Search

The world's newest quasi-axisymmetric stellarator CFQS is constructed as a joint project of the National Institute for Fusion Science, Japan and Southwest Jiaotong University, the People's Republic of China. The project is named “NSJP for CFQS”. The CFQS has a major radius of 1 m, toroidal periods of 2, and aspect ratio of 4. The toroidal magnetic field strength will be increased up to 1 T. The CFQS equilibrium is characterized by low-aspect ratio and quasi-axisymmetry, having both natures of two-dimensional tokamak and three-dimensional stellarator. The CFQS offers new opportunities and fields to increase understanding of toroidal fusion plasmas.

In this paper, a novel scheme of generating terahertz radiation is proposed, which considers a relativistic electron beam being modulated by its self-excited plasma wakefield in a plasma-filled slow-wave structure (SWS). Both the dispersion relation and the particle-in-cell simulation show that the self-modulation of the beam can act as a mode-selection method to excite the high-harmonics of the SWS and generate terahertz electromagnetic radiation. The present work gives a new thought for researching high-power and tunable terahertz sources.

The Chinese First Quasi-axisymmetric Stellarator (CFQS) is a new quasi-axisymmetric experimental device planned for construction at South West Jiaotong University (SWJTU), China. This is a joint project of the National Institute for Fusion Science (NIFS) and the SWJTU. The present paper discusses the equilibrium configuration of the CFQS with a major radius of 1.0m, a toroidal magnetic field strength of 1.0 T, and an aspect ratio of 4.0. As the CFQS is a quasi-axisymmetric stellarator, a tokamak-like bootstrap current is expected. The magnitude of the bootstrap current was estimated by BOOTSJ code. Next, the effects of the bootstrap current on the quasi-axisymmetric property and the neoclassical diffusion coefficient were estimated. The bootstrap current little affected the quasi-axisymmetric property, and a good neoclassical transport property was maintained.

Kasper et al. have found that solar-wind helium could be heated to be nearly 7 times hotter than hydrogen on average from the observation of the Wind spacecraft. The stochastic Fermi mechanism is employed to investigate this phenomenon via the ion-cyclotron resonant process (Kasper J. C. et al., Phys. Rev. Lett., 110 (2013) 091102). Due to strong ion cyclotron resonances caused by counterpropagating Alfven waves, the helium could be thermalized to be 7 times hotter than hydrogen. In this paper, a new aspect, the non-resonant interaction between thermal non-equilibrium particles and turbulent Alfven waves, is utilized to illustrate the above observation analytically and numerically. The result of our model is broadly consistent with the observational result. Additionally, this paper predicts that the various temperature anisotropies of ions may exist in the solar-wind core which different thermal non-equilibrium factors lead to. This work builds up a close relation among non-resonant heating of thermal non-equilibrium ions, differential flow, and temperature anisotropy.

An experiment on edge localized modes (ELMs) during the vertical swing is performed in HL-2A tokamak. ELM mitigation is observed when the plasma moves upward. The plasma column expands accompanied by a decrease in both lower triangularity and the absolute value of local magnetic shear at the outer mid-plane during the upward moving. The critical pressure gradient in pedestal is reduced for the mitigating case according to the linear simulation based on peeling-ballooning theory. The physics of the ELM crash process is also explored through nonlinear simulation. For the mitigating case, small current drive for low-n modes and a weak ion diamagnetic effect on high-n modes lead to a higher dominant mode and a wider growth rate spectrum, which result in weak turbulence transport and a small ELM size.

Shaping effects of the E-fishbone in tokamaks are investigated. Coordinates related to the Solov'ev configuration are used to calculate the precession frequency and kinetic contribution. It is shown that elongation does not change the precession frequency and the kinetic energy. Growth rates of the E-fishbone vary with elongation which essentially has destabilizing effects. For elongated tokamaks, triangularity has a stabilizing effect on the modes which play a compensative role. The results may apply to Sunist.

