Xufeng Peng

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%.

We developed a two-dimensional self-consistent model of the CRAFT RF-driven dual-driver negative ion source to quantify how RF power deposition, gas inflow, neutral heating, and pressure evolution jointly determine the precursor-plasma conditions. The model couples charged-particle drift-diffusion transport, neutral diffusion, inductive RF power deposition, hydrogen plasma chemistry, gas heating, and boundary conditions that include secondary electron emission, surface recombination, and neutral inflow and outflow. Unlike models that prescribe the filling pressure, the present formulation uses the gas inflow as the external control parameter and computes the neutral pressure self-consistently during discharge. The model is validated against Langmuir-probe measurements over 33–125 kW, reproducing the electron density distribution near the plasma grid (PG) with an overall root-mean-square deviation of 22%. The simulations capture localized plasma generation in the RF drivers, diffusive expansion into the chamber, strong molecular dissociation, and substantial gas heating in the expansion region. They further show that source operation is controlled by the coupled balance between RF power deposition and neutral replenishment. At reduced gas inflow, the discharge enters a depletion-limited regime, in which the electron density saturates with increasing RF power. Rovibrational excitation and relaxation of hydrogen molecules provide an additional channel of neutral-gas heating and contribute to the non-monotonic dependence of gas temperature and plasma parameters on RF power and gas feed. These results provide a physically consistent framework for interpreting precursor-plasma conditions in the CRAFT negative ion source and for identifying operating regimes favorable for stable high-density plasma and enhanced atomic-hydrogen production near the PG.

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.

Neutral beam injection is an effective way for auxiliary heating in tokamak fusion research. To enhance the efficiency of extracting hydrogen positive ions for improving the injected power, research on negative bias voltage has been conducted through numerous experiments in the testbed of an arc ion source. Experimental results of extracted positive ion density are agreed with results predicted by sheath theory. The current of extracted hydrogen positive ions increased from 7.8 A to 8.7 A with negative bias voltage. The positive ion current remains constant when negative bias voltage exceeds 15 V. It has been observed that electron density is higher and electron temperature is lower compared to no bias conditions. The uniformity of electrons in the vicinity of extraction area is better by adding negative bias voltage. However, the plasma temperature on the extraction area was detected in the experiment against results predicted by sheath theory. An imperative phenomenon, which the electron temperature and density are increasing with negative bias voltage, has been seen. Ultimately, electron density remains constant and the change in electron temperature is less. The changes in plasma parameters will be explained by electron energy distribution function approach, since the research work of plasma sheath theory for plasma characteristic is relatively less.

A magnetic filter field can be used to reduce negative ion collisional loss in the negative ion source for fusion. However, the plasma can be drifted by the magnetic field that is produced by current flowing through the plasma grid (PG), and the non-uniformity plasma may influence the safety of ion source operation with a long pulse. PG with return wires is applied to optimize the plasma uniformity practically, but the implications on the negative ion source have not been thoroughly studied. The effect of a magnetic filter field with and without return wires is carried out in a radio frequency (RF) negative ion source by comparison of magnetic field distribution, plasma parameters, and beam uniformity. Simulation results indicate that the curvature of the magnetic field in the extraction region is reduced by 40%, which means the drift can be reduced to improve plasma uniformity with return wires. Also, the uniformity of electron density is increased to 95.9% while the electron temperature still maintains at around 1.5 eV according to the experimental results. The beam uniformity collected by a copper calorimeter with an infrared camera shows that the variance of temperature is decreased by 36.8%. The application of a magnetic filter field with return wires can improve the uniformity of plasma. The research gives a better understanding of the influence of the magnetic filter field on plasma and provides ideas for future upgrades and design of the RF negative ions sources.

Due to the high stability and less maintenance, the radio frequency (RF) driven ion source is preferred for the neutral beam injection (NBI) system. In a popular design of the RF ion source for NBI application, a Faraday shield (FS) is installed inside the RF plasma driver to protect the discharge tube. However, the FS also brings some drawbacks, such as lowering the RF power transfer and increasing the processing difficulty. A prototype of the RF plasma driver without FS and with mature manufacturing technology has been developed by using a water-cooled discharge tube. After basic testing, this prototype was further tested under the RF plasma discharge experiments in views of high power, long pulse, and long term. The reliability and its plasma characteristics were the focus of these experiments, also for the hidden issues. The results show that the prototype could generate stable and high-density plasma without any damage or sputtering mark. An RF plasma discharge of 50 kW and 20 s has been achieved. The expected frequency tuning was equally effective on the prototype. Moreover, compared to the RF plasma driver with FS, the prototype could produce higher electron density in the extraction region under the same RF power. Of course, some of the shortcomings of the prototype have also been exposed in the experiments and will be improved in subsequent experiments.

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.

In the negative hydrogen ion sources, the positive bias of the plasma grid with respect to the discharge chamber is often used to suppress co-extracted electrons. Because most of the co-extracted electrons are magnetically deflected onto the extraction grid, the difference between the extraction and acceleration current (i.e., Iext − Iacc) is commonly used to represent the number of co-extracted electrons. In the single-driver RF negative ion source at ASIPP, the variation of Iext − Iacc with the plasma grid bias voltage was accidentally found to have different trends with and without the Cs seeding. When the current density of Iacc (regarded as JH-) was larger than 120 A/m2 during Cs conditioning, the Iext − Iacc fell first and then rose with the increasing bias voltage. The previous research results from NIFS that the beam optics were changed by the bias voltage can be used to explain the phenomenon. In this paper, the electron temperature (Te) in front of the plasma grid was found to increase from the Langmuir probe measurement, when the bias voltage was higher than a threshold of 4.5 V to the float potential. Interestingly, the bias voltage corresponding to the trough of Iext − Iacc was higher than the float potential—a similar value of 4.1 V to the threshold of Te increasing. Since the negative hydrogen ions are easily destroyed by the high-energy electrons, the increasing of Te may also cause the change of beam optics and the increase in co-extracted electrons. The electron energy distribution function was used to explain the change of plasma parameters in front of the plasma grid.