Accelerator Grid Research Articles

Effects of translation misalignment on ion optics with slit apertures

For ion thrusters, the deflection of the ion beam caused by the relative translation of the grids is one of the primary factors limiting the erosion lifetime of the ion optical system. A two-dimensional (2D) simulation package of the ion optic system is developed to investigate the ion sputtering corrosion due to grid translation misalignment. For benchmark cases, the 2D simulation package shows a reasonable consistency compared to the experimental method in ion beam deflection angle and drain-to-beam current ratio, indicating the effectiveness of the simulation package. The deviation between the simulated deflection angle and the experimental value is within 8.86%. Furthermore, this 2D package is employed to analyze the variation patterns of ion beam, ion collection, and the distribution of ion sputtering rates caused by grid translation. The simulation results indicate that the ion beam deflection occurs in the direction opposite to grid translation. The number of ions collected at different positions on the acceleration grid shows different tendencies. Only the upstream surface Sy− and the aperture surface Sx+ will be subjected to energetic ion impingement. The sputtering of energetic ions on these two surfaces becomes the dominant factor limiting the grid’s ion corrosion lifetime when the grid translation is significant. The sputtering rate of energetic ions can exceed that of charge exchange (CEX) ions by more than 10 times. The proportion of the region where energetic ions contribute to sputtering expands with increasing grid misalignment, reaching up to 52.5% on surface Sx+. Additionally, the downstream surface Sy+ and the aperture surface Sx− are only subjected to CEX ion sputtering regardless of grid translation. Moreover, the CEX ion sputtering regions on surfaces Sx− and Sx+ expand as the grid misalignment distance increases. The peak in the ion sputtering rate distribution profile on surface Sy+ becomes more prominent due to grid translation, with the sputtering rate at the peak position reaching approximately 1.65 times that of the surrounding lower-rate regions. The analysis of the electric field indicates that local electric field variations caused by grid misalignment are the underlying reason for the ion erosion characteristics.

Optimizing the ITER NBI ion source by dedicated RF driver test stand

The experimental fusion reactor ITER will feature two (or three) heating neutral beam injectors (NBI) capable of delivering 33(or 50) MW of power into the plasma. A NBI consists of a plasma source for production of negative ions (extracted negative ion current up to 329 A/m2 in H and 285 A/m2 in D) then accelerated up to 1 MeV for one hour. The negative ion beam is neutralized, and the residual ions are electrostatically removed before injection. The beamline was designed for a beam divergence between 3 and 7 mrad.The ion source in ITER NBIs relies on RF-driven, Inductively-Coupled Plasmas (ICP), based on the prototypes developed at IPP Garching; RF-driven negative-ion beam sources have never been employed in fusion devices up to now. The recent results of SPIDER, the full size ITER NBI ion source operating at NBTF in Consorzio RFX, Padova, measure a beamlet divergence minimum of 12mrad and highlighted beam spatial non-uniformity. SPIDER results confirmed the experimental divergence found in smaller prototype sources, which is larger compared to filament-arc ion sources. Although prototype experiments have shown that the extracted current requirement can be achieved with minor design improvements, the beamlet divergence is expected to marginally achieve the design value of 7 mrad, which in multi-grid long accelerators results in unexpected heat loads over the accelerator grids. A contributor to the beam divergence is the energy/temperature of the extracted negative ions, so it is believed that plasma differences between the two source types play a role. Research is focused on the plasma parameters in the ion source.One RF driver, identical to the ones used in SPIDER, installed in a relatively small-scale experimental set-up, inherently more flexible than large devices, is starting operations devoted to the investigation of the properties of RF-generated plasmas, so as to contribute to the assessment of negative ion precursors, and of their relationship with the plasma parameters, particularly when enhancing plasma confinement. The scientific questions, that have arisen from the preliminary results of SPIDER, guided the design of the test stand, which are described in this contribution, together with the diagnostic systems and related simulation tools. The test stand, which shares with the larger experiment all the geometrical features and constraints, will allow technological developments and optimized engineering solutions related to the ICP design for the ITER NBIs.