When an intense relativistic electron beam propagates through the plasma background, the electron beam can expel the plasma electrons and leave the plasma ions which can be seen as static. In this process, the ion channel can be formed and the electromagnetic radiation can be observed. This paper uses the method called particle-in-cell to study on the influence of different valence state under same particles on electromagnetic radiation in beam-plasma system.

The ideal ballooning mode model in the presence of the externally applied resonant magnetic perturbation (RMP), developed by Bird and Hegna [Nucl. Fusion 53, 013004 (2013)], is extended to include the non-ideal effects, i.e., the finite resistivity and diamagnetic drift effects. Using the eigenvalue equation with the non-ideal effects, a comprehensive analysis is carried out to elucidate the impact of the RMP on ballooning mode stability. Finite resistivity is shown to reduce the line bending stabilization in low to intermediate toroidal mode number (n), resulting in the more peaked growth rate spectrum with respect to n. It is shown that the combination of finite resistivity and the RMP-induced local shear modulation have a strong influence on line bending stabilization, leading to an interesting threshold behavior of the ballooning mode stability. This signifies the importance of the RMP to the line bending stabilization coupled with finite resistivity, as well as the local shear modulation.

The Chinese First Quasi-axisymmetric Stellarator (CFQS) is a joint project of international collaboration. It is designed and fabricated by Southwest Jiaotong University (SWJTU) in China and National Institute for Fusion Science (NIFS) in Japan. In this work, a plasma boundary and modular-coil system for the CFQS have been designed and optimized via scan of the magnetic configurations with various aspect ratios. The present target parameters of the CFQS are as follows: toroidal periodic number, aspect ratio, magnetic field strength and major radius are 2, 4.0, 1.0 T, and 1.0 m, respectively. The VMEC and NESCOIL codes are employed to obtain the optimum coil design via minimizing the normal component of the magnetic field on the target plasma boundary surface generated by the modular coils. In order to estimate the accuracy of the filament coils, the shape of the plasma boundary, rotational transform, depth of magnetic well and main Fourier components of magnetic field strength produced by the modular-coil system are compared with the target plasma boundary, rotational transform, depth of magnetic well and main Fourier components, correspondingly. This comparison between properties of the coil induced configuration and the target configuration shows a good agreement.

This paper will discuss the design and construction of BESIII, which is designed to study physics in the tau-charm energy region utilizing the new high luminosity BEPCII double ring e+e- collider. The expected performance will be given based on Monte Carlo simulations and results of cosmic ray and beam tests. In BESIII, tracking and momentum measurements for charged particles are made by a cylindrical multilayer drift chamber in a 1 T superconducting solenoid. Charged particles are identified with a time-of-flight system based on plastic scintillators in conjunction with dE/dx (energy loss per unit pathlength) measurements in the drift chamber. Energies of electromagnetic showers are measured by a CsI(Tl) crystal calorimeter located inside the solenoid magnet. Muons are identified by arrays of resistive plate chambers in the steel magnetic flux return. The level 1 trigger system, Data Acquisition system and the event filter system based on networked computers will also be described., 57 pages, 75 figures

The traditional cylinder resonant cavity in a gyrotron is replaced with a photonic-band-gap resonant cavity (PBGC) with a structure of triangular arrays of metal posts, and the cold character of the TE operating modes in the PBGC are analysed and calculated. Under the consideration of the cavity structure features, a research method for the mode theory on PBGC is provided, single-mode operation in the cavity is found, and the distribution characteristics of the azimuthally inhomogeneous electromagnetic modes are reported. The study provides a physical theory fundament which reveals possibilities for changing the essential operating status of a gyrotron.

This particle-in-cell simulation study finds that the system of a long relativistic electron beam passing through an overdense plasma can produce high harmonic terahertz electromagnetic radiation. In this process, the electron beam is self-modulated to create a periodic electromagnetic structure which will resonate with the plasma wakefield and excite the electromagnetic instability in the nonlinear stage of the self-modulation. The study also finds that when the radiation is achieved, the self-modulated electron beam will be destroyed due to the self-consistent interaction among the radiation, the electron beam, and the plasma. Meanwhile, the radiation will gradually attenuate, which also coincides with the physical explanation of the radiation.

