Yongjian Xu

Radio-frequency (RF) negative ion sources are essential component for neutral beam injection systems in magnetic confinement fusion devices. In this work, the RF power coupling efficiency of the Comprehensive Research Facility for Fusion Technology dual-driver RF negative ion source was experimentally measured on the Hefei Open-facility for Negative-ion Source Research test facility. The effects of selected external configurations were evaluated, including the relative RF coil current direction, the diameter of the electromagnetic shielding (EMS) for the RF driver coil, and the cusp magnet configuration. Under the reference configuration without EMS or cusp magnet systems, the RF power coupling efficiency was approximately 55%. Changing the relative RF coil current direction had only a marginal effect on the coupling efficiency and electron density. The installation of EMS improved the plasma density uniformity in front of the plasma grid (PG), while the EMS diameter affected both network resistance and RF coupling efficiency. The cusp magnet system mounted on the backplate of the RF driver reduced the RF coupling efficiency and electron density near the extraction region, whereas the additional sidewall cusp magnet system increased the electron density in front of the PG despite a further decrease in coupling efficiency. The results show that the evaluated external configurations can increase the RF power coupling efficiency by up to approximately 10% and increase the plasma density in the extraction region by about 6%.

Neutral beam injection (NBI) is a key auxiliary heating technology used in magnetic confinement fusion devices. With the scale up of the device, the requirement of beam energy is higher. A negative ion source-based neutral beam injection (NNBI) system is an inevitable choice, but the NNBI system presents significant engineering complexity and technical challenges. To investigate and master core NNBI technologies, an NNBI test facility is currently being developed under the Comprehensive Research Facility for Fusion Technology (CRAFT) in China. The initial operational targets for the CRAFT NNBI system are to achieve a beam with energies in the range of 200400 keV, the neutral beam power of 2 MW, and the pulse duration of 100 s. In the negative ion source, the beam divergence is one of the important parameters that determines the pulse duration and energy of the beam. A large beam divergence will cause heavy thermal load on the electrode grids and additional heat load on the beamline components. These can cause the breakdown of grids and interrupt the acceleration process. The accelerator beam optics design for the CRAFT NNBI dual-driver negative ion source is based on IBSimu. Two diagnostic methods, beam emission spectroscopy (BES) and secondary electron emission (SEE), are adopted to analyze the optical characteristics of beam in the experiments. The experimental results verify the simulation results calculated by IBSimu, confirming the limitations and reliability of the simulation program.

The uniformity of beam, for large RF negative ion source, is closely related to the plasma uniformity in the extraction region. The six electrostatic probes were installed above the plasma grid (PG) 5 mm to evaluate the plasma uniformity. The influence of RF power, source pressure, magnetic filter field, and bias voltage on the plasma uniformity in the extraction region were analyzed. In the absence of magnetic filter field and bias voltage, the plasma density exhibits good uniformity (>90%), but the uniformity of electron temperature (T _e) is poor (>60%). It is mainly caused that the magnetic field was generated by the permanent magnet inside extraction grid (EG). The plasma density and T _e can be increased by increasing RF power, but it has little effect on the plasma uniformity. Increasing source pressure can effectively lower T _e and slightly improve its uniformity, but the uniformity of plasma density has deteriorated by increasing source pressure. While increasing the PG current effectively reduces the T _e in the extraction region, it simultaneously degrades the uniformity of both T _e and plasma density due to drift effects. Increasing the bias voltage can increase T _e in the extraction region when the PG current is 1200 A, but T _e at the bottom increases more significantly, thereby worsening the uniformity of T _e. In addition, the electron density (N _e) at the top decreases, while the N _e at the bottom increases, thereby improving the uniformity of N _e. The influence of the magnetic filtered field and bias voltage on the non-uniformity of T _e in the extraction region, as observed in the manuscript, is similar to the effect on the non-uniformity of the extracted beam current reported in the literature. These results provide theoretical foundations for optimizing the beam uniformity in large RF negative ion sources.

Low-temperature plasma is widely used in thin-film deposition, circuit etching, material surface modification, and other applications in which plasma parameters can significantly affect the outcomes. Compared to single-source excitation, dual-source excitation can enhance the efficiency of energy utilization and expand the range of parameter variations. A low-pressure tube filled with a mixture of 95% Ar and 5% N₂ was developed to study the plasma properties under DC excitation and dual-source excitation with DC superimposed nanosecond pulse. The effects of different pulse amplitudes and pulse widths on the plasma properties were explored. The experimental results show that the superposition of pulses can enhance the electron density in the afterglow phase, and the enhancement of electron density in the afterglow phase is more sensitive to the pulse amplitude. The electron density can be maintained at a high level after the pulse ends, with a maximum value reaching 10¹⁷ m⁻³. In addition, the electron density in plasma under dual-source excitation can be two orders of magnitude higher than under single DC excitation, while the gas temperature decreases by approximately 40 K, thereby increasing the range of plasma parameters variations.

The plasma characteristics of the extraction region in high-power RF negative ion source have a significant impact on the production and extraction of negative hydrogen ions. This study utilized electrostatic probe to investigate the effect of RF power, source pressure, magnetic filter field and bias voltage on the plasma parameters of the extraction region (without cesium), and also studied the variation of plasma parameters with position and time. The results indicate that as RF power increases, the plasma density in the extraction region significantly rises, but it also leads to an increase in the electron temperature ( T e ) of the extraction region (which increases the loss of negative hydrogen ions); increasing the source pressure can effectively increase the electron density ( N e ) in the extraction region and reduce T e as expected; increasing the magnetic filter field can effectively reduce the T e in the extraction region, but after the plasma grid current exceeds 1900 A, the change in T e is not significant; although increasement of the bias voltage can effectively suppress the N e in the extraction region, it causes T e to rise; moreover, the distribution of plasma parameters in the extraction region is relatively uniform; in addition, the variation of plasma parameters with time is not obvious. These findings not only deepen the understanding of the physical mechanisms of plasma behavior in the extraction region of negative ion sources, but also provide important theoretical and experimental bases for the optimization of high-power RF negative ion sources.