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A concept of achieving trifocal terahertz (THz) radiation is discussed based on the application of a periodic electric field in addition to two dark hollow laser beams impinging into collisional plasma. Nonlinear plasma medium is engineered for altering the dynamics of electrons under such a periodic electric field and obtaining an additional transverse component of nonlinear current. This additional component modifies the optics associated with the emitted radiation, leading to trifocal THz radiation. The parameters associated with the doughnut intensity of dark hollow laser beams, such as beamwidth and beam order, control the position and peak of the emitted THz field. Interestingly, the efficiency of the emitted THz field can also be tuned by appropriate combinations of the laser beam orders. [ABSTRACT FROM AUTHOR]

In order to reconstruct the plasma boundary directly and independently, a new method based on visible imaging diagnostics is developed for experimental advanced superconducting tokamak (EAST); its effectiveness has been verified by conducting offline experiments based on EAST’s historical data. The offline boundary reconstruction results, including X-points, are compared to offline equilibrium fitting code (offline EFIT), which is a commonly used reconstruction method. With an average error of 1.5 cm, the total processing time for one frame is less than 2 ms by current software and hardware. When the camera sensor is not saturated, the algorithm is robust for different image intensities of the plasma discharge. The results of the boundary reconstruction by visible imaging diagnostics also show potential for real-time plasma control in a tokamak. [ABSTRACT FROM AUTHOR]

High-resolution optical emission spectroscopy and automated Langmuir probe are applied to examine the production of active species in the pure nitrogen plasma excited by single (40.68 MHz) and dual high-frequency (HF) radio frequency (RF) power sources (40.68/2.1 MHz). The emission intensities of the spectral bands correspond to the (0, 2) transition of the second positive system (SPS) of N2 ( \(\lambda =380.50\) nm) and ( \(0, 0\) ) transition of the first negative systems (FNSs) of N2+ ( \( \lambda =391.40\) and 427.81 nm) have been measured and compared with plasma parameters using the Langmuir probe to investigate the dependence of their radiative states on operating conditions for both single and dual HF RF capacitively coupled plasma (CCP) discharge. It was found that the high power of the low-frequency (LF) source and the gas pressures are the most effective parameters in dual RF CCP discharge system. Thus, both SPS intensity decreases and FNS intensity increases with the increasing LF power at constant pressure. Furthermore, the emission intensity of the first negative band heads increases at least four times in dual RF mode in comparing with the single RF CCP discharge. In contrast, the intensity of the second band heads decreases dramatically to its half values. Likewise, it was found that the transition pressures for the electron heating mode (with increasing the gas pressure) and the transition from \(\alpha \) to \(\gamma \) mode (with increasing the RF power) is close to 0.3 torr in single RF mode, but these transitions are about 0.5 torr for dual RF CCP mode. [ABSTRACT FROM PUBLISHER]

The W -for tungsten- environment in steady-state tokamak (WEST) project consists in transforming the French tokamak Tore Supra into an X-point divertor one and aims at testing the ITER technology of actively cooled high heat flux tungsten components during ITER-relevant long pulses. In addition to the integration in the vacuum vessel of a lower divertor and an upper one, this metamorphosis has a strong impact on diagnostics initially present in Tore Supra. Indeed, the introduction of a divertor in the upper part of the machine leads to block the lines of sight (LOS) of diagnostics previously installed in the upper ports. Therefore, new diagnostics playing a paramount role for the safe tokamak operation need to be implemented, such as an infrared (IR) thermography diagnostic for the protection of plasma-facing components (PFCs) and heating antennas, and a visible spectroscopy system for the protection of tungsten PFCs. Besides, several existing diagnostics need to be modified to adapt their LOS to the new divertor targets and plasma shape. This paper presents the main issues linked to the integration of diagnostics deriving from the WEST project. First, the changes involved by the upgrade on the set of diagnostics currently implemented in Tore Supra are described. Then, a particular focus is put on the integration of the two new optical systems, namely the IR system and the visible spectroscopy one. These two systems being located within the vacuum vessel, a special care must be paid to their design, to allow them to withstand their harsh environmental conditions. Other new or updated fundamental diagnostics are also briefly presented. Finally, the resulting new implementation of diagnostics envisaged to reach the scientific and technological goals of the WEST project is described. [ABSTRACT FROM PUBLISHER]

