Lars D. Ludeking

1.4: Design of a high-gain wideband high-power 220-GHz multiple-beam serpentine TWT

A new concept for a compact high-gain multiple-beam TWT amplifier is introduced. To illustrate the concept, we present the design of a three-beam 220-GHz serpentine TWT amplifier. MAGIC-3D particle-in-cell code simulation results predict that the device should be capable of a peak power of 73 W and saturated gain of 42 dB over an instantaneous bandwidth of 50 GHz (23%), when powered by three 100 mA, 20 kV, well-focused electron beams. This performance can be achieved in a very compact circuit length of only 1.5 cm - a significant advantage for THz electron beam devices where issues of fabrication tolerances, beam alignment, and electron interception are of critical importance.

Sheet-Beam 90 GHz and 220 GHz Extend-Interaction-Klystron Designs

Circuit designs for 91 GHz and 220-GHz sheet-beam extended-interaction klystrons (EIKs) are presented. The W-band circuit employs a relatively simple two-cavity configuration to demonstrate beam transport and high peak power RF generation, which will serve as the basis for future scaling to higher gain and frequencies. Uniform (solenoidal) magnetic focusing is employed in both cases. Simulations with MAGIC-3D indicate peak RF powers of 10.5 kW and >400 W are feasible with a 3 A, 19.5 kV beam and a 0.5 A, 16.5 kV beam at 91 GHz and 220 GHz, respectively. Proof of principle experimental demonstrations are planned.

High-efficiency relativistic backward wave oscillator: theory and design

Backward wave oscillators (BWOs) driven by high-current relativistic electron beams are capable of producing high-power coherent radiation in the centimeter and millimeter wavelength regions. However, the efficiency of these devices is usually limited to 15-20% when a homogeneous slow-wave structure is used. Utilizing a two-section slow-wave structure, where the spatial period of the second section is larger than that of the first section, a BWO efficiency of greater than 50% was calculated. A conceptual design of a high-efficiency S-band BWO driven by a 500-kV 5-kA electron beam has been developed and analyzed.

Design Methodology and Experimental Verification of Serpentine/Folded-Waveguide TWTs

The general electromagnetic properties and design methodology for serpentine/folded-waveguide (FW) amplifiers are presented. In addition, hybrid-waveguide circuit topologies, which permit greater design flexibility than the basic serpentine/FW topologies, are also introduced, and their dispersion characteristics are discussed. Experimental validation of design methodology and tools is provided via test results of the recently demonstrated wideband 220-GHz serpentine amplifier, which embodies the design methodology described herein. Particular attention will be paid to the comparison between code prediction and experimental data, which are in excellent agreement.

Linearity performance of multi-stage TWT amplifiers: Cascade vs. series

Key performance characteristics that guide the selection of an amplifier for various applications are gain, power, linearity, and efficiency. In this paper, these parameters are contrasted for two amplifier configurations: a single-beam amplifier with stages placed in series vs. a multi-beam amplifier with cascading stages. Based on MAGIC-3D simulations, while gain and efficiency are comparable, the novel cascading amplifier configuration provides significant improvements in terms of power for a given level of linearity, both AM/AM and intermodulation distortions, over those of a conventional serial approach.

15.4: Design of terahertz Extended Interaction Klystrons

The development of terahertz power amplifiers presents significant new challenges as it brings into focus design, fabrication, and measurement issues that are not important factors at lower frequencies. In this paper, we describe our design approach to meet these challenges with particular emphasis on a 0.67 THz Extended-Interaction Klystron (EIK). This device is being designed to generate a peak power of ∼0.5W with gain of ∼23 dB over an instantaneous bandwidth of 15 GHz. The circuit will be driven by a 100 mA, 25kV electron beam confined in a 0.005″diameter beam tunnel with ∼1 Tesla magnetic field. This choice of beam size ensures that the same electron gun can be employed for all program development phases culminating in a 1.03 THz amplifier. A highly efficient depressed collector has also been designed to meet the efficiency requirement.

Design of a wideband high-power W-band serpentine TWT

The design of a W-band serpentine TWT with >200 W of power over a 4 GHz bandwidth (>100 W over 7 GHz) is presented. The amplifier is driven by a 122mA, 20 kV electron beam generated by a slightly modified version of the demonstrated 670 GHz beamstick at a reduced magnetic field. The design was performed by both the established MAGIC-3D and the recently validated NRL code Neptune, with good agreement between the two codes. Predicted RF peak power is 245 W, corresponding to 10% electronic efficiency.

Development of high-power broadband Ka-band cascaded-TWT

Progress on the demonstration of a high power broadband three-beam Ka-band cascaded traveling-wave tube (TWT) prototype is presented. The amplifier will be driven by an electron gun (0.6 A × 3, 20 kV) adapted from our 18-beam multiple-beam klystron gun design. Both serpentine and folded-waveguide versions of the cascaded-TWT have been evaluated. The expected peak RF output power is 4.5 kW with 100 W of drive power at a frequency of 30 GHz. The minimum output power is 3 kW over a 5 GHz frequency band (4 kW over > 2 GHz).

Well Matched Electromagnetic Boundary in FDTD-PIC for Charged Particle Penetration

The authors address the Matched Phase Velocity method for bounding an electromagnetic (EM) domain in the MAGIC finite difference-time domain (FDTD) particle-in-cell (PIC) EM code. Reflected power is shown to be reduced 18 dB compared to the conventional port treatment. Boundary quality is preserved under penetration by charged particles. fields. This approach is versatile and provides self consistent interaction between particles and fields. The MAGIC software includes a broad variety of boundary and material properties as well as particle and field algorithms. We will make use of only a selected subset to illustrate the behavior of the improved CMPV approach. The centered MPV method for bounding an electromagnetic domain will be shown to be effective in reducing unwanted boundary reflections in the MAGIC EM code and has the pleasing symmetry of application that waves may be injected and removed from the simulation via the same numerical methodology, and does not exhibit the unstable behavior of other ABC methods when penetrated by charged particles.

High-power broadband cascaded-TWT development

The design of a Ka-band three-beam cascaded serpentine travelling-wave tube (TWT) amplifier is presented. Simulations predict that a peak power of 3.5 kW with saturated gain of 16 dB and an instantaneous bandwidth of 30% can be achieved in a circuit length of ≤ 4.5 cm driven by three 20 kV beams with 0.5-A each. A plan for a proof-of-principle experiment will also be discussed.