Chiping Chen

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

We report room‐temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1‐nm‐thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound‐state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak‐to‐valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak‐to‐valley ratio, should allow oscillations up to about 1 THz.

Guiding optical light in air using an all-dielectric structure

The emergence of a dielectric omnidirectional multilayer structure opens new opportunities for low loss broad-band guiding of light in air. We demonstrate the effectiveness of such an approach by fabricating a broad-band, low-loss hollow waveguide in the 10-/spl mu/m region and measuring its transmission around a 300 bend. The generality of the solution enables the application of the method to many wavelengths of interest important in telecommunication applications as well as for guiding high-power lasers in medical and other fields of use.

Low resistance Pd/Ge/Au and Ge/Pd/Au ohmic contacts to n‐type GaAs

We have fabricated Pd/Ge/Au and Ge/Pd/Au sintered ohmic contacts on n‐type GaAs. These contacts have specific resistances similar to those of conventional Ni/Ge/Au alloyed ohmic contacts, but their surfaces are much smoother and their edges are well defined. The fabrication procedure is compatible with GaAs technology, and the sintered contacts are thermally stable. The properties of these low resistance contacts are insensitive to the sintering temperature and to the thickness of the Pd and Ge layers or their order of deposition.

Self‐field‐induced chaoticity in the electron orbits in a helical‐wiggler free‐electron laser with axial guide field

The motion of a relativistic electron is analyzed in the field configuration consisting of a constant‐amplitude helical wiggler magnetic field, a uniform axial magnetic field, and the equilibrium self‐electric and self‐magnetic fields produced by the non‐neutral electron beam. By generating Poincare surface‐of‐section maps, it is shown that the equilibrium self‐fields destroy the integrability of the motion, and consequently part of phase space becomes chaotic. In particular, the Group I and Group II orbits can be fully chaotic if the self‐fields are sufficiently strong. The threshold value of the self‐field parameter e=ω2pb/4Ω2c for the onset of beam chaoticity is determined numerically for parameter regimes corresponding to moderately high beam current (and density). It is found that the characteristic time scale for self‐field‐induced changes in the electron orbit is of the order of the time required for the beam to transit one wiggler period. An analysis of the first‐order, self‐field‐induced resonanc...

Experimental and theoretical studies of a 35 GHz cyclotron autoresonance maser amplifier

Experimental and theoretical studies of a cyclotron autoresonance maser (CARM) amplifier are reported. The measurements are carried out at a frequency of 35 GHz using a mildly relativistic electron beam (1.5 MeV, 130 A, 30 nsec) generated by a field emission electron gun followed by an emittance selector that removes the outer, hot electrons. Perpendicular energy is imparted to the electrons by means of a short bifilar helical wiggler. The entire system is immersed in a uniform axial magnetic field of 6–8 kG. With an input power of 17 kW at 35 GHz from a magnetron driver, the saturated power output is 12 MW in the lowest TE11 mode of a circular waveguide, corresponding to an electronic efficiency of 6.3%. The accompanying linear growth rate is 50 dB/m. When the system operates in the superradiant mode (in the absence of the magnetron driver) excitation of multiple waveguide modes is observed. A three‐dimensional simulation code that has been developed to investigate the self‐consistent interaction of the ...

Numerical study of relativistic magnetrons

The relativistic multiresonator magnetron is analyzed in full cylindrical geometry, using the two‐dimensional particle‐in‐cell simulation code magic. Detailed comparisons are made between the simulation results and the classic experiments by Palevsky and Bekefi for the relativistic A6 magnetron configuration [Phys. Fluids 22, 986 (1979)]. The computer simulations show the same dependence of microwave power on axial magnetic field as measured in the experiment. It is found that the electron flow in the preoscillation regime differs substantially from ideal Brillouin flow, and that the nonlinear regime is characterized by large‐amplitude spoke formation. By analyzing a class of relativistic magnetrons for a wide range of operating voltage and axial magnetic field, it is found that the optimal microwave power exhibits an approximately cubic dependence on the applied diode voltage. Moreover, the simulations indicate that relativistic magnetrons with small aspect ratio oscillate predominantly in the π mode, wh...

Computer simulation of relativistic multiresonator cylindrical magnetrons

The relativistic multiresonator magnetron is analyzed in cylindrical geometry, using the two‐dimensional particle‐in‐cell simulation code magic. Detailed comparisons are made between the simulation results and the experiments by A. Palevsky and G. Bekefi [Phys. Fluids 22, 986 (1979)] using the A6 magnetron configuration. Within a constant scale factor, the computer simulations show a similar dependence of microwave power on magnetic field, with dominant excitations in the π and 2π modes. In the preoscillation regime, the electron flow in the simulations differs substantially from the ideal Brillouin flow model. In the nonlinear regime, the saturation is dominated by the formation of spokes.

Linear and nonlinear theory of cyclotron autoresonance masers with multiple waveguide modes

The interaction of multiple waveguide modes with a relativistic electron beam in an overmoded, single‐frequency, cyclotron autoresonance maser amplifier is analyzed using a nonlinear self‐consistent model and kinetic theory. It is shown analytically, and confirmed by simulation, that all of the coupled waveguide modes grow at the spatial growth rate of the dominant unstable mode, but suffer different launching losses which depend upon detuning. The phases of coupled modes are locked in the exponential gain regime, and remain approximately locked for some finite interaction length beyond saturation. The saturated power in each mode is found to be insensitive to the input modal radio‐frequency (rf) power distribution, but sensitive to detuning. Simulations indicate that the saturated fractional rf power in a given mode reaches a maximum at its resonant magnetic field, and then decreases rapidly off resonance. Good agreement is found between the simulations and the kinetic theory in the linear regime.

Confinement criterion for a highly bunched beam

The nonrelativistic motion is analyzed for a highly bunched beam propagating through a perfectly conducting cylindrical pipe confined radially by a constant magnetic field parallel to the conductor axis. In the present analysis, the beam is treated as either a thin rod distribution representing a continuous (unbunched) beam or periodic collinear point charges representing a highly bunched beam. Use is made of a Green’s function to compute the electrostatic force on the beam due to the induced surface charge in the conductor wall. By analyzing the Hamiltonian dynamics, a criterion is derived for the confinement of unbunched and bunched beams. It is shown that for the confinement of beams with the same charge per unit length, the maximum value of the effective self-field parameter is 2ωp2/ωc2≅2a/L for a highly bunched beam with a≪L. This value is significantly lower than the Brillouin density limit for an unbunched beam 2ωp2/ωc2=1. Here, a is the radius of the conducting cylinder, and L is the periodic spac...

Chaotic particle motion and halo formation induced by charge nonuniformities in an intense ion beam propagating through a periodic quadrupole focusing field

A test‐particle model is used to investigate the charged‐particle dynamics in an intense matched ion beam with nonuniform density profile propagating through an alternating‐gradient quadrupole focusing field in the space‐charge‐dominated regime. It is shown that self‐field nonlinearities due to the transverse nonuniformity in the beam density profile not only can result in chaotic ion motion but also can cause halo formation by the ejection of particles from the beam core. The structure of the particle phase space is studied. It is shown that the process of halo formation can occur on a fast time scale—on the order of a few lattice periods. The halo size is found to be determined by a Kolmogorov–Arnold–Moser (KAM) surface on a time scale much shorter than that of Arnold diffusion.