查看更多>>摘要：Linear eigenmode analysis of slab ion-temperature-gradient (ITG) instabilities in weakly reversed magnetic shear configurations is performed based on a Gyro-Landau-Fluid model. Numerical results show that the linear features of the ITG instabilities are different in weakly reversed shear plasmas with double, single and non-rational surfaces, respectively. For the case with double rational surfaces separated far away, mode structures exhibit a global symmetry or antisymmetry versus the minimum of the safety factor q ( min ) with locally odd or even parity around each resonant surface. Linear properties are mainly determined by local magnetic shear as in normal magnetic shear case. With the distance between two surfaces decreasing, linear instability properties vary non-monotonically. Various high order eigenmodes are driven unstable with a moderate distance. The mode structures are eventually localized around q ( min ) surface in the single rational surface case, even nonresonant one. When there is no rational surface, the nonresonant modes are strongly driven unstable with growth rates even larger than the resonant modes. In addition, similar results in a cylinder plasma indicate the importance of non-resonant ITG instability in the short wavelength regime with weakly reversed magnetic shear.
查看更多>>摘要：A beam of ultrarelativistic charged particles in a plasma can reach equilibrium with its own radial wakefield and then propagate with little change in shape. If some co-moving perturbation appears ahead of the beam, it may or may not destroy the beam with its wakefield, depending on the phase and amplitude of the wakefield. We numerically study which perturbations can destroy a single short bunch or a train of many short bunches at the parameters of interest for plasma wakefield acceleration in an axisymmetric configuration, and how fast. We find that there are particularly dangerous wakefield phases in which the beam can be destroyed by perturbations of very low amplitude. We also find that perturbations with an amplitude larger than the wakefield of a single bunch in the train are always destructive.
查看更多>>摘要：A novel polarization modulation method is proposed, for a possible application in the measurement of the electron density and magnetic field profiles by an interferometer-polarimeter diagnostic in the Divertor Test Tokamak (DTT) device. Starting from the output of a CO2 pumped CHCOH3 far-infrared (FIR) laser (lambda = 118.8 mu m), three waves with frequencies omega, omega - delta omega, and omega + delta omega are generated and coherently combined to produce a polarization modulated laser beam suitable to probe the DTT plasma in a multichord, double-pass scheme. A second, coherently pumped, FIR cavity operating at the slightly detuned omega ' frequency, acts as a local oscillator for the interferometric measurement. By this polarization modulation method, it is possible to simultaneously measure the interferometric phase, the Faraday rotation angle, and the Cotton-Mouton effect, all by a single detector, while keeping to an acceptable value the perturbation of the interferometric phase due to the time modulated polarization. In this paper, we describe the principles of the method and discuss its possible application in the poloidal interferometer-polarimeter diagnostic of the DTT device. A single chord mock-up experiment is in preparation to experimentally test the technique.
查看更多>>摘要：Although experiments have demonstrated a feasibility of the electron-cyclotron-wave (ECW) non-inductive current start-up, the physical mechanism of the current start-up remains unclear, particularly the role of the preferential confinement of collisionless tail electron currents (j(TPC)) and vertical charge-separation currents (j(VTF)). Therefore, j(TPC) and j(VTF) were observed using limiter probes during the ECW non-inductive current start-up on an ENN XunLong-50 (EXL-50) spherical torus to understand the significance of these values. It was found that j(TPC) increased and j(VTF) decreased during start-up, indicating a dominant role played by j(TPC) in this process. Furthermore, it was found that the product of the gas pressure and vertical magnetic field has a limit for the ECW non-inductive current start-up. Finally, we found that the ECW non-inductive current start-up can be realized only for sufficiently low vertical fields and gas pressures in EXL-50.
查看更多>>摘要：Detachment control based on ion saturation current I-sat measurements from Langmuir probes (LPs) is implemented in the KSTAR tokamak and shown to be capable of following dynamic and constant target trajectories with good accuracy, in H-mode, by moderating the flow rate of nitrogen or deuterium. I-sat controllers normalize I-sat in order to form attachment fraction (A(frac)) as their control parameter. The KS TAR implementation of A(frac) control differs from previous work in that it continuously calculates a model for attached I sa r and uses that as the denominator in A(frac), whereas prior implementations either record peak I-sat at rollover as they pass it or take estimated I-sat,I-rollover as a manual input prior to the shot. The KSTAR controller therefore does not need to keep track of rollover status and keep separate targets for pre- and post-rollover states, and it can automatically adapt to changes in scenario at any time. It is also less vulnerable to noise as it will not lock in an outlier as a rollover point.
