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Generation of the Harmonic Structure of Upper Hybrid and Electron Cyclotron Waves Driven by Energetic Electrons
Get PDFORCID 0000-0001-8894-3430This is the first report of the harmonic structure of upper hybrid waves (UHWs). Using a one-dimensional electromagnetic particle-in-cell simulation, it is demonstrated that the harmonic structure of UHWs can be generated by energetic electrons through non-linear wave-wave coupling, similar to that of lower hybrid waves by energetic ions. After the saturation of UHWs, furthermore, it is found that electron cyclotron waves (ECWs) gradually grow due to the injection of energetic electrons. The involvement of ECWs in non-linear wave-wave coupling can make the harmonic structure of UHWs more complex.journal articl
Enhanced Classical Radiation Damping of Electronic Cyclotron Motion in the Vicinity of the Van Hove Singularity in a Waveguide
Get PDFORCID 0000-0001-6636-8500We study the damping process of electron cyclotron motion and the resulting emission in a waveguide using the classical Friedrichs model without relying on perturbation analysis such as Fermi’s golden rule. A Van Hove singularity appears at the lower bound (or cutoff frequency) of the dispersion associated with each of the electromagnetic field modes in the waveguide. In the vicinity of the Van Hove singularity, we found that not only is the decay process associated with the resonance pole enhanced (amplification factor ∼104) but the branch-point effect is also comparably enhanced. As a result, the timescale on which most of the decay occurs is dramatically shortened. Further, this suggests that the non-Markovian branch-point effect should be experimentally observable in the vicinity of the Van Hove singularity. Our treatment yields a physically acceptable solution without the problematic runaway solution that is well known to appear in the traditional treatment of classical radiation damping based on the Abraham–Lorentz equation.journal articl
Local momentum balance in electromagnetic gyrokinetic systems
Get PDFORCID 0000-0001-5444-1758The Eulerian variational formulation is presented to obtain governing equations of the electromagnetic turbulent gyrokinetic system. A local momentum balance in the system is derived from the invariance of the Lagrangian of the system under an arbitrary spatial coordinate transformation by extending the previous work [H. Sugama et al., Phys. Plasmas 28, 022312 (2021)]. Polarization and magnetization due to finite gyroradii and electromagnetic microturbulence are correctly described by the gyrokinetic Poisson equation and Ampère's law which are derived from the variational principle. Also shown is how the momentum balance is influenced by including collisions and external sources. Momentum transport due to collisions and turbulence is represented by a symmetric pressure tensor, which originates in a variational derivative of the Lagrangian with respect to the metric tensor. The relations of the axisymmetry and quasi-axisymmetry of the toroidal background magnetic field to a conservation form of the local momentum balance equation are clarified. In addition, an ensemble-averaged total momentum balance equation is shown to take the conservation form even in the background field with no symmetry when a constraint condition representing the macroscopic Ampère's law is imposed on the background field. Using the WKB representation, the ensemble-averaged pressure tensor due to the microturbulence is expressed in detail and it is verified to reproduce the toroidal momentum transport derived in previous works for axisymmetric systems. The local momentum balance equation and the pressure tensor obtained in this work present a useful reference for elaborate gyrokinetic simulation studies of momentum transport processes.journal articl
Investigation of Cs27LiYCl6:Ce scintillator energy response for D-D fusion neutron spectrometer
No full textDuring plasma experiments on EAST (Experimental Advanced Superconducting Tokamak), the discharges heated with neutral beam injection (NBI) for plasma results in the production of 2.45 MeV deuterium-deuterium (D-D) fusion neutrons. In order to measure the energy spectrum of D-D fusion neutrons, a new Cs27LiYCl6:Ce (CLYC-7) crystal with 7Li-enrichment has been developed and fabricated into a CLYC-7 scintillation detector. Before utilizing this detector for studying neutron energy spectra, it is crucial to explore the response function of the detector within a relevant neutron energy range. Therefore, the detector has been experimented using the 4.5 MeV electrostatic accelerator at Peking University to investigate its response to fast neutrons, energy resolution, and pulse discrimination capability in the neutron energy range 2.07 MeV–5.51 MeV. The neutron reactions occurring within the CLYC-7 scintillator were simulated using MCNPX, focusing on the reactions 35Cl(n,p) and 35Cl(n,α), as well as the energy response curve and the contributions of different particles (protons, alphas) to the neutron spectrum. The experimental results were successfully validated on EAST, where the 2.45 MeV fusion neutron energy spectrum was measured. That demonstrates the ability of the CLYC-7 scintillation detector to diagnose the fusion neutron energy spectrum for deuterium plasma discharge.journal articl
