Atsushi Komuro

Numerical simulation for production of O and N radicals in an atmospheric-pressure streamer discharge

A streamer discharge model is developed to analyse the characteristics of a pulsed positive streamer discharge in point-to-plane electrodes filled with oxygen–nitrogen mixed gas at room temperature and atmospheric pressure. In this paper we study the mechanisms of O and N radical production in an atmospheric-pressure streamer discharge. To confirm the validity of the simulation model, the discharge emission of light and the discharge current are compared with experimental data at several voltages in gas mixtures with 2–20% oxygen concentrations. The calculated streak picture and the axial distribution of streamer luminous intensity are in good agreement with our previous experimental results. After demonstrating the reliability of the model, we performed a numerical study on radical production by the streamer discharge. The experimentally obtained axial distributions of oxygen radical production in O2(20%)/N2 and nitrogen radical production in O2(2%)/N2 are successfully reproduced in our simulation. For the production of nitrogen radicals, two-step dissociation through the vibrationally excited states is predominant.

Two-dimensional simulation of fast gas heating in an atmospheric pressure streamer discharge and humidity effects

Gas heating in an atmospheric-pressure streamer discharge was analysed by a two-dimensional streamer discharge simulation model describing internal molecular energy transfer. Our two-dimensional streamer simulation model incorporates concepts from the fast gas heating mechanism proposed by Popov (2011 J. Phys. D: Appl. Phys. 44 285201) and our self-developed state-to-state vibrational kinetics. In dry air, gas heating occurs mainly from electron-impact dissociation reactions of O2 molecules and from quenching processes of electronically excited N2(B 3Πg, C 3Πu) molecules and O(1D) atoms. In humid air, rapid vibration-to-translation transitions of H2O and the exothermicity of the OH formation reactions additionally increase the gas temperature. It is shown that gas heating during the discharge pulse increases with humidity.

Effect of source diameter on helicon plasma thruster performance and its high power operation

Thrust imparted by a helicon plasma thruster is experimentally measured for two different diameter source tubes. The measurements demonstrate that the thrust-over-power of the helicon thruster can be increased by enlarging the source diameter. This result is qualitatively understood with a simple analysis connecting a global model in the source and a one-dimensional magnetic nozzle model, where the model does not include the magnetic field effect in the source and the cross-field diffusion effect in the magnetic nozzle. A mass flow rate of propellant argon and a magnetic field strength are experimentally surveyed; then the thrust of ~18 mN is obtained for the rf power of 1 kW, the 95 mm diameter source, and the largest solenoid current being tested, while the maximum thrust for the 26 mm diameter source is only 5 mN. Furthermore the rf power is increased up to ~6 kW and a thrust close to 60 mN is obtained.

Radio frequency ion source operated with field effect transistor based radio frequency system

Characteristics of radio frequency (RF) plasma production are investigated using a field effect transistor inverter power supply as an RF wave source. With the frequency of around 0.3 MHz, an electron density over 10(18) m(-3) is produced in argon plasma. Although lower densities are obtained in hydrogen plasma, it drastically increased up to 5x10(18) m(-3) with an axial magnetic field of around 100 G applied in the driver region. Effects of the magnetic field and gas pressure are investigated in the RF produced plasma with the frequency of several hundred kilohertz.

Measurement of plasma momentum exerted on target by a small helicon plasma thruster and comparison with direct thrust measurement

Momentum, i.e., force, exerted from a small helicon plasma thruster to a target plate is measured simultaneously with a direct thrust measurement using a thrust balance. The calibration coefficient relating a target displacement to a steady-state force is obtained by supplying a dc to a calibration coil mounted on the target, where a force acting to a small permanent magnet located near the coil is directly measured by using a load cell. As the force exerted by the plasma flow to the target plate is in good agreement with the directly measured thrust, the validity of the target technique is demonstrated under the present operating conditions, where the thruster is operated in steady-state. Furthermore, a calibration coefficient relating a swing amplitude of the target to an impulse bit is also obtained by pulsing the calibration coil current. The force exerted by the pulsed plasma, which is estimated from the measured impulse bit and the pulse width, is also in good agreement with that obtained for the steady-state operation; hence, the thrust assessment of the helicon plasma thruster by the target is validated for both the steady-state and pulsed operations.