In quasi-optical transmission systems, it is usually required to transform a certain Gaussian beam into another one characterized by a different set of parameters. Mirrors with quadratic surface contour functions, such as paraboloidal, ellipsoidal, and toroidal mirrors, are widely used for such a transformation. However, quadratic surface contours do not provide sufficient conversion efficiencies needed for the transformation of off-axis Gaussian beams into desired Gaussian millimeter-wave field distributions. Therefore, it is necessary to develop a new method for the transformation of off-axis Gaussian beams. This paper presents such a method for the synthesis of beam-shaping mirrors for off-axis Gaussian beam transformation. Based on this new method, a matching optics unit consisting of two mirrors has been designed to transform a Gaussian-mode-like field distribution into a Gaussian beam with desired parameters. [ABSTRACT FROM PUBLISHER]

A circular waveguide loaded with periodic dielectrics is considered. Two theoretical methodologies have been developed to investigate the wave dispersion characteristics for such structure. The feasibility and accuracy of the theoretical approaches have been verified by high frequency structural simulator (HFSS) simulations. Using the theoretical approaches, effects of the material properties and structure dimensions on the dispersion characteristics, power distribution, coupling impedance, and wave attenuation have been analyzed, and results obtained for different structures have been compared. Examples of W-band structures with periodic dielectric loading that can be used in various microwave devices have been given. The presented methodologies and results can be used for design optimization of microwave devices with dielectric loading. [ABSTRACT FROM PUBLISHER]

The ion sources play an important role in the framework of the nuclear fusion, as a fundamental part of the neutral beam injection systems needed for the auxiliary heating of the plasma magnetically confined in fusion devices. In the case of large experiments, such as ITER and beyond, the use of negative ions was found necessary to increase the neutralization efficiency of high-energy beams, calling for an additional effort in the research on the physical mechanisms regulating their formation and extraction as a beam from the source. This paper summarizes the analyses investigating the physical phenomena occurring in the source and their effect on the optics of the extracted beam. The results, carried out by means of advanced numerical tools, are particularly useful to yield a better evaluation of the real performances of the system, and particularly of the accelerator, whose modeling usually neglect some characteristics of the source. These analyses were performed for the specific case of the SPIDER experiment (Source for Production of Ion of Deuterium Extracted from an RF plasma), representing the major test bed for negative ion source worldwide. The choice of a specific device for the source modeling was found useful to support its operational phase and to help the experimental data interpretation and the identification of dangerous operating conditions. Nonetheless, the validity of the results here presented still holds in more general cases. [ABSTRACT FROM PUBLISHER]

The ion sources play an important role in the framework of the nuclear fusion, as a fundamental part of the neutral beam injection systems needed for the auxiliary heating of the plasma magnetically confined in fusion devices. In the case of large experiments, such as ITER and beyond, the use of negative ions was found necessary to increase the neutralization efficiency of high-energy beams, calling for an additional effort in the research on the physical mechanisms regulating their formation and extraction as a beam from the source. This paper summarizes the analyses investigating the physical phenomena occurring in the source and their effect on the optics of the extracted beam. The results, carried out by means of advanced numerical tools, are particularly useful to yield a better evaluation of the real performances of the system, and particularly of the accelerator, whose modeling usually neglect some characteristics of the source. These analyses were performed for the specific case of the SPIDER experiment (Source for Production of Ion of Deuterium Extracted from an RF plasma), representing the major test bed for negative ion source worldwide. The choice of a specific device for the source modeling was found useful to support its operational phase and to help the experimental data interpretation and the identification of dangerous operating conditions. Nonetheless, the validity of the results here presented still holds in more general cases. [ABSTRACT FROM PUBLISHER]

Tests of up to three simultaneously firing 40-cm NASA's Evolutionary Xenon Thrusters took place at NASA Glenn Research Center. In addition to numerous engineering tests, the experiment was instrumented to measure plasma properties in the interacting plumes, charge exchange return current to the active and dormant thrusters, and current and plasma properties at various locations. This paper describes the use of Nascap-2k to calculate plume potentials and charge exchange ion return currents to active and dormant ion optics and to diagnostic probes. Calculations were performed varying the numbers of active thrusters, thruster power levels, and gimbaling. The results were extrapolated to space conditions. Despite several simplifying assumptions, we obtained good agreement with experimental results. No major problems associated with multiple thruster operation were found. [ABSTRACT FROM AUTHOR]

