The Globus-M2 spherical tokamak is the considerably upgraded Globus-M facility. Its technical parameters were increased as much as possible to achieve the promising range of physical parameters ...(sub-fusion temperatures and collisionality of much less than unity). These parameters will be achieved in a compact magnetic configuration similar to that of the Globus-M tokamak, the plasma current and toroidal magnetic field amounting to 0.5 MA and 1 T, respectively. The demand to increase the magnetic field and plasma current in the Globus-M2 resulted in the need for a complete redesign of the electromagnetic system because the plasma equilibrium requirements have changed and the mechanical and thermal loads have considerably increased as compared to the Globus-M. The vacuum vessel and the in-vessel components of the new Globus-M2 tokamak remain the same. Power supplies were upgraded to provide the required currents in the toroidal field coil and the central solenoid. The Globus-M2 tokamak was build up and preliminary tests were carried out. New auxiliary heating systems and diagnostics were developed and installed to be used in future experiments. Fist plasma was achieved at the Globus-M2 in April 2018.
We present the results of studies of the plasma source based on the coaxial accelerator with the slothole channel geometry for plasma acceleration and working gas inlet into the accelerator via the ...electrodynamic valve. The plasma parameters at the output of the accelerator are measured. The slot-hole channel of the accelerator created higher jet pressure, as compared to the coaxial channel, especially at large distances from the source. The jet pressure reached 10
6
N/m
2
at a distance of 0.7 m. The source created moderately pure plasma for a current below 80 kA. The density was (2.5–5) × 10
22
m
–3
, which was higher than the density obtained with the coaxial gun.
Neutron diagnostic system at the Globus-M2 tokamak Iliasova, M.V.; Shevelev, A.E.; Khilkevitch, E.M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2022, Volume:
1029
Journal Article
Peer reviewed
A neutron diagnostic system was developed at the Ioffe Institute as part of the Globus-M2 tokamak to optimize NBI heating conditions and evaluate heating efficiency. The system contains two compact ...neutron spectrometers based on the liquid organic scintillator BC-501A and two gas-discharge counters based on a 10B isotope. The BC-501A spectrometers were calibrated by measuring neutron emission produced in a 9Be(α,n)12C nuclear reaction on the cyclotron facility at the Ioffe Institute. In addition, in situ calibrations of the system, including the neutron spectrometers and the gas-discharge counters, was carried out using an Am–Be neutron source to provide accurate measurements of the total neutron yield from the plasma of the Globus-M2 tokamak. During the plasma experiments at the Globus-M2 tokamak, a deuterium beam was injected into the deuterium plasma that causes a yield of the DD-neutrons with ∼2.45 MeV energy. The neutron spectrometry diagnostic system was used to provide neutron measurements and detect the DD-neutrons in these experiments. The neutron yield and the DD-reaction rate during plasma discharges were evaluated. The energy distributions of neutrons emitted from plasma during discharges with neutron beam injection were reconstructed from the measured neutron spectra.
Studies were performed on the possibility of using the coaxial plasma jet accelerator to facilitate breakdown and the plasma current start-up in the Globus-M2 and KTM tokamaks. The results of ...experiments on the jet injection into the Globus-M2 tokamak with a magnetic field of 0.7 T showed that the discharge current rise began 1–2 ms earlier than it did in experiments using gas puffing and inductive breakdown of the working gas. In this case, the breakdown voltage decreased approximately 2 times. The results of experiments on plasma jet injection through the equatorial diagnostic duct into the KTM tokamak with a magnetic field of 0.9 T showed that the gas breakdown occurred up to 6 ms earlier than the breakdown initiated using the inductive method. In this case, the loop voltage decreases approximately 1.5 times. In almost all shots using the accelerator, the gas breakdown occurred, while in its absence, sometimes the gas breakdown could not be obtained.
We report on experimental results on multiple action of hydrogen, deuterium, and helium plasmas produced by a plasma gun and the Globus-M tokamak on tungsten. The surface temperature in the course of ...irradiation is measured with a bichromatic pyrometer with a time resolution of ⩾1 μs. The morphology of the surface layer is investigated and X-ray structure analysis of tungsten exposed to multiple radiations by the plasma under various conditions is carried out. A slight decrease in the lattice parameter in the sample subjected to the maximal number of irradiation cycles is detected. It is shown that the morphology of the tungsten surface irradiated by the hydrogen plasma from the gun and by the deuterium plasma from the Globus-M tokamak changes (the structure becomes smoother). The characteristic depth of the layer in which impurities have been accumulated exceeds 0.5 μm. This depth was the largest for the sample exposed to 1000 shots from the gun and 2370 shots from the tokamak. It is shown that the helium jet from the plasma gun makes it possible to simulate the action of helium ions on the International Thermonuclear Experimental Reactor (ITER) diverter, producing a layer of submicrometer particles (bubbles).
We have studied the cyclic action of a plasma gun jet, which modeled nonstationary thermal load related to the edge localized modes (ELMs), on the surface of tungsten elements intended for the ITER ...divertor. The results revealed significant changes in both the structure and morphology of a surface layer. After 100 irradiation cycles, a columnar or “ridged” structure of elements perpendicular to the initial irradiated surface is formed. Exposure to 1000 cycles changes the mechanism of material degradation and radically modifies the character of the surface topography, since the ridges are melted with the formation of droplet structures. The surface becomes rough, a loose material layer appears under the irradiated surface, and a 100-μm-thick recrystallized layer is formed.
Experimental results on the generation and maintenance of the toroidal current in the Globus-M spherical tokamak by using waves in the lower hybrid frequency range without applying an inductive ...vortex electric field are presented. For this purpose, the original ridge guide antennas forming a field distribution similar to that produced by multiwaveguide grills were used. The high-frequency field (900 MHz) was used for both plasma generation and current drive. The magnitude of the generated current reached 21 kA, and its direction depended on the direction of the vertical magnetic field. Analysis of the experimental results indicates that the major fraction of the current is carried by the suprathermal electron beam.
Experiments with the limiting density of a plasma generated in the Globus-M spherical tokamak operating in the ohmic heating regime show that, when the plasma density reaches a certain value at the ...center of the plasma filament, snake-type strong magnetohydrodynamic instability exciting the
m/n
= 1/1 mode arises. A further rise in the density causes an internal disruption of the plasma. Numerical simulation of the instability dynamics is performed, and the results of simulation are compared with experimental and theoretical data. Experimental and theoretical data are in good agreement. It is shown that the instability is associated with an increased amount of impurities in the plasma and that the limit in density is of a “technological,” rather than of a fundamental, character. With the vacuum chamber cleaned more thoroughly, this limit disappears and the snake instability is not observed. Accordingly, a plasma filament density of 1 × 10
20
m−3
was attained with a Greenwald number as high as 95%.
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