T. A. Spencer

University of Michigan, Ann Arbor, MI, United States

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Publications (149)38.5 Total impact

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    Dataset: IR&MM02
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    ABSTRACT: Experiments have been performed testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high power microwave performance. Three high permeability wires were embedded beneath the emission region of a 1.27 cm diameter cathode, spaced 120 degrees apart (for pi-mode symmetry in an 6 vane magnetron) to perturb both the axial and radial magnetic fields near the emission region of the cathode. Magnetic priming was demonstrated to increase the percentage of pi-mode shots by 15% over the baseline case. Mean peak power for pi-mode shots was found to be higher in the magnetically primed case by almost a factor of 2. Increases in mean microwave pulse width were also observed in the magnetically primed case when compared to the unprimed case (66 ns primed versus 50 ns unprimed). Magnetron starting current for the magnetically primed pi-mode exhibited a reduction to 69% of the unprimed baseline starting current. Earlier research by Neculaes (2005) and recent simulation work performed utilizing MAGIC PIC and the Magnum magnetostatics code suggest that using permanent magnets with radially-directed remanence fields centered under the cathode emission region instead of high permeability wires can yield improved magnetron performance. Simulations of magnetically primed magnetrons utilizing permanent magnets with radially-directed remanence fields demonstrated improved performance as compared to simulations of axially-directed remanence fields. Both simulation and experimental results will be presented for the magnetic priming cases described.
    Plasma Science, 2008. ICOPS 2008. IEEE 35th International Conference on; 07/2008
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    ABSTRACT: Experiments have been performed in testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high-power microwave performance. Magnetic priming consists of N/2 azimuthal magnetic perturbations applied to an N-cavity magnetron for rapid generation of the desired number of electron spokes for the pi-mode. Magnetic perturbations were imposed by utilizing three high-permeability nickel-iron wires embedded beneath the emission region of the cathode, spaced 120 apart. Magnetic priming was demonstrated to increase the percentage of pi-mode shots by 15% over the baseline case. Mean peak power for -mode shots was found to be higher in the magnetically primed case by almost a factor of two. Increases in mean microwave pulsewidth were also observed in the magnetically primed case when compared to the unprimed case (66-ns primed versus 50-ns unprimed). Magnetron starting current for the magnetically primed pi-mode exhibited a reduction to 69% of the unprimed baseline starting current.
    IEEE Transactions on Plasma Science 07/2008; · 0.87 Impact Factor
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    [show abstract] [hide abstract]
    ABSTRACT: Experiments have been performed testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high power microwave performance. Magnetic perturbations were imposed utilizing three, high-permeability nickel-iron wires embedded beneath the emission region of a 1.27 cm diameter cathode, spaced 120 degrees apart (for N/2 symmetry in an N (6) cavity magnetron). These three, high-permeability wires perturb both the axial and radial magnetic fields near the emission region of the cathode. Magnetic priming was demonstrated at UM to increase the percentage of p-mode shots by 15% over the baseline case in the relativistic magnetron. Improvements in microwave power, pulse width and start-oscillation time were also observed. Earlier experimental research by Neculaes and recent simulation work suggest that using permanent magnets with radially-directed remanence fields centered under the cathode emission region instead of high permeability wires can yield improved magnetron performance.
