Hiroshi Amemiya

Probe Diagnostics in Negative Ion Containing Plasma

The effect of negative ions on the probe characteristics is studied theoretically and experimentally by using iodine and iodine-argon mixture plasmas. For a small ratio of negative ion to electron densities, the negative ion density and temperature can be determined from a sharp peak on the second derivative of the probe characteristics. At a higher ratio, the ratio of saturation currents at positive and negative probe biases should be used by calibrating the ratio with that of the case without negative ions. For a negative ion dominating plasma, a change of the Bohm criterion by negative ions should be considered and the parameters can be obtained from the probe characteristics which are nearly symmetric at positive and negative biases.

Production of microwave plasma in narrow cross sectional tubes: Effect of the shape of cross section

A microwave plasma is produced in a conducting tube with a cross section smaller than the cutoff value. Waves of 2.45 GHz are launched perpendicularly to the multicusp magnetic field formed by permanent magnets surrounding the tube. Circular and square cross sectional tubes are tested. Overdense plasmas with a density of (0.8–2.0)×1011 cm−3 are obtained in the range of 10−4 Torr for powers of 200–360 W. The electron temperature is 6–12 eV. Under the same experimental conditions, the plasma density and the electron temperature are higher for the circular cross section.

Production of pulsed microwave plasma in a tube with a radius below the cut-off value

A plasma is produced by pulsed microwaves in a circular tube with a conducting wall and a radius below the cut-off value for the fundamental circular waveguide mode (TE11 ). The discharge tube is surrounded with a multicusp magnetic field constructed with permanent magnets which provide a minimum-B configuration. Short pulse (0.05-1.0 µs) microwaves of 3 GHz and 60-100 kW peak power with a repetition frequency of 10-500 Hz are launched into the narrow tube with the propagation vector perpendicular to the cusped magnetic field in a pressure range of 10-2 -10 Torr. It has been found that, in addition to occurring at the entrance, a plasma is produced at a location of about one-third length from the tube exit. Plasmas generated at the two locations diffuse mostly along the tube axis with the radial diffusion strongly suppressed by the multicusp field. The density uniformity is better at lower pressures. The spatio-temporal evolution of electrons and ions shows a density growth after the end of a microwave pulse for a few to tens of microseconds depending on the axial position, followed by decay. The profiles have been explained by numerical simulation based upon a model that charged particles are driven by electrostatic and ponderomotive forces and diffuse along the longitudinal direction. The interpulse regime has an electron temperature of about 10 eV. This indicates that the plasma is still active between the pulses, in distinct contrast with the usual afterglows.

Microwave Plasma in a Multicusp Circular Waveguide with a Dimension below Cutoff.

A microwave plasma is produced in a nearly circular multicusp waveguide with a cross section smaller than the cutoff value. Permanent magnets are used to form the multicusp. Plasma density above the cutoff value was obtained in the range of 10-4 Torr at a power density of 6 – 10 W/cm2 for 2.45 GHz. The plasma production in the narrow waveguide and its characteristics are discussed.

Experiments on the Energy Distribution Function in Hydrogen Plasmas

The electron energy distribution function has been measured in typical hydrogen plasmas in a multipole plasma device by an improved alternating-current probe method. The function is found to be non-Maxwellian, containing primary electrons, thermal electrons and groups due to inelastic scattering of the primaries. A characteristic peak due to negative ions in the distribution has been detected and is used to determine the density of negative hydrogen ions. The variations of their density with the densities of thermal electrons and primary electrons have been studied and interpreted. The dependence of the relative intensity of the negative ion peak on the probe material, probe shape, and frequency of the superimposed ac signal has been investigated. The optimum conditions have been determined for using the probe method to measure the negative ion density in plasmas.

A new method to determine ion temperatures in magnetized plasmas by means of an electrical probe

The particle flow to a probe in a magnetized plasma is anisotropic. Because of this effect a probe pin generates a shadow in the plasma. From the measurement of the density profile of this shadow the ion temperature can be inferred for specified plasma conditions by comparison with results from Monte Carlo calculations. The probe presheath and sheath are accounted for in the calculations only by use of the Bohm criterion. This implies that the calculated results of the present article pertain strictly only for plasmas with an ion temperature that is large compared to the electron temperature, although the principle of the method is not restricted to these temperature ratios. The space resolution of the measurement in the direction perpendicular to the magnetic field is a few times the ion gyro radius and the time resolution is <100 ms. We present an application of this method of the ion temperature measurement for the scape‐off layer of the tokamak TEXTOR.

Production of Electron-Free Plasma by Using a Magnetic Filter in Radio Frequency Discharge

A source plasma is generated in a multipole-type device in oxygen by a radio frequency discharge at 13.56 MHz. A SmCo magnet array is mounted between source and target plasma sections as a magnetic filter. In the target section, almost electron-free plasma is obtained. The device can be operated in the pressure range of 10 −3 to 10 −1 Torr. The plasma densities in source and target plasmas are 10 9 -10 10 cm −3 and 10 8 -10 9 cm −3 , respectively

Measurements of Energy Distributions in ECR Plasma

The electron energy distribution is measured in an ECR device for H2 and N2 plasmas as a function of pressure. The maximum measurable energy and the high-energy tail increase with decreasing pressure. However, the energy distribution at the high-energy part progressively departs from a Maxwellian shape as the pressure is increased. The electron temperature obtained from the low-energy part increases with decreasing pressure. The positive ion temperature is of the order of a few eV and increases with the electron temperature. A mechanism of ion acceleration should exist.

Quasisteady state interpulse plasmas

The generation of quasisteady state plasmas in the power off phase, by short pulses [pulse duration (τp)∼0.5–1.2μs] of intense (60–100kW) microwaves in the X band (9.45GHz) is observed experimentally. The steady state is sustained from a few to tens of microseconds and depends upon the ionization processes in the interpulse phase and the characteristic diffusion length. The results are explained by a model, which considers the electron acceleration effects by the large amplitude of the field, the energy losses, and the characteristic electromagnetic field decay time. The effects of wave frequency, microwave power density, and particle diffusion on the steady state are investigated. A striking difference with conventional afterglows of pulsed discharges is pointed out.

Plasma buildup by short-pulse high-power microwaves

The buildup of a plasma produced by short-pulse (0.05–1.2 μs), high-power (60–100 kW) microwaves is studied in a pressure range of 10 mTorr–10 Torr, by measurements of the temporal variation of the current and the optical intensity. The plasma is produced in a cylindrical tube and confined by a minimum-B field. The buildup of the electron current and the optical intensity are found to continue beyond the end of the pulse, for a few to tens of μs depending upon the pressure, and a minimum in their peak values and buildup times occur around 1 Torr. Increase in microwave pulse duration increases the buildup rate and peak current, whereas the pulse repetition frequency (10–500 Hz) has only a weak influence. The results are discussed from the growth of electron temperature during the pulse, and the following plasma evolution after the end of pulse. Collisional wave absorption, electron cyclotron heating, and diffusion are found to play important roles in plasma production and maintenance over the pressure range.