Studies of pure and nitrogen gas mixed xenon ECR plasma: Signature of abundance dependent unusual trends in isotope anomaly

Abstract

We report the effect of N 2 gas-mixing in the xenon electron cyclotron resonance (ECR) plasma, and abundance-dependent novel, exciting and unusual trends of the isotope anomaly. The xenon plasma was produced using a 10 GHz all-permanentmagnet NANOGAN ECR ion source, and the charge state distributions of naturally abundant six stable xenon isotopes with and without N 2 gas-mixing (at 25%, 50%, and 75%) were recorded. The intensity ratio of the heavier to lighter isotope, where the heavier isotope is less abundant, showed a clear signature of the isotope anomaly as explained by the linear Landau wave damping theory. Contrary to the theoretical prediction that the isotope anomaly should vanish with a relatively large fraction of the heavier isotope in mixed plasmas, the trends of intensity ratios observed in such cases are very unusual and have almost the mirror-symmetrical shapes of those trends recorded with less abundant heavier isotope. Further, the effect of relative mass difference on the isotope anomaly was also evidenced. The N 2 gas-mixing of the xenon plasma at 25% and 50% shifted the entire charge state distribution toward the higher intensity side owing to the supply of additional electrons that caused high ionization efficiency. However, a prominent gas-mixing effect was observed at 75% of N 2 mixing in the xenon plasma beyond the +7 charge state. The abundancedependent unusual trends in isotope anomaly have been explained by considering different ionic temperatures, ion heating by the wave damping, and Coulomb scattering in the core of the plasma.charge state distribution, electron cyclotron resonance (ECR) plasma, gas mixing effect, isotope anomaly, low energy accelerator, mass analyzer | INTRODUCTIONThe capability of producing multiply charged positive ions via the electron-impact ionization without using any cathode/filament made the electron cyclotron resonance ion sources (ECRIS) very popular among the accelerator physicists. [1][2][3][4][5][6][7] The intensities in ECRIS are mainly governed by the ion-confinement using the magnetic fields. In a typical structure, multipole permanent magnets are used for the radial confinement of the ions, whereas the axial confinement of the ions is achieved either by the permanent magnets or the electromagnets. 8,9 The fourth generation high-frequency ECRIS utilized superconducting magnetic coils for the axial confinement of the ions. [10][11][12] The resonant electrons, which have a gyration frequency equals to the frequency of the input electromagnetic wave, gain enough kinetic energy to ionize the gaseous atom/molecules and yield intense plasma. 13