    Vacuum Electronics Conference, 2008. IVEC 2008. IEEE International; 05/2008
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    ABSTRACT: Magnetic priming^1,2 experiments on the UM/ L-3Titan relativistic magnetron (100 MW's in L-band, -300 kV, ˜3 kGauss), have shown suppression of unwanted modes and reductions in starting currents for the pi-mode. Data from continuing simulations and experiments on magnetic priming at the cathode will be presented, as well as data on magnetic priming at the cathode and anode. Data show that magnetic priming at the cathode significantly lowers (average factor of 2.5) the starting current for pi-mode generation. The percentage of pi-mode shots was also increased by magnetic priming at the cathode by as much as 60% over unprimed shots. Experiments and simulation results will be reported concerning the effects of magnetic priming at both cathode and anode. References [1] V.B. Neculaes, R.M. Gilgenbach, and Y.Y. Lau, US Patents 6,872,929 (3/29/2005) and 6,921,890 (7/26/2005); Appl. Phys. Lett., 83, 1983 (2003). [2] M.C. Jones, V.B. Neculaes, W. White, Y.Y. Lau, and R.M. Gilgenbach, Appl. Phys. Lett., 84, p1016, (2004) Acknowledgements: This research was supported by AFOSR, AFRL and the AFOSR-MURI Program on Cathode and Window Breakdown for High Power Microwave Sources
    11/2007;
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    ABSTRACT: Magnetic priming experiments on the UM/ L-3-Titan relativistic magnetron (100 MW in L-band, -300 kV, ~3 kGauss), have shown suppression of unwanted modes and major reduction in starting currents for the pi-mode. Data from continuing experiments on magnetic priming at the cathode will be presented, as well as preliminary data on magnetic priming at the cathode and anode. Azimuthally-varying, axial-magnetic-perturbations are generated by three, 4 cm or 6 cm-long Mu-metal wires located just below the surface of 0.86 mm wall-thickness stainless steel tubing. The electron-emitting surface of the stainless steel is laser machined for field emission. Magnetic perturbations applied only at the cathode decay with increasing radius, hi order to maintain magnetic perturbations across the entire A-K gap, we also install three, magnetic-priming, Mu-metal wires inside holes drilled in the anode structure. Magnetostatics calculations have been performed for the case of magnetic wires embedded in the cathode and in the anode. Simulation results show strong perturbations at the cathode surface, which fall off slightly at small radii, but grow in intensity as the anode surface is approached. Data demonstrate that magnetic priming at the cathode significantly lowers (average factor of 2.5) the range of starting currents for pi-mode generation. The percentage of pi-mode shots was also increased by magnetic priming at the cathode by as much as 60% over unprimed shots. Experiments are reported concerning the effects of magnetic priming at both cathode and anode.
    Plasma Science, 2007. ICOPS 2007. IEEE 34th International Conference on; 07/2007
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    ABSTRACT: Magnetic priming consists of N/2 azimuthal perturbations applied to the axial magnetic field in an N-cavity magnetron. When employed on the UM/L-3-Titan, 6-cavity, relativistic magnetron (L-band, -300 kV, ~3 kGauss), magnetic priming has shown enhanced performance over uniform-B- field operation. Magnetic perturbations are imposed utilizing Mu-metal wires embedded in either the cathode or both the cathode and anode of the magnetron.
    01/2007;
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    ABSTRACT: Magnetic priming [1] experiments performed on the UM/Titan relativistic magnetron (6-vane, -300kV, 5-10kA, 0.3-0.5 mus) have shown improvements in magnetron performance over baseline operation. In the current experimental setup, three, 4-cm long magnetic wires (Mu-Metal) are located within the cathode structure, centered beneath the emission region, and spaced 120 degrees apart. These wires produce magnetic perturbations with N/2 azimuthal-symmetry (for pi-mode in an N vane magnetron). Because of the close proximity of the priming structures to the cathode surface, the magnetic perturbations are strongest in the region where the electrons are emitted into the magnetron interaction space.
    Infrared Millimeter Waves and 14th International Conference on Teraherz Electronics, 2006. IRMMW-THz 2006. Joint 31st International Conference on; 10/2006
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    ABSTRACT: Rapid startup, increased pulsewidth, and mode locking of magnetrons have been explored experimentally on a relativistic magnetron by radio frequency (RF) priming. Experiments utilize a -300 kV, 2-8 kA, 300-500-ns electron beam to drive a Titan six-vane relativistic magnetron (5-100 MW output power in each of the three waveguides). The RF priming source is a 100-kW pulsed magnetron operating at 1.27-1.32 GHz. Tuning stubs are utilized in the Titan structure to adjust the frequency of the relativistic magnetron to match that of the priming source. Experiments are performed on rising sun as well as standard anode configurations. Magnetron start-oscillation time, pulsewidth, and pi-mode locking are compared with RF priming versus the unprimed case. The results show significant reductions in microwave output delay and mode competition even when Adler's Relation is not satisfied
    IEEE Transactions on Plasma Science 07/2006; · 0.87 Impact Factor
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    ABSTRACT: Magnetic priming [1] was applied to the UM/L-3 Titan relativistic magnetron (6-vane, -300kV, 5-10KA, 0.3-0.5 mus). Three 4-cm-long Mu-Metal were inserted within the cathode, centered beneath the emission region, and spaced 120 degrees apart. These wires produce magnetic perturbations with N/2 azimuthal symmetry (for pi-mode, N vane magnetron). Experimental results using the non-symmetric waveguide load array showed dramatic reduction in pi-mode starting current. Magnetic priming increased the percentage of pi-mode shots from 35% to 58%. Preliminary data also yielded increases in pi-mode peak power and mean pulse width. Symmetric waveguide load array data showed similar trends in magnetron performance improvement. A second series of experiments using three 6-cm wires within the cathode showed an 11% increase in the probability of pi-mode shots over the baseline case. MAGIC simulations combining magnetic priming on the cathode and anode have shown faster startup than the baseline case without magnetic priming, as well as improvements over magnetic priming applied only on the cathode or anode. [1] V.B. Neculaes, R.M. Gilgenbach, and Y.Y. Lau, US Patents 6,872,929 and 6, 921,890 (2005).