Numerical simulation for the production of chemically active species in primary and secondary streamers in atmospheric-pressure dry air

Production of chemically active species in primary and secondary streamers is investigated using a two-dimensional axisymmetric numerical simulation model. The production processes of N2(v = 1), O(3P) and N(4S), which each have different threshold energies, are simulated using experimentally obtained pulsed voltages with peak values, Vpeak, of 18, 24 and 30 kV in dry air at atmospheric pressure. As Vpeak increases, the simulated length of the secondary streamer increases, although there is little change in the primary streamer characteristics. This means that the ratio of the secondary streamer phase to the primary streamer phase increases for increasing Vpeak. The simulated results show that as Vpeak increases, the energy efficiency of O(3P) production increases and that of N2(v = 1) production decreases. On the other hand, the energy efficiency of N(4S) production has reduced dependence on Vpeak. These characteristics can be explained by the spatiotemporal variations of the reduced electric field in the primary and secondary streamer.

Experimental identification of an azimuthal current in a magnetic nozzle of a radiofrequency plasma thruster

The azimuthal plasma current in a magnetic nozzle of a radiofrequency plasma thruster is experimentally identified by measuring the plasma-induced magnetic field. The axial plasma momentum increases over about 20 cm downstream of the thruster exit due to the Lorentz force arising from the azimuthal current. The measured current shows that the azimuthal current is given by the sum of the electron diamagnetic drift and drift currents, where the latter component decreases with an increase in the magnetic field strength; hence the azimuthal current approaches the electron diamagnetic drift one for the strong magnetic field. The Lorentz force calculated from the measured azimuthal plasma current and the radial magnetic field is smaller than the directly measured force exerted to the magnetic field, which indicates the existence of a non-negligible Lorentz force in the source tube.

Standing Helicon Wave Induced by a Rapidly Bent Magnetic Field in Plasmas.

An electron energy probability function and a rf magnetic field are measured in a rf hydrogen helicon source, where axial and transverse static magnetic fields are applied to the source by solenoids and to the diffusion chamber by filter magnets, respectively. It is demonstrated that the helicon wave is reflected by the rapidly bent magnetic field and the resultant standing wave heats the electrons between the source and the magnetic filter, while the electron cooling effect by the magnetic filter is maintained. It is interpreted that the standing wave is generated by the presence of a spatially localized change of a refractive index.

Low-pressure, high-density, and supersonic plasma flow generated by a helicon magnetoplasmadynamic thruster

A high density magnetoplasmadynamic (MPD) plasma under a magnetic nozzle is produced with a low gas flow rate of argon by combining helicon and MPD plasma sources, where a cathode and an anode are located upstream and downstream of the helicon source, respectively. Once the high density helicon plasma is produced in the source tube, a pulsed current of a few kA is triggered between the cathode and anode. A plasma density above 1020 m−3 and a supersonic plasma flow (Mach number of ∼1.8) are obtained at ∼10 cm downstream of the source exit. As the thrust efficiency estimated from the measured plasma parameters is much higher than that of the simple MPD thruster, the helicon MPD thruster being proposed and tested potentially provides more efficient high-power plasma thruster.

Operating a magnetic nozzle helicon thruster with strong magnetic field

A pulsed axial magnetic field up to ∼2.8 kG is applied to a 26-mm-inner-diameter helicon plasma thruster immersed in a vacuum chamber, and the thrust is measured using a pendulum target. The pendulum is located 30-cm-downstream of the thruster, and the thruster rf power and argon flow rate are fixed at 1 kW and 70 sccm (which gives a chamber pressure of 0.7 mTorr). The imparted thrust increases as the applied magnetic field is increased and saturates at a maximum value of ∼9.5 mN for magnetic field above ∼2 kG. At the maximum magnetic field, it is demonstrated that the normalized plasma density, and the ion flow energy in the magnetic nozzle, agree within ∼50% and of 10%, respectively, with a one-dimensional model that ignores radial losses from the nozzle. This magnetic nozzle model is combined with a simple global model of the thruster source that incorporates an artificially controlled factor α, to account for radial plasma losses to the walls, where α = 0 and 1 correspond to zero losses and no magnetic field, respectively. Comparison between the experiments and the model implies that the radial losses in the thruster source are experimentally reduced by the applied magnetic field to about 10% of that obtained from the no magnetic field model.