Kiyoyuki Yambe

Normal Modes and Slow-Wave Instabilities in Oversized Coaxial Slow-Wave Structure With Rectangular Corrugations

Normal modes and slow-wave instabilities in an oversized coaxial slow-wave structure (SWS) are numerically analyzed for the case of a weakly relativistic annular beam. The coaxial SWS has outer and inner rectangular corrugations with K-band dimensions. The inner corrugations have a cylindrical surface wave, which is the fundamental mode and exists even if the outer conductor is removed. The outer corrugations generate higher-order waveguide modes. Both transverse and longitudinal beam perturbations are considered in the beam interactions, leading to slow cyclotron instabilities as well as Cherenkov instabilities. The instabilities of axisymmetric and nonaxisymmetric modes have almost the same growth rates. The relative position between the outer and inner corrugations affects the normal modes and may allow wide frequency tunability.

Influence of flowing helium gas on plasma plume formation in atmospheric pressure plasma

We have studied atmospheric pressure plasma generated using a quartz tube, helium gas, and a foil electrode by applying RF high voltage. The atmospheric pressure plasma in the form of a bullet is released as a plume into the atmosphere. The helium gas flowing out of quartz tube mixes with air, and the flow channel is composed of the regions of flowing helium gas and air. The plasma plume length is equivalent to the reachable distance of flowing helium gas. Although the amount of helium gas on the flow channel increases by increasing the inner diameter of quartz tube at the same gas flow velocity, the plasma plume length peaks at around 8 m/s of gas flow velocity, which is the result that a flow of helium gas is balanced with the amount of gas. The plasma plume is formed at the boundary region where the flow of helium gas is kept to the wall of the air.

Relation between plasma plume density and gas flow velocity in atmospheric pressure plasma

We have studied atmospheric pressure plasma generated using a quartz tube, helium gas, and copper foil electrode by applying RF high voltage. The atmospheric pressure plasma in the form of a bullet is released as a plume into the atmosphere. To study the properties of the plasma plume, the plasma plume current is estimated from the difference in currents on the circuit, and the drift velocity is measured using a photodetector. The relation of the plasma plume density nplu, which is estimated from the current and the drift velocity, and the gas flow velocity vgas is examined. It is found that the dependence of the density on the gas flow velocity has relations of nplu ∝ log(vgas). However, the plasma plume density in the laminar flow is higher than that in the turbulent flow. Consequently, in the laminar flow, the density increases with increasing the gas flow velocity.We have studied atmospheric pressure plasma generated using a quartz tube, helium gas, and copper foil electrode by applying RF high voltage. The atmospheric pressure plasma in the form of a bullet is released as a plume into the atmosphere. To study the properties of the plasma plume, the plasma plume current is estimated from the difference in currents on the circuit, and the drift velocity is measured using a photodetector. The relation of the plasma plume density nplu, which is estimated from the current and the drift velocity, and the gas flow velocity vgas is examined. It is found that the dependence of the density on the gas flow velocity has relations of nplu ∝ log(vgas). However, the plasma plume density in the laminar flow is higher than that in the turbulent flow. Consequently, in the laminar flow, the density increases with increasing the gas flow velocity.

Experimental Study on Generation of Electron Beam Utilizing Cold Cathode in the Weakly Relativistic Energy Region

Electron beam generation in a weakly relativistic region of less than 100 kV is investigated utilizing rectangular and disk cold cathodes made of copper. Electron emission areas are located on the edges of the cathodes. For uniform electron emission, some surface roughness is required on the emission surface, which is obtained by scratching the surface with sandpaper. A uniform sheet beam with a size of about 2 × 40 mm and an annular beam with a diameter of about 25 mm and a thickness of about 2 mm are demonstrated in the weakly relativistic region. Stable electron beams of sheet and annular shapes are experimentally demonstrated.

Experimental Study on Favorable Properties of Compound RF Discharge Plasmas with a Tapered Shape Hollow Cathode Compared with a Plane Cathode

An experimental investigation on the characteristics of compound RF discharge plasmas with a tapered shape hollow-cathode (HC) compared with a plane cathode is presented for the development of processing plasma. Dense plasmas are shown to be generated in the wide range of working pressures, 3< p<90 Pa. It is clarified that the higher RF power yields the greater HC effect to increase the ratio of electron density with the HC to that without it. The HC effect is shown to work well for decreasing the self-bias potential compared with the plane cathode. It is clarified that the dense plasma with relatively low electron temperature produced by the compound RF discharge results from the synergetic effect of the lowered input RF power density and the effective increased discharge area due to the HC effect.

