Yasuyoshi Yasaka

Production of Large-Diameter Uniform Plasma in mTorr Range Using Microwave Discharge

A large-diameter uniform plasma is obtained by microwave discharge without the use of magnetic fields at pressures in the mTorr range. Microwave fields at 2.45 GHz are radiated from a multislotted planar antenna located a short distance above the glass window of a discharge chamber. Overdense plasmas are produced with ± 3–4% uniformity of ion saturation current over 30 cm diameter for wide ranges of microwave power and gas pressure. The discharge can be started up at pressures as low as 0.5 mTorr. The efficient production of overdense plasmas is investigated by measuring microwave field propagation in the system.

Role of Helicon Waves on High-Density Plasma Production

Plasma production by rf fields in the helicon wave frequency range is investigated by using the rotating field antenna which can select the azimuthal mode number of the applied rf. The calculated dispersion relation shows that the m = -1 mode helicon wave propagates whereas the m =+1 mode helicon wave is cut off. The produced helium plasma density in the case of the m = -1 mode rf application is 1 ×1013 cm-3, which is more than 5 times greater than in the case of the m = +1 mode rf application. The two-dimensional numerical simulation of the rf plasma production also qualitatively gives the same result as in the experiment. The plasma for the m = +1 mode rf is sustained by the antenna near field, while for the m = -1 mode rf, it is sustained by the power absorption from the propagating helicon wave. From both the experiment and the simulation, the helicon wave is revealed to increase the plasma density from the order of 1012 cm-3 to 1013 cm-3 by efficient power deposition to electrons.

Performance of Electron Cycrotron Resonance Plasma Produced by a New Microwave Launching System in a Multicusp Magnetic Field with Permanent Magnets

A multi-ring-cusp-type surface magnetic field is tested for production of large-diameter electron cyclotron resonance (ECR) plasma. The density and its distribution are improved by means of a new launching system consisting of an adjustable antenna and a microwave suppressor inserted into the vacuum chamber, which forces the microwave to propagate into the plasma from the stronger side of the surface magnetic fields.

Simulation of Plasma Production and Chemical Reaction in an Oxide Deposition Apparatus Using Electron Cyclotron Resonance Plasma

A two-dimensional fluid simulation code is developed which can predict plasma properties and chemical species transport in gas phase in a realistic configuration of a production-line apparatus. The code is applied to an electron cyclotron resonance plasma reactor for oxide deposition on 20-cm wafers. The predicted density in Ar plasma is in good agreement with the experimental result. Two-dimensional distributions of radical densities in plasmas with gas mixtures of Ar/O 2 /SiH 4 and Ar/O 2 /SiF 4 are obtained, which are in accordance with the measured deposition rates of SiO 2 and Sio x F y , respectively.

Experiment on direct energy conversion from tandem mirror plasmas by using a slanted cusp magnetic field

A direct energy converter (DEC) designed for thermal ions escaping from a linear or near-linear device consists of a cusp magnetic field and decelerating electrodes. The electrons are deflected along the field lines and consequently separated from ions that are not fully magnetized. The ions are led to the decelerating electrodes to produce dc power. This type of DEC, the CUSPDEC, is applied to the GAMMA 10 tandem mirror in order to investigate the capability of separation of charged particles as well as to demonstrate energy conversion from ions. The separation of electrons and ions with energies of the order of kilo-electron Volt is achieved by using a slanted cusp magnetic field for the first time. It is also demonstrated that the separated ions are decelerated by the electric field in front of ion collectors and flow into the collectors at a high potential to produce electricity.

Plasma Production and Wave Propagation in a Plasma Source Using Lower Hybrid Waves

The lower hybrid wave, which is in the same frequency range as the helicon wave, is resonant at the lower hybrid frequency and does not penetrate into the higher density side of the plasma. This creates the localized ionization region at the outer plasma radii and leads to uniform plasma production. Typical plasma densities in the plasma source designed to take advantage of this characteristic of the lower hybrid wave are of the order of 1011cm-3 in the source region for 10 mTorr He and 1 kW radio frequency (rf) power. The radial density profile can be controlled by changing the location of the lower hybrid resonance by changing the magnetic field B0. We measured the radial dependence of rf electric and magnetic fields to determine if lower hybrid wave and helicon wave were present.

Control of plasma profile in microwave discharges via inverse-problem approach

In the manufacturing process of semiconductors, plasma processing is an essential technology, and the plasma used in the process is required to be of high density, low temperature, large diameter, and high uniformity. This research focuses on the microwave-excited plasma that meets these needs, and the research target is a spatial profile control. Two novel techniques are introduced to control the uniformity; one is a segmented slot antenna that can change radial distribution of the radiated field during operation, and the other is a hyper simulator that can predict microwave power distribution necessary for a desired radial density profile. The control system including these techniques provides a method of controlling radial profiles of the microwave plasma via inverse-problem approach, and is investigated numerically and experimentally.

A Plasma Source Using Waves in a Lower Hybrid Frequency Range

Plasma production using the waves in a lower hybrid frequency range is investigated numerically and experimentally. The radial profiles of wave field and power absorption by electrons are calculated using a one-dimensional kinetic wave code. The two-dimensional fluid simulation code is coupled with the output of the kinetic wave code to predict the plasma production in the designed plasma source. The power deposition profile, and hence the produced plasma density profile, can be controlled by selecting the wave number and the magnetic field. It is observed in the experiment that plasma is produced in the outer radii of the source chamber, and diffuses to form a radially uniform profile in the diffusion chamber in accordance with the simulation.

Spatial structure of waves and plasma uniformity in planar microwave discharges

The spatial structure of waves and plasma uniformity in microwave discharges using a multi-slotted planar antenna are investigated experimentally and by calculation. The wave field amplitude and phase are measured in the axial and azimuthal directions, and are compared with three-dimensional finite-difference time-domain calculation result. The wave mode in the plasma does not change with various densities, and, as a consequence, spatially uniform plasmas are obtained for a wide range of parameters being free from abrupt changes associated with surface wave eigenmodes.

NUMERICAL ANALYSES OF AN EXPERIMENTAL DEVICE OF TRAVELLING WAVE DIRECT ENERGY CONVERTER

A travelling wave type direct energy converter has been proposed as a converter in a D- 3 He nuclear fusion reactor. Basic experiments have been carried out in a small-scale experimental device. We have performed numerical analyses of the device with the one-dimensional approximation in order to improve the performance, especially the efficiency. We have firstly compared the numerical results with the experimental results on the efficiency and the energy distribution. The numerical results are well corresponded to the experimental results. On the basis of these results, we have suggested the optimum distribution of electrodes in the device to convert the kinetic energy efficiently, and examined the effects. By the optimization, the efficiency is expected to become over 58% when the number of electrodes is sixteen.