Current density distributions and sputter marks in electron cyclotron resonance ion sources

Abstract

Most electron cyclotron resonance ion sources use hexapolar magnetic fields for the radial confinement of the plasma. The geometry of this magnetic structure is then--induced by charged particles--mapped onto the inner side of the plasma electrode via sputtering and deposition. The resulting structures usually show two different patterns: a sharp triangular one in the central region which in some cases is even sputtered deep into the material (referred to as thin groove or sharp structure), and a blurred but still triangular-like one in the surroundings (referred to as broad halo). Therefore, both patterns seem to have different sources. To investigate their origins we replaced the standard plasma electrode by a custom-built plasma electrode acting as a planar, multi-segment current-detector. For different biased disc voltages, detector positions, and source biases (referred to the detector) we measured the electrical current density distributions in the plane of the plasma electrode. The results show a strong and sharply confined electron population with triangular shape surrounded by less intense and spatially less confined ions. Observed sputter- and deposition marks are related to the analysis of the results. Our measurements suggest that the two different patterns (thin and broad) indeed originate from different particle populations. The thin structures seem to be caused by the hot electron population while the broad marks seem to stem from the medium to highly charged ions. In this paper we present our measurements together with theoretical considerations and substantiate the conclusions drawn above. The validity of these results is also discussed.

Spectroscopic study of ion temperature in minimum-B ECRIS plasma

“…the contribution of the ion temperature on the transverse emittance could be more than 20 % instead of the negligible few percent as previously thought. For high charge states that are shown to be even hotter and are extracted near the source axis from an effective radius smaller than the extraction aperture [42,43], the contribution of the thermal emittance is even larger.…”

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

“…However, the ions are bound to the magnetic field lines indirectly through the dynamics of the magnetized electron population, as the experimental results reported in a recent study [30] indicate. In that study the ion and electron current density distributions were spatially resolved near the extraction region of an 11 GHz ECR ion source.…”

The longitudinal energy spread of ion beams extracted from an electron cyclotron resonance ion source

“…Figure 1 presents the initial and final horizontal (𝑥, 𝑥 ) phase space distributions of the beam with three components. All particles are 16 O + . The beam components have total kinetic energies of 20 keV, 19.94 keV and 20.06 keV, i.e.…”

“…The varied parameters were the ion charge state, ion mass, initial longitudinal energy (representing the plasma potential), initial transverse energy spread (representing the ion temperature), and the emitting disc diameter. The disc diameter was varied to change the effective extraction radius, which is known to depend on the charge state of the ions [15,16]. The ion velocity components were recorded at the end of the simulation domain (𝑧 = 1000 mm) and their longitudinal and transverse energies were calculated from these values.…”