Development of Acceleration Grid Power Supply of NNBI System for CRAFT

Development of Acceleration Grid Power Supply of NNBI System for CRAFT

Electron heating mode in magnetic field diffusion region of microwave discharge ion thruster

In magnetic field design principle of microwave discharge ion thruster, it is universally received that enlarging the magnetic mirror region can confine more electrons to acquire better energy utilization rate, while reducing the magnetic field diffusion region can prevent electrons from losing at wall to reduce the discharge loss. However, recently the integrated simulation proposes a hypothesis that electrons can also be heated in the magnetic field diffusion region when the Child-Langmuir sheath is considered as a constraint condition for electrons. Therefore, herein a magnetic field structure for the magnet array microwave discharge ion thruster is designed to verify the hypothesis, in which the magnetic field diffusion region is located near the screen grid. Then, an integrated simulation is conducted for studying the initial discharge and ion beam extraction stages of the thruster. The simulation results show that in the magnetic field diffusion region, the electron temperature is 4–8 eV when the grid system voltage is not applied, while the electron temperature is 4–12 eV when the the grid system voltage is applied. And the plasma density in the latter case has one order of magnitude higher than that in the former case. It means that electrons are obviously heated in the magnetic field diffusion region when they are confined among the Child-Langmuir sheath, the plasma sheath at antenna surface, and magnetic mirror. This electron heating mode produces more high-energy electrons outside the magnetic mirror region to generate plasma in front of the grid system, which can significantly increase the plasma density and ion beam current density. The result shows that under the conditions of 0.3 sccm (1 sccm = 1 mL/min) xenon gas flow, 1 W input microwave power, 300 V screen grid voltage and –50 V acceleration grid voltage, the ion beam current and its density are 0.47 mA and 0.60 mA/cm<sup>2</sup> for the magnet array microwave discharge ion thruster, while the ion beam current and its density are 1.2 mA and 0.38 mA/cm<sup>2</sup> for the 2-cm microwave discharge ion thruster. The ion beam current density increases by 57.9%. Through the integrated simulation, a new electron heating mode in the magnetic field diffusion region is proved theoretically, which provides a theoretical basis for the magnetic field structure optimization of microwave discharge ion thruster.

An integrative mathematical model of microwave ion thruster for mHz-frequency thrust noise analysis

Many high-precision space missions need thrusters to produce thrust with low noise to compensate for disturbances and ensure satellite platform stability. Microwave ion thruster is characterized with a wide thrust range and potential for these missions. A cost-effective and accurate mathematical model is crucial for mHz-frequency thrust noise analysis and feedback controller design. The Particle-In-Cell (PIC) and global models are two common simulation tools. The PIC model is characterized with high accuracy but huge computation cost, which is difficult to analyze long-time performance characteristics. Now, the global model is only used for the discharge chamber with low accuracy and cannot reflect ion extraction properties. In this paper, an integrative mathematical model is built for a 1-cm microwave ion thruster and can reflect ion beamlet divergence and impingement on the Accelerator Grid (AG). Simulation results show good agreement with experiments at 0.06 sccm. However, the model demonstrates worse consistency with experiments when the flux increases to 0.1 sccm, which may be because the influence of neutral gas on the Electron Cyclotron Resonance (ECR) is not considered in the model. A long-time (1000 s) simulation is conducted with this model under 35 μN. It takes 3 hrs, and the thrust noise reaches 1 μN/Hz0.5 at 1 mHz.

Model improvement and analysis of NBI power supply under breakdown condition

Breakdown commonly occurs in the acceleration grids of the NBI system. It will cause destructive effects on the accelerating grids, and the generated transient overvoltage and current will threaten the normal operation of the acceleration grid power supply (AGPS). Therefore, it is very necessary to model the circuit to analyze the effects of breakdown on the NBI system. However, the current models are not accurate enough and are likely to have insufficient knowledge of the consequences of breakdown. In this paper, a series of improvements are made to the model by analyzing in detail the change process of AGPS during breakdown and correcting the model parameters in conjunction with the existing AGPS design scheme. Subsequently, the key indicators of the NBI system under breakdown are listed. The impact of breakdown and the need for some protection are assessed based on the results of these indicators. The results of this paper are instructive for the design and breakdown protection of future NBI systems.

