Signature of gas mixing and isotope anomaly in neon ECR plasma

Figure 1: A typical beam profile; beam before analysis (left), ²⁰Ne⁺⁴ after analysis (right). The analyzed beam spot was ~ 3 mm in diameter

Figure 2: The layout of LEIBF. The flux measurements of isotopic neon have been carried out in 90⁰ beam line using Faraday cup installed just before materials science experimental chamber.

Figure 3: At E/q = 100, 200 and 300 kV; the charge state distribution of ²⁰Ne (top), the charge state distribution of ²²Ne (middle) and the intensity ratio of ²²Ne to²⁰Ne for various charge states (bottom).

Figure 4: At E/q = 200 kV, the intensity ratio of ²²Ne to ²⁰Ne as a function of charge states for pure natural and oxygen mixed (at 25, 50 and 75 %) ECR plasma

Figure 5: At E/q = 200 kV, the intensity ratio of ²²Ne to ²⁰Ne as a function of charge states for pure natural and helium mixed (at 25, 50 and 75 %) ECR plasma

Figure 6:  At E/q = 200 kV, the charge state distribution of pure ²⁰Ne plasma and its comparison with that of mixed with O₂ and He; with 25 % mixing (top), with 50 % mixing (middle) and with 75 % mixing (bottom)

Figure 7: At E/q = 200 kV, the charge state distribution of pure ²²Ne plasma and its comparison with that of mixed with O₂ and He; with 25 % mixing (top), with 50 % mixing (middle) and with 75 % mixing (bottom)

Figure 8: The dependence of neon isotope anomaly as a function of the charge state. A polynomial of degree 2 fits the experimental data well.  

 We have reported the isotope anomaly and the gas mixing (He & O₂) effects in Ne ECR plasma populated with maximum charge state of +7 using a 10 GHz all-permanent-magnet NANOGAN ECR ion source. The ion intensity ratio of ²²Ne to ²⁰Ne in pure plasma condition showed clear signature of the anomalous effect, which starts quenching with increasing the fraction of mixing gases. The charge state dependence of the isotope anomaly follows a polynomial of degree 2. Compared to O₂, the insertion of helium gas in the neon plasma has strong impact on deterioration of the isotope anomaly. The measured beam intensities of different charge states of both highly abundant neon isotopes (²²Ne & ²⁰Ne) were high at high E/q (300 kV). However, the anomalous effect was prominent at E/q = 200 kV owing to mass dependent focusing properties of the ion optical lenses. In comparison to helium, O₂ gas mixing was noticed to be more effective for neon plasma. The mixing effect increases with increases fraction of O₂ in the plasma. Further, ²⁰Ne showed mixing effect at charge state lower than that of ²²Ne. Contrary to previous study on Kr plasma which could only be explained considering nonlinear effects, the observed trends in the isotope anomaly of Ne plasma are in good agreement with the prediction of linear Landau damping theory. The previous studies with Xe and Kr plasma and present results further confirm the strong influence of the relative mass difference on the isotope anomaly. The establishment of generalized selective ion heating mechanism in the plasma still requires enough experimental results on several isotopes with different mixture ratio.