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 (5 mT), the plasma-sheath resonance was found to occur at about 12 MHz 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 5 mT 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 1 mT was required to obtain a sufficient ion extraction efficiency. To obtain the same extraction time as when appling a -3 kV dc voltage in the electrostatic method, the rf resonance method needed a voltage more than 70 V-rms, in which the dc bias was -1 kV. Therefore, we confirmed that this method is feasible for the ion extraction from the photoionized plasma.