Research on plasma electron density distribution based on microwave diffraction

In this paper, a non-contact plasma microwave diffraction measurement method is proposed, which can obtain the electron density at different diameters of the cylindrical plasma. There is a lot of diffraction when a non-focused antenna is used to transmit plasma. As we all know, when the frequency of the incident microwave is lower than the characteristic frequency of the plasma, the microwave cannot be transmitted through the plasma, so this interface can be regarded as a metal. According to the microwave diffraction of the plasma, the size of the plasma correspond-ing to the characteristic frequency can be obtained. Furthermore, by sweeping the incident elec-tromagnetic wave, the size of plasma with different characteristic frequencies can be obtained, and the distribution of electron density can be obtained. To verify the method, a cylindrical plasma was measured by microwave diffraction, in which the electron density of the plasma column gradually decreased along the increase in radius. According to the diffraction of the plasma column at different frequencies, the distribution of the electron density along the diame-ter is obtained. And compared with the transmission diagnosis method, the validity and accuracy of this method are verified. In non-uniform high-temperature plasma, the diffraction method greatly improves the accuracy of spatial diagnosis compared with traditional transmission diag-nosis.

Study of electric currents excitation in the plasma of the L-2M stellarator with its electronic cyclotronic creation and heating

The results of measuring the longitudinal electric current excited in the toroidal plasma of the L-2M stellarator as a result of powerful pulsed microwave heating (power up to 600 kW, pulse duration up to 20 ms) are presented. In the experi-ments, to create and heat plasma in the stellarator, microwave radiation of gyro-trons with a frequency of 75 GHz, equal to the frequency of the 2nd harmonic of electron cyclotron resonance for a magnetic field with induction B = 1.34 T at the center of the plasma column, was used. To measure the currents in the plasma, di-agnostic systems of the stellarator were used, designed to record changes in time of the transverse and poloidal magnetic fields. It is shown that the presence of an ohmic heating iron transformer in the stellarator design significantly affects the temporal development of equilibrium currents due to the significant inductance of the toroidal plasma. When compensating the inductance of these devices, the ex-pected value of the current excited in the plasma can reach a value of about 7 kA.

Radial distribution of the plasma potential in a cylindrical plasma column with a longitudinal magnetic field

The possibility of controlling the electrostatic field distribution in plasma has yielded wide prospects for modern technologies. As a magnetic field primarily allows for creating electric fields in plasma, it serves as an additional obstacle for the current flow through a medium. In the present paper, an axially symmetric system is considered in which the magnetic field is directed along the axis and concentric electrodes are located at the ends. The electrodes are negatively biased. A model which solves the problem of the radial distribution of the plasma potential inside the cylindrical plasma column supported by the end electrodes is proposed. The most commonly encountered configurations of the electrical connection for the end electrodes are considered, and the particular solutions to the problem of the radial distribution are presented. The contribution of ions and electrons to the transverse conductivity is evaluated in detail. The influence of a thermionic element on the radial profile of the plasma potential is considered. To verify the proposed model, an experimental study of the reflex discharge is carried out with both cold electrodes and a thermionic element on the axis. A comparison of the computational model results with experimental data is given. The presented model makes it possible to solve the problem concerning the plasma potential distribution in the case of an arbitrary number of end electrodes, and also to take into account the inhomogeneity of the distribution of plasma density, neutral gas pressure and electron temperature along the radius.

Rotation and shear control of a weakly magnetized plasma column using current injection by emissive electrodes

The evolution of the radial profile of the rotation of a weakly magnetized plasma column is investigated experimentally in a radio-frequency argon plasma at low pressure when a strong electron current is emitted by large emissive cathodes. Current injection from large emissive cathodes over a background plasma column (with a plasma density of a few $10^{18}\ \textrm {m}^{-3}$ ) is characterized. Radial scans of the ion velocity show that a continuous control of the rotation profile may be obtained using two spatial configurations for the locations of the emissive cathodes (either in the centre or at the edge of the plasma column). The rotation profile results from the electric drift velocity, damped by the drag exerted on the ions. We demonstrate that the evolution of the rotation profile with the injected current is then controlled by the modification of the plasma potential profile in the presence of strongly emissive cathodes.