The poloidal electric filed, which is the drive field of poloidal rotation, has been observed and increases obviously after the injection of electron cyclotron resonance wave in HL-2A experiment, and the amplitude of the poloidal electric field is in the order of 103 V/m. Through theoretical analysis using Stringer rotation model, the observed poloidal electric field is of the same order as the theoretical calculation value. In addition, the magnetic pump damping which would damp the poloidal rotation is calculated numerically and the calculation results show that the closer to the core plasmas, the stronger the magnetic pump damping will be. Meanwhile, according to the value of the calculated magnetic pump damping, the threshold of the poloidal electric field which could overcome magnetic pump damping and drive poloidal rotation in tokamak plasmas is given out. Finally, the poloidal rotation velocity over time at different minor radius is studied theoretically.

The plasma cladding waveguide, which is a cylindrical dielectric core surrounded by a plasma cladding, is developed, and the guided modes and their characteristics of this waveguide are displayed through the present detailed theoretical research. The conditions of the single mode existing in the plasma cladding waveguide have been given, and the defined forbidden gap of frequency is discussed. It is found that the usage characteristics of the plasma cladding waveguide vary strongly with plasma frequency, and changing the plasma parameters can control the propagation mode. This paper focuses on exhibiting the basic characteristics and the potential applications of this new type of waveguide.

A new plasma maser scheme, the ion-channel electron cyclotron maser (ICECM) is investigated, and the electromagnetic instability in the ICECM is studied. Using three-dimensional perturbation theory, the dispersion relation of the ICECM is derived in this paper. Analysis finds that three typical regions of electromagnetic instability exist in the ICECM after considering the influence of the plasma wave, and some potential physical mechanisms for the instability are identified. In the Compton regime, an approximate calculation example for the instability growth rate is given. As a high-power short-wavelength electromagnetic radiation source, some evident advantages of ICECM in comparison with other electron cyclotron masers are shown.

From a significant view, considering the thermal non-equilibrium factor, we investigate Kappa (κ) ion temperature anisotropies induced by a low-frequency parallel propagating Alfven wave by combining quasi-linear theory and test particle simulation. Analytic expressions for the ion temperature ratios and are derived for the solar wind, where and denote the perpendicular temperature of species i, parallel temperature of species i, and perpendicular temperature of species j, respectively. The results of our model are broadly consistent, compared to observations of solar-wind measurements. Solar wind helium that meets the condition for strong core heating is nearly seven times hotter than hydrogen, on average. Furthermore, we note that we are able to predict the temperature anisotropies of ions based on their thermal non-equilibrium factors.

The simulation study finds that the relativistic electron beam propagating through the plasma background can produce electromagnetic (EM) radiation. With the propagation of the electron beam, the oscillations of the beam electrons in transverse and longitudinal directions have been observed simultaneously, which provides the basis for the electromagnetic radiation. The simulation results clearly show that the electromagnetic radiation frequency can reach up to terahertz (THz) wave band which may result from the filter-like property of plasma background, and the electromagnetic radiation frequency closely depends on the plasma density. To understand the above simulation results physically, the dispersion relation of the beam-plasma system has been derived using the field-matching method, and the dispersion curves show that the slow wave modes can couple with the electron beam effectively in THz wave band, which is an important theoretical evidence of the EM radiation.

A new kind of open-resonator for a quasioptical gyrotron—an axisymmetrical quasioptical cavity of oblique roiation at arbitrary angle (AQCORAA)—is proposed. Using scalar diffraction theory and an approximate method, the field distributions in the AQCORAA are derived, and the influence of rotating angle on the field distributions is analysed. The stored field energy in the cavity is also obtained.

