allows a reduction in volume, weight, and prime power requirements that makes for a much more practical and fieldable system. Texas Tech University has developed a vircator-based laboratory HPM system that has a high end-to-end energy efficiency relative to other HPM systems. The system utilizes a sealed-tube vircator source that requires no external vacuum pumping subsystem. For rep-rate use, the tube contains an integrated low-power sputter ion pump. The lack of need for an externally applied magnetic field saves complexity, weight, and potentially power. The vircator tube is simple and robust. The source is driven by a Marx generator utilizing pulse forming networks (PFNs) instead of lumped capacitors. Utilizing PFNs allows the driver to apply a more ideal pulse shape to the source than a traditional Marx, enhancing the performance of the source and reducing the amount of energy wasted in the rise and fall of the pulse. The system has demonstrated greater than 1% energy efficiency from the energy stored in the Marx to the radiated HPM, with potential for improvement. The peak total radiated output power of the system is between 50 and 100 MW.
    01/2011;
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    ABSTRACT: During the development and optimization of a compact sealed tube virtual cathode oscillator (vircator) at Texas Tech University, it has become apparent that processes at the anode have a significant impact on tube performance. The impact of the high energy, high current density (100–200 A/cm2 or higher) beam on the anode will cause outgassing, plasma production, and anode melting and material ejection. The emitted material expands, eventually impacting the anode transparency and (combined with the plasma formed at the cathode) shorting out the anode-cathode gap. This expansion limits the maximum radiated pulse width, and can also limit the peak output power. The residual evolved gas also negatively impacts the maximum repetition rate of the tube. An effort is underway to study the thermal behavior, gases evolved, and transparency versus time for different vircator anode materials and material treatments. Several different anode materials are under investigation, including stainless steel, copper tungsten, tantalum, nickel, and molybdenum. The effect of different treatments on the anodes before tube assembly is also being studied. The gases that are evolved during operation have been characterized utilizing pressure and residual gas analyzer measurements. The pre-shot background pressure in the tube is in the ultra-high vacuum range (10−8 to 10−9 Torr), and the vircator is not pumped on during firing. The data collected for the different materials is presented.
    01/2011;
  • A. Young, A. Neuber, M. Kristiansen
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    ABSTRACT: An investigation aimed at optimizing the integration between a capacitor based prime power source and flux-trapping helical flux compression generator (FT-HFCG) is presented. An FT-HFCG simulation code, previously benchmarked with single and multi-pitch generators, was employed to study the kilo-joule class explosive system for this purpose. The details of this effort, which include the optimization of the field coil and stator coupling, as well as an examination of the effect of field coil parameters on the system performance, will be described in this document. For the simulated parameter space, the choice of field coil configuration caused the system energy gain to vary by 300%, and the optimum field coil configuration was found to be a single Litz wire conductor that had an axial length which was approximately 60% of the stator axial length.
    Pulsed Power Conference (PPC), 2011 IEEE; 01/2011
  • P. Kelly, J. Mankowski, M. Kristiansen
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    ABSTRACT: The development of a high power in-line limiter utilizing varactor-loaded metamaterial structures is presented. A metamaterial structure is an artificial structure engineered to provide electromagnetic properties not available in nature, more explicitly defined as a material having simultaneously negative permittivity and negative permeability. A singly-negative material (SNG) structure, the split-ring resonator (SRR), is a negative permeability material which acts as a notch filter with resonant frequency f0. The resonant frequency of the SRR filter yields itself to tuning since the capacitance between the SRR and transmission lines is easily changeable through the use of varactors. At nominal power levels, f0 is significantly offset from the receiving frequency such that the receiving frequency is unattenuated. When an in-band high power microwave (HPM) is incident upon the filter, a DC bias is applied to several varactors and shifts the resonant frequency of the filter to that of the receiving frequency due to the change in capacitance of the varactors. This effectively attenuates the incident HPM. The filter uses a microwave rectifying circuit to extract a DC voltage from the in-band HPM, which serves as the DC bias voltage across the varactors. Ansoft's HFSS was used to accurately model and design the SRR structure to minimize the E-field and maximize resonant effects. Both high and low power continuous wave testing verified minimal insertion loss as well as verification that the use of varactors in conjunction with a split ring would effectively shift the resonant frequency of the notch filter.
    01/2011;
  • C. Lynnx, J. Walter, A. Neuber, M. Kristiansen
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    ABSTRACT: Vacuum diodes utilizing explosive emission cathodes generally suffer from non-uniform emission, gap closure by anode and cathode plasma expansion, and outgassing. Also, the Child-Langmuir relation does not apply at the edge of the emission area. This results in a high current density sheath at the edge of the emission area. Each of these phenomena presents its own technical challenge in HPM source design and optimization. Diagnostic techniques and particle in cell simulations for a vacuum diode, to be operated as a greater than 100 MW class vircator, have been used in order to compare the characteristics of the electron beam produced by various cathode materials and geometries. Uniform current density plays a key role in vircator performance and efficiency, as well as in the lifetime of the diode. The diode under investigation has an emission area of 20.3 cm2, and operates at a current density on the order of 300 A/cm2 at 200 kV. The background vacuum level and the associated adsorbed and absorbed gases at the surface play a major role in the behavior of an explosive emission cathode. This vircator source is operated as a sealed tube, requiring no vacuum pumping until the device is repetitively operated. The small amount of gas generated during operation is pumped down in between shots, if needed, utilizing an integrated small sputter-ion pump. The background pressure in the diode is in the ultra-high vacuum (UHV) range, on the order of 10−9 Torr to 10−8 Torr. Particle in cell simulations were performed to investigate current density distributions at the surface of both the anode and cathode due to various field shaping profiles. Additionally, scintillator images were taken to compare the beam profile of a machined aluminum cathode with a CsI coated carbon fiber cathode.
    01/2011;

Publication Stats

1k Citations
214.76 Total Impact Points

Institutions

  • 1971–2012
    • Texas Tech University
      • • Department of Electrical and Computer Engineering
      • • Pulsed Power Lab
      • • Department of Physics
      Lubbock, TX, United States
  • 2004–2005
    • Huazhong University of Science and Technology
      • School of Electrical and Electronic Engineering
      Wuhan, Hubei, China
  • 2002–2003
    • Kwangwoon University
      • Department of Electrophysics
      Seoul, Seoul, South Korea
  • 2001
    • University of Texas at Austin
      • Institute for Advanced Technology (IAT)
      Austin, TX, United States
  • 1994–2001
    • Nagaoka University of Technology
      Нагаока, Niigata, Japan
  • 1989
    • Lubbock Christian University
      Lubbock, Texas, United States