Takao Tanikawa

Development of high-density helicon plasma sources and their applications

We report on the development of unique, high-density helicon plasma sources and describe their applications. Characterization of one of the largest helicon plasma sources yet constructed is made. Scalings of the particle production efficiency are derived from various plasma production devices in open literature and our own data from long and short cylinder devices, i.e., high and low values of the aspect ratio A (the ratio of the axial length to the diameter), considering the power balance in the framework of a simple diffusion model. A high plasma production efficiency is demonstrated, and we clarify the structures of the excited waves in the low A region down to 0.075 (the large device diameter of 73.8 cm with the axial length as short as 5.5 cm). We describe the application to plasma propulsion using a new concept that employs no electrodes. A very small diameter (2.5 cm) helicon plasma with 1013 cm−3 density is produced, and the preliminary results of electromagnetic plasma acceleration are briefly de...

Development of very large helicon plasma source

We have developed a very large volume, high-density helicon plasma source, 75 cm in diameter and 486 cm in axial length; full width at half maximum of the plasma density is up to ∼42 cm with good plasma uniformity along the z axis. By the use of a spiral antenna located just outside the end of the vacuum chamber through a quartz-glass window, plasma can be initiated with a very low value of radio frequency (rf) power (<1 W), and an electron density of more than 1012 cm−3 is successfully produced with less than several hundred Watt; achieving excellent discharge efficiency. It is possible to control the radial density profile in this device by changing the magnetic field configurations near the antenna and/or the antenna radiation-field patterns.

Development of Electrodeless Plasma Thrusters With High-Density Helicon Plasma Sources

Helicon plasma sources are very useful in many aspects and are applicable to many fields across science and technology, as they can supply high-density (~1013 cm-3) plasmas with a broad range of external operating parameters. In this paper, developed, featured sources with various sizes are characterized along with discussions on their particle production efficiency. This paper aims to develop systems that can realize schemes with completely electrodeless plasma production and acceleration. This is expected to mitigate the existing problems of the finite lifetimes inherent in electric plasma propulsion tools. Experimental and theoretical approaches that implement such schemes are presented.

Large-area high-density helicon plasma sources

High-density (10 12 ‐10 13 cm −3 ) helicon plasmas produced using two different but similar devices whose diameters are very large, 40cm and 74cm, respectively, have been characterized. A scaling of the particle production efficiency has been investigated in detail into a low aspect ratio (the ratio of the diameter to the axial length) region down to 0.075. It has been found that reducing this ratio manifests the standing wave-like patterns of the excited radio frequency (rf) fields. Spatial profiles of the electron density and the rf wave fields are measured, showing the effectiveness of the control of these profiles by changing the magnetic field configurations near the excitation antenna. Finally, high-beta plasmas with up to β ∼ 0.8, that exhibit diamagnetic character, where β is the ratio of the plasma pressure to the magnetic pressure, have been produced with low magnetic fields. (Some figures in this article are in colour only in the electronic version)

Anomalous Cross-Field Transport of Electrons Driven by the Electron-Ion Hybrid Instability Due to the Velocity Shear in a Magnetized Filamentary Plasma

The effect of cross-field velocity shear of electrons on the plasma transport has been experimentally investigated in a steady-state filamentary plasma whose effective column radius rp is smaller than the ion Larmor radius ?i and much larger than the electron Larmor radius ?e (?e?rp<?i). When the shear frequency ?S (=v0/LE, where v0 is the maximum flow velocity in the sheared flow and LE is the shear scale length), which is the measure of the strength of the velocity shear, becomes much larger than the lower hybrid frequency, electrostatic plasma fluctuations of the lower hybrid range of frequency are excited in the significantly sheared flow. It is observed that these enhanced fluctuations induce the anomalous, non-ambipolar diffusion of electrons across magnetic field lines. The estimated cross-field diffusion coefficient of electrons obeys the Bohm scaling. The observed instability can be identified as the electron-ion hybrid shear instability through comparison between experimental and numerically obtained results.

