A novel design of a continuous toroidal solenoid for the small spherical tokamak MEPhIST-0 is presented. The solenoid's geometry is optimized in regard to stray magnetic fields perpendicular to the toroidal fields based on Maxwell's equations and interface conditions for magnetic fields. The solutions obtained may in theory provide the level of stray fields similar to 10(-10) of the toroidal fields. The design of the solenoid is tested by means of COMSOL modeling. The observed ratio of stray fields to the toroidal component of magnetic fields, B-p/B-T is approximately similar to 0.4% at the plasma center. A ripple of the toroidal magnetic field at the edge of the plasma of delta similar to 0.8% is observed. The possibility of plasma breakdown with the chosen toroidal solenoid configuration was evaluated using the DINA calculation code.

In this work, the profiles of craters obtained by the irradiation of a picosecond laser with a wavelength of 1064 nm and energy density in the range of 0.4–7.0 J/cm2 of tungsten films were measured using energy-dispersive X-ray spectroscopy, as well as a contact profilometry. The presence of several mechanisms of surface erosion at given irradiation parameters is shown. The influence of the craters shape on the signal of laser-assisted quadrupole mass spectrometry during irradiation of deuterium-containing tungsten films is analyzed.

In the MEPhIST-0 tokamak, toroidal magnetic coil system is implemented as a single continuous toroidal solenoid. This design ensures synchronized operation of each segment of the solenoid and requires only one power supply line to operate. However, this configuration has the downside of introducing stray vertical and radial magnetic field components, which result from the inclination of the solenoid segments. Such stray fields can be measured using an electron beam, as previously demonstrated in experiments on T-15, TEXTOR, KSTAR, and SST-1. This paper describes the parameters of the experiment and presents the main results obtained from the MEPhIST-0 tokamak. Two experimental techniques for measuring the vertical magnetic field are discussed: one with a change in the current direction in the toroidal solenoid and one without. The ratio of the stray vertical field to the toroidal field was found to be approximately 0.5%. The conducted experiments highlight the necessity of compensating for the stray vertical field using additional poloidal coils.

The paper compares the laser-assisted quadrupole mass spectrometry for two laser pulses (15 ns and 80 ps) for W-D films and Ti–V alloy with deuterium. A technique for the production of samples with high concentrations of trapped deuterium, which makes it possible to obtain a good signal repeatability on a quadrupole mass spectrometer for several laser shots, is presented, which can be used to align a diagnostic laser system. The experimental dependence of the total deuterium yield from the studied materials on the energy density is obtained. The high sensitivity of the technique, as well as a weak dependence on the laser pulse duration, is demonstrated. A significant change in the deuterium concentration in the near-surface layer of the tungsten film during contact with the atmosphere is noted.

The article is devoted to the calculation of equilibrium configurations of the MEPHIST-0 tokamak plasma with a poloidal magnetic system taking into account the part responsible for compensating the magnetic field of the central solenoid using the DINA plasma physics code. The parameters of the poloidal magnetic system necessary for the formation of initial equilibrium after the discharge breakdown are obtained. The results of calibration experiments without taking into account the vacuum chamber using a pulsed current source are presented, on the basis of which the dynamic problem of plasma equilibrium calculation is formulated. The simulation results are compared with the experimental data, and the influence of the chamber on the results is considered.

Laser induced background spectroscopy (LIBS) is a very useful instrument for material surface characterization. Its usage for plasma facing tokamak components is envisaged in future machines. The calibration-free LIBS is also actively tested on existing tokamaks including Li covered surfaces. However, application of the most frequently used calibration-free Saha-Boltzmann technique for interpretation of LIBS results assumes local thermodynamic equilibrium (LTE) and plasma transparency for observed spectral lines. To investigate validity of this approach for Li plasma, we developed a hybrid model using 3DLINE, FEOS and THERMOS codes describing Li plasma formation, expansion, and radiation from the plasma plume. Using the model we reproduced quite well experimental measurements of Li lines radiation dynamics. Calculations show that transparency conditions can be very easily violated for Li LIBS plasmas, which absorbs up to 97% of 671 nm (Li I) line radiation. Other two easily observed in experiments lines, 548 nm (Li II) and 610 nm (Li I), remain relatively less affected by radiation absorption. We also notice that sharper focusing of the laser beam onto the surface makes the plasma more transparent. We believe that these conclusions are important to take into account while using LIBS for characterization of Li coated surfaces.

