Harmonic mode competition in a terahertz gyrotron backward-wave oscillator Phys. Plasmas 19, 103103 (2012) Effect of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency Phys. Plasmas 19, 092105 (2012) Low-order-mode harmonic multiplying gyrotron traveling-wave amplifier in W band Phys. Plasmas 19, 093103 (2012) The Ka-band 10-kW continuous wave gyrotron with wide-band fast frequency sweep Rev. Sci. Instrum. 83, 074706 (2012) Selective mode suppression in a W-band second harmonic coaxial-waveguide gyrotron backward-wave oscillator

The thermal and field emission of electrons from protrusions on metal surfaces is a possible limiting factor on the performance and operation of high-gradient room temperature accelerator structures. We present here the results of extensive numerical simulations of electrical and thermal behavior of protrusions. We unify the thermal and field emission in the same numerical framework, describe bounds for the emission current and geometric enhancement, then we calculate the Nottingham and Joule heating terms and solve the heat equation to characterize the thermal evolution of emitters under RF electric field. Our findings suggest that, heating is entirely due to the Nottingham effect, that thermal runaway scenarios are not likely, and that high RF frequency causes smaller swings in temperature and cooler tips. We build a phenomenological model to account for the effect of space charge and show that space charge eliminates the possibility of tip melting, although near melting temperatures reached.

Chemically induced rearrangements of amphifunctional molecules have been demonstrated using strong nonuniform electric fields (10 8-10 10 V/m) induced in the vicinity of nanoscale asperities. Electrostatic interactions utilizing these rearrangements of alkylthiolates assembled on Au(111) result in the nanopatterning of raised nanostructure (1.5-9 nm high, 15-100 nm wide) arrays on a second time scale by manipulating an atomic force microscope (AFM) tip above the monolayer. It is suspected that, as a result of the oxidative cleavage initiated by a weak bias of the tip, the S end of the alkylthiolate chain carrying a sulfenium cation is attracted to the (lifting) tip, forming bi-and higher-layer structures in the vicinity of the tip apex. Stabilization of the multiplelayered structures is accomplished via mutual attraction and entanglement of hydrocarbon chains. The rearrangements suggest a novel and general approach for nanoscale architecture in self-assembled systems. Water condensation is shown to have a major influence on electric charge transport and nanostructure formation in polymer-, and semiconductor-thin-film surfaces in the proximity of a biased Atomic Force Microscope (AFM) tip. The water forms a meniscus bridge between the AFM tip and the surface to form a three-component system comprised of the AFM tip, water meniscus, and the surface. The associated electric field in the meniscus is spatially non-uniform and has a magnitude of the order of 10 8-10 10

High-gradient accelerating structures should operate reliably for a long time. Therefore studies of various processes which may lead to disruption of such an operation are so important. In the present paper, the dissipation of rf electromagnetic energy in metallic microparticles is analyzed accounting for the temperature dependence of the skin depth. Such particles may appear in structures, for example, due to mechanical fracture of irises in strong rf electric fields. It is shown that such microparticles with dimensions on the order of the skin depth, being immersed in the region of strong rf magnetic field, can absorb enough energy in long-pulse operation to be melted. Then, the melted clumps can impinge on the surface of a structure and create nonuniformities leading to field enhancement and corresponding emission of dark current. Results are given for several geometries and materials of microparticles.

Nanoscale surface modification is reported for styrene butadiene rubber using an electrically biased conducting atomic force microscope tip. Under appropriate bias conditions, the local electric field magnitude is of the order of 10 8 -10 9 V m −1 , which is sufficiently large to initiate cross-linking in the rubber. Peaklike surface features, surrounded by a circular trough and a raised ring, are created by careful and controlled retraction of the biased tip. The features' aspect ratios can be controlled by modifying the tip retraction protocol, tip geometry, and bias voltage. Typical feature dimensions reported here vary from approximately ͑0.5-10͒-nm high and up to several hundreds of nanometer in diameter. Although the temperature of the rubber is above the glass transition and the rubber is in a viscous state, the features are stable over a period of several days once created-which is believed to be due to cross-linking of the rubber during feature formation. Modeling of the electric field distribution in the vicinity of the tip is presented, which strongly supports the assertion that the resulting nonuniform electric field induces nanostructure formation and initiates cross-linking. A mechanism is proposed whereby source material is redistributed in the proximity of the tip/surface region to form the observed features.