Negative hydrogen ion production by the tandem-type microwave plasma source

Neutral Beam Injection (NBI) systems have been considered the most reliable method for plasma heating and are prospected so in the future fusion devices. However, for the beam particle energies over 100keV, the neutralization efficiency of the positive deuterium ion beam drops below 50% and gradually decrease to zero when ion beam energy approaches 1000keV. At the same time the neutralization efficiency for the negative hydrogen ion beam drops to 60% at 100keV and then does not change with increase of the ion beam energy. Obviously, the only way to create high-energy neutral hydrogen beams for plasma heating in the future fusion machines is the neutralization of the negative deuterium ion beam. Compared to positive hydrogen ion sources, negative hydrogen ions production is much more complicated. The efficiency of the negative ion production determines the efficiency of the whole NBI system. That is why the development of the negative hydrogen ion sources is so important. At present hot cathode arc discharge plasma sources are used as a negative ion source for the negative ion beam-based NBI systems. Those plasma sources being very efficient and easy in construction still have some problems including filament sputtering which leads to plasma contamination and short lifetime of filaments. Because of the short lifetime, frequent maintenance of the negative ion sources is necessary and it is practically impossible in the radioactive environment of the future fusion reactors. In this thesis I propose a new concept of the Tandem Microwave Plasma Source for negative ion production, which should eliminate those problems. I separate plasma production chamber with high magnetic field and hot non-uniform plasma from the confinement chamber where negative ion production occurs. A special reversed-field coil is eliminating residual magnetic field of the production chamber in the negative ion production region of the confinement chamber. In the production chamber high-electron-temperature plasma is ideal for the excitation of the rotational-vibrational levels of the hydrogen molecules. Those excited molecules diffuse into the confinement region. Confinement region has a low electron temperature and the process of the dissociative attachment results in the negative hydrogen ion production. To investigate the possibility of using this new approach to the negative hydrogen ion production, I have constructed and tested a plasma source of the tandem type. Plasma source consists of two chambers: plasma production chamber (cylindrical shape, 20cm long and 6cm in diameter) and confinement chamber (rectangular, 26×26cm square crossection and 30cm in depth). Production chamber is placed into the axial magnetic field of about 1800G and the microwaves (2.45GHz, 5kW) are introduced through the quartz window along the magnetic field lines. Confinement chamber is surrounded by the magnetic cusp field, which is providing field-free uniform plasma area of 20×20cm near the extraction region. Non-uniform high-temperature plasma is generated in the production chamber and expands into the confinement chamber where it gets uniform and electron temperature decreases through the diffusion process. Plasma flux is controlled by conditioning coil around the confinement chamber, which also eliminates magnetic field from the plasma production region in the area of negative ion production. For the input microwave power of 5kW uniform plasma was generated with the plasma density of 3×l012cm-3 for argon and 3×l011 cm-3 for hydrogen in a wide rectangular area of 20×20cm. Plasma parameters uniformity is within 3%. Electron temperature in the plasma grid region is reduced to 1eV and with the help of the conditioning coil may be controlled in the range of 1-4eV. To confirm the negative ion production the negative ion beam was extracted at voltage of 5kV through a single-hole (diameter 1cm) extraction system. To measure the negative hydrogen beam, 9-channel Faraday Cup array unit was designed and manufactured. Source adjustment with continuous monitoring of negative ion beam current was carried out. Pressure of 4mTorr and B=0 at the extraction region were found optimal for the negative hydrogen ions production. Dependence of the H- production on the magnetic field strength at the extraction grid area shows that the obtained H- current is proportional to the plasma density when the electron temperature is constant and when the electron temperature becomes high, the H- current decreases. We extracted 1.0mA negative hydrogen ion beam current from a single hole 1cm in diameter and that corresponds to 1.3mA/cm2 of the negative hydrogen ion current density at the plasma grid area. Considering that we have a wide uniform plasma area of 20×20-cm and input power of 5kW, total power efficiency of this ion source is higher and the operation pressure is lower than that of the conventional arc discharge and RF ion sources. The new source may be considered as an alternative for the NBI system