Maxim Ignatenko

Absorption of Microwave Energy by a Spherical Nonmagnetic Metal Particle

A single particle model is used to provide simple physical insight into the absorption of microwave energy by nonmagnetic metal powders. For this, Mie theory is applied to the microwave heating of a highly conductive spherical particle of micron size. The frequency of the electromagnetic wave is 2.45 GHz. Energy absorption by the highly conductive particle arises from Joule heating by eddy currents which occur as a result of the magnetic component of the electromagnetic wave. Since the electric field component is screened out by conduction electrons, the particle ineffectively couples with this component. The necessary condition for complete separation of dielectric and magnetic losses is provided. Scaling of the induced current with particle size is considered. The region, where the single particle model is valid, is outlined.

Numerical study of neutral gas transport in linear plasma device

Possible methods of controlling the neutral gas pressure in a linear plasma device are numerically investigated. For this purpose, the neutral gas transport in argon plasma is calculated by means of Monte Carlo method. The code takes into account the self-elastic collisions along with the elastic neutral–ion and the electron impact ionization collisions. The effects of a baffle plate, the electron temperature, the pump speed, and plasma flow on the neutral density are evaluated within a range of experimental conditions. The baffle plate is found to affect the amount of neutral gas when the electron temperature is low and the plate is located close to a neutral source. As the electron temperature increases, electron ionization strongly enhances the recycling of neutral gas, thus making the role of the baffle plate weaker. It is clarified that the electron ionization is a key factor of the neutral transport in a linear plasma device with low density and low electron temperature.

Reduction effect of neutral density on the excitation of turbulent drift waves in a linear magnetized plasma with flow

The importance of reducing the neutral density to reach strong drift wave turbulence is clarified from the results of the extended magnetohydrodynamics and Monte Carlo simulations in a linear magnetized plasma. An upper bound of the neutral density relating to the ion-neutral collision frequency for the excitation of drift wave instability is shown, and the necessary flow velocity to excite this instability is also estimated from the neutral distributions. Measurements of the Mach number and the electron density distributions using Mach probe in the large mirror device (LMD) of Kyushu University [S. Shinohara et al., Plasma Phys. Control. Fusion 37, 1015 (1995)] are reported as well. The obtained results show a controllability of the neutral density and provide the basis for neutral density reduction and a possibility to excite strong drift wave turbulence in the LMD.

Numerical study of microwave imaging reflectometry for measurements of density fluctuations in a tandem mirror plasma

In this paper, numerical experiment is used to study the imaging properties of microwave imaging reflectometry for the case of tandem mirror device geometry. First of all, the alignment of the experimental setup is performed and then the imaging system is applied for density fluctuations measurements. Fluctuations employed in this study have a nonshifted Gaussian wavenumber spectrum with equal poloidal and radial widths. The size of the optics is shown to be a main parameter limiting the performance of the imaging system. Space-imaging and time-imaging modes are considered. In the latter case, for model conditions imaging and conventional (without optics) reflectometers demonstrate comparable performance in general. When root-mean-squared amplitude of the density fluctuations is large (σ n = 0.06 and 0.09) the imaging system shows a wider range of measurable parameters.

Effects of asymmetry and target location on microwave imaging reflectometry

In this article we perform a numerical study of microwave imaging reflectometry (MIR) and compare it with conventional reflectometry system. As an approximation to the reflections by real plasma fluctuations, a corrugated wheel is used. As far as general performance is concerned, our simulations confirm the results by Munsat et al. [Plasma Phys. Controlled Fusion 45, 469 (2003)] that the MIR system reproduces shape of corrugation far from the wheel while conventional systems fail to do so. We addressed the effects of asymmetry and defocusing of the wheel-reflectometer system as well as spectral sensitivity of the imaging reflectometer. For a particular geometry we estimated the deterioration of the MIR performance due to misalignments and existence of broadband fluctuations.

Ultrashort-pulse reflectometer on LHD

We have applied an ultrashort-pulse reflectometer to large helical device plasmas for density profile measurement. The frequency range of the impulse is up-converted into the R band, which corresponds to the cutoff frequency of edge plasma region in the ordinary mode. The reflected wave from plasma is received and directly recorded by a sampling scope. This system can be controlled and monitored from remote site (Kyushu University) by using ultrawideband science information network (super-SINET) that ideal bandwidth reaches up to 1 Gbps. As a result of the measurement, we have confirmed reflection wave from the plasma when the plasma density reaches steady state during the measurement.

Development of new detector for millimeter wave imaging

We have been developing a detector element which includes dipole antennas and a doubly balanced mixer fabricated on a dielectric substrate. This detector is designed for millimeter-wave imaging system on fusion device experiment in order to measure electron temperature distribution. Prototype detector which works in X-band shows ideal characteristics such as axisymmetric field pattern and detection sensitivity as an imaging element. Recently, we have fabricated same type detectors which work in E-band range. We have confirmed that the detectors have sensitivity in this frequency range.

Development of Quasi-Optical Technology for Microwave Diagnostics in LHD

This paper presents development of quasioptical microwave technology for ECE and reflectometry diagnostics in the Large Helical Device (LHD). The ECE for the electron temperature measurement is transmitted with the corrugated wave guide system. Technology for Microwave imaging, such as imaging reflectometry and ECE imaging, has been developed to visualize 3D view of turbulence

Application of ultrashort-pulse reflectometry to Large Helical Device

Summary form only given, as follows. The ultrashort-pulse reflectometer (USRM) system is being prepared to apply to Large Helical Device (LHD). At the present time, we preliminary applied this USRM system to a metal plate as a target, and confirmed that detection time of the reflected pulse is delayed proportionally as increasing in double path length. Recently, we are constructing the USRM system at National Institute for Fusion Science (NIFS). The experiment using this USRM system will be performed since February this year. The USRM system for LHD is as follows. The system utilize an impulse generator as a source, which can transmit an impulse with a pulse width of 22 ps and a voltage amplitude of 3 V. The impulse is fed to an active frequency doubler via a WD-750 waveguide and a low loss coaxial cable to produce higher frequency component suitable for high density plasma such as LHD plasma. The chirped millimeter wave is amplified by a wide band power amplifier and transmitted by a conical horn in ordinary (O) mode. We utilize optical lenses in front of the transmitter and the receiver. The reflected wave is received by the receiver horn and amplified by low-noise amplifiers. The signal is recorded directly by a high-speed sampling scope (50 GSample/s). The repetition rate of impulse transmission is fixed at 1GHz, and data acquisition is completed for 10 ms.