Michael Lambrecht

Design, Characterization, and Optimization of a Waveguide-Based RF/MW Exposure System for Studying Nonthermal Effects on Skeletal Muscle Contraction

A waveguide-based exposure system for studying in vitro nonthermal radio-frequency/microwave (RF/MW) effects on skeletal muscle contraction in the frequency range of 0.75-1.12 GHz was designed, characterized, and optimized. The design includes a vertical organ bath (OB) placed inside the waveguide for suspending an intact skeletal muscle from the hind limb of the mouse, i.e., flexor digitorum brevis, in an oxygenated Tyrode solution during the RF/MW exposures. Muscle contraction is stimulated by an electric field generated between two platinum electrodes and continuously measured before, during, and after RF/MW exposure by a force transducer. A temperature feedback system that controls the perfusion rate of the Tyrode solution works in conjunction with the outer water jacket of the OB to maintain the temperature of the solution to within plusmn0.2degC. Characterization and optimization of the RF/MW exposure setup were accomplished by a detailed numerical computation of the RF/MW fields and specific absorption rate (SAR) inside the inner chamber of the OB, where the muscle is suspended, using the finite-difference time-domain (FDTD) method. Analysis of the computed RF/MW fields within and immediately surrounding the skeletal muscle showed that the RF/MW fields and SAR exhibit the level of homogeneity required for performing well-controlled RF/MW exposure experiments

Design and Simulation of a Mega-Watt Class Nonrelativistic Magnetron

Numerical simulations of a prototype conventional magnetron capable of an RF output power exceeding 1.0 MW are presented. Magnetron design evaluation is carried out via numerical simulation using the 3-D Improved Concurrent Electromagnetic Particle-in-Cell code. The magnetron was capable of oscillating in the π mode with little mode competition at 655 MHz over a range of magnetic fields extending from B = 0.186 to B = 0.261 T and voltages ranging from 40 to 64 kV. RF Output power ranged from 400 kW to 2.1 MW over these voltages with efficiencies typically at 60%. RF power propagation upstream was identified as a major source of loss in the design.

Numerical Simulations of a Relativistic Inverted Magnetron

A new design for an inverted magnetron is presented and modeled both analytically, using a single particle smooth bore relativistic approach, and numerically, using a massively parallel electromagnetic particle-in-cell code, Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC) code. Analysis and simulation confirm that the inverted magnetron design presented here is capable of oscillating in the π mode at axial magnetic fields of the order of ~0.1 T. ICEPIC simulations demonstrate that the inverted magnetron is capable of fast start-up, mitigation of mode competition, π-mode dominance, and high output power, of the order of 1 GW in some cases. Moreover, these performance features spanned over a variety of magnetic fields and input voltages. In simulations, the inverted magnetron design presented here demonstrated that end-loss current, a common source of energy leakage in relativistic magnetrons, has been eliminated as a source of energy loss. However, radio frequency output power efficiencies only remained comparable with standard relativistic designs. This was due to poor energy exchange between the particle and field. Thus, a refinement of the slow wave structure may be necessary.

A High-Efficiency Megawatt-Class Nonrelativistic Magnetron

Numerical simulations of a prototype conventional magnetron capable of an RF output power exceeding 1.3 MW at peak efficiency greater than 87% for relatively low diode voltages of ~ 40 kV are presented. Virtual prototyping of the magnetron design is carried out on massively parallel architecture utilizing the 3-D improved concurrent electromagnetic particle-in-cell code. Simulations demonstrate that the magnetron is capable of stable and robust oscillations in the π mode at saturation with negligible mode competition at 912 MHz over a range of magnetic fields extending from B = 0.18 T to B = 0.275 T and voltages ranging from 37-56 kV. RF Output power ranged from 400 kW-1.5 MW over these voltages with efficiencies typically above 85%. Oscillations in the π mode follow the Buneman-Hartree resonance curve for all magnetic fields sampled with a window of π-mode oscillations typically extending over 6 kV. Electron back bombardment of the cathode as well as collisions with the slow wave structure acted as major loss mechanisms.

