Tandem type hydrogen negative ion (H$^-$) sources are generally designed to have two plasma regions of different average electron energies. The higher and lower electron energy regions are called “driver region” and “extraction region”, respectively . The driver region should contain electrons with energy high enough to produce excited hydrogen molecules for H$^-$ production. On the other hand, the extraction region is generally located in the vicinity of a beam extraction hole in order to enhance H$^-$ beam current, while avoiding the H$^-$ destruction due to the higher energy electrons over 1 eV. The separation of the two plasma regions is realized with the filter magnetic field which is introduced to the extraction region. Therefore, the low-energy electron transport in the filter field determines the plasma parameters at the extraction region which decides the performance of H$^-$ sources.
In this study, we aim to understand mechanism of the low-energy electron transport at the extraction region through experiments, especially transport across the filter magnetic field, using numerical simulations in order to obtain basic knowledge for development of more efficient H$^-$ sources.
In the experiment, we use a test stand with a small cylindrical-shape ion source whose diameter and length are 9 cm and 11 cm, respectively . The ion source generates a DC hydrogen plasma with a pair of tungsten filament installed at the driver region. Here, to study the low-energy electron transport, we introduced another filament system  inside the extraction region. The additional filament system enhances density of low-energy electrons in the extraction region. The injected electrons diffuse around the filament and the electron density increases locally. The transport of these low-energy electrons in the extraction region can be studied through an analysis of the local change on the density profile. Thus, we measured the spatial distribution of electron density with a Langmuir probe under different experimental conditions. The experimental results were analyzed with two-dimensional position and three-dimensional velocity Particle-In-Cell (2D3V PIC) simulations to understand the low-energy electron transport.
In our early experiments, we have already confirmed the local-density enhancement in electron density profiles. In a poster, we will report the results of experiment and analysis with the PIC simulation.
 K. N. Leung and W. B. Kunkel, Phys. Rev. Lett. 59, 787 (1987).
 Y. Matsumoto, M. Nishiura, M. Sasao, H. Yamaoka, K. Shinto, and M. Wada, Rev. Sci. Instrum. 79, 02B909 (2008).
 K. N. Leung, K. W. Ehlers, and R.V. Pyle, Rev. Sci. Instrum. 57, 321 (1986).