L.A. Bowers

3-D ICEPIC simulations of the relativistic klystron oscillator

Improved concurrent electromagnetic particle-in-cell (ICEPIC), developed at the US Air Force Research Laboratory, is a three-dimensional (3-D) particle-in-cell (PIC) code specifically designed for parallel high-performance computing resources. ICEPIC simulates collisionless plasma physics phenomena on a Cartesian grid. ICEPIC has several novel features that allow efficient use of parallel architectures, including automated partitioning, dynamic load balancing, and an advanced parallel PIC algorithm. Even though ICEPIC is still in development, it has successfully simulated real-world high-power microwave (HPM) experimental devices, such as the magnetically insulated line oscillator (MILO) and the relativistic klystron oscillator (RKO), and the results from recent RKO simulations will be presented.

Virtual prototyping of directed energy weapons

This paper gives an overview of how RF systems, from pulsed power to antennas, may be virtually prototyped with the improved concurrent electromagnetic particle-in-cell (ICEPIC) code. ICEPIC simulates from first principles (Maxwells equations and Lorenzs force law) the electrodynamics and charged particle dynamics of the RF-producing part of the system. Our simulations focus on gigawatt-class sources; the relativistic magnetron is shown as an example. Such simulations require enormous computational resources. These simulations successfully expose undesirable features of these sources and help us to suggest improvements

3D PIC simulations of the Relativistic Klystron Oscillator

Summary form only given. The Relativistic Klystron Oscillator (RKO) is a gigawatt class microwave source that is being investigated within the Air Force Research Laboratory (AFRL). In this talk, results of 3D simulations of the AFRL RKO using the particle-in-cell software ICEPIC (Improved Concurrent Electromagnetic Particle-In-Cell) are presented. Full 3D simulations of the RKO are challenging for a number of reasons, not the least of which is the sheer size of the problem. ICEPIC is unique in its ability to perform very large simulations and is an appropriate tool for solving problems related to the RKO. Other challenges include the correct treatment of emission from the knife-edge cathode and proper modeling of the Q of the RKO cavities.

Three Dimensional PIC Simulations of Novel Cathodes in the Michigan and AFRL Relativistic Magnetrons

Several novel shaped cathodes for a relativistic magnetron were modeled and optimized using a massively parallel electromagnetic particle-in-cell code ICEPIC. The effect of using a shaped cathode to enhance key performance parameters such as output power, power efficiency and impedance, was examined. In simulations we saw a dramatic increase in the range of magnetic field in which the magnetron functions, an increase of output power, an increase in efficiency, an elimination of mode competition, and immediate start up

Virtual Prototyping of Directed Energy Weapons on Thousands of Processors

This paper documents the changes required to permit ICEPIC to scale efficiently to the thousand CPU range. Substantial changes were made to the communication paradigm within the code, so that only one synchronization point is now required. This led to increase of a factor four in the number of processors ICEPIC can productively use on real world problems

Three dimensional PIC simulations of the transparent and eggbeater cathodes in the Michigan relativistic

Summary form only given. A novel relativistic magnetron priming technique consisting of either a main center cathode and several satellite cathodes (eggbeater design) or just the satellite cathodes (transparent cathode design [Fuks and Schamiloglu, 2005]) is investigated using the three-dimensional electromagnetic, particle in cell code ICEPIC. Both these cathode designs rely on RF field penetration into the cathode region to enhance performance. This priming technique is thought to allow for larger amplitudes of the synchronous Ee field in the electron hub region which in turn hastens the capture of electrons into spokes. This technique effectively eliminates mode competition with the Pi mode amplitude dominating the other modes by at least a factor of 25. The A6-3 Michigan magnetron was used for all simulations [Lopez 2002]. For both cathode designs the number, radial placement and orientation of the satellite strips were allowed to vary over several axial magnetic field values so that an optimization with respect to output power and efficiency could be determined at a given magnetic field. Performance optimization was achieved for both cathode configurations at B=3.2 kG with nine satellite cathodes at a radial placement of 1.75 times the main cathode radius. A power output efficiency of 35% and an output power exceeding 550 MW was measured for both designs at optimum parameters. Efficiency and output power decreased as satellite radial placement was increased from optimum. Moreover, it was observed in some cases that satellite cathode orientation (with respect to the slow wave structure) provided an additional ~50 MW of output power. Transparent and eggbeater cathode designs were simulated with 3, 6 and 9 satellite cathodes. It was found that the number of satellite cathodes did affect the magnetron output performance characteristics. For example, unlike the 6 and 9 satellite cathode simulations, the 3 cathode transparent design was robust with respect to cathode placement. Power efficiencies for satellite cathode radial placements at 1.75, 2.0 and 2.3 times the cathode radius all yielded efficiencies in the 30% range and output powers ~500 MW. Only after satellite cathode radius exceeded 2.3 times the main cathode radius did efficiencies start to drop off significantly for the transparent 3 cathode design