Examination of two-stream instability for high power broadband RF amplifiers

Summary form only given. Exploiting the two-stream instability to amplify radio frequency signals is an attractive concept because the interaction is extremely broadband and co-propagating beams of different energies make the instability convective instead of absolute, thus enabling operation as an amplifier. This holds out the possibility of bandwidth performance that rivals that of helical traveling wave tubes (helix TWTs) at much higher powers. Previous attempts1 at two-stream amplifiers were abandoned due to poor efficiency compared to helix TWTs. More recent work2 3 has indicated that the use of relativistic beams can increase the efficiency, perhaps as high as 50%.

Virtual prototyping of a 5 MW conventional magnetron

Researchers at the Air Force Research Laboratorys (AFRL) Directed Energy Directorate have begun work on designing a conventional magnetron to produce 5 MW of peak power at 3GHz. This magnetron will be capable of delivering long pulse average power with a duty cycle of >10%. The Improved Concurrent Electromagnetic Particle in Cell (ICEPIC) code is used to perform numerical simulations of this source. ICEPIC is a massively parallel finite-difference time-domain electromagnetic solver capable of performing simulations with the high resolution and particle counts required for virtual prototyping. The baseline design will be a 900 MHz 1.5 MW magnetron previously developed by AFRL that will require extensive scaling and geometrical modifications to meet the new power and frequency requirements. Design challenges include the operational magnetic field and keeping electric fields within the smallest gaps of the magnetron below breakdown levels.

Design and simulation of a megawatt class conventional magnetron

Summary form only given. A Megawatt class conventional strapped magnetron (diode voltages <; 100kV) design is simulated using the Improved Concurrent Electromagnetic Particle-in-Cell code (ICEPIC). ICEPIC is designed to run on massive parallel architecture, consequently field and particle resolution requirements for these simulations are satisfied. Simulations are carried out at over 22 million grid cells with a resolution of one grid length = 0.5 mm or less. Numerical results yield a design capable of a mean RF output power exceeding 1.5 MW at greater than 85% efficiency at ~53 kV. The magnetron consistently oscillates in the π mode with no mode competition at ~900 MHz across a range of magnetic fields and voltages extending from 0.18-0.3 T and 45-60 kV. RF output power extends from 500 kW to ~ 4 MW over this range. Moreover RF output efficiency remains stable above 85% over these voltages and magnetic fields. Oscillations at these field parameters were consistent with single particle Buneman-Hartree analysis. For all cases, RF power is extracted axially through three conducting rod excitations. Field stresses remained below the Kilpatrick limit for simulations with diode voltage <; 52 kV and exceeded the limit above 53 kV. Field stress maximums were observed between the straps and the anode as well as between the straps. Major power loss mechanisms were particle collisions with the slow wave structure and cathode, which accounted for a loss of over 10% of the input power.

Predictive, optimized numerical simulation of microwave generation in complex geometries with real materials

Summary form only given. Predictive numerical simulation of microwave generation and plasma interactions requires resolution of multiple length and time scales in complex 3-dimensional geometries. The Particle-in-Cell (PIC) method for the numerical formulation of the basic governing equations is a well-understood and reliable approach if we are able to assume that short-range collisional effects are on average small. Particle collisions can be included in the PIC formulation through the use of Monte Carlo Collision models, and non-linear materials for which the permittivity depends on the strength of applied fields can be incorporated into the PIC formulation if proper attention is paid to issues of numerical stability of the expanded set of governing equations. Accurate modeling of time-dependent physics in complex geometries requires careful formulation of the numerical equations and the dynamic ability to evolve the image of a decomposed domain across thousands of computational cores.

Development of a 1.5 MW conventional magnetron via numerical simulation

Summary form only given. Researchers at the Air Force Research Laboratorys (AFRL) Directed Energy Directorate and magnetron engineers at Communications and Power Industries (CPI) have designed a conventional magnetron intended to deliver 1.5 MW peak power output at 920 MHz. The source was virtually prototyped using AFRLs state-of-the-art computer simulation software package, the Improved Concurrent Electromagnetic Particle In Cell (ICEPIC) code, a highly parallelized full-wave electromagnetic PIC code that is capable of running on thousands of processors in parallel. The magnetron design was based on a commercially available 75kW magnetron, and then optimized for operating frequency, mode separation, and efficient power extraction. Many design variants were explored including 14, 16, and 18 vanes, strap placement and configuration, axial length, number of RF extraction rods, and cathode radii. The process has yielded an optimum design that meets the established requirements for this new source. Thermal analysis and manufacturability studies have begun prior to fabrication of an experimental prototype.