Improving the transported negative ion beam current in NIO1
In order to reduce the beam losses of the negative ion beam in the NIO1 experiment, the vacuum system was improved by installing an additional cryogenic pump. In the upgraded configuration, the negative ion beam can be operated in two regimes of high and low background pressures. The gas flow in the improved configuration was simulated, characterizing the existing pumping system and verifying the best configuration for the improved pumping scheme. The charge state of the negative ion beam moving through the background hydrogen gas is calculated, thus providing the stripping losses inside the accelerator and in the transport region. The expected values are compared with the experimental data obtained with a diagnostic beam dump, a one-dimensional carbon fiber calorimeter, and other beam-plasma diagnostics measuring the parameters of the secondary plasma. The collected electric current and the heat flux reconstructed from thermal measurements are compared with the expected beam particle flux. The measurements indicate as expected an increase of negative ion current at full acceleration and a decrease of the plasma electron contribution.