Mode-selective amplification in a waveguide free electron laser with a two-sectioned wiggler

One important issue in waveguide free electron lasers (FELs) involves an interaction of the electron beam with one waveguide mode at two different resonant frequencies. Since the low-frequency mode often has a higher pain, the usually preferred high-frequency mode is suppressed as a result of mode competition. In this paper, possible control of this mode competition is considered using a nonstandard wiggler magnet consisting of two cascaded wiggler sections with different periods and field strengths. It is demonstrated that with an appropriate differentiation between the two wiggler sections the high-frequency mode may be amplified preferentially. This mode-selective amplification may be used to suppress the low-frequency mode. A small signal gain formulation is developed for a waveguide FEL with such a two-sectioned wiggler arrangement and numerical examples are used to demonstrate its applicability to mode control in waveguide FELs. Effects of wiggler field errors and electron energy spread are also considered. It is shown that the requirement for wiggler field errors and electron energy spread in the two-sectioned wiggler arrangement is similar to that in the usual straight wiggler configuratio

Gas Plasma Pre-Treatment Increases Antibiotic Sensitivity and Persister Eradication in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of serious nosocomial infections, and recurrent MRSA infections primarily result from the survival of persister cells after antibiotic treatment. Gas plasma, a novel source of ROS (reactive oxygen species) and RNS (reactive nitrogen species) generation, not only inactivates pathogenic microbes but also restore the sensitivity of MRSA to antibiotics. This study further found that sublethal treatment of MRSA with both plasma and plasma-activated saline increased the antibiotic sensitivity and promoted the eradication of persister cells by tetracycline, gentamycin, clindamycin, chloramphenicol, ciprofloxacin, rifampicin, and vancomycin. The short-lived ROS and RNS generated by plasma played a primary role in the process and induced the increase of many species of ROS and RNS in MRSA cells. Thus, our data indicated that the plasma treatment could promote the effects of many different classes of antibiotics and act as an antibiotic sensitizer for the treatment of antibiotic-resistant bacteria involved in infectious diseases

Mode transition in radio-frequency atmospheric argon discharges with and without dielectric barriers

In this letter, basic characteristics of glow modes and their mode transition are studied for radio-frequency rf atmospheric argon discharges with bare and dielectrically insulated electrodes. Through input power control, large-volume rf atmospheric argon discharges with bare electrodes are achieved in the mode via an abrupt transition from a constricted mode, whereas dielectrically insulated electrodes result in large argon discharges in both the and modes with gradual mode transition. Current dependence of the 750 nm line intensity and of the gas temperature are shown to capture clearly the signature of mode transition

Mode characteristics of radio-frequency atmospheric glow discharges

Building on recent experimental and numerical evidence of different glow modes in atmospheric plasmas, this paper reports a systematic study of these modes in radio-frequency (RF) glow discharges in atmospheric helium. Using a one-dimensional (1-D) hybrid computer model, we present detailed characterization of three glow modes, namely the α mode, the α - γ transitional mode, and the γ-mode in a 13.56-MHz atmospheric glow discharge over a wide range of root mean square (RMS) current density from 5 m A / cm2 to 110 m A / cm2. Our focus is on sheath dynamics through spatial and temporal profiles of charged densities, electric field, electron mean energy, sheath thickness, and sheath voltage, and when appropriate our results are compared against experimental data of atmospheric glow discharges and that of glow discharges at reduced gas pressure below 1 torr. Fundamental characteristics of the three glow modes are shown to be distinctively different, and these can be used as a hitherto unavailable route to tailor the operation of radio-frequency atmospheric glow discharges to their intended applications

Prospects for treating foods with cold atmospheric gas plasmas

In this review the potential applications of cold atmospheric gas plasmas are presented with particular reference to the problems of contamination of foods by biological agents. In addition to the accidental contamination of food, the very real threat arising from the deliberate contamination of the human food chain is also considered. The evidence that has been gained for the efficacy of cold plasmas in inactivating a wide range of biological agents is briefly surveyed. This is followed by an examination of previous work in which various types of foodstuffs have been successfully treated using cold gas plasmas. The need to demonstrate that the quality attributes of treated foods is not adversely affected is stressed. Finally, the role which gas plasmas may have in decontaminating food processing equipment is considered

Impact of surface discharge plasmas on performance of a metallized film capacitor

Surface breakdown discharges are one probable failure mechanism of metallized polymeric film capacitors used in power systems, traction drives, and other technological applications. To assess whether surface breakdown discharges may undergo considerable elongation on the electrode surface to affect significantly capacitor performance, an equivalent electric circuit model is developed for metallized polymer film capacitors under the thermal equilibrium condition. With the aid of a surface field gradient mechanism, propagation of surface plasmas is studied and the necessary condition for their possible elongation is obtained. Numerical examples of a metallized film capacitor are used to demonstrate that surface breakdown plasmas and their elongation are unlikely to affect capacitor performance in a significant fashion. Then the generic problem of plasma propagation is restudied under thermally nonequilibrium conditions. Based on a heat conduction formulation in the one-dimensional limit, a temperature gradient mechanism is proposed to explain the possible elongation of breakdown plasmas on an electrode surface. Numerical examples are again used to deduce that thermally nonequilibrium surface plasmas are unlikely to evolve into catastrophic flashover arcs to fail film capacitors

Nonthermal atmospheric plasmas sustained without dielectric barrier in the kilohertz range

We report observation of nonthermal atmospheric discharges produced between two bare metallic electrodes over a wide frequency range from 20 to 260 kHz, in which generation of stable atmospheric glow discharges has so far necessitated dielectric barrier to be added to at least one electrode. Measured current and voltage characteristics suggest a distinctively different plasma-sustaining mechanism from that of atmospheric dielectric-barrier discharges. This is confirmed by hydrodynamic simulation

Special Issue on Plenary and Invited Papers From ICOPS 2012

The 39th International Conference on Plasma Science (ICOPS) was held in Edinburgh, Scotland in July 2012. This was the third time that the conference was organized outside North America, the previous ones being in Karlsruhe (Germany) in 2008 and Jeju (S. Korea) in 2003. The technical programme combined seven technical areas of plasma science and technology covering a wide range of topics. The conference featured a wide range of advances in innovative plasma and beam science and applications, and served as a venue for an international community to meet and discuss their ideas and research results. More than 800 abstracts were received in 35 different topical areas, with more than half the papers originating outside the United State. The conference was attended by over 600 delegates and enjoyed the participation of over 200 registered students

Nonlinear amplification in a second-harmonic waveguide free-electron laser

This paper describes the results of numerical simulation of second-harmonic waveguide free-electron lasers (FELs) from the small-signal regime to the large-signal regime. Aimed at reducing the size and hence the cost of compact waveguide FELs operated from the microwave to the far infrared, these unconventional waveguide FELs can substantially decrease the minimum electron energy required for strong FEL radiation at a given frequency while increasing the small-signal gain. This contribution focuses on their saturation behaviors, taking into consideration variation in wiggler field and electron-energy spread. Depending on the roundtrip power loss within the FEL cavity and the initial electron-energy spread, the computed relationship between interaction gain and in-cavity power can be used to maximize the output power at a given electron current. Furthermore, it is found that gain degradation due to electron-energy spread remains relatively unchanged regardless of radiation power and wiggler field