Kazuki Tsuchida

Ion extraction from plasma by using a radio frequency resonant electric field

In order to raise an ion extraction efficiency from a plasma, a new method using an rf field has been proposed and demonstrated. The resonant frequencies of the rf field were theoretically evaluated to excite the eigenwave of the plasma. The lower frequency of the two plasma‐sheath resonances under the magnetic field was selected because it has hardly any dependence on the plasma density when the density is over a critical value. Verification of this method was carried out using Xe discharge plasma (electron density, 1×1016 m−3; electron temperature, 8 eV) between the parallel plate electrodes (length, 0.5 m). The resonance was found at about 10 MHz, which agreed with the theoretical result. The ion current at the resonance was anisotropic and was twice as large as the ion saturated current, which is the limiting value of the conventional electrostatic method.

Particle simulation of ion extraction from plasma by a radio frequency resonance method

The mechanism of ion extraction from plasma by rf resonance has been studied by one-dimensional particle simulation. The plasma-sheath resonance under the weak magnetic field occurs at the theoretically predicted frequency in the simulation and it has durability. In the rf period at the resonance, the large electric field penetrates into the plasma and the electrons move collectively due to a polarization drift and E×B drift. Two processes are proposed for the ion extraction mechanism. In the first, the rectified electron current in the resonance causes the plasma potential to be higher. Consequently, ions are extracted to a pair of parallel plate electrodes, which sandwich the plasma. In the second, the time-averaged electric field in the plasma region causes ions to accelerate to both electrodes. This means that the restriction of the plasma shielding effect is overcome by the time-averaged electric field.

Electric field distribution in plasma during ion extraction by a radio frequency resonance method

An ion extraction method using plasma-sheath resonance in a weak magnetic field has been employed to measure the electric field distribution and the time-resolved potential distribution experimentally. The Xe discharge plasma is sandwiched by parallel plate electrodes, which are set parallel to the magnetic field. When resonance occurs at 10 MHz, the electric field perpendicular to the magnetic field has its peak at the midpoint of the electrodes’ length. Therefore, a standing wave seems to be induced in the electrodes. When measuring the time-resolved potential distribution perpendicular to the magnetic field at the midpoint of the electrodes’ length, potential gradients are formed and their directions are reversed in one rf period. It is, therefore, verified that the rf electric field penetrates to the plasma. Moreover, simulation results show that the electric field strength in the resonance increases, but nonlinearly, with the applied voltage.

Cross sections of charge transfer between uranium atoms and ions produced by autoionization

A technique to reduce charge transfer has been studied to cut enrichment loss during atomic vapor laser isotope separation. The charge transfer cross sections of uranium were experimentally examined. The cross sections of charge transfer for case A (between ground-state ions and atoms) and, cases B and C (between ions populated in some excited states and atoms) were relatively measured in the energy range of 100–2,000 eV by the cross-beam method. The charge transfer cross sections for cases A, B and C do not depend on impact energy, and the cross sections for case A are smaller than those for cases B and C. These results are in fairly good agreement agreed with theoretical predictions. It is confirmed experimentally and theoretically that the uranium charge transfer cross section depends on its initial ionic state, therefore ion loss by charge transfer will be reduced by controlling the initial state of the ions.

Ion extraction characteristics from photoionized plasma by radio-frequency resonance method

In order to raise ion extraction efficiency in laser isotope separation, we have developed a radio-frequency (rf) resonance method. Then, to confirm feasibility of this method to a photoionized plasma, we experimentally studied the ion extraction characteristics. When the rf frequency was swept under a weak magnetic field (5mT), the plasma-sheath resonance was found to occur at about 12MHz which was almost the same value as the theoretical one. Moreover, it was confirmed that the ion extraction time at the resonance frequency became the minimum. When the magnetic field strength decreased from 5mT to zero, the ion extraction time became long. From the simulation results, this was because the plasma potential decreased with the magnetic field strength. Therefore, a magnetic field strength of more than 1mT was required to obtain a sufficient ion extraction efficiency. To obtain the same extraction time as when applying a −3kV dc voltage in the electrostatic method, the rf resonance method needed a voltage mo...

Evaluation of minimum anode area for rapid ion extraction from photo-ionized plasma

The characteristies of ion extraction from photo-ionized plasma (ion species: Gd + , typical density: 1 × 10 16 m -3 ; electron temperature: 0.4 eV) in a simplified M-type electrode configuration were measured when the ratio of the anode area to the cathode one (α) was changed between 0.04% and 5%. The ion extraction was as rapid as the original M-type electrode configuration when α≥0.5%, but it was longer when α≤0.1%. The plasma potential was measured by analyzing the energy of the ions extracted from the plasma. The rise of the plasma potential at α=0.1% was delayed more than that at α=0.5%. Therefore, the delay of the rise of the plasma potential caused the delay of the ion extraction when α≤0.1%. The anode area for rapid ion extraction is evaluated to be more than 0.5% of the cathode area. An anode area large enough to raise the plasma potential to the anode one ensures that the ion extraction is as rapid as that of the original M-type electrode system.