Characteristics of a large volume, helicon plasma source

A high-density helicon plasma source with very large volume, 75cm in diameter and 486cm in axial length, has been developed, and its characteristics have been investigated. Furthermore, the radial density profile control has been successfully demonstrated by utilizing two techniques: (1) by changing the magnetic field configurations near the antenna and (2) by changing the antenna radiation-field patterns. Using two types of large-diameter spiral antennae, plasma with density exceeding 1012cm−3 has been produced with several hundreds of watts of radio frequency power. By changing the magnetic field configuration near the antenna, the threshold power and the degree of the density change in a density jump can be varied. The electron density reaches the maximum away from the antenna; then decays weakly along the axial direction.

High-Density Helicon Plasma Sources: Basics and Application to Electrodeless Electric Propulsion

The development of unique, high-density helicon plasma sources is described. Characterization of the largest and the smallest source sizes is made along with a discussion of particle production efficiency using Ar gas. Next, we describe an application of helicon sources to plasma propulsion using a new advanced concept without any eroding electrodes, as a review of our Helicon Electrodeless Advanced Thruster (HEAT) project.

On the Electrodeless MPD Thruster Using a Compact Helicon Plasma Source

Helicon wave excitation is one of the promising RF plasma production methods. It produces high density plasma (~10 cm) that enables electromagnetic acceleration. We prepared the acceleration coil or 2 pairs of copper plates around the glass tube. These acceleration methods are called a repetitious coil current acceleration and a continuous “Lissajous” acceleration, respectively. When the repetitious coil acceleration was applied, the maximum plasma velocity of 3.6 km/s was 76% velocity increment compared with before acceleration with the absorbed plasma production + acceleration power of (400+180) W and the applied magnetic field strength of 1,450 gauss. As for the “Lissajous” acceleration, the plasma velocity was 2.2 km/s with the absorbed plasma production + acceleration power of (290+200) W. However it is suggested that these accelerations remain in the thermal acceleration regime judging from the electron temperature or density increase.

A segmented multi-loop antenna for selective excitation of azimuthal mode number in a helicon plasma source

A flat type, segmented multi-loop antenna was developed in the Tokai Helicon Device, built for producing high-density helicon plasma, with a diameter of 20 cm and an axial length of 100 cm. This antenna, composed of azimuthally splitting segments located on four different radial positions, i.e., r = 2.8, 4.8, 6.8, and 8.8 cm, can excite the azimuthal mode number m of 0, ±1, and ±2 by a proper choice of antenna feeder parts just on the rear side of the antenna. Power dependencies of the electron density ne were investigated with a radio frequency (rf) power less than 3 kW (excitation frequency ranged from 8 to 20 MHz) by the use of various types of antenna segments, and n(e) up to ~5 × 10(12) cm(-3) was obtained after the density jump from inductively coupled plasma to helicon discharges. Radial density profiles of m = 0 and ±1 modes with low and high rf powers were measured. For the cases of these modes after the density jump, the excited mode structures derived from the magnetic probe measurements were consistent with those expected from theory on helicon waves excited in the plasma.

Characteristics of low-aspect ratio, large-diameter, high-density helicon plasmas with variable axial boundary conditions

A low-aspect ratio, high-density helicon plasma source with a large-diameter of ∼74 cm that utilizes an end-launch flat-spiral antenna has been characterized under three different axial boundary conditions. Whereas one end of the device is a quartz-glass window through which an excitation rf wave is injected, the other end is a movable plasma terminating plate of three different kinds: (1) metal with small holes, (2) solid metal, and (3) solid insulator. Using this movable plate, the device aspect ratio A (device axial length/device diameter) can be reduced to ∼0.075 corresponding to the device axial length of 5.5 cm. The plasma production efficiency (PPE, defined as the ratio of the total number of electrons in the plasma to the input rf power) and helicon wave structures are examined for plasmas with various aspect ratios and boundary conditions to characterize our helicon device. Even for the lowest aspect ratio case (A ∼0.075), a plasma with the electron density of 7.5 × 1011 cm−3 can be produced. The...