Experimental Advanced Superconducting Tokamak Neutral Beam Injector Research Articles

Beam Optics Analysis for the 120-keV Multiaperture Accelerator of Neutral Beam Injector

Neutral beam injection is an important auxiliary heating method for magnetic confinement nuclear fusion. To further optimize the steady-state operation of the high-temperature plasma in the Experimental Advanced Superconducting Tokamak (EAST), a higher beam energy is required for the high-power, long-pulse neutral beam injection system. The most critical technology to achieve this goal is the upgrade of the ion source accelerator. This paper analyzes the beam optics of the multiaperture accelerator for the extraction of a deuterium ion beam of 120 keV/50 A with a divergence angle of less than 1 deg root-mean-square. The effects of the number of grids, shapes of plasma grids (PGs), grid gaps, voltage ratio, and plasma parameters on beam optics were assessed by numerical simulation. The results allowed optimization of the PG shape; they also indicated that the tetrode configuration can reduce the beam divergence angle effectively while the triode configuration can extract a higher current. These conclusions provide guiding significance for the selection and parameter design of the EAST neutral beam injector ion positive source with a 120-keV accelerator.

Neutronic response of the neutral beam after EAST NBI upgraded

In order to improve the heating effect of neutral beam injector, the neutral beam injector (NBI) of the Experimental Advanced Superconducting Tokamak (EAST) is planned to be upgraded. After the EAST NBI is upgraded, the neutron emission intensity will increase. Neutron leaking from the neutral beam port penetrate the beamline, resulting in the material radiation damage. The neutronic response has been investigated by using the Monte Carlo particle transport code MCNP with the two dimensional RZ model of the EAST NBI. The results demonstrate that a 40% increase of the neutron emission rate has been attained with the upgraded EAST NBI. However, the simulations for the activation of different materials, displacement damage and nuclear heating of the neutral beam still satisfy the requirement of material safety performance. The calculations of nuclear response show that the materials can meet the requirements of the NBI steady state operation after the EAST NBI updated.

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

The designed neural networks are trained to appraise the risk of ion sources breakdown events in the neutral beam injector (NBI) experimental device using several offline diagnostic signals as inputs. A saliency analysis proves the reasonableness of the chosen inputs, some of which are helpful to improve the network performance. The experimental tests that were carried out refer to data collected from successfully-terminated and breakdown-terminated shots performed during three years of experimental advanced superconducting tokamak NBI experiments, from 2016 to 2019. Results show that it is very possible to develop a predictor base on neural network that intervenes well in advance to avoid ion sources breakdown or mitigate its effects during the beam extraction in the high-power and long-pulse mode.

Development of Test System for Arc Power Control of EAST Neutral Beam Injector Ion Source

Neutral beam injection is recognized as one of the most effective methods for plasma heating. According to the requirements for physics experiments of the Experimental Advanced Superconducting Tokamak (EAST), two beamlines for neutral beam injection were built in 2014. In order to meet the diverse requirements of magnetic confinement experiments, recent attempts have been carried out for arc power control of a neutral beam injection ion source. A test system was added to the arc power control system. It provides a new method to smoothly change arc power during a discharge. According to the experimental results of controlling arc power, the fesibility of varying the arc power was verified in the duration of discharge. Meanwhile, the test system achieves the remote setting of arc power parameters.

Analysis of Effects of the Arc Voltage on Arc Discharges in a Cathode Ion Source of Neutral Beam Injector**supported by the National Magnetic Confinement Fusion Science Program of China (No. 2013GB101000) and National Natural Science Foundation of China (No. 11405207)

A hot cathode bucket ion source is used for the EAST (experimental advanced superconducting tokamak) neutral beam injector. The thermal electrons emitted from the surface of the cathode are extracted and accelerated by the electric field formed by the arc voltage, which is applied between the arc chamber of the ion source and the cathode. This paper analyzes the effects of arc voltage on the arc discharge in a hot cathode high current ion source.

Performance of positive ion based high power ion source of EAST neutral beam injector.

The positive ion based source with a hot cathode based arc chamber and a tetrode accelerator was employed for a neutral beam injector on the experimental advanced superconducting tokamak (EAST). Four ion sources were developed and each ion source has produced 4 MW @ 80 keV hydrogen beam on the test bed. 100 s long pulse operation with modulated beam has also been tested on the test bed. The accelerator was upgraded from circular shaped to diamond shaped in the latest two ion sources. In the latest campaign of EAST experiment, four ion sources injected more than 4 MW deuterium beam with beam energy of 60 keV into EAST.

Upgrade of accelerator of high current ion source for EAST neutral beam injector

A 4MW high current ion source was developed for Experimental Advanced Superconducting Tokamak (EAST) neutral beam injector. In order to optimize the beam performance of high current ion source, the plasma grid of accelerator is replaced from circular cross section grid to diamond cross section grid. A new high current ion source with modified diamond plasma grid was designed and tested on the test bed. The results shows that, the mean arc efficiency is increased from 0.47A/kW to 0.52A/kW, and the divergence angle decreased from 1.8° to 1.4° with optimum beam perveance of 2.75μperv and 2.3μperv, respectively. The ion source with modified plasma grid can improve the performance of ion source effectively.