A study of grid failure mode drivers and methods for accelerated life testing of a 30 cm diameter ion thruster

In view of the high cost caused by the 1:1 lifetime verification test of ion thrusters, the lifetime acceleration test should be considered. This work uses the PIC-MCC (Particle-in-Cell Monte-Carlo Collision) method to analyze the five failure factors that lead to the failure of the accelerator grid of a 30 cm diameter ion thruster under the working mode of 5 kW. Meanwhile, the acceleration stress levels corresponding to different failure factors are obtained. The results show that background pressure has the highest stress level on the grid’s erosion. The accelerator grid aperture’s mass sputtering rate under the rated vacuum degree (1 × 10−4 Pa) of 5 kW work mode is 8.78 times that of the baseline vacuum degree (1 × 10−6 Pa), and the mass sputtering rate under worse vacuum degree (5 × 10−3 Pa) is 5.08 times that of 1 × 10−4 Pa. Under the influence of the other four failure factors, namely, the voltage of the accelerator grid, upstream plasma density, the screen grid voltage and mass utilization efficiency, the mass sputtering rates of the accelerator grid hole are 2.32, 2.67, 1.98 and 2.51 times those of the accelerator grid hole under baseline condition, respectively. The ion sputtering results of two 30 cm diameter ion thrusters (both installed with new grids assembly) after working for 1000 h show that the mass sputtering rate of the accelerator grid hole under vacuum conditions of 5 × 10−3 Pa is 4.54 times that under the condition of 1 × 10−4 Pa, and the comparison error between simulation results and test results of acceleration stress is about 10%. In the subsequent ion thruster lifetime verification, the working vacuum degree can be adjusted according to the acceleration stress level of background pressure, so as to shorten the test time and reduce the test cost.

Direct current measurements of the SPIDER beam: a comparison to existing beam diagnostics

For negative ion beam sources there are several methods of measuring the accelerated beam current, most commonly electrical measurements at the power supply and calorimetric measurements. On SPIDER, the ITER Heating Neutral Beam full-scale beam source prototype, electrical measurements at the acceleration grid power supply (AGPS) are complemented by polarizing the diagnostic calorimeter STRIKE to provide an additional electrical measurement of the accelerated current. This is in addition to the calorimetric measurements provided by STRIKE. These diagnostics give differing measurements of the beam current. Exploiting the reduced number of open apertures on SPIDER a new beam diagnostic has been installed to measure the individual beamlet currents directly. The so called Beamlet Current Monitor (BCM) has been used to measure the current of five beamlets during the most recent SPIDER campaign. This work compares the BCM current to the electrical measurements at the AGPS and STRIKE. The average BCM current agrees well with the STRIKE electrical measurements, indicating that the AGPS overestimates the beam current. The individual beamlets are compared to the STRIKE calorimetric measurements, showing similar current trends with the source parameters.

A modular multilevel DC active power filter scheme based on optimal parameter design for the CFETR N-NBI accelerator grid power supply

A DC active power filter (DAPF) based on a modular multilevel converter scheme is proposed for the accelerator grid power supply (AGPS) in a neutral beam injector system, to reduce the DC bus voltage ripple of AGPS. It works better than adding capacitors directly to the DC bus. This will help reduce the output voltage ripple of the AGPS at rated operating conditions, improve the focus state of the extraction beam, and reduce the occurrence of breakdown conditions. The scheme is special in that each module can compensate one frequency component of the ripple current, and the maximum switching frequency is not more than 4 times the ripple frequency. The electrical parameters of all DAPF modules (switching states and module capacitor voltages) are optimal values obtained through an optimal parameter design algorithm. In the process of applying the algorithm, two types of APF modules are distinguished, which have different switching frequencies and ripple suppression effects to adapt to different occasions. The simulation results show that each module can reduce the relative frequency component of the ripple by about 80% under the optimal parameters. Similar results are obtained by the 400V AGPS prototype and 20 V DAPF prototype. The ripple reduction scheme can be applied to other high-power applications with DC bus.

