Lawrence M. Earley

Beam Line Design, Beam Alignment Procedure, and Initial Results for the

A gain experiment was performed at Los Alamos using a 120-keV 2-A cylindrical electron beam with a ridged waveguide slow-wave structure at 94 GHz, demonstrating 22 dB of amplification through a traveling-wave interaction. The structure was planar with a gap of 0.75 mm and a length of 5 cm. The 2-A electron beam was confined in a 3.2-kG axial magnetic field, with roughly a 0.5-mm diameter. The electron beam was aligned along the magnetic axis of the solenoid by scribing out its cyclotron motion on a novel optical diagnostic using a procedure that depends on varying the solenoidal field strength. The transport through the structure was verified by letting the beam drill holes in a series of thin metallic foils before insertion of the structure

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The transport of planar electron beams is a topic of increasing interest for applications to high-power, high-frequency microwave devices. This paper describes two- and three-dimensional simulations of electron-beam transport in a notched wiggler magnet array. The calculations include self-consistent effects of beam-generated fields. The simple notched wiggler configuration can provide vertical and horizontal confinement of high-perveance sheet electron beams with small transverse dimensions. The feasibility calculations address a beam system to drive a 95-GHz traveling-wave tube experiment under construction at Los Alamos National Laboratory, Los Alamos, NM.

-Band Gain Experiment at Los Alamos

The dual-axis radiographic hydrotest facility at Los Alamos National Laboratory will employ two linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. The accelerator cell design for a 3-kA, 16-20-MeV, 60-ns flattop, high-brightness electron beam is presented. The cell is optimized for high-voltage stand-off while also minimizing the transverse impedance. Measurements of high-voltage RF characteristics are summarized.<<ETX>>

Focusing of high-perveance planar electron beams in a miniature wiggler magnet array

LANL has developed a new vane loaded waveguide RF structure for a sheet electron beam traveling wave tube (TWT). The goal was to create a new class of wideband RF structures that allow simple mechanical fabrication and have geometry suitable for interaction with sheet electron beams. We have concentrated on structures at 94 GHz. We have achieved 6% bandwidth and believe that 10% is possible. We have performed 3D electromagnetic simulations using the codes Microwave Studio and HFSS, and fabricated several aluminium cold models of RF structures at 10GHz to confirm the design. Agreement between the 10 GHz cold test data and computer simulations was excellent. An RF structure at 94GHz was fabricated using electrical discharge machining (EDM) with a 0.004 inch wire and cold tested.

Cell design for the DARHT linear induction accelerators

We have designed, fabricated, and tested with low power a novel W-band traveling-wave tube (TWT) structure based on a slow-wave cylindrically symmetric photonic band gap (PBG) structure, or an ldquoomniguide.rdquo The development of wideband millimeter-wave power amplifiers is underway at Los Alamos National Laboratory. PBG TWT structures have great potential for very large bandwidth and linear dispersion. In addition, being cheap to fabricate, the PBG structures enhance commercial transferability of W-band TWT technology. The omniguide structure was designed and fabricated with silica dielectric with a copper harness. Cold-test results were found to be in excellent agreement with the design. A bandwidth of more than 10% was demonstrated. This structure is designed to generate millimeter-wave RF when driven by a 2-A 120-keV electron beam.

Wideband RF Structure for Millimeter Wave TWTs

We have designed, fabricated, and tested a novel photonic band gap (PBG) channel-drop filter (CDF) operating at around 240 GHz. A PBG CDF is a device that allows the channeling of selected frequencies from continuous spectra into separate waveguides through select defects in a PBG structure. It is compact and configurable, and thus, it can be employed for millimeter-wave spectrometry with applications in communications, radio astronomy, and radar receivers for remote sensing and nonproliferation. In this paper we present the design, modeling, and fabrication methods used to produce a silicon-based PBG CDF, and demonstrate its ability to filter the frequency of 240 GHz with a linewidth of approximately 1 GHz and transmission of 25 dB above background.

Design, Fabrication, and Low-Power Tests of a W-band Omniguide Traveling-Wave Tube Structure

The nonlinear electromagnetic response is one of the foundations of modern technology and it arises in natural materials at the atomic scale. We briefly present some of the fundamentals of nonlinearity in natural materials and then we present experimental studies of analogous behavior in meta-atoms, the fundamental building block of metamaterials. Specifically tunnel-diode loaded, microwave split-ring resonators are shown to enable various nonlinear phenomena including self-sustained oscillation, harmonic/comb generation, frequency locking/pulling, and quasi-chaos generation. We discuss the possible adaptation of these unit cells to create bulk nonlinear metamaterials.

First experimental demonstration of a photonic band gap channel-drop filter at 240 GHz

Currently ongoing at Los Alamos National Laboratory is a program to develop high-power, planar 100–300GHz traveling-wave tubes. An enabling technology for this effort is a sheet electron beam source and much of our effort has been geared toward understanding sheet beam generation and transport. Toward this end we have developed a robust, high resolution optical diagnostic for measuring the transverse density profiles of our electron beams. The diagnostic consists of a thin metal foil followed by an YAG:Ce or YAP:Ce scintillator crystal, both mounted on a vacuum actuator that allows us to position the foil/scintillator combination at arbitrary positions along the beam’s longitudinal axis. The electron beam strikes the metal foil and is stopped, generating Bremsstrahlung x rays that are imaged by the scintillator crystal. This image is then captured by an optical system using a high-speed, intensified gated camera. Using this diagnostic, we have measured beam profiles with resolutions as low as 0.05mm from ...

Tunnel-diode loaded split-ring resonators as a foundation for nonlinear metamaterials

We report the progress on the design and cold test of metallic photonic band gap (PBG) resonators and PBG accelerator structures. First, an 11 GHz PBG resonator, which can be utilized as an accelerator cavity with reduced long‐range wakefields, was constructed and tested. The higher‐order‐modes suppression was proved in the cold test. A Q‐factor of 5000 was achieved for a brazed resonator. Next, a 6‐cell 17.137 GHz PBG accelerator structure with reduced long‐range wakefields was designed. A high Q 6‐cell 17.137 GHz PBG accelerator was electroformed and is currently cold tested. A PBG structure will be tested at high power and will be used to accelerate an electron beam at Massachusetts Institute of Technology.

Optical beam profile diagnostic for low energy, long pulse, moderate current electron beams

Simulations have indicated that emerging electron sheet‐beam technology can drive simple rippled and stepped waveguide traveling‐wave tubes with extremely high gain. There are many design possibilities that need to be evaluated. The interaction can be made with the n=−1 (backward wave), n=0 (fundamental forward wave), or n=+1 (first space harmonic forward wave) interactions. In this paper, we discuss some of the fundamental design issues.