查看更多>>摘要：It is observed in EAST that the non-axisymmetric helical currents induced by lower hybrid waves (LHWs) in the scrape-off layer (SOL) can produce resonant magnetic perturbations (RMPs), hence mitigate edge-localized modes (ELMs) significantly and cause the strike points splitting. In this work, the non-axisymmetric SOL current driven by an alternative method, i.e. biased electrode, is shown to influence the plasma boundary, especially the strike points, in the divertor configuration on J-TEXT. With positive bias to the electrode, the upper strike points on the divertor target plate is observed to split as captured by the visible camera. Such splitting is usually observed with the RMPs. In addition, the SOL plasma parameters measured by the reciprocating probe, such as floating potential (V (f)), electron temperature (T (e)), and ion saturation current (I (s)), are significantly altered by applying modulation bias, which is closely related to the SOL current. A SOL current model has been extended from the limiter configuration version to the poloidal divertor configuration, and the magnetic field generated by SOL current and the change of magnetic topological structure of plasma boundary can be simulated. The simulation results shows that the magnetic perturbation generated by the SOL current can induce the lobes near the X-point, which is three-dimensional asymmetric structure, and shows good agreement with the experimental observation. The active control of the boundary magnetic topology indicates that the SOL current is capable of generating strong magnetic perturbations, and hence might be sufficient for the control of ELMs.
查看更多>>摘要：A previously presented model for generating 2D estimates of the divertor plasma conditions at JET from deuterium Balmer line intensity ratios, obtained from tomographic reconstructions of divertor camera images, was amended to consider also the Balmer emission arising from molecular processes. Utilizing the AMJUEL and H2VIBR atomic and molecular databases of EIRENE enabled also inference of the molecular divertor density from the distinguished molecularly induced emission. Analysis of a JET L-mode density scan suggests the molecularly induced emission accounting for up to 60%-70% and 10%-20% of the Balmer D-alpha and D-gamma intensities, respectively, at the onset of detachment, while electron-ion recombination becomes increasingly dominant with deepening detachment. Similar observations were made by post-processing EDGE2D-EIRENE simulations, which indicated significant roles of molecular D-2(+) ions and vibrational excitation of the D-2 molecules as precursors for the molecularly induced emission. The experimentally inferred molecular density at the outer strike point was found to increase monotonously with decreasing strike point temperature, reaching approximately 30%-50% of the local electron density at n(mol,osp) = 1-2 x10(20) m(-3) at T-e,T-osp approximate to 0.7 eV. A further steep increase by a factor of 3-5 was observed with decrease of T-e,T-osp to 0.5 eV. The observations are in qualitative and reasonable quantitative agreement with EDGE2D-EIRENE predictions of n(mol,osp) within the uncertainties of the experimental data.
查看更多>>摘要：A novel nonlinear functional relation of turbulence potential intensity, zonal flow potential intensity, and ion thermal diffusivity that accurately reproduces nonlinear gyrokinetic simulations of toroidal ion temperature gradient (ITG) driven turbulence is proposed. Applying mathematical optimization techniques to find extremal solutions in high-dimensional parameter space, the optimal regression parameters in the functional form are determined to be valid for both near- and far-marginal regime of the ITG stability including the Dimits-shift. Then, the regression error of similar to 5% is accomplished. In addition, it is clarified that the intensity ratio of the zonal flow and turbulence potential intensity is a crucial factor to determine the reproduction accuracy.
查看更多>>摘要：In laser wakefield accelerators, guiding of drive laser pulses in preformed plasma channels plays a key role to overcome laser diffraction for effective acceleration. Different from guiding schemes studied previously, where a Gaussian laser pulse and a parabolic plasma channel were investigated, here we investigate the guiding of Laguerre-Gaussian (LG) pulses in plasma channels. Analytical studies and three-dimensional particle-in-cell simulations show that the matched conditions still exist for high order laser pulses and high order plasma channels. For usual Gaussian and high order LG pulses, the second order parabolic channel gives the best guiding. Although the laser pulse can also be guided in even higher order channels, its envelope deforms during propagation. For laser pulses with combined multi-LG modes, determined by their azimuthal orbit angular momenta, there is axisymmetric or non-axisymmetric evolution for the transverse laser intensity profile. The preformed plasma channel can guide the combined pulses but the transverse intensity profile of the laser pulses always evolves.
查看更多>>摘要：Impurity temperatures have been determined by a spectroscopic line shape analysis for several species in the divertor scrape-off-layer of the stellarator Wendelstein 7-X (W7-X). Examples include spectral lines from intrinsic elements (C II and C III, He I) as well as from seeded impurities (Ar II, N II) through the divertor gas inlet system. Both Doppler broadening and Zeeman splitting are found to contribute significantly to the impurity line shapes. Zeeman splitting arises due to the confining magnetic field in W7-X and complicates the line shape appearance. By attributing Doppler widths to each of the various Zeeman components, however, we demonstrate that reliable ion temperature values can be derived provided that the presence of the magnetic field is properly accounted for. The spectrally highly resolved lines are analyzed by means of a multi-parameter, least-squares fit routine, which accounts for Doppler broadening, Zeeman splitting, as well as the instrumental broadening of the spectrometer used to measure the spectral line shapes. By spectral fitting of the Zeeman features, it is also found that the line shape analysis can yield values for the local magnetic field, which can be used to localize the impurity radiation approximately provided that the line emission is dominant in a small area intersected by the lines of sight of the spectrometer.
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