Jizhong Sun

Probabilistic and Ensemble Representations of the Uncertainty in an IR/Microwave Satellite Precipitation Product

Abstract While current satellite techniques are theoretically capable of producing precipitation estimates to image pixel resolutions, significant uncertainty is present in such high-resolution products. This uncertainty is frequently difficult to characterize using scalar measures of additive error. This paper describes the development of a methodology to more fully represent the uncertainty in satellite precipitation retrievals. The methodology derives conditional probability distribution functions of rainfall on a pixel-by-pixel basis. This array of distribution functions is then combined with a simple model of the spatiotemporal covariance structure of the uncertainty in the precipitation field to stochastically generate an ensemble precipitation product. Each element of the ensemble represents an equiprobable realization of the precipitation field that is consistent with the original satellite data while containing a random element commensurate with the uncertainty in that field. The technique has be...

Plasma density enhancement in atmospheric-pressure dielectric-barrier discharges by high-voltage nanosecond pulse in the pulse-on period: a PIC simulation

A particle-in-cell (PIC) plus Monte Carlo collision simulation is employed to investigate how a sustainable atmospheric pressure single dielectric-barrier discharge responds to a high-voltage nanosecond pulse (HVNP) further applied to the metal electrode. The results show that the HVNP can significantly increase the plasma density in the pulse-on period. The ion-induced secondary electrons can give rise to avalanche ionization in the positive sheath, which widens the discharge region and enhances the plasma density drastically. However, the plasma density stops increasing as the applied pulse lasts over certain time; therefore, lengthening the pulse duration alone cannot improve the discharge efficiency further. Physical reasons for these phenomena are then discussed.

Modelling of hydrogen isotope inventory in mixed materials including porous deposited layers in fusion devices

Hydrogen isotope inventory (HII) is a key issue for fusion devices such as ITER. Simultaneous use of Be, W and C as the wall material for different parts of plasma-facing components (PFCs) will bring in material mixing issues, which compound that of hydrogen isotope retention. To simulate the hydrogen inventory in the PFCs, we have developed a flexible standalone model called HIIPC (Hydrogen Isotope Inventory Processes Code). The particlebalance-based model for reaction–diffusion and HII in metal and porous media (mainly carbon and co-deposited layers) is presented, coupled with a heating model which can calculate the temperature distribution. Some sample results are given to illustrate the model’s capabilities and show good qualitative agreement with the experiment. (Some figures may appear in colour only in the online journal)

Properties of plasma sheath with ion temperature in magnetic fusion devices

The plasma sheath properties in a strong magnetic field are investigated in this work using a steady state two-fluid model. The motion of ions is affected heavily by the strong magnetic field in fusion devices; meanwhile, the effect of ion temperature cannot be neglected for the plasma in such devices. A criterion for the plasma sheath in a strong magnetic field, which differs from the well-known Bohm criterion for low temperature plasma sheath, is established theoretically with a fluid model. The fluid model is then solved numerically to obtain detailed sheath information under different ion temperatures, plasma densities, and magnetic field strengths.

Characteristics of nanosecond pulse needle-to-plane discharges at high pressure: a particle-in-cell Monte Carlo collision simulation

A model of one dimensional in position and three dimensional in velocity space self-consistent particle in cell with Monte Carlo collision technique was employed to simulate the argon discharge between the needle and plane electrodes at high pressure, in which a nanosecond rectangular pulse was applied to the needle electrode. The work focused on the investigation of the spatiotemporal evolution of the discharge versus the needle tip size and working gas pressure. The simulation results showed that the discharge occurred mainly in the region near the needle tip at atmospheric pressure, and that the small radius of the needle tip led to easy discharge. Reducing the gas pressure gave rise to a transition from a corona discharge to a glowlike discharge along the needle-to-plane direction. The microscopic mechanism for the transition can arguably be attributed to the peak of high-energy electrons occurring before the breakdown; the magnitude of the number of these electrons determined whether the breakdown can take place.

Numerical analysis of two homogeneous discharge modes at atmospheric pressure with a self-consistent model

Two homogeneous discharge modes, Townsend discharge and glow discharge, can be obtained in dielectric barrier discharges at atmospheric pressure when an external voltage with an appropriate frequency is applied to the electrodes. In this paper, a one-dimensional self-consistent model was used to investigate the transition and the difference in characteristics of these two modes. The simulation results showed that the spatiotemporal distributions of the electron temperature in the two discharge modes differed noticeably. In the glow discharge, the electron temperature in the cathode fall was several times higher than that in any of the other regions; in contrast, the electron temperature in the Townsend discharge was approximately spatially uniform. The electron energy distribution functions (EEDFs) at different locations in the discharge gap at the discharge current peaks were given and analyzed. In the glow discharge, the EEDF in the cathode fall region contained the largest percentage of high energy in ...

