Numerical simulations of gas mixing effect in electron cyclotron resonance ion sources

The particle-in-cell Monte Carlo collisions code nam-ecris is used to simulate the electron cyclotron resonance ion source (ECRIS) plasma sustained in a mixture of Kr with ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, Ar, Ne, and He. The model assumes that ions are electrostatically confined in the ECR zone by a dip in the plasma potential. A gain in the extracted krypton ion currents is seen for the highest charge states; the gain is maximized when oxygen is used as a mixing gas. The special feature of oxygen is that most of the singly charged oxygen ions are produced after the dissociative ionization of oxygen molecules with a large kinetic energy release of around 5 eV per ion. The increased loss rate of energetic lowly charged ions of the mixing element requires a building up of the retarding potential barrier close to the ECR surface to equilibrate electron and ion losses out of the plasma. In the mixed plasmas, the barrier value is large ($\sim 1\mathrm{V}$) compared to pure Kr plasma ($\sim 0.01\mathrm{V}$), with longer confinement times of krypton ions and with much higher ion temperatures. The temperature of the krypton ions is increased because of extra heating by the energetic oxygen ions and a longer time of ion confinement. In calculations, a drop of the highly charged ion currents of lighter elements is observed when adding small fluxes of krypton into the source. This drop is caused by the accumulation of the krypton ions inside plasma, which decreases the electron and ion confinement times.

Figure 1

Charge state distribution of the extracted krypton ions for the electron temperatures of 8 and 16 keV.

Figure 2

Charge state distribution of the extracted oxygen ions for the electron temperatures of 8 and 16 keV.

Figure 3

Charge state distribution of the extracted krypton ions in the krypton discharge (red) and in the mix of krypton and oxygen (green).

Figure 4

Extracted current of ${\text{Kr}}^{18+}$ ions for different oxygen contents.

Figure 5

Mean charge state of the krypton and oxygen ions inside the ECR volume for different oxygen contents.

Figure 6

Potential dip $\mathrm{\Delta }\phi$ (red circles, left scale), temperature of ${\text{Kr}}^{17+}$ ions (black squares, left scale), and the ratio between these values (line with blue squares, right scale) for different oxygen contents.

Figure 7

Confinement time of ${\text{Kr}}^{17+}$ ions for different oxygen contents. The fit with the Rognlien-Cutler times is shown as a line.

Figure 8

Ion pressure profiles along the source $z$ axis: pressure of the oxygen ions (red line), pressure of the krypton ions at $\text{Kr}=95\%$, $\mathrm{O}=5\%$ (black line), and pressure of the krypton ions at $\text{Kr}=100\%$ (blue line).

Figure 9

Charge state distribution of extracted oxygen ion currents for oxygen plasma and for the mix with krypton ($\text{Kr}=5\%$, $\mathrm{O}=95\%$).

Figure 10

Charge state dependence of the ion temperature for krypton ions in the pure krypton discharge (left scale, solid black squares) and in the mix of krypton and oxygen (right scale, red circles). Temperatures of oxygen ions are shown as open blue squares (right scale).

Figure 11

Confinement times of krypton ions for the krypton plasma (black squares) and in the mix with oxygen (red circles) as a function of the ion charge state. Confinement times of oxygen ions in the mix are shown as the open blue squares. The Rognlien-Cutler fits are shown as the lines.

Figure 12

Density of the krypton ions with the charge states greater and equal to ($17+$) along the $x$ axis in the middle of the source ($z=14\mathrm{cm}$) for the mix with oxygen ($\mathrm{O}=85\%$, black) and nitrogen ($\mathrm{N}=85\%$, red) and with no mix ($\text{Kr}=100\%$, blue).