The instability caused by the interaction among the electron beam, plasma, and electromagnetic (EM) waves in a radially non-uniform ion channel is studied. The dispersion relation of the interaction is derived in this paper. The study shows that the EM instability can be induced nearby the center of the beam through the coupling between the fast plasma wave and forward EM wave. On the other hand, the beam mode instability is found in the region relatively far from the center, which means the beam mode can couple with the slow plasma wave so that the electron beam can provide energy to the plasma wave. The distributions of the instability with radius and plasma density are given, respectively, by the numerical simulation, and the result shows that the instability frequency is increased exponentially with the increase of the plasma density. When the plasma density reaches 1×1024m−3, the scale of the frequency can be 10 THz, which provides a theoretical basis for developing the new kind of high-power teraher...

Using kinetic coupling mode theory of three waves, the effect of longitudinal momentum spread in ion-ripple laser (IRL), which is an advanced new scheme to generate tunable-coherent radiation with frequency ranging from microwaves to x-rays, is investigated. For a Gaussian distribution in beam longitudinal momentum, the results show that the growth rate of IRL falls off rapidly with increasing momentum spread, and the momentum spread causes linear damping of beam space-charge wave, weakening the beam-radiation mode coupling and lowering the output radiation frequency apart from broadening the radiation spectrum. The dispersion relation and phase matching conditions are also obtained.

The range for rationality of ideal boundary condition is discussed at length. Plasma antenna is always out of the range, but many studies based on this ideal boundary condition. So we use a more appropriate method to cope after rigid boundary conditions are derived. In some cases, the results tally with those by usual method, and in other cases, some conclusions are obtained which cannot get from the usual way but reasonable theoretically. The dispersion relation of plasma antenna is given finally.

A novel CARM oscillator with axisymmetrical quasi—optical cavity of oblique rotation at arbitrary angle (AQCORAA) is studied using the kinetic theory. By means of the method of local field expansion in the coordinates of the guiding centre and the Laplace transform, the beam—wave interaction power, starting current density and frequency shift are derived. The formulas presented here may have some potential applications for more further studies of the CARM oscillator with AQCORAA.

In view of the fact that the large quantities of data contained in the medical image do not have a favorable setting for effective storage and transmission in the existing network, we have developed a JPEG2000 DICOM image compression kit using VC++, and we have realized the medical image storage service according to the C-STORE rule of DICOM standard. The results showed that the medical image processed by JPEG2000 compression algorithm not only greatly saved the storage space, but also reduced the band width needed by transmission. In this paper is discussed the DICOM image compressed by JPEG2000 compression algorithm in DICOM strorage service application. The significance of JPEG2000 compression algorithm in the development of picutre archiving and communication systems (PACS) and telemedicine is also explained.

The heating of non-equilibrium non-resonant ions with a Kappa velocity distribution via turbulent Alfven waves is discussed by means of both quasi-linear theory and test particle simulation. The interesting results obtained are as follows: the different thermal non-equilibrium factors (κ factors) possess differing heating efficiencies in the directions perpendicular and parallel to the background magnetic field. This can be confirmed from the simulation result which is consistent with the quasi-linear theory. On the other hand, it is revealed from the physical mechanism that turbulent Alfven waves can lead to the pitch-angle scattering of non-resonant ions and make the ions stochastic, giving rise to effective heating.

A hollow core photonic crystal resonant cavity for a terahertz gyrotron oscillator is proposed. The dispersion, mode structure, and confinement loss in this structure are theoretically derived. For advanced applications in the terahertz regime, the utilization of high-order modes, as well as mode competition issues, is discussed. The mode selectivity of the hollow core photonic crystal structure is demonstrated by utilizing a narrow bandgap to confine the design mode; meanwhile the competing modes are designed to have high confinement loss and weak beam-wave interaction efficiency. According to the mode selectivity, a mode selective hollow core photonic crystal terahertz gyrotron oscillator is designed.

