Anthony T. Lin

Gain and bandwidth of the gyro-TWT and CARM amplifiers

Issues concerning the interpretation of gain and bandwidth from the dispersion relation are examined for the gyrotron traveling wave tube (gyro-TWT) and the cyclotron auto-resonance maser (CARM) amplifiers. A general method for the determination of the critical current for oscillation is illustrated. Despite the broad bandwidth predicted for the CARM amplifier by the commonly employed dispersion relation, it is seen in particle simulation that single-particle interaction. Rather than collective amplification, prevails over much of the band. Reasons for the discrepancy are analyzed. >

High-power harmonic gyro-TWT's. I. Linear theory and oscillation study

A linear theory using Laplace transforms which is applicable to both gyrotron traveling wave amplifiers (gyro-TWTs) and gyrotron backward-wave oscillators (gyro-BWOs) is presented. The validity of the linear theory is verified by comparing it with an existing nonlinear self-consistent theory based on a different approach. In conjunction with a time-dependent multimode particle simulation code, the linear theory is applied to study the stability of harmonic gyro-TWTs. It is shown that a harmonic gyro-TWT can be made stable to all forms of spontaneous oscillations by employing a multistage interaction structure and that it can generate power levels far in excess of those possible for a fundamental gyro-TWT. The linear bandwidth of a second-harmonic gyro-TWT amplifier is also calculated. >

Marginal stability design criterion for gyro-TWTs and comparison of fundamental with second harmonic operation

Abstract Stability properties of both the fundamental and second harmonic gyrotron travelling wave amplifier (gyro-TWT) are examined with multi-mode particle simulations. The second harmonic cyclotron interaction with an axis-encircling electron beam is found to be more stable to oscillations and can yield significantly greater power than the fundamental harmonic gyro-TWT. A multiple stage interaction structure based on a marginal stability criterion is proposed and illustrated with examples of a 128kW fundamental gyro-TWT and a 532 kW second harmonic gyro-TWT, Stable amplification at much higher power levels is in principle possible.

Design of a W-band TE/sub 01/ mode gyrotron traveling-wave amplifier with high power and broad-band capabilities

A high-power gyrotron traveling-wave amplifier operating in the low-loss TE/sub 01/ mode has been constructed at the University of California, Davis that will be driven by a 100-kV, 5-A electron beam with a pitch angle (/spl upsi//sub /spl perp////spl upsi//sub z/) of unity and velocity spread of 5%. The amplifier is predicted by large-signal simulations to generate 140 kW at 92 GHz with 28% efficiency, 50-dB saturated gain and 5% bandwidth. The stability of the amplifier from oscillation has been investigated with linear codes. The threshold current for the absolute instability of the TE/sub 01/ operating mode for the chosen operating parameters is predicted to be 10 A. To suppress the potential gyro-backward-wave oscillator interactions, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with distributed loss so that the single-pass attenuation is 90 dB at 93 GHz. The coaxial input coupler has a predicted and measured coupling of 1 and 2 dB, respectively.

High-power harmonic gyro-TWT's. II. Nonlinear theory and design

For pt.I, see ibid., vol.20, no.3, p.155-162 (1992). Based on an analytical study of the stability problems of gyrotron traveling wave amplifiers (gyro-TWTs), an extremely high power second-harmonic gyro-TWT has been designed, evaluated and optimized with a self-consistent nonlinear numerical simulation code. The design, which is based on the magnetron-injection-gun (MIG)-type beam, is presented. Using a 100 kV, 25 A MIG beam with alpha =1 and an axial velocity spread of 5%, nonlinear self-consistent analysis of a three-stage second-harmonic gyro-TWT amplifier predicts a peak output power of 533 kW, peak efficiency of 21.3% and a 7.4% saturated bandwidth, which verifies the theoretical predictions that a stable harmonic gyro-TWT can generate power levels an order of magnitude higher than those possible from a fundamental gyro-TWT. It is shown that the positioning of the electron beam is very important. A multistage structure is used to recover the loss in gain resulting from shortening the interaction sections to ensure stability. >

InGaAs heterostructure formation in catalyst-free GaAs nanopillars by selective-area metal-organic vapor phase epitaxy

We investigate axial GaAs/InGaAs/GaAs heterostructures embedded in GaAsnanopillars via catalyst-free selective-area metal-organic chemical vapor deposition. Structural characterization by transmission electron microscopy with energy dispersive x-ray spectroscopy(EDS) indicates formation of axial In x Ga 1 − x As ( x ∼ 0.20 ) inserts with thicknesses from 36 to 220 nm with ±10% variation and graded Ga:In transitions controlled by In segregation. Using the heterointerfaces as markers, the vertical growth rate is determined to increase linearly during growth.Photoluminescence from 77 to 290 K and EDS suggest the presence of strain in the shortest inserts. This capability to control the formation of axial nanopillarheterostructures is crucial for optimized device integration.

Unified theory and comparative study of cyclotron masers, ion‐channel lasers, and free electron lasers

A unified small‐signal amplification theory is developed to compare growth mechanisms responsible for a number of relativistic radiation generators. The theory is formulated from the basis that the electron resonance frequency produced by the external fields of the devices depends on γ−q, where γ is the beam Lorentz factor and q is a constant (q=1 for cyclotron masers, q=1/2 for ion‐channel lasers, and q=0 for free electron lasers). It is concluded that for wave amplification, the sign of the electron mismatch frequency is required to be the same as the sign of bunching parameter that is determined by the total bunching both axial and azimuthal; this depends on the q value. The two bunching mechanisms exist, not only in the single electron resonance regime, but also in the collective gain regime. Competition or reinforcement between the two bunching mechanisms is determined by the q value, the electron axial velocity, and the wave phase velocity.

The stability and tunability of a CARM amplifier

Theoretical analysis and computer simulations have been carried out to examine the stability and tunability of a cyclotron auto resonance maser (CARM) amplifier. It is shown that frequency tunability over one octave can be achieved by magnetic field tuning within the stability range. Tunability can be further enhanced if the waveguide wall is made lossy to increase the threshold of the absolute instability. However, beam momentum spread is found to have a strong deteriorating effect on the tunability and gain.

Visible light emission from self-catalyzed GaInP/GaP core-shell double heterostructure nanowires on silicon

The authors report on the formation, structural analyses, and optical properties of GaInP/GaP self-catalyzed core-shell double heterostructure nanowires (NWs) grown on Si(111) substrates. The NW growth is initiated with the formation of Ga droplets as catalysts, followed by the growth of GaP core and GaInP double heterostructure shells. Structural analyses elucidate the existence of interfaces among GaP core and GaInP double heterostructure shells. Light emissions at 640 and 800 nm are observed at 77 K from GaInP core-shell double heterostructure NWs and surface states of GaInP layers, respectively. The signal from the surface state can be mitigated via surface passivation with ammonium sulfide solution. These results will enable the realization of novel NW-based light-emitting diodes or nanolasers grown on Si substrates utilizing mature Si technologies.

Nonlinear dynamics of the gyrotron traveling wave amplifier

Mode interactions in the gyrotron traveling wave tube amplifier (gyro‐TWT) are examined with particle simulations and compared with experimental observations. Studies focus on the competition between the absolute and convective instabilities. Detailed diagnostics on mode structures, field growth profiles, and evolution of electron distribution in momentum and energy spaces allow an in depth look at the basic physical processes, such as the mechanism of mode competition and suppression. Mutual verifications between the theory and experiment have strengthened understanding of the multimode behavior and added to the predictability of high‐power gyro‐TWT’s.