    01/2006;
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    ABSTRACT: Summary form only given. Initial magnetic priming experiments performed on the UM/Titan relativistic magnetron (6-vane, -300 kV, 5-10 kA, 0.3-0.5 mus) have shown improvements in magnetron performance over baseline operation. In the current experimental setup, three, 4-cm magnetic wires (Mu-Metal) are located within the cathode structure, centered beneath the emission region, and spaced 120 degrees apart. These wires produce magnetic perturbations with N/2 azimuthal symmetry (for pi-mode in an N vane magnetron). Because of the close proximity of the priming structures to the cathode surface, the magnetic perturbations are strongest in the region where the electrons are emitted into the magnetron interaction space. Results from experiments using the non-symmetric extraction-waveguide load array showed a dramatic reduction in pi-mode starting current (2.4 kA primed vs. 3.4 kA unprimed). Magnetic priming increased the percentage of pi-mode shots from 35% (unprimed) to 58% (primed). Preliminary data also show significant reductions in magnetron startup time as well as time to peak power for the pi-mode. Pi-mode peak power was increased by almost a factor of 2 in the magnetically primed case. The mean pulse width of pi-mode shots showed a slight, but statistically significant increase in the magnetic priming case (90 ns unprimed vs. 105 ns primed). Data obtained using the symmetric extraction-waveguide load array showed similar trends in magnetron performance improvement. Pi-mode mean starting current was reduced by magnetic priming (3.6 kA unprimed vs. 1.4 kA primed). Magnetic priming increased the percentage of pi-mode shots (46% unprimed vs. 50% primed). The series-2 magnetic priming case also showed a modest reduction in the time to peak power of the pi-mode shots. With the available number of shots, reductions in magnetron pi-mode startup time in the series-2 primed case versus the unprimed case could not be proven to be statistically significant using a t-test. Futu- re simulations and experiments will utilize different Mu-Metal wire lengths and diameter, as well as magnetic priming structures in the relativistic magnetron anode
    01/2006;
  • B. W. Hoff, T. A. Spencer, D. Price
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    ABSTRACT: Magnetic priming utilizes N/2 azimuthal variations in the axial magnetic field of an N-cavity magnetron to prebunch N/2 spokes for π-mode. [1] Positive results have been obtained in magnetic priming of the UM/Titan, relativistic magnetron (-300kV, 2-10kA, 0.3-0.5μs). Priming fields were created by three, axial, mu-metal wires within the cathode. Modeled magnetic field data were imported into 3-D MAGIC PIC and run for the A6 relativistic magnetron. Simulations showed faster startup and enhanced pi-mode control compared to the unprimed baseline. Initial experiments were performed in the UM/Titan magnetron with 3, 4 cm-long mu-metal wires embedded in the cathode, centered beneath the emission region. This primed magnetron yielded increased π-mode shots (57% primed vs. 35% unprimed) and statistically significant decreases in startup time (114 ns primed vs. 156 ns unprimed) and time to peak power (241 ns primed vs. 277 ns unprimed); mean peak power increased (11 MW primed vs. 6.5 MW unprimed, measured from 1 of 3 outputs). Additional concepts include longer cathode wires and wires in the anode. [1] V.B. Neculaes, R.M. Gilgenbach and Y.Y. Lau, US Patents 6,872,929 and 6,921,890
    10/2005;
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    ABSTRACT: Summary form only given. Research is underway to develop a viable technique to implement magnetic priming in the UM/Titan relativistic magnetron (-300 kV, 5-10 kA, 0.3-0.5 mus) for rapid startup and mode control. Magnetic priming involves imposing N/2 azimuthal variations in the axial magnetic field of an N-vane magnetron operating in the pi-mode. This type of magnetic priming has been demonstrated in -4 kV kilowatt magnetrons. Simulations of a highly idealized magnetic priming scheme using the MAGIC PIC code showed rapid growth of the pi-mode and suppression of mode competition in a relativistic magnetron. In the case of the kilowatt oven magnetrons, axial magnetic variations were imposed using external perturbation magnets. Relativistic magnetrons are bulkier and utilize large, pulsed electromagnets, making external priming magnets less practical. Because of this, new magnetic priming techniques are required. A new magnetic priming concept is proposed to alleviate this problem utilizing internal ferromagnetic structures to shape the magnetic field within the magnetron. Physically realizable magnetic field geometry was modeled using the field precision Magnum magnetostatics code. The resulting magnetic field data were then imported into MAGIC PIC and run for the case of a 6-vane relativistic magnetron. Preliminary fully three dimensional (3-D) simulations using MAGIC PIC have shown potential for faster relativistic magnetron startup. Magnetic field measurements will be compared to simulations and experiments on the relativistic magnetron
