Volume 80, Issue 1, January 2009
Index of content:
- NUCLEAR PHYSICS, FUSION AND PLASMAS
80(2009); http://dx.doi.org/10.1063/1.3049379View Description Hide Description
The effective diffusion coefficient of radon is a very important factor in estimating the rate of radon exhalation from the ground surface. In this study, we developed an experimental system that overcomes technical problems in previous studies to accurately evaluate the effective diffusion coefficient. The radon source used for this system was the National Institute of Radiological Sciences radon chamber. This chamber is a calibrated international standard facility that can produce stable radon concentrations for long periods of time. Our tests showed that leakage of radon from the system was negligible. After the leakage test, we evaluated the effective diffusion coefficient in free-space and in dry porous materials at porosities of 35% and 45%. To ensure that the porous material in the column was as homogeneous as possible, we filled the column with an artificial soil with controlled grain size and grain composition. The measured values and theoretical calculations agreed well, which indicate that the proposed system can be used to accurately and quickly evaluate the effective diffusion coefficient.
Three point method to characterize low-pressure electronegative discharges using electrostatic probe80(2009); http://dx.doi.org/10.1063/1.3065089View Description Hide Description
Electrostatic probe measurements for low-pressure inductively coupled plasmas are performed. From the current-voltage curves of probe, the saturation currents of the positive ions and electrons and the electron temperature are measured. The electronegativity and the negative ion density are deduced by using the ratios of these parameters at three adjacent pressure points. The positive ion density is calculated by the orbital-motion-limited theory, and the electron temperatures are given either by the slope of the curves or by the electron energy distribution function with the second derivative of curves. The variations in the charged species density with pressure and power are investigated.
80(2009); http://dx.doi.org/10.1063/1.3064917View Description Hide Description
Set of experiments has been developed to study existing runaway electrons in “Damavand” tokamak plasma upon characteristics of hard x-ray emissions produced by collision of the runaway electrons with the plasma particles and limiters. As a first step, spatial distribution of hard x-ray emissions on the equatorial plane of the torus was considered. Obtained spectra of hard x-ray emissions for different alignments of shielded detector indicate isotropic emissivity in the equatorial plane. This is in agreement with wide angle cone of bremsstrahlung radiations, deduced from the mean value of energy of the runaway electrons. The mean energy was calculated from the slope of the energy spectrum of hard x-rayphotons. In the second stage in order to investigate time evolution of energy of the runaway electrons, similar technique were applied to obtain hard x-ray energy in every 3 ms intervals, from the beginning to the end of plasma. The mean energy of the runaway electrons increases during the ramp up phase and reaches its maximum between 3 and 9 ms after plasma formation. Also considering the time dependence of the counted photons in each energy range shows that energetic photons are emitted during the ramp up phase of the plasma current in Damavand tokamak.
80(2009); http://dx.doi.org/10.1063/1.3073735View Description Hide Description
Electron cyclotron heating(ECH) is widely used in magnetic fusion devices, and the polarization of the injected millimeter-wave beams plays a crucial role in the propagation and absorption of the beam energy by the plasma. This polarization can be adjusted by gratingmirrorpolarizers placed in the transmission lines which carry the microwaves from the power source to the plasma. In long-pulse devices such as the Large Helical Device(LHD) and ITER, it is desirable to track changes in the plasma and adjust the polarization of the ECH in real time such as to keep the absorption as high as possible and avoid shine-through which may lead to overheating of vessel components. For this purpose a real-time feedback control scheme is envisioned in which a measure of the absorption efficiency can be used to adjust the orientation of the polarizing mirrors toward an optimum. Such a setup has been tested in a low-power test stand as preparation for future implementation in the LHDECH system. It is shown that a simple search algorithm is efficient and can in principle be used to control either the absorption efficiency or the linear polarization angle.