Key issues for the filament-arc ion source of EAST neutral beam injector toward high-power and long-pulse operation

Abstract

Neutral beam injection is an effective plasma heating and current drive method for magnetic confinement devices. For supporting the high-confinement plasma operation of the fully superconducting tokamak EAST, a high-power and long-pulse neutral beam is foreseen to be injected into EAST. To achieve such a mission, the most important task is to develop a qualified ion source which can launch a positive ion beam for pulse length up to 100 s with the beam power larger than 2 MW. During the research and development on the EAST positive ion source, several key issues have been studied, including the heat load on electrode grids, the backstreaming electrons deposition, and the gas discharge efficiency. The capability of heat removal of the electrode grids has been demonstrated via a series of simulations and experiments. However, the backstreaming electrons deposition was extremely dense on the electron dump of the ion source, leading to its serious melting damage. According to the simulation of the trajectories of backstreaming electrons, the central magnets on the electron dump were removed as a preliminary solution. The melting damage was successfully avoided without the central magnets and the modified ion source has been operated without malfunction until now. But the total deposition of backstreaming electrons on the electron dump was increased and the magnetic confinement of primary electrons was reduced. Consequently, the arc efficiency was lower and the fraction of the neutral atoms with half beam energy was higher for the modified ion source.

The negative beam source with single driver for CRAFT NNBI: design and conditioning results

“…A negative beam source based neutral beam injection (NNBI) system is one of the most important devices to be developed for CRAFT. The target of the NNBI system is the ability to deliver a powerful hydrogen beam with beam energy of 200-400 keV, beam power of 2 MW and beam duration of 100 s. A radio frequency (RF) beam source [1] is a good choice to be operated with a long pulse due to its long lifetime and no tungsten contamination compared with hot cathode arc source, even if it was used worldwide [2][3][4][5]. So, the RF based negative beam source, which was also the reference source for the ITER-NBI system [6,7], was employed for the CRAFT NNBI system [8].…”

Enhancing ion extraction with an inverse sheath in negative hydrogen ion sources for NBI heating

“…An ion source is indispensable for the NBI system, and both positive and negative hydrogen ion sources have been developed for the NBI systems. Positive-ion-based NBI is commonly employed in most present-day fusion devices [4][5][6]. A negative ion source has been tested for NBI in JT-60U, JT-60SA, LHD [7][8][9][10], and a prototype of a negative ion source for ITER-NBI is being developed [11,12].…”

Breakdown prediction of ion source at EAST-NBI using neural network

“…• The initiating events, which can be internally driven (for example, the mismatch of operating parameters or bad vacuum [11,12]), or externally driven (for example, a failure of control equipment); • The precursor phase events that depends on the specific initiating events (for example, when the ion optical properties are in a non-optimal state, the ion beam bombards the ion source electrode, accumulating a large number of secondary electrons, and the secondary electrons collide with the ion beam [13][14][15][16][17][18]. Figure 3 shows the movement and collision of ions between the electrodes of the ion source); • The events of over current.…”