    IEEE International Conference on Plasma Science 01/2005;
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    ABSTRACT: Summary form only given. Research is underway to investigate radio frequency (RF) priming of relativistic magnetrons for rapid startup and reduced mode competition. Experiments utilize the MELBA-C to drive the Titan 6-vane relativistic magnetron at parameters: V=-300 kV, I=1-10 kA, e-beam pulselength=0.5 musec, microwave power=100-500 MW, microwave frequency=1-1.3 GHz (with tuning). MELBA-C's ceramic insulator enables base vacuum down to 8.5E-8 torr. The RF priming source is a 230 kW, 3 musec, pulsed magnetron from AFRL operating at 1.27-1.32 GHz. Tuning stubs are utilized in the Titan tube to adjust the frequency of the relativistic magnetron to match that of the priming magnetron. Microwaves are injected into 1 of the 3 open coupling slots in the MELBA-C relativistic magnetron. Time frequency analysis (TFA) of the magnetron's microwave output shows a clear effect of the RF priming on the relativistic magnetron. Frequency locking is observed when the priming RF frequency is sufficiently well matched to that of the relativistic magnetron. When the priming RF frequency is detuned, mode-beating is observed in agreement with the theory. Concurrent with the experiments are simulations utilizing the MAGIC 3D code to investigate the effects of RF priming on the relativistic magnetron.
    IEEE International Conference on Plasma Science 01/2005;
  • W. M. White, T. A. Spencer, D. Price
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    ABSTRACT: Magnetron start-oscillation time, pulsewidth and pi-mode locking are experimentally compared for RF priming versus cathode priming on the Michigan-Titan relativistic magnetron (-300 kV, 2-10 kA, 300-500 ns). Cathode priming [1, 2] is an innovative technique first demonstrated experimentally at UM. In this technique, the cathode is fabricated with N/2 emitting strips or N/2-separate cathodes (for an N-cavity magnetron), which generate the desired number of spokes for pi-mode. Cathode priming yields 13% faster startup with more reproducible pi-mode oscillation. Radio Frequency (RF) priming is investigated as the baseline priming technique for magnetrons. The external priming source is a 100kW, 3mus pulsewidth magnetron on loan from AFRL. RF priming reduced startup delay by 15% and increased pulsewidth by 9%. [1] M.C. Jones, V.B. Neculaes, R.M. Gilgenbach, W.M. White, M.R. Lopez, Y.Y. Lau, T.A. Spencer, and D. Price, Rev. Sci. Inst., 75, 2976 (2004) [2] M.C. Jones, Doctoral Dissertation, University of Michigan, 2005
    01/2005;
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    ABSTRACT: Summary form only given. Experiments have been performed on a relativistic magnetron using innovative monolithic metal cathodes (Al). These new types of cathodes are fabricated utilizing KrF laser ablation of metal surfaces to enhance the local E field. Two new types of cathodes invented are: projection ablation lithography (PAL) and ablation line focused (ALF) cathodes. PAL cathodes are fabricated by ablating a demagnified-projected pattern onto the metal surface thereby creating "metal cloth", ALF cathodes use a cylindrical lens to focus the laser beam into a single line; these focused laser lines are then ablated into patterns using a stepper motor controlled with LabVIEW. A novel technique for priming N-cavity magnetrons has been invented at the University of Michigan; this technique, "cathode priming", utilizes N/2 discrete, azimuthal, electron emission regions around the cathode circumference to prebunch electrons into pi-mode symmetry. Cathode priming experiments and simulations are performed on a relativistic, 6-vane, magnetron. This magnetron is powered by the MELBA-C accelerator with parameters: 0.3 MV, 1-10 kA and 300-500 ns pulselength. Microwave start-oscillation time for PAL cathode priming decreased an average of 10 ns when compared to the unprimed case. Simulations of cathode priming have been performed using the 3D particle-in-cell, MAGIC code. These simulations show pi-mode start-up times decrease from 28 ns with no priming down to 14 ns when cathode priming is imposed. Also, 3D simulations show suppression of undesired magnetron modes (2pi/3), for effective pi-mode-locking