The linear and nonlinear theory of a plasma-filled relativistic Cerenkov generator with coaxial slow-wave structure (SWS) is developed. Linear analysis includes the space-charge limiting current, dispersion relations, dispersion curves, and coupling impedance. In nonlinear theory, the electron beam space-charge effect, nonsynchronous interaction, and complex reflection coefficients of the electromagnetic wave at the boundaries of the SWS are taken into account. With the assumption of quasi steady state, the self-consistent equations are numerically calculated using the fourth-order Runga-Kutta method. The effects of the relative phase between inner and outer corrugations, phase and amplitude of the complex reflection coefficient at the boundaries of the SWS, and plasma density on device efficiency are discussed. Efficiencies of up to 37% without plasma and about 59% with plasma density of np = 3-7 × 1011 cm-3 are obtained. [ABSTRACT FROM AUTHOR]

Periodic structures in plasmas form unique dispersions of electromagnetic waves, which are examined in numerical calculation. Above electron plasma frequency, where waves equivalently propagate in dielectric media, plasma arrays with 2-D periodicity produce frequency regions of forbidden propagation, like band gaps in a photonic crystal. Below electron plasma frequency, where waves are usually forbidden to propagate due to cutoff phenomenon, bulk plasmas with periodic holes can become wave-propagating media in which localized surface modes play an important role. Such features give rise to concepts of dynamic wave controllers that change their inner electron density in time. [ABSTRACT FROM AUTHOR]

This paper presents the investigation of a mirror system of a quasi-optical (QO) mode converter for a high-power coaxial-cavity gyrotron. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical one. Based on the Katsenelenbaum-Semenov Algorithm (KSA), the second and the third mirrors are iteratively optimized as adapted phase-correcting mirrors to transform the outgoing wave beam into a fundamental Gaussian structure. The investigation shows that the focal length of the quasi-elliptical mirror has a great influence on the optimized conversion efficiency, and, hence, it should be chosen to match the asymptotic beam growth (ABG) angle well in order to obtain high conversion efficiency. The design of a mirror system has been performed for a 2 MW, continuous wave (CW), 170 GHz, and TE34,19-mode coaxial-cavity gyrotron, which is under development at Forschungszentrum Karlsruhe, Germany. Taking into account the size of the mirrors and the conversion efficiency, a mirror system has been designed with a conversion efficiency of 98.3%. [ABSTRACT FROM AUTHOR]

Magnesium-oxide (MgOx) films have been prepared by a cathodic-vacuum-arc ion deposition system operated in an intermediate frequency pulse mode of substrate bias voltage at a mixed atmosphere of O2 and Ar. The O2 flow was adjusted in a range of 180-240 sccm at a fixed O2/Ar flow rate of 20. The structure, composition, morphology, and optical properties of the samples were analyzed by X-ray diffraction, Rutherford backscattering technique, atomic force microscopy, and UV-visible absorption spectra, respectively. Results show that the crystal orientation and grain size of the samples strongly depends on O2 flow rate. Growth of Magnesium-oxide films prefers MgO (200) and MgO (220) orientation at a smaller and larger O2 flow rate of 180 and 220 sccm, respectively. Transparent MgOx films used as the protective layer for alternating-current plasma display panel are obtained at optimal deposition conditions. [ABSTRACT FROM AUTHOR]

Phase contrast imaging (PCI) is an internal reference beam interferometry technique which provides a direct image of line integrated plasma density fluctuations. The method has been used with great success to measure waves and turbulence in magnetically confined high temperature plasmas. The principle of PCI was developed in optics in the 1930s by the Dutch physicist Zernike, leading to the development of phase-contrast microscopy [1], [17]. The technique allows one to detect the variation of the index of refraction of a dielectric medium (such as a plasma) due to the presence of waves or turbulent fluctuations. The image produced by the introduction of a phase plate in the beam path, and subsequently imaging the expanded laser beam onto a detector array can be used to calculate wavelengths and correlation lengths of fluctuations in high temperature plasmas. In this paper, the principle of PCI is summarized and examples of measurements from the DIII-D and Alcator C-Mod tokamak plasmas are given. [ABSTRACT FROM AUTHOR]

