Speaker
Description
The cesium (Cs)-free negative-ion neutral-beam injection (NNBI) system is technical issue on the development of magnetic fusion reactors. The progress toward realizing a high-performance Cs-free negative-ion source based on volume production in a magnetized sheet plasma device (TPDsheet-U) is reported [1,2]. The volume production of negative hydrogen ions in the produced magnetized sheet plasma is proposed to be developed as a negative-ion source using Test Plasma produced by Direct current for Sheet plasma Upgrade (TPDsheet-U). High-density magnetized sheet plasmas are suitable for producing H- ions in dissociative attachment processes because of the narrow space (e.g., 10–30 mm) between high-energy (10–15 eV) and low-energy (~1 eV) electron regions. Therefore, H- is formed by the dissociative attachment (DA) of low-energy electrons e (slow) (Te ~1 eV) to highly excited molecules H2 (v” > 5), which are vibrated by the impact of high-energy electrons e (fast) (Te > 15 eV) in the plasma.
In the previous work, the results for the single-aperture grid are described as follows. (i) The negative- ion current density JC(H-) was ~7.5 mA/cm2 at an extraction voltage of 10 kV, a discharge current Id of 90 A, and a gas pressure of 0.3 Pa. The performance of JC(H-) without Cs on TPDsheet-U is approximately one-fourth that of the negative-ion source on ITER-NNBI with Cs. To achieve the current value required by ITER-NNBI, the current density of negative ions, i.e. the negative ion density, should be increased by a factor of four [1].
In the present work, we have demonstrated to control cold electrons and increase negative ion density by increasing the cold electron density with setting electron emitters in the plasma periphery region. The JC(H-) is increased from 0.64 to 0.87 mA (36% increase) at a draw voltage of 10 kV, discharge current of 50 A and gas pressure of 0.3 Pa by increasing the filament temperature of the electron emitter from 800 K to 1400 K. In addition, JC(H-) increased from 0.32 to 0.67 mA (109% increase) by applying a negative bias voltage from 0 to -30 V against the electron emitter. These results indicate that this can facilitate the negative ion generation process, which is expected to further improve the performance of the Cs-free negative-ion source using this electron emitter.
