On the proton flux toward the plasma grid in a RF-driven negative hydrogen ion source for ITER NBI

The transport of protons in a RF-driven negative hydrogen ion source for ITER NBI was investigated by means of a 3D Monte Carlo transport code. As input for the code a consistent axial profile of the electrostatic potential for the complete ion source was constructed. The potential difference between the plasma generation volume and the plasma grid is in the range 10–20 V and depends on the RF power, the filling pressure and the plasma grid bias voltage. The mean free path for proton collisions with the background gas is in the range of several centimeters which means that the protons are noticeably decelerated and scattered. The velocity distribution function of the protons impinging the plasma grid resembles much more a low-temperature Maxwell distribution function than the high energetic proton beam which would be expected by looking only at the maximum value of the potential in the plasma generation volume. This finding is of high relevance for further analysis of the negative hydrogen ion production on the plasma grid surface as well as the ion transport and beam formation.