Numerical simulation of the RF plasma discharge in the Linac4 H$^−$ ion source

This paper presents a Particle-In-Cell Monte Carlo Collision simulation of the Radio-Frequency (RF) plasma heating in the Linac4 H$^−$ ion source at CERN. The model self-consistently takes into account the electromagnetic field generated by the RF coil, the external static magnetic fields and the resulting plasma response, including a kinetic description of the charged species (e$^−$, H$^+$, H$^+_2$, H$^+_3$, H$^−$), as well as the atomic and molecular (vibrationally resolved) populations. The simulation is performed for the nominal operational condition of 40 kW RF power and 3 Pa H2 pressure. Results show that the plasma spatial distribution is non-uniform in the plasma chamber, with a density peak of $n_e = 5 \cdot 10^{19} m^{−3}$ in the RF coil region. In the filter field region the electron density drops by two orders of magnitude, with a substantial reduction of the electron energy as well. This results in a ratio $e/H^− \approx 1$ in the extraction region. The vibrational population is characterized by a two temperature distribution, with the high vibrational states showing a factor 2 higher termperature. A very good agreement is found between the simulation results and optical emission spectroscopy measurement performed on a dedicated test stand at CERN.