Nano-Architecture of nitrogen-doped graphene films synthesized from a solid CN source

New synthesis routes to tailor graphene properties by controlling the concentration and chemical configuration of dopants show great promise. Herein we report the direct reproducible synthesis of 2-3% nitrogen-doped ‘few-layer’ graphene from a solid state nitrogen carbide a-C:N source synthesized by femtosecond pulsed laser ablation. Analytical investigations, including synchrotron facilities, made it possible to identify the configuration and chemistry of the nitrogen-doped graphene films. Auger mapping successfully quantified the 2D distribution of the number of graphene layers over the surface, and hence offers a new original way to probe the architecture of graphene sheets. The films mainly consist in a Bernal ABA stacking three-layer architecture, with a layer number distribution ranging from 2 to 6. Nitrogen doping affects the charge carrier distribution but has no significant effects on the number of lattice defects or disorders, compared to undoped graphene synthetized in similar conditions. Pyridinic, quaternary and pyrrolic nitrogen are the dominant chemical configurations, pyridinic N being preponderant at the scale of the film architecture. This work opens highly promising perspectives for the development of self-organized nitrogen-doped graphene materials, as synthetized from solid carbon nitride, with various functionalities, and for the characterization of 2D materials using a significant new methodology

Practical implementation of a dead zone inverse on a hydraulic wrist

ABSTRACT This paper presents practical aspects of implementing a dead zone inverse on a hydraulic wrist. A dead zone occurs over a range of small input values for which a system does not respond. It was desirable to use the most straightforward method available to achieve improved system performance while requiring the least amount of modification to the controller. Thus a fixed parameter dead zone inverse (DZI) was added to an existing proportionalintegral (PI) controller. First, the parameters of the dead zone were characterized from open loop testing. These parameters are the break points, or input values between which the system does not respond at all, and the slope of the system's response just outside the break points. The DZI augments the PI signal input to the plant, effectively adding or subtracting a constant equal to the size of the dead zone break points and scaling the input by its slope. Simulations predicted perfect system tracking, but implementation on the hardware revealed several practical issues. First, the dead zone slope parameters vary throughout the robot's workspace. Overestimation can lead to non-ideal system performance, but the more extreme problem is underestimation, which effectively increases control loop gain and can lead to system instability. However, performance is not affected significantly unless these parameters are off by an order of magnitude. Overall the system performance is relatively robust to modeling errors in the slope parameters. The second issue is that noise can be magnified by the dead zone inverse and cause chattering. This problem was very noticeable in the wrist when the estimated dead zone break points were used in the DZI. This problem can be eliminated by reducing the dead zone break points or reintroducing a small artificial dead zone back into the control loop to envelope the expected noise level. The requirements for successful implementation of the DZI were found to be a basic characterization of the dead zone and an understanding of practical system issues that can be accentuated by its use. The effectiveness of the technique was tested through simulations and experiments on the wrist

Synchronously injected amplifiers, a novel approach to high-average-power FEL

Two new FEL ideas based on synchronously injected amplifiers are described. Both of these rely on the synchronous injection of the optical signal into a high-gain, high-efficiency tapered wiggler. The first concept, called Regenerative Amplifier FEL (RAFEL), uses an optical feedback loop to provide a coherent signal at the wiggler entrance so that the optical power can reach saturation rapidly. The second idea requires the use of a uniform wiggler in the feedback loop to generate light that can be synchronously injected back into the first wiggler. The compact Advanced FEL is being modified to implement the RAFEL concept. We describe future operation of the Advanced FEL at high average current and discuss the possibility of generating 1 kW average power

A high-power compact regenerative amplifier FEL

The Regenerative Amplifier FEL (RAFEL) is a new FEL approach aimed at achieving the highest optical power from a compact rf-linac FEL. The key idea is to feed back a small fraction (< 10%) of the optical power into a high-gain ({approximately}10{sup 5} in single pass) wiggler to enable the FEL to reach saturation in a few passes. This paper summarizes the design of a high-power compact regenerative amplifier FEL and describes the first experimental demonstration of the RAFEL concept

Characterization of a ramped gradient DTL: Experiment and theory

An experimental demonstration confirming the beam-dynamics of a Ramped-Gradient Drift-Tube Linac (RGDTL) was performed at Los Alamos National Laboratory. The RGDTL was designed to optimize on the requirements of maximum beam acceleration, minimum longitudinal and transverse emittance growth, and acceptable wall power loss. At low beam energies, transverse magnet focusing is weak and the rf defocusing must be minimized. As the beam energy increases, stronger rf defocusing can be tolerated and the rf electric field gradient can increase. A detailed comparison of theory and experiment was carried out. Beam longitudinal centroids (output energy and phase), transverse and longitudinal emittances were measured as a function of RGDTL rf field amplitude and phase. The longitudinal centroids were also studied as functions of input beam current, energy, and degree of bunching. Comparison between experimental data and theory was found to be in good agreement. 12 refs., 1 fig