    IEEE International Conference on Plasma Science 01/2005;
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    ABSTRACT: Experiments and simulations of cathode priming of a relativistic magnetron using KrF laser projection ablation lithography (PAL) cathodes are reported. The new UM technique of cathode-priming of N-cavity magnetrons utilizes N/2 azimuthally periodic electron emission regions spaced around the cylindrical cathode[1]. The PAL cathode is an all metal cathode which has advantages over fabric cathodes; 1) little or no out-gassing, 2) emission regions can be micro-machined to prescribe electric field enhancement, and 3) emission regions are heat-sinked to the cathode. Cathode-priming experiments on a 6-vane Titan relativistic magnetron with a Tri-PAL cathode show that microwave pulselength increased, startup-time and starting-currents decreased, while microwave output power remained constant. Plasma diode closure has been reduced and, in some cases, eliminated with PAL cathodes. [1] M.C. Jones et. al., Rev. Sci. Instrum., Sept. 2004. *This research is supported by AFOSR, AFRL and the AFOSR-MURI Program on Nanophysics of Electron Emission and Breakdown for High Power Microwaves
    11/2004;
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    ABSTRACT: We investigate 2 priming techniques in relativistic magnetrons for rapid startup and mode-locking: RF priming experiments with 0.1-1 MW from a 2nd magnetron; Magnetic-priming simulations by azimuthally-varying-axial magnetic field. Experiments utilize MELBA-C with a Titan 6-vane magnetron: V = -300kV, I = 1-10kA, e-beam T = 0.5 μs, microwave power = 100-500 MW, f= 1-1.3 GHz, base vacuum= 8.5 x 10-10 Torr. The AFRL RF priming magnetron is at 0.1-2 MW, 3 μsec, 1.27-1.32 GHz. About 0.2-0.3 MW is injected into 1 of 3 open coupling slots in the relativistic magnetron. Analysis of the relativistic magnetron's microwave output shows a clear effect of RF priming. Simulations of magnetic priming in the pi-mode are run in MAGIC code by imposing N/2 azimuthal-variations in the axial magnetic field of an N-vane magnetron. Faster startup and mode-locking are simulated by rapid-electron spoke formation and excitation of RF fields.
    10/2004; -1:1121P.
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    ABSTRACT: Summary form only given. Research is underway to investigate two techniques for priming of relativistic magnetrons for rapid startup and reduced mode competition: 1) RF priming experiments with a 2 MW magnetron signal 2) Magnetic-priming simulations by an azimuthally-varying axial magnetic field. Experiments utilize the MELBA-C (Titan) 6-vane, relativistic magnetron which operates with parameters: V=-300 kV, I=1-10 kA, e-beam pulselength=0.5 μsec, microwave power=100-500 MW, microwave frequency in L-band: 1-1.3 GHz. The ceramic insulator enables operation down to 8.5 E-8 Torr. The RF priming source is a 2 MW, 2.2 μsec, pulsed magnetron from AFRL operating at 1.3 GHz. The microwaves are injected into 1 of the 3 open coupling slots in the MELBA-C relativistic magnetron. Magnetic priming consists of imposing N/2 azimuthal variations in the axial magnetic field of an N-vane magnetron. Such optimal magnetic priming has been demonstrated in low voltage experiments and high voltage simulations to cause rapid startup of magnetrons by pre-bunching the electrons into the N/2 electron spokes desired for the pi-mode. A highly idealised model of magnetic priming uncovered a parametric instability, which draws electrons into N/2 spokes that extend to the anode even in the absence of RF fields.
    Plasma Science, 2004. ICOPS 2004. IEEE Conference Record - Abstracts. The 31st IEEE International Conference on; 08/2004

Publication Stats

183 Citations
141 Downloads
38.50 Total Impact Points

Institutions

  • 1989–2008
    • University of Michigan
      • Department of Nuclear Engineering and Radiological Sciences
      Ann Arbor, MI, United States
  • 1999–2006
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 1996–2004
    • Air Force Research Laboratory
      Washington, Washington, D.C., United States
  • 1994
    • University of New Mexico
      • Department of Electrical and Computer Engineering
      Albuquerque, NM, United States
  • 1991
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States