Ion noise is an important figure of merit in microwave tube. It behaves manifest as a relaxation oscillation of phase. In this paper, we used hybrid model to analyze the noise phenomena and the way of depressing and eliminating the ion noise. To reduce the ion noise, besides the commonly way such as improving the vacuum degree and electronic beam focus level, another more practicability technique of adding electrode ring in tube is discussed and the numeric simulation (PIC) proved it is a well method.

The noise caused by ion relaxation oscillation becomes a research focus in the field of microwave tubes recently because of the improvement of radars and communications requirements. In this paper, electron beam is described by beam envelope equation, and ions generated from ionization of the background gas in microwave tubes are treated as discrete macro-particles. One dimensional particle-in-cell (PIC) simulation code was developed, and time series of ion relaxation oscillation are obtained by the presented method. The ion relaxation oscillation is treated as the response of a complex nonlinear dynamical system, and the time series is analyzed by power spectrum; restructure phase diagram and Lyapunov exponent. From the analysis results, we find that the ion relaxation oscillation has chaotic character at the first time. The reason of chaos was attributed to the dissipative structure of the system. This would be useful for depressing relaxation oscillation and processing signal with relaxation oscillation noise.

The synergy between electron cyclotron current drive (ECCD) and lower hybrid current drive (LHCD) is investigated numerically with the parameters of the HL-2A tokamak. Based on the understanding of the synergy mechanisms, a high current driven efficiency or a desired radial current profile can be achieved through properly matching the parameters of ECCD and LHCD due to the flexibility of ECCD. Meanwhile, it is found that the total current driven by the electron cyclotron wave (ECW) and the lower hybrid wave (LHW) simultaneously can be smaller than the sum of the currents driven by the ECW and LHW separately, when the power of the ECW is much larger than the LHW power. One of the reasons leading to this phenomenon (referred to as negative synergy in this context) is that fast current-carrying electrons tend to be trapped, when the perpendicular velocity driven by the ECW is large and the parallel velocity decided by the LHW is correspondingly small.

The dispersion relation for high-n ballooning modes is obtained from the customary drift gyro-kinetic equation employing the energy-dependent Lorenz collision operator. There are significant differences between our results and those with the Krook collision operator. The present results show that collision seems to have much weaker stabilizing effects, if any, on the high-n ballooning modes than those with the Krook collision operator when the collisionality, υ*e, is smaller than a critical value. However, contrary to the phenomenon, the collision shows strong stabilizing effects on the ballooning mode once the collisionality exceeds a critical value. These differences are supposed to arise from boundary layer behaviour which refers to the electron perturbation at the boundary between trapped and passing electrons in velocity space, but the physical process cannot be described by non-conserving Krook collision operators. This destabilizing mechanism of boundary layer behaviour is discussed in this paper. In addition, the critical value shows a considerably strong dependence on ηe (ηe ≡ d ln T e/d ln N e).

A plasma-based multistage virtual cathode radiation is proposed. Multistage virtual cathode can be formed as an electron beam passes through a high dense ion background. The reflected electrons can be coupled with transverse magnetic wave, and electromagnetic radiation is detected. Unlike the traditional virtual cathode devices, the beam current can be decreased greatly due to the effect of ions, and single mode operation can be achieved by adjusting the beam density. Besides, the radiation frequency, which is proportional to the beam density, covers from 10 to 100 GHz. The output power flux density reaches a magnitude of GWm−2.

The features of the waves and the instability induced by the interaction of electromagnetic (EM) wave with plasma wave in the electron beam-ion channel system are studied. Characteristics of waves in the system are displayed through the dispersion relation and two kinds of EM instabilities are found. The first one is excited by the interaction of the fast plasma wave and the forward EM wave, and the other one is induced by the coupling of the fast plasma wave and the backward EM wave, the numerical results show that this is caused by the virtual cathode oscillation. The conditions of these EM instabilities are given and the related physical mechanisms have been discussed in detail.