Preliminary experimental study of deuterium beam species for the EAST neutral beam injector by Doppler shift spectroscopy

Deuterium neutral beam injection is playing an important role in magnetic confinement fusion research at present. A preliminary result of beam species is obtained by using Doppler shift spectroscopy on the Experimental Advanced Superconducting Tokamak (EAST) neutral beam injector. Typical spectral experimental results are presented to illustrate the performance of the ion source. The results show that the full energy particle is up to 80%—reaching its design specifications.

Preliminary Experimental Study of Shine-Through Power for EAST-NBI by Infrared Pyrometer

Deuterium neutral beam injection is employed for assistant heating of plasma on experimental advanced superconducting tokamak (EAST). A set of infrared pyrometers is used for fast interlock protection system on EAST neutral beam injector. Typical experimental results are presented to illustrate the performance of beam shine through. During the high power injection, the maximal surface temperature will reach up to about 600 °C. If the temperature is higher than the limited value, the beam pulse is interrupted in about 20 ms. The shine through power is explained qualitatively with the performance of the plasma.

Ion beam optics analysis and beam transmission simulation in the reflection magnet of the EAST NBI system

Detailed trajectories of the residual ion passing through a reflection magnet, which is installed in the Experimental Advanced Superconducting Tokamak (EAST) neutral beam injector (NBI) system to remove the residual ion from the beam path, are reported. Monte Carlo simulation has been performed to obtain estimations of the beam density on the high heat flux elements of reflection magnet. On magnet pole shielding, there are three high beam power density regions, resulting from beam line focus and corresponding to the full energy, half energy and third energy. These high heat flux regions will bring a serious challenge for steady-state operation. The simulation result indicates that the re-ionization loss is about 2.43% and the magnet pole shielding encounters a problem of beam line focus, which should be taken seriously during the steady-state operation.

Analysis and development of a high performance core snubber based on deltamax

The core snubber is one of the most important devices in the EAST (Experimental Advanced Superconducting Tokamak) with NBI (Neutral Beam Injector) system. This device is designed to protect the NBI system during periods of high voltage breakdown. The theory for the core snubber was developed by Fink, Baker and Owen (the FBO model), who considered only resistance effects which generates a snubber design based on saturation conditions. The paper presents an approach to limit high voltage breakdown effects based on the concepts of the equivalent resistance and inductance implemented on a parallel circuit model with Deltamax cores. The EAST NBI circuit fault simulation code simulates fault conditions using a time-varying resistance and inductance in the parallel circuit model. The code was calibrated with the experimental results. Based on the simulation results an ion source core snubber using Deltamax was designed. This snubber was tested at 50 kV in a real NBI system, which has a large gradient capacitor, and a peak short current less than 400 A. The results show that testing verify the modeling method and two such snubbers can protect ion source reliability at 100 kV.

Development and conditioning of the high current ion source for the EAST neutral beam injector

A 4MW high current ion source is developed for the Experimental Advanced Superconducting Tokamak (EAST) neutral beam injector (NBI). The EAST-NBI ion source is conditioned and the primary results and analysis are reported in this paper. The plasma current of plasma generator reaches 1100A and maximum arc power reaches 90kW. Plasma density uniformity is measured by Langmuir probes and is estimated as 90%. The hydrogen ion beam extracted from a tetrode multi-slot accelerator with beam energy of 80keV. The mean arc efficiency is 0.5A/kW and beam pervereance is 2.5μp. The experimental results show that, the high current ion source is developed and conditioned successfully.

Analysis and Application of the Series Core Snubber

The transformer core snubber (CS), as one of the most important components in the EAST (experimental advanced superconducting tokamak) NBI (neutral beam injector) system, is designed to limit grid damage and protect the ion source during periods of electrical breakdowns. A transformer core snubber is analyzed in detail in this paper. Several kinds of soft magnetic cores are presented and compared. With analysis and experiment on the basic characteristics of the cores, the most suitable materials are suggested. The circuit simulation code is established which could simulate faulty conditions with concentrated and distributed CS concepts. Based on the above work, an ion source CS is developed with series type of distributed topology. The CS has been subjected to experimental validation at 80 kV with a peak short-current of approximately 400 A in a real NBI system, which proves the accuracy of the adopted assumptions and the analysis method.

Gas Flow in the Neutralization Region of EAST Neutral Beam Injector

The gas flow in the neutralization region affects the neutralization efficiency as well as the beam transition efficiency of whole neutral beam injector, which will be applied as a high power auxiliary heating and non-inductive current driver system for the Experimental Advanced Superconducting Tokamak (EAST). The neutralization region of EAST neutral beam injector includes not only the gas cell neutralizer in traditional sense, but also part of the ion source downstream the electrode grid, the gate valve, and the transitional piping and fitting. The gas flow in this neutralization region has been modeled and researched using an adjusted Direct Simulation Monte Carlo code to understand the formation mechanism of gas target thickness, which determines the neutralization efficiency. This paper presents the steady-state, viscous and transition region flowfields, the gas density distribution and the various centerline profiles including Knudsen number, temperature, pressure and axial velocity by injected the deuterium gas from the arc chamber and neutralizer, respectively. The target thickness in the neutralization region as a function of gas inlet quantity is also given in the absence of beam for future operation of EAST neutral beam injector.