Commissioning of a fast fine-step electron-energy-scan system for electron-ion crossed-beams experiments

Abstract

We report on the commissioning of a fast electron-energy scan system for measurements of electron-impact ionization cross sections. The Giessen crossed-beams experiment employs a high-power electron gun which has been developed over recent years and which permits wide variations of the beam parameters. A multi-electrode design enables the decoupling of the electron energy from the electron density. The newly implemented control system, which governs the various electrode potentials, is described together with the salient technical features.

Figures (11)

• Figure 1: CAD drawing of the Giessen electron-ion crossed-beams setup. The blue arrows indicate the direction of the ion beam in the various sections of the setup. The footprint of the entire setup is about $4\times4$ m$^2$.
• Figure 2: Photograph of the 3500-eV electron gun before its insertion into the crossed-beams chamber (Fig. \ref{['fig:setup']}). Each electrode is individually electrically contacted. The stainless-steel tubes in the foreground belong to the cooling water circuit of the anode in the background.
• Figure 3: Section along the electron beam direction providing an overview over the various electrodes of the 3500-eV electron gun. Electrodes of the same color are on the same electric potential. A detailed circuit diagram can be found in the appendix (Fig. \ref{['fig:circuit']}).
• Figure 4: Simulated electron trajectories in the 3500-eV electron gun. Top: Mode HE10 with $U_e$ = 1000 V and $I_\mathrm{cath} = 147.351$ mA. Bottom: Mode HC6 with $U_e = 20$ V and $I_\mathrm{cath} = 7.14$ mA. The color encodes the electron energy. The simulations were carried out with the software CST Studio Suite CSTSpachmann2006.
• Figure 5: Measured (panel a) emitted electron currents $I_\mathrm{cath}$ and (panel b) electron current correction factors $f_\mathrm{ecc}$ (Eq. \ref{['eq:efcc']}) as functions of electron energy for the operation modes from Tab. \ref{['tab:modes']}, i.e., HE10 (orange up-triangles), HE15 (blue down-triangles), and HC6 (green circles). The insets zoom into the low-energy regions. The full lines in the left panels represent fits of Eq. \ref{['eq:perv']} to the data points, which result in the tabulated values for the effective perveance in Tab. \ref{['tab:modes']}.