Ion behavior and gas mixing in electron cyclotron resonance plasmas as sources of highly charged ions

Abstract

This article deals with ion confinement in small open-ended magnetic devices, the electron cyclotron resonance ion sources (ECRIS) that were developed for multicharged ion production. The ECRIS are basically ECR-heated plasma confinement machines with hot electrons and cold ions. The main parameters of the ion population in ECRIS plasmas are successively analyzed, temperature, collisions, losses, ionization, confinement times, charge state distribution equilibrium, followed by the analysis of the gas mixing effect, a specific technique to improve the performance as an ion source. A series of experiments is described for the systematic analysis of the phenomena related to gas mixing. It is shown that high charge state optimization by gas mixing relies on a compromise between three criteria, ion losses, mass effect, and ionization rates. The article stresses the role of some fundamental plasma parameters for the next generation of high charge state/high intensity ion sources.

A new plasma potential measurement instrument for plasma ion sources

“…The thermal energy of the a) Electronic mail: olli.tarvainen@phys.jyu.fi ions in an ECRIS plasma has been theoretically estimated to be a few electron volts. 11 Consequently, the effect of the thermal energy on the total energy of the ion beam is small compared to the effect of the plasma potential.…”

Studies of emittance of multiply charged ions extracted from high temperature superconducting electron cyclotron resonance ion source, PKDELIS

“…4 for Ar 6+ and Ar 9+ at low magnetic field mode corresponding to B inj = 1.2 T and B ext = 0.77 T. According to the theoretical calculations by Melin et al, 15 the ion temperature is expected to increase with rf power. This behavior is seen for Ar 6+ and Ar 9+ .…”

The effect of plasma electrode collar structure on the performance of the JYFL 14GHz electron cyclotron resonance ion source

“…the ions are not magnetized, if on an average f ii =f ci 4 1, where f ii is the ion-ion collision frequency. In ECRIS plasma the f ii can be estimated with the equation presented by Melin et al [6] and Girard et al [6,29], showing that f ii ∝Q 2 Q eff n e =T 3=2 i , where Q is the charge state of the ion, Q eff the effective (mean) charge state of the ion population in the plasma, n e the electron density and T i the ion temperature. Using reasonable assumptions, obtained from experiments with ECRIS plasmas [6,29], the collision frequencies of 40 Ar charge states 1+ and 12+ are 4:3 Â 10 5 and 6:2 Â 10 7 s −1 , for example.…”

“…In ECRIS plasma the f ii can be estimated with the equation presented by Melin et al [6] and Girard et al [6,29], showing that f ii ∝Q 2 Q eff n e =T 3=2 i , where Q is the charge state of the ion, Q eff the effective (mean) charge state of the ion population in the plasma, n e the electron density and T i the ion temperature. Using reasonable assumptions, obtained from experiments with ECRIS plasmas [6,29], the collision frequencies of 40 Ar charge states 1+ and 12+ are 4:3 Â 10 5 and 6:2 Â 10 7 s −1 , for example. The corresponding ion gyrofrequencies range from 1:9 Â 10 5 to 3:8 Â 10 5 s −1 for 1+ and from 2:3 Â 10 6 to 4:6 Â 10 6 s −1 for 12+, when the magnetic field varies between 0.5 and 1 T. With 0.5 T the collision frequency is more than an order of magnitude higher than the gyrofrequency with charge states Q ≥5þ.…”

“…Numerous techniques have been developed to improve the performance of ECR ion sources. These include the use of a negatively biased disk at the injection end of the plasma chamber [2][3][4], the gas mixing technique in which lighter gas is introduced into the plasma to boost the production of the high charge states of the heavier element [5,6], the use of two (or multiple) microwave frequencies to produce additional resonance surfaces [7][8][9][10], the introduction of metal-dielectric (MD) surfaces with high secondary electron emission coefficients into the plasma chamber [11,12] and, more recently, the fine tuning of the heating microwave frequency [13][14][15][16][17].…”