Observation and Identification of W19+-W23+ Spectra in the EUV Wavelength Region in the Vicinity of 200Å
Get PDFTungsten (W) is one of the major impurities in ITER and future DEMO reactors. However, diagnosing ion density, temperature, and spatial distribution for tungsten ions in low charge states such as W17+-W27+ is difficult due to a lack of spectral line data. In this study, we observed tungsten Unresolved Transition Array (UTA) spectra around W20+ in Large Helical Device. Furthermore, the emission spectra of tungsten ions ranging from W19+-W23+ were also measured using Compact electron Beam Ion Trap (CoBIT). Two spectral peaks were detected in the CoBIT experimental setup. Subsequently, these peaks were theoretically identified as 5s-5p and 5p3/2-5d transitions using Flexible Atomic Code (FAC). The identified peaks are useful for impurity diagnostics of ITER edge plasma.journal articl
A reduced ideal MHD system for nonlinear magnetic field turbulence in plasmas with approximate flux surfaces
Get PDFORCID 0000-0002-2973-0635This paper studies the nonlinear evolution of magnetic field turbulence in proximity of steady ideal Magnetohydrodynamics (MHD) configurations characterized by a small electric current, a small plasma flow, and approximate flux surfaces, a physical setting that is relevant for plasma confinement in stellarators. The aim is to gather insight on magnetic field dynamics, to elucidate accessibility and stability of three-dimensional MHD equilibria, as well as to formulate practical methods to compute them. Starting from the ideal MHD equations, a reduced dynamical system of two coupled nonlinear partial differential equations for the flux function and the angle variable associated with the Clebsch representation of the magnetic field is obtained. It is shown that under suitable boundary and gauge conditions such reduced system preserves magnetic energy, magnetic helicity, and total magnetic flux. The noncanonical Hamiltonian structure of the reduced system is identified, and used to show the nonlinear stability of steady solutions against perturbations involving only one Clebsch potential. The Hamiltonian structure is also applied to construct a dissipative dynamical system through the method of double brackets. This dissipative system enables the computation of MHD equilibria by minimizing energy until a critical point of the Hamiltonian is reached. Finally, an iterative scheme based on the alternate solution of the two steady equations in the reduced system is proposed as a further method to compute MHD equilibria. A theorem is proven which states that the iterative scheme converges to a nontrivial MHD equilbrium as long as solutions exist at each step of the iteration.journal articl
First Attempt at Photoionized Plasma Production with VUV Radiation in Synchrotron Light Source UVSOR-III
Get PDFPhotoinduced processes such as photoionization and photoexcitation in the vacuum ultra violet (VUV) energy range are considered important for the divertor region in nuclear fusion reactors and in interstellar space because the cross sections (photoionization, photoexcitation) of these processes in relevant species (hydrogen, helium, neon, argon, and biomolecules) become large in that energy range. Herein, a photoionization experiment was conducted for the first time in the synchrotron light source UVSOR-III with VUV photon energies. The synchrotron light source has the advantage of capability to change photon energy continuously over a wide range and high beam repetition rates. These features allow the simulation of the divertor region and interstellar radiation field to systematically investigate photoinduced processes. Using argon as the sample gas, plasma production was evidenced by the detection of electron current in Langmuir probe measurements. Although an accurate evaluation of the plasma parameters was challenging because of the large scatter of probe data, the possible ranges of plasma parameters are discussed based on a 0D model of photoionization plasmas [R.M. van der Horst et al., J. Phys. D: Appl. Phys. 48, 285203 (2015)] and a newly proposed 1D model in the steady state. Analysis result indicates that plasma density is in the range of 1010 - 1011 m-3. Additionally, the further development of experiments is discussed for realizing higher plasma densities and for studying photoinduced processes in the divertor region in nuclear fusion reactors and interstellar plasma in terms of the chemical evolution of biomolecules.journal articl
Tensile and creep properties of small specimens of reduced-activation ferritic steel F82H, and the correlation to standard specimen data
No full textORCID 0000-0002-5000-1269Background
A reduced-activation ferritic/martensitic (RAFM) steel, F82H steel, is the primary candidate structural material for fusion reactor blanket. Small specimen test technique is essential to develop the blanket materials using limited irradiation volume in high flux neutron field. An international collaboration activity “Towards the Standardization of Small Specimen Test Techniques for Fusion Applications” has been initiated under the framework of the International Atomic Energy Agency Coordinated Research Project for Phase I from 2017 to 2021, and Phase II from 2022 to 2026. The present paper reports the preliminary results on tensile and creep tests as a summary of the above Phase I activity.
