Yuwen Yang

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%.

In a neutral beam injection (NBI) system, the accelerated ions should be neutralized before being injected into the magnetically confined plasma. Plasma neutralizer is promised for the future negative ion-based NBI system due to its higher neutralization efficiency. A plasma neutralizer prototype for the MW-level negative ion beam is under development under the project of the Comprehensive Research Facility for Fusion Technology (CRAFT). A global plasma model and a three-dimensional (3D) charged particle tracing model were used to evaluate the neutralization performance of the conceptual design of the plasma neutralizer prototype. The global plasma model indicated that, based on the CRAFT dual-driver RF negative ion source, the ionization degree of the beam-driven plasma was less than 1%, and the neutralization efficiency of 59% did not reveal its advantage. An external discharge power of 10 kW via filament arc discharge could obviously enhance the generated plasma density and increase the corresponding neutralization efficiency to 72%. Via the 3D charged particle tracing model, the energy loss and the uniformity of plasma distribution were studied by calculating the motion trajectories of electrons. The Halbach array of confining magnets showed the best performance in terms of the survival time and the distribution of the primary electrons.

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.

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.

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.

Design processes of a large-scale negative ion accelerator for neutral beam injection (NBI) application involve a series of physics and engineering issues, which include high voltage holding, background gas and stripping losses, beamlets optics and steering, particle and power flux on the grid electrodes, heat removal, thermal deformation, and stress of the grids. A self-consistent design model covering all these critical issues has been developed, where the results of one design aspect can be directly plugged into another one as the input conditions with little approximation or assumption. This design model has been applied to the negative ion accelerator of the NBI test facility of CRAFT (Comprehensive Research Facility for Fusion Technology), which is designed to produce a negative hydrogen ion beam of 25 A with the particle energy of 400 keV and the pulse duration of 3600 s. The accelerated current density is required to be 210 A/m² from 768 apertures with a diameter of 14 mm. The evaluated results of the CRAFT accelerator design are quantitatively analyzed. Additionally, the modeling is applied to a large-scale and relatively complete structure of the multi-grid electrodes. Hence, some nonuniformities or special distributions appear in different design issues, which were not noticed in the reference works.