25.06.2007
At the ion accumulation experiment the mass of accumulated ions and thereby the measurement uncertainty is governed by the value of the ion current at the collector. In order to achieve the maximal ion current with the existing apparatus simulations and measurements were performed for optimizing the parameters gas pressure and extraction voltage at the ion source as well as the transmission of the dipole magnet.
At the ion accumulation experiment it is essential to transport as much as possible ion current from the ion source to the place of accumulation, the collector.
The maximal current, jCL, which can be extracted from the source is given theoretically by the Child-Langmuir-Equation:
Thereby, q is the charge and m the mass of the ions. U is the extraction voltage, d is the distance between extraction- and screening electrode and e0 is the electric constant. According to this equation approximately 4 mA of singly charged ions can be extracted using usually an extraction voltage of 25 keV by the given extraction system (triode 1-hole system). A nearly parallel extraction of the ion beam in respect to the optical axis (“matched case”) is a necessary condition for a further optimal transportation. Figure 1 shows a simulation of the extraction system of the ion source. Case (a) is the so called “matched case” where the ions leave the source in parallel trajectories. Case (b) and (c) show simulations where the plasma density is a factor 10 less, (b), a factor 10 larger, (c), respectively. The optimal plasma density is only valid for certain voltages at the extraction system. If the voltage is changed even at the optimal plasma density then one gets the situation according cases (d) and (e).
Figure 1: Calculated ion trajectories in the extraction system of the ion source using the simulation code IGUN. The figure shows half of the cross section of the triode extractor and ion trajectories starting from the plasma meniscus.
According the simulation the optimal extraction depends crucially from the plasma density and in this way from the working gas pressure. At optimal transmission the beam has the lowest divergence and passes the magnet with smallest possible losses.To adjust the matched case, transmission measurements were performed through the doubly focussing dipole magnet of the ion beam set up. Thereby the ion current is measured by use of a dc current transformer (Bergoz MCP) after the ion source and the magnet respectively. The gas flow into the ion source is changed by a needle valve which is driven by a step motor. The pressure is measured directly behind the source using a pressure sensor (Pfeiffer-Vacuum PKR 251).
Figure 2 shows the transmission measurement as a function of pressure for different extraction voltages. The maximum transmission is roughly 40 % according to these investigations. The very sensitive pressure dependence of the extraction is one of the important facts obtained from these measurements.
Figure 2: Measured transmission of the ion current through the dipole magnet of the ion beam set up as a function of the vacuum pressure just behind the ion source at different extraction voltages.
The maximal current as a function of the extraction voltage which can be obtained by the measurements and simulations is summarised in Figure 3. The experimental values are the currents, which were measured at the maximum of the transmission, the data points show the theoretical function according to the Child-Langmuir equation. The dashed line shows the theoretical behaviour according the Child-Langmuir-Law. Both experimental and simulated data reproduce the theoretical expectation very well. Thus we succeeded to obtain the maximal possible current out of the ion source through the transmission optimisation.
Figure 3: Maximal ion current for the “matched case” as a function of the extraction voltage which can be extracted out of the ion source.
Contact person:
Ch. Schlegel, FB 1.2, AG 1.24, christian.schlegel@ptb.de