Abstract
The electrical and optoelectronic properties of nanometer-sized ZnO structures are highly influenced by its native point defects. Understanding and controlling these defects are essential ...for the development of high-performance ZnO-based devices. Here, an electrical device consisting of a polycrystalline ZnO-coated silica nanospring was fabricated and used to characterize the electrical and photoconductive properties of the ZnO layer using near-UV (405 nm) and sub-bandgap (532 and 633 nm) excitation sources. We observe a photocurrent response with all three wavelengths and notably with 532 nm green illumination, which is the energy associated with deep oxygen vacancies. The polycrystalline ZnO-coated silica nanospring exhibits a high responsivity of 1740 A W
−1
with the 405 nm excitation source. Physical models are presented to describe the photocurrent rise and decay behavior of each excitation source where we suggest that the rise and decay characteristics are highly dependent on the energy of the excitation source and the trapping of electrons and holes in intermediate defect levels in the bandgap. The energy levels of the trap depths were determined from the photoconductive decay data and are matched to the reported energy levels of singly and doubly ionized oxygen vacancies. A phenomenological model to describe the dependence of the saturation photocurrent on excitation intensity is presented in order to understand the characteristics of the observed breaks in the slopes of the saturation photocurrent versus excitation intensity profile.
The engineering part of the GLOBSYS code is presented, and the parameters of the Globus-3 facility, which is a development of the Globus program, are analyzed. The facility is primarily designed to ...provide a long pulse, a large toroidal magnetic field and strong heating. The concepts of searching for Globus-3 parameters under physical and engineering limitations are described. Obviously that reliable confinement and a large part of noninductive current are necessary to ensure existence of a plasma for a long time. Engineering constraints are involved in the choice of parameters in a more complex way: in some cases, it is overheating of the coils, in other cases, it is the total power supply, or the limit on the flux provided by the ohmic solenoid, or the strength of the constructions. The parameters of the Globus-3 spherical tokamak were preliminarily selected for the cases of a “warm” copper EMS (Electromagnetic system) and the EMS precooled to liquid nitrogen temperature. The exceeding of the duration of the plasma current plateau Δ
t
plateau
over the characteristic settling time of the plasma profiles τ
L
/
R
was chosen as the key condition. At values of the toroidal magnetic field
B
t
0
= 3 T, the condition Δ
t
plateau
> τ
L
/
R
cannot be attained even for precooled EMS. At
B
t
0
= 2 T, only options with precooled EMS can be considered acceptable, but the facility dimensions are fairly large. For the field
B
t
0
= 1.5 T, the options with “warm” EMS correspond to the duration of the plasma current plateau ~3 s (Δ
t
plateau
/τ
L
/
R
~ 1–1.5). In the case of precooled EMS, the duration of the plateau can increase to 12–13 s (Δ
t
plateau
/τ
L
/
R
~ 5). In the latter case, as a basis for further development of the Globus-3 facility, options with the following geometric dimensions are reasonable:
R
0
~ 0.6–0.7 m,
a
~ 0.35–0.4 m,
А
≤ 1.7–1.8,
k
95
~ 1.7–1.8. The minimum allowable value of the plasma current
under the condition of effective absorption of the input power of neutral injection has been calculated. In the Globus-3 facility,
I
p
≈ 0.8 MA was chosen as the base value.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The paper is devoted to calculations of anisotropic spectra of nuclear fusion products in a compact tokamak. The knowledge of these spectra is essential for estimations of the first wall load by ...neutron radiation and other fast particle fluxes from the plasma, computations of the plasma heating profiles, and correct analysis of experimental data, obtained on the present-day and prospective tokamaks and other controlled fusion devices. Anisotropic analytical S- and L-formulae found earlier for nuclear fusion product distributions are computationally demanding. In this paper improved anisotropic S- and L-algorithms are introduced. The new simplified formulae retain the generality and reduce the calculation time without changing the results. Their application for the calculation of energetic and angular distributions of products of both neutron and proton branches of the deuterium-deuterium nuclear fusion reaction in the neutral beam heated plasma of the Globus-M2 spherical tokamak is described. Notwithstanding the low neutral beam energy, the obtained fusion product spectra are strongly anisotropic and may be observed experimentally.