Toward the Design of High Power and High Efficiency Relativistic Magnetrons using Novel Cathode Geometries

Summary form only given. A novel relativistic magnetron priming technique consisting of either a shaped cathode, a main center cathode with several satellite cathodes (Eggbeater design), or only the satellite cathodes (transparent cathode design) is investigated using the three-dimensional electromagnetic particle in cell code ICEPIC. This priming technique allows for perturbations of the diode electric field such that a DC azimuthal electric field component, Ethetas, is introduced within the electron hub region. Additionally, for both the transparent and Eggbeater designs, a high amplitude azimuthal RF electric field component develops within the interaction region as well. These azimuthal field components in turn hasten the capture of electrons into spokes and initiate oscillations.

Virtual Prototyping of Directed Energy Weapons

This paper reports on radio frequency (RF) sources that have been virtually-prototyped with the Improved Concurrent Electromagnetic Particle-in-Cell (ICEPIC) code. ICEPIC simulates from first-principles, Maxwells equations and Lorenzs force law, the electrodynamics and charged particle dynamics of the RF-producing part of the system. Our simulations focus on several proposed variants of the L-band A66 half height (A66-hh) relativistic magnetron HPM source. The variations of the A66-hh include higher frequency designs (C-band, S-band and X-band), and a design that uses permanent magnets at L-band instead of electromagnets. Such simulations require enormous computational resources. Simulations exposed undesirable features in the first iteration of these designs and by the visualization and analysis of the simulation solutions became apparent. In the next iteration of the design, the simulations did not have these undesirable features.

Simulation of a MILO with self-consistent plasma formations and comparison to experiment

Summary form only given, as follows. Experimental and computer simulation results on a magnetically insulated transmission line oscillator (MILO) have indicated that the large beam current density emitted from each end of the cathode leads to anode plasma formation. This initiates bipolar space-charge flow in the anode-cathode (A-K) gap that severely perturbs the electron flow at the launch point. The result is significant microwave power reduction on a 600-ns time scale. The field-shaper cathode, used previously to extend the MILO RF pulse duration beyond 400 ns, is shown to have several deficiencies concerning anode plasma formation. We report on simulations that explain the power reduction in the MILO and possible ways to minimize anode plasma.

Three-dimensional PIC simulations of a load-limited MILO

Summary form only given, as follows. The magnetically insulated transmission line oscillator (MILO) is a gigawatt class high power microwave (HPM) source. By relying on magnetic insulation, it permits large input powers, (10s of GW) without electrical breakdown or external magnets. Despite these advantages, magnetic insulation in conjunction with a slow wave structure results in highly non-linear RF generation, mode structure, and mode competition. While this phenomena is not generally amenable to analytic treatment, much insight can be gained from particle-in-cell (PIC) simulation. ICEPIC, a fully three-dimensional, parallel, electromagnetic PIC code developed at AFRL, allows this complex device to be simulated from end-to-end including a Vlasov antenna. By including the full geometry of the device, simulation enables the transient behavior, such as mode competition, to be studied in detail. Given the excellent agreement with experimental data, these simulations point to the importance of return current paths and antenna design in the development of unwanted non-axisymmetric modes during the start-up phase of microwave generation. The saturation and quenching of these modes and the growth of the desired operating mode are examined in detail.