Vacuum System Optimization for EAST Neutral Beam Injector

The neutral beam injector (NBI) generates a high-energy ion beam and neutralizes it, and then relies on drift transmission to inject the formed neutral beam into the fusion plasma to increase the plasma temperature and drive the plasma current. In order to better cooperate with the Experimental Advanced Superconductive Tokamak (EAST), part of the Chinese major national scientific and technological infrastructure, in carrying out long-pulse high-parameter physics experiments of 400 s and above, this paper considers the optimization of the current design and operation of the NBI beam line with a pulse width of 100 s. Based on an upgraded and optimized NBI vacuum chamber and the structure of the beam-line components, the gas-source characteristics under the layout design of the NBI system are analyzed and an NBI vacuum system that meets relevant needs is designed. Using Molflow software to simulate the transport process of gas molecules in the vacuum chamber, the pressure gradient in the vacuum chamber and the heat-load distribution of the low-temperature condensation surface are obtained. The results show that when the NBI system is dynamically balanced, the pressure of each vacuum chamber section is lower than the set value, thus meeting the performance requirements for the NBI vacuum system and providing a basis for subsequent implementation of the NBI vacuum system upgrade using engineering.

Recent results from deuterium experiments on the Large Helical Device and their contribution to fusion reactor development

In recent deuterium experiments on the Large Helical Device (LHD), we succeeded in expanding the temperature domain to higher regions for both electron and ion temperatures. Suppression of the Energetic particle driven resistive InterChange mode (EIC) by a moderate electron temperature increase is a key technique to extend the high temperature domain of LHD plasmas. We found a clear isotope effect in the formation of the Internal Transport Barrier (ITB) in high temperature plasmas. A new technique to measure the hydrogen isotope fraction was developed in the LHD in order to investigate the behavior of the isotope mixing state. The technique revealed that the non-mixing and the mixing states of hydrogen isotopes can be realized in plasmas. In deuterium plasmas, we also succeeded simultaneously realizing the formation of the Edge Transport Barrier (ETB) and the divertor detachment. It is found that Resonant Magnetic Perturbation (RMP) plays an important role in the simultaneous formation of the ETB and the detachment. Contributions to fusion reactor development from the engineering point of view, i.e. Negative-ion based Neutral Beam Injector (N-NBI) research and the mass balance study of tritium, are also discussed

Study of RF Stray Currents in ITER Neutral Beam Test Facilities

The operation of SPIDER (Source for the Production of Ions of Deuterium Extracted from Radio-frequency plasma), full-scale prototype of ITER NBI (Neutral Beam Injector) radio-frequency ion source, pointed out deleterious effects caused by stray Radio-Frequency (RF) currents flowing in the electrical equipment not included in the RF power system. MITICA (Megavolt ITER Injector and Concept Advancement), the full-scale prototype of ITER NBI, is characterized by a similar design in terms of layout of the power supplies and connections to the beam source; thus, it is expected to be subject to the RF stray currents problem. SPIDER RF system is composed of four RF generators, four coaxial lines and four RF loads. Each RF generator is rated for operation at 200 kW in the frequency range 0.9 ÷ 1.1 MHz. The power is delivered to the four loads via as many RF coaxial lines, housed inside a multiconductor transmission line. Each load consists of a capacitive matching network and two plasma drivers in series. Due to the presence of stray connections at the generator and beam-source side (e.g., parasitic capacitances), unwanted RF currents can flow through alternative paths and negatively affect the components not intended for transmission of RF power, the output stages of power supplies and several diagnostics installed in the High-Voltage Deck (HVD) and at the beam source. This paper presents the development of a circuital model used to estimate the RF stray currents in SPIDER electrical system; the understanding of this phenomenon and the development of a model with predictive capabilities is fundamental for the assessment of the performance of both SPIDER and MITICA and, in general, of alternative RF system layouts with respect to the stray currents issue.

ECR discharge sustained by millimeter waves as a source of dense plasma flux

The results of the investigation of the dense ECR discharge hydrogen plasma flux formation in the single solenoid magnetic field are presented in this work. The transversal flux profile obtained at the optimal system parameters is shown. The possibility of the formation of homogeneous plasma fluxes with density of 750 mA/cm2 and total current of 5 A is demonstrated. The results of the first experiments of the hydrogen ion beam extraction from the ECR discharge plasma in the single magnetic coil are presented. The record values of the ion current density higher than 1.5 A/cm2 were obtained. The results of the research presented in this paper show the prospects of the proposed system for applications of the neutral beam injector development for the plasma heating in the controlled thermonuclear fusion installations.