The Globus-M spherical tokamak has demonstrated practically all of the project objectives during the 15-year period of operation. The main factor limiting further progress in plasma performance is a ...relatively low toroidal magnetic field. The maximum toroidal magnetic field achieved on Globus-M was 0.4 T with the exception of a limited number of shots with 0.55 T, which led to damage of the toroidal field coil in 2002. The increase of the magnetic field up to 1.0 T together with the plasma current up to 0.5 MA will result in the significant enhancement of the operating parameters in the upgraded Globus-M2 machine. The experimental program will be focused on plasma heating and non-inductive current drive and will contribute to the creation of a physical and technological base for the compact fusion neutron source development. In the article a brief overview of the physical background for the machine upgrade is outlined. The current status of the project implementation is described. First experimental results on moderate magnetic field increase from 0.4 T up to 0.5 T in the existing Globus-M machine are discussed. The improvement of plasma confinement as well as enhancement of efficiency of the beam driven current is observed.
The behaviour of the fast particle population during 18 keV hydrogen and 26 keV deuterium neutral beam injection in deuterium plasmas is investigated. Experiments reveal large fast ion losses. The ...experimental results are confirmed using different types of modelling: simulation using the NUBEAM module, solution of the Boltzmann kinetic equation and solution of the 3D fast ion tracking algorithm. The dynamics of the energetic particle redistribution and losses during sawtooth oscillation and toroidal Alfvén eigenmodes are studied. A method to decrease fast ion losses under the current conditions (0.4 T, 0.2 MA) is shown. The influence of the plasma parameters on the energetic ion confinement rate is investigated. Modelling for the Globus-M2 conditions (1 T, 0.5 MA) is performed.
A significant improvement of the response characteristics of a redox chemical gas sensor (chemiresistor) constructed with a single ZnO coated silica nanospring has been achieved with the technique of ...lock-in signal amplification. The comparison of DC and analog lock-in amplifier (LIA) AC measurements of the electrical sensor response to toluene vapor, at the ppm level, has been conducted. When operated in the DC detection mode, the sensor exhibits a relatively high sensitivity to the analyte vapor, as well as a low detection limit at the 10 ppm level. However, at 10 ppm the signal-to-noise ratio is 5 dB, which is less than desirable. When operated in the analog LIA mode, the signal-to-noise ratio at 10 ppm increases by 30 dB and extends the detection limit to the ppb range.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The description of the zero-dimensional engineering-physical code GLOBSYS (Globus spherical tokamak system code), designed for parametric analysis of the next step of the program Globus-M, Globus-M2, ...is given. Within the framework of the zero-dimensional approximation, the definitions of the main scaling parameters of the plasma (poloidal beta, the fraction of bootstrap current, the energy lifetime of the plasma), as well as the specifics of calculating the inductance and resistance of the plasma in spherical tokamaks, are refined. The results of calculations of the plasma parameters by the code were compared with the experimental data of one of the Globus-M2 discharges (no. 38800) with neutral beam heating and showed good agreement. It is proposed to perform a comparison of calculations based on the code with the achieved and predicted parameters of the spherical tokamaks NSTX, NSTX-U, MAST, MAST-U, and ST40 in a separate paper. The goals of the next step (Globus-3) are formulated, the main ones of which are long pulse, high toroidal field, and powerful heating, which allow us to consider Globus-3 as a hydrogen prototype of a neutron source. The infrastructural restrictions on the Globus-3 parameters are given, which require further analysis of various versions of the electromagnetic system. Using the example of Globus-M2 discharge no. 38800, the effect of restrictions on the flow balance and heating of the elements of the electromagnetic system is shown.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The GLOBSYS code was developed for analysis and prediction of parameters of the Globus-M2 tokamak and its modifications. In 1, preliminary selection of correlations which connect physical and ...technical parameters was made. In this paper, the verification of the code using the achieved and predicted data from the installations NSTX, NSTX-U, MAST, MAST-U, and ST40 is given. As a whole, there is good agreement between simulations and plasma parameters at the discharge plateau. The best agreement is observed if ITER confinement scaling is used for energy confinement time with the enhancement factor
H
y
, 2
= 1–1.2. Simulations with other confinement scalings (Globus-2021, NSTX scalings) give good agreement with plasma parameters for the toroidal field
B
t0
~ 0.5 T. For increasing
B
t0
, more optimistic predicted plasma parameters are obtained for the Globus-2021 and NSTX scalings in comparison with the ITER confinement scaling. The condition of reaching the plasma quasistationary regime (or the time of establishment of quasistationary plasma profiles τ
L
/
R
) is estimated for NSTX, NSTX-U, MAST, MAST-U and ST40 discharges. This time is compared with two technical restrictions, which are connected with the times of toroidal field coil heating and poloidal flux capacity. Verification of the GLOBSYS code using the data from the aforementioned spherical tokamaks is the basis for the prediction of parameters of the next step of Globus-M program.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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