Methods
Tensile and creep tests were conducted at 550 and 650°C, using flat-plate SSJ type small specimens with various gauge thickness ranged from 0.14 to 1.2 mm, while gauge length and width are 5 and 1.2 mm, respectively. In addition, round bar type standard specimens with a gauge geometry of 6 mm in diameter and 30 mm in length was also tested for comparison.
Results
Tensile yield stress, ultimate tensile strength and uniform elongation were independent of the gauge thickness of SSJ specimens, and agreed with the data from the standard size specimens. On the other hand, total elongation was decreased with decreasing the thickness. In creep tests, rupture time was decreased with decreasing the gauge thickness of SSJ specimens. Standard size specimens exhibited shorter rupture time than the SSJ specimens.
Conclusions
The SSJ type specimens provided similar tensile parameters to those from the standard specimen, except total elongation. Creep rupture time of the SSJ specimens were different from the standard specimen, and decreased with decreasing the gauge thickness.journal articl
Effect of oxygen or copper impurities on the work function of cesiated surfaces under co-existence with hydrogen: A study about negative hydrogen ion sources for neutral beam injection systems
Get PDFThis study investigates the effect of oxygen and copper impurities on the work function of the Cs/H/Mo (001) surface based on the Density Functional Theory (DFT) method. First, the deposition of 2/16 θ impurities on the Cs0.5/H0.5/Mo (001) surface (the subscript 0.5 represents 8/16 θ coverage) is studied. It is found that the deposition of impurities beneath the cesium layer and oxygen impurities as atoms, and in a high-symmetry configuration, both lower the surface system energy. As the impurity coverage increases, it is observed that impurity deposition with a coverage of less than or equal to 8/16 θ effectively reduces the work function of the Cs0.5/H0.5/Mo (001) surface. However, as the cesium and hydrogen coverage increases, the impurities are no longer effective at reducing the work function. Moreover, when the cesium coverage reaches 16/16 θ, the surface work function approaches that of a cesium metal surface. Another important discovery is that when oxygen impurities are deposited above the cesium layer on the Cs0.5/H0.5/Mo (001) surface, a reduction in the surface work function is observed. This anomalous phenomenon is attributed to significant charge polarization caused by the deposition of oxygen impurities. The migration barriers of impurities traversing the dense cesium layer to the Mo (001) substrate are evaluated using the climbing image nudged elastic band (CI-NEB) method. Oxygen impurities tend to traverse the cesium layer in molecular form, facing a minimum barrier of 0.52 eV, while copper atoms encounter a lower barrier of 0.09 eV when traversing the cesium layer.journal articl
Fast Charge Exchange Spectroscopy for Measurements of Ion-Velocity-Space Distribution Function
Get PDFORCID 0000-0002-5113-9710Fast charge exchange spectroscopy of C VI (Δn = 8−7, λ = 529.05 nm) has been developed to measure the ion-velocity-space distribution function. The fast charge exchange spectroscopy system consists of an F2.8 lens spectrometer, image intensifier, and high-speed camera. To maximize photon count, we efficiently accumulate light from 50 optical fibers (400 µm core) into one spatial channel. By applying a curved multi-slit, one spectrometer has four spatial channels. This system has good time resolution with a frequency of 10 kHz beyond that of ion-ion collision and excellent accuracy (signal-to-noise ratio), enough to detect the deformation of carbon ion-velocity distribution from Maxwell-Boltzmann distribution.journal articl