Influence of Gas Flow on Plasma Length in Atmospheric Pressure Plasma Jet

Recently, plasma techniques under atmospheric pressure circumstances have been adapted for industrial, medical and biological applications. Atmospheric pressure plasmas, such as a plasma jet are typically produced using an arc discharge and a dielectric barrier discharge (DBD). Atmospheric pressure plasma of DBD is intermittently generated using a quartz tube as a dielectric, rare gas and metal electrode by applying radio frequency and high voltage [1]. A plasma jet is released to the atmosphere and is a small bullet-like volume of plasma traveling at unusually high velocities. The length of plasma traveling influences a gas flow [2]. The gas flow is summarized in a laminar, transition and turbulent modes. In this work, we examine the dependence of length of plasma traveling on the gas flow rate. The dependence of plasma length on gas flow rate and the pictures of plasma shape in each mode are shown in Fig. 1. In this configuration, the laminar mode is the region of less than 6 slm and the turbulent mode is the region of more than 10 slm. It is found that the tip of plasma jet in three cases is disturbed. Therefore, the Reynolds number at the tip of plasma jet increases due to the decrease of helium ratio on the cross-section.

Investigation of helium plasma temperature in atmospheric-pressure plasma plume using line pair method

We have studied the atmospheric pressure plasma generated using a quartz tube, helium gas, and foil electrode by applying AC high voltage. The plasma plume is released into the atmosphere from inside the quartz tube. The helium plasma temperature is investigated using a line pair method. The excitation temperature of the electrons in the helium plasma is estimated at about 700 K along the plasma plume (column). The excitation temperature of the electrons decreases slightly towards the tip of the plasma plume. Because the emitted light of the helium ion (He I) excitation at a wavelength of 706 nm is weak at the tip of the plasma plume, the electron excitation temperature is overestimated. In the atmosphere region near the tip of the plasma plume, collisions between helium gas and air increase. Therefore, the electron excitation temperature determined using the observed emitted light from helium ion excitation inside of the quartz tube filled with a helium gas.

Dependence of Plasma Plume Formation on Applied Voltage Waveform in Atmospheric-Pressure Plasma

We have studied an atmospheric-pressure plasma generated using a quartz tube, helium gas, and a copper foil electrode by the application of a radio-frequency high voltage. The atmospheric-pressure plasma, which is generated in the form of a bullet, is released as a plume into the atmosphere. To study the properties of plasma plume formation, we examine the relationships among the plasma plume length and charge and the applied voltage waveform. The dependences of the waveform on the plasma plume length and charge are not clear. However, the plasma plume length increases with increasing plasma plume charge in laminar flow. Consequently, we find that the plasma plume length depends on the plasma plume charge becoming an energy source so that electric charge travels the plasma.

Investigation of turbulence in reversed field pinch plasma by using microwave imaging reflectometry

Turbulence in the reversed field pinch (RFP) plasma has been investigated by using the microwave imaging reflectometry in the toroidal pinch experiment RX (TPE-RX). In conventional RFP plasma, the fluctuations are dominated by the intermittent blob-like structures. These structures are accompanied with the generation of magnetic field, the strong turbulence, and high nonlinear coupling among the high and low k modes. The pulsed poloidal current drive operation, which improves the plasma confinement significantly, suppresses the dynamo, the turbulence, and the blob-like structures.

Measurement of Fast Magnetic Fluctuations in Edge Region of TPE-RX Reversed-Field Pinch Plasma

Fast magnetic fluctuation (δB) levels are measured using a newly developed complex edge probe (CEP) in the edge region of TPE-RX reversed-field pinch plasma. The CEP is installed inside a vacuum vessel and is sensitive to fast δB. The δB levels measured using the CEP are compared with those measured using an extensive magnetic measurement system (MMS), which is located outside the vacuum vessel and has little sensitivity to fast δB. The δB levels before and after the appearance of the locked mode (LM) are compared in both the CEP and MMS signals. It was found that the rapid increase in the δB signal level obtained using the MMS just after the appearance of LM is mainly caused by the slowing of plasma rotation.