A common approach to modeling kinetic problems in plasma physics is to represent the plasma as a set of Lagrangian macro-particles which interact through long-range forces. In the well-known particle-in-cell (PIC) method, the particle charges are interpolated to a mesh and the fields are obtained using a fast Poisson solver. The advantage of this approach is that the electrostatic forces can be evaluated in time O(N log N), where N is the number of macro-particles, but the scheme has difficulty resolving steep gradients and handling nonconforming domains unless a sufficiently fine mesh is used. The current work describes a grid-free alternative, the boundary integral/treecode (BIT) method. Using Green's theorem, we express the solution to Poisson's equation as the sum of a volume integral and a boundary integral which are computed using particle discretizations. The treecode replaces particle-particle interactions by particle-cluster interactions which are evaluated by Taylor expansions. In addition, the Green's function is regularized and adaptive particle insertion is implemented to maintain resolution. Like PIC, the operation count is O(N log N), but BIT avoids using a regular grid, so it can potentially resolve steep gradients and handle complex domains more efficiently. We applied BIT to several bounded plasma problems including a one-dimensional (1-D) sheath in direct current (dc) discharges, 1-D virtual cathode, cold two-stream instability, two-dimensional (2-D) planar and cylindrical ion optics, and particle dynamics in a Penning-Malmberg trap. Some comparisons of BIT and PIC were performed. These results and ongoing work will be reviewed. [ABSTRACT FROM AUTHOR]

A new kind of coaxial vircator with a three-mirror quasi-optical resonate cavity is proposed and studied in this paper. Theoretical study and particle-in-cell (PIC) simulation are both included. Through two-and-a-half-dimensional PIC code MAGIC, using an input voltage pulse with peak value of 600 kV the average voltage obtained on the diode is 460 kV and the diode current is 50 kA, the instantaneous output power with peak value over 4 GW and the average power 2 GW are obtained with frequency 7.0 GHz. The beam-wave conversion efficiency is reaching 8.7%. [ABSTRACT FROM AUTHOR]

Transverse gaps are introduced in thin-wire conductors to extend the region of phase variations of currents that are induced on them by an incident electromagnetic wave. Periodically placed gaps lead to qualitative changes in the properties of reradiated waves, such as the appearance of resonances corresponding to total reflection and total transmission of the incident wave. It is shown that total reflection occurs in a narrow band and depends very weakly on the incident angle of the illuminating radiation. This idea leads to novel filters and switches, whose operation is practically independent of the structure of the incident wave. Total resonant reflection can occur for conductors with high optical transparency, a property that is useful for various microwave diagnostic devices. Because of the increased radiation resistance of such structures, the resonant effects under consideration depend weakly on conductivity. For gratings made using these conductors with gaps stacked above a metal ground plane, it is possible to controllably collimate the rays. Expansion of the scattered fields in terms of Rayleigh's series, together with Poisson's summation formula, are used for numerical simulations of such structures. [ABSTRACT FROM AUTHOR]

A method of quickly and accurately getting the dispersion curves for both symmetric and asymmetric modes in arbitrary slow-wave structures is investigated analytically. A universal dispersion equation is derived by utilizing the field theory and expressing the slow-wave structure's profile in a finite Fourier series. In principle, this method can be applied to arbitrary axisymmetric profiles. As examples, numerical results for three typical slow-wave structures (namely, rippled-wall, disk-loaded, and modified disk-loaded structures) are presented. [ABSTRACT FROM AUTHOR]

We have performed two-dimensional, relativistic, electromagnetic, particle-in-cell simulations for an axially extrated vircator. With a copper foil anode, the output frequency of the vircator is found to be strongly dependent upon the variation of foil transparency with electron energy. Using an average transparency for all electron energies yields results that are markedly different from those obtained using the actual variation. However, the output power shows only mild sensitivity. Using the full energy-dependent form of the transparency, we get fairly good agreement with published experimental results. Increasing the energy resolution for the calculation of foil transparency increases the accuracy of calculation of the dominant frequency, although the frequency tends to saturate beyond a certain resolution. However, an increase in the resolution must necessarily be accompanied by an increase in the number of simulation particles, in order to limit statistical fluctuations to an acceptable level. A physical explanation has also been provided for these trends. [ABSTRACT FROM AUTHOR]