The technique and results of measuring the soft X-ray spectra of plasma on the small spherical MIFIST-0 tokamak are presented. The measurements were carried out by the gray filter method using a multichannel spectrometer based on LiF (Mg, Ti) thermoluminescent lithium fluoride detectors. This technique made it possible to study X-ray radiation in the energy range of quanta 0.2−15 keV. The spectrum of the soft X-ray component was obtained.

Microwave preionization is a common technique used for electron-cyclotron resonance assisted plasma start-up in spherical tokamaks. The purpose of this work is to test the developed MEPhIST-0 tokamak preionization system and to study the preliminary preionization plasma. The gas discharge parameters are measured using Rogowski coils, optical spectroscopy, and Langmuir probes. A fast CCD camera is additionally used to capture the emission during the discharge. The results show that the preliminary plasma discharge is localized at a certain distance inside the vacuum chamber, which satisfies the condition for the existence of electron cyclotron resonance. The estimated plasma density and electron temperature are 5.5 × 1016 m–3 and 8 eV, respectively.

A system for creating a toroidal magnetic field in the MEPHIST tokamak is described. The system is a single toroidal solenoid consisting of 12 copper loops made from a copper strip of variable width. The shape of the poloidal cross section is selected based on the zero torque condition. The distribution of the angle of inclination of the loops in the toroidal direction along their length was selected empirically by means of numerical modeling designed to minimize the scattered fields. The expected corrugation of the magnetic field at the large radius of the tokamak is ~0.6%. The inductance of the solenoid and the current supply lines is ~ 37 μH, the ohmic resistance is ~12 mΩ. When the toroidal field was raised, the loop voltage of ~ 3 V was observed.

In this paper, electrode sputtering of the miniature linear accelerator (MLA)is investigated. The main part of this study is numerical simulations of the deposited metal layers formation on the insulator. It consist of three parts: ion beam trajectory analysis in MLA using COMSOL Multiphysics, simulations of surface sputtering by BCA codes (SRIM, SDTrimSP, and SCATTER) and sputtered particles trajectory analysis in MLA using COMSOL Multiphysics. The main sources of deposited particles are identified using numerical simulations of the surface sputtering by the ions extracted from the Penning ion source (PIS) and charge-exchange neutrals originated inside the ion-optical system (IOS) and subsequent analysis of the sputtered particle trajectories. Experimental data were obtained by digitizing MLA photos during operation by ImageJ software. Simulations of the electrodes sputtering of MLA were performed. The simulations allowed to determine the erosion zones responsible for the deposited metal layer formation on the insulator surface. The experimental data are in good qualitative agreement with the simulated ones.

В данной работе приводятся расчет движения ионного пучка в ионно-оптической системе (ИОС) миниатюрного линейного ускорителя (МЛУ). Результаты моделирования используются для оценки распыления электродов и напыления их материала на высоковольтный изолятор ИОС. Приведены оценки скорости напыления, преимущественной области запыления изолятора и их сравнение с экспериментальными данными.

The sensitivities of laser-induced breakdown spectroscopy and quadrupole mass spectrometry are compared to determine the deuterium retained in titanium films using a nanosecond Nd:YAG laser. Simultaneous measurements are made in the 1–18 J cm −2 range of energy densities. Considerably higher sensitivity of mass spectrometry is revealed, along with the possibility of non-destructive analysis at low energy densities.

The radiation dynamics of an adiabatic self-similar expanding spherical plasma plume is considered. It is shown that the radiation intensities evolution strongly depends on the adiabatic index γ. For example, it is proven that the radiation intensity in a highly ionized plasma plume increases over time if γ = 5/3 and has a minimum in time if γ < 5/3. The analytical results are compared to measurements of the radiation from lithium laser-produced plasma. A lithium sample was irradiated by a powerful (42 J/cm2) laser pulse, and radiation of produced lithium plasma was registered by a monochromator. Reasonable agreement with the experiments is reached for γ = 5/4.

Controlling the thickness of thin layers of lithium when they are deposited on plasma-facing elements is an important task in the field of plasma-surface interaction in fusion devices. It is shown the keV‑energy proton scattering spectroscopy can be used for this purpose. The sensitivity of technique to chemical reactions on a surface is demonstrated, and estimates of the charge state of hydrogen ions reflected from a lithium surface are given.