Scaling of divertor power footprint width in RF-heated type-III ELMy H-mode on the EAST superconducting tokamak

Dedicated experiments for the scaling of divertor power footprint width have been performed in the ITER-relevant radiofrequency (RF)-heated H-mode scheme under the lower single null, double null and upper single null divertor configurations in the Experimental Advanced Superconducting Tokamak (EAST) under lithium wall coating conditioning. A strong inverse scaling of the edge localized mode (ELM)-averaged power fall-off width with the plasma current (equivalently the poloidal field) has been demonstrated for the attached type-III ELMy H-mode as λq ∝ I −1.05 p by various heat flux diagnostics including the divertor Langmuir probes (LPs), infra-red (IR) thermograph and reciprocating LPs on the low-field side. The IR camera and divertor LP measurements show that λq,IR ≈ λq,div-LPs/1.3 = 1.15B −1.25 p,omp , in good agreement with the multi-machine scaling trend during the inter-ELM phase between type-I ELMs or ELM-free enhanced Dα (EDA). H-mode. However, the magnitude is nearly doubled, which may be attributed to the different operation scenarios or heating schemes in EAST, i.e., dominated by electron heating. It is also shown that the type-III ELMs only broaden the power fall-off width slightly, and the ELM-averaged width is representative for the inter-ELM period. Furthermore, the inverse Ip (Bp) scaling appears to be independent of the divertor configurations in EAST. The divertor power footprint integral width, fall-off width and dissipation width derived from EAST IR camera measurements follow the relation, λint ∼ λq +1.64S, yielding λ EAST = (1.39±0.03)λ EAST +(0.97±0.35) mm. Detailed analysis of these three characteristic widths was carried out to shed more light on their extrapolation to ITER.

Numerical investigation on operation mode influenced by external frequency in atmospheric pressure barrier discharge

The influence of external driving frequency on the discharge mode in the dielectric barrier discharge was investigated with a two-dimensional, self-consistent fluid model. The simulation results show that the helium discharge exhibits three operation modes: Townsend, homogeneous glow, and local glow discharges from the lower frequency (1 kHz) to the higher frequency (100 kHz) under discharge parameters specified in this work. The discharge operates in a Townsend mode when the driving frequency varies from 1 to about 7 kHz; while it exhibits homogenous glow characteristics in an approximate range from 7 to 65 kHz; when the external frequency exceeds 65 kHz, it turns into a local glow discharge. The effects of external driving frequency on the discharge mode were revealed and the physical reasons were discussed.

Particle-in-cell simulation of hydrogen discharge driven by combined radio frequency and pulse sources

A particle-in-cell plus Monte Carlo collision model is employed to investigate the low pressure hydrogen capacitive discharge driven by combined radio frequency (rf) and pulse sources. This work focuses on the evolutions of electron energy and density in the discharge to illustrate the role that a short pulse source plays. The simulation results show that an extra short pulse source can modulate the electron energy effectively: in the early and late pulse-on times, the electron energy is much higher than that in the single rf source discharge; during the pulse-off time, the electron energy can drop gradually to a low value. It is also found that a few peaks of attenuated electron energy appear periodically just after the pulse voltage drops to zero. The similar phenomena can also be found in the production rate of highly vibrationally excited hydrogen molecules. Physical mechanisms responsible for these phenomena are discussed.

SOLPS modeling of lithium transport in the scrape-off layer during real-time lithium injection on EAST

Edge fluid?plasma/kinetic?neutral SOLPS [1, 2] modeling of lithium (Li) transport and its effect on the edge plasma during real-time Li injection H-mode discharge on the EAST tokamak are analysed in this work. Since Li has strong chemical activity, deuterium (D) recycling is suppressed by a Li coated plasma-facing wall. By comparing the simulated edge plasma parameters between the no Li case and the Li injection case, it is found that both of the D atom and molecule densities in the divertor region are reduced with the Li injection. It is also found that most of the radiated power is radiated in the divertor. The simulation provides and analyzes the distributions of each Li ion charge state, and the evolution of Li impurity distribution. The simulation shows that the Li+ prefers to accumulate on the high-field side than on the low-field side, which is in qualitative agreement with the experimental measurements on EAST. The possible reason for the Li+ preferential accumulation is discussed in this study.