An all-metal disc-loaded cylindrical waveguide excited in TE modes was cold-analyzed in the fast-wave regime for dispersion characteristics, keeping in view of its potential application as an interaction structure for wide-band gyro-traveling-wave tubes (TWTs). The analysis was carried out considering the standing and propagating waves in the disc-occupied and disc-free regions, respectively, using three approaches which differ from one another with respect to how they process the boundary conditions at the interface between these two regions. One such approach is capable of including higher order harmonics in both the structure regions. An adequate number of harmonics with reference to the two regions was taken in the calculation to ensure the convergence of results. The results have been validated against available published results based on different other approaches. The passband and shape of the dispersion characteristics both depend on the disc-hole radius and periodicity, being more sensitive to the latter. The adjustment of disc parameters led to the widening of the straight-line portion of ω-β dispersion characteristics, for wide-band coalescence between the beam-mode and waveguide-mode dispersion characteristics of a gyro-TWT as required for wide-band device performance. [ABSTRACT FROM AUTHOR]

Experimental results on Z-pinch acceleration of massive cylindrical Copper liners for generation of pressure pulses in the range of 20-40 GPa for investigation of the graphite-to-diamond phase transition are reported. Pulse pressure was created when the liner struck a steel tube filled with graphite or graphite-containing composite, The crystal structure of the substance after pulse compression was cleans by chemical methods and investigated using electron microscopy analysis. As a result of the investigation, it was found that about 3% of the substance initially present in the sample was transformed to a diamond phase with an average crystallite size of 1-2 μm. [ABSTRACT FROM AUTHOR]

Presents a study which used electrical and optical diagnostics in surface flashover under vacuum. Classification of unipolar flashover on dielectric surfaces; Results and discussion; Conclusion.

Presents a study which performed optical diagnostics on helix flux compression generators. Limitations of the efficiency of magnetic flux compression generators; Results and discussion; Summary.

An optical tomographic diagnostic instrument has been fielded at the Dual-Axis Radiographic Hydrodynamic Test Facility at Los Alamos National Laboratory. Four optical lines of sight create projections of an image of an electron beam on a Cerenkov target, which are relayed via optical fiber to streak cameras. From these projections, a reconstruction algorithm creates time histories of the beam’s cross section. The instrument was fielded during and after facility commissioning, and tomographic reconstructions reported beam parameters. Results from reconstructions and analysis are noted. [ABSTRACT FROM PUBLISHER]

We present time-resolved laser-induced breakdown spectroscopy (LIBS) spectra of atomic (In I) and ionic (In II) indium lines recorded in time ranges from 500 ns to \(20~\mu \) s. Optical spectra have been acquired using a streak camera. Since it is assumed that the available supply of indium will slowly but inevitably diminish, the recycling of indium and finding the new natural sources are more and more important as the price of indium rises. The study of indium lines showed in this paper could be useful for making the LIBS detecting devices less expensive and more portable. [ABSTRACT FROM PUBLISHER]

Rotating gliding arc plasma reactor performs two operation modes according to supply power (dc and ac modes). According to the images of operating states and the emission spectrum of the two modes, the characteristics of dc mode and ac mode are well distinguished and investigated. The dc mode owns platform-type plasma region, high temperature, and energy consumption, suitable for fuel conversion field. The ac mode has a few advantages in gaseous organic pollutants treatment field, because of its relatively low gas atmosphere temperature, little by-products (NOx) production, and low energy consumption. [ABSTRACT FROM PUBLISHER]

Three-dimensional images of absolute radiance as a function of position and wavelength are obtained for shock-heated plasmas traveling in a shock tube at velocities near Mach 30. Vacuum optics coupled to a shock tube are used to image the shock onto four separate spectrometers, which, in turn, disperse the radiation in wavelength onto intensified CCD arrays covering selected spectral ranges from the vacuum ultraviolet (> 120\ \nm) through midwave infrared (<5000\ \nm). Quantitative radiometry performed in this fashion is used to benchmark radiative heating codes used in sizing spacecraft thermal protection systems. [ABSTRACT FROM AUTHOR]

This paper investigates the appearance of sprite streamer discharges imaged at different exposure times. Both observational and modeling results are presented to illustrate that the formation of luminous filamentary channels in sprites is caused by bright and fast-moving sprite streamer heads. [ABSTRACT FROM AUTHOR]

Arrays of ellipsoidal microcavities have been fabricated in glass by a micropowder blasting technique. Optical microscopy of the plasmas generated in arrays of these cavities provides evidence of coupling between each microplasma and its nearest neighbors. Scattering of incoherent atomic radiation from one microplasma into adjacent microcavities is clearly observed. After correcting for field-of-view distortion and saturation, the images also appear to reveal the perturbation of each microplasma by its neighbor in the form of an elongation of the emission distribution along the axis joining the two cavities. [ABSTRACT FROM AUTHOR]