Modeling grid erosion in the NEXT ion thruster using the CEX2D and CEX3D codes

NASA’s Evolutionary Xenon Thruster (NEXT) is a candidate for future deep space missions that offers high efficiency and specific impulse over a large power throttling range. One of the key life-limiting components is the ion accelerator system, which is subject to sputter erosion by low energy discharge plasma ions incident on the upstream screen grid and higher energy charge exchange ions that impact the downstream accelerator grid. The grid erosion codes CEX2D and CEX3D were validated with data from tests of NEXT as well as the NSTAR ion thruster and then used to assess the time to failure in space due to screen grid erosion and electron backstreaming caused by accelerator grid aperture erosion. Screen grid erosion was found to be important only at the lowest throttle levels, and was conservatively estimated to lead to failure after processing over 900 kg of xenon. The first failure mode at high power levels was found to be electron backstreaming due to accelerator grid hole wall erosion, which would occur after processing over 700 kg of propellant.

Three-dimensional analysis of RF-biased ion optics with misalignments of apertures

Accelerator grid hole shift is a critical reason for erosion failure of optic systems in ion thrusters, which may also cause an unexpected roll torque about the ion beam axis. A three-dimensional (3D) model of ion optics is developed based on a particle-in-cell Monte Carlo collision method to investigate the plasma dynamics and performance of radio frequency (RF) grid systems with misalignments of apertures, which are compared with those in the direct current (DC) grid system. For the benchmark case, the 3D model gives a better agreement with the experiments in the ion energy distribution function (IEDF) compared with the two-dimensional model from a previous publication, with 36.4% and 47.9% relative error reduction of the peak position and full width at half maximum (FWHM), respectively, indicating the effectiveness of the developed 3D model. Simulations show that in the RF grid system the ion beamlet is deflected in the direction opposite to the shift of the accelerator grid hole, while electrons move first in the hole shift direction, and then deflect in the opposite direction. The average ion beamlet deflection angle calculated is consistent with the predictions from linear optical theory in both the RF and DC grid system. The amplitude of beamlet deflection angle fluctuation with time decreases with the increase of RF frequency. When the grid holes shift, the ion beamlet will deflect with the divergence angle almost unchanged in the DC grid system, while the beamlet divergence angle increases in the RF grid system. When RF frequency is low, the big vortex-like structure in the electron velocity phase diagram breaks into small vortices, showing a reduced oscillation intensity. The hole shift also causes high-frequency oscillation in the shift direction. In terms of performance, the RF grid system is more sensitive to grid hole shift than the DC grid system.

Ion thruster accelerator grid erosion mechanism under extreme conditions of cylindrical erosion and chamfer erosion

In this paper, the abnormal experimental phenomenon on barrel erosion under extreme working conditions in the ultra-long life experiment (>10000 h) of ion thruster ion optics is studied by the Immersed-Finite-Element Particle-In-Cell Monte-Carlo-Collision (IFE-PIC-MCC) method and the grid erosion evaluation model. The transport process of beam ions and Charge Exchange (CEX) ions in the grid system, and the characteristics and mechanisms of the aperture barrel erosion under extreme erosion conditions (i.e. the cylindrical erosion and chamfer erosion) were systematically studied. Thanks to the advantage of the IFE method for dealing with complex boundaries in structured mesh, the aperture barrel erosion morphology of the accelerator grid is reconstructed accurately based on the experimental results. The results show that, with the evolution of working conditions, the mechanism of the aperture barrel erosion changes significantly, which relies heavily on the accelerator grid morphology. The change of the accelerator grid aperture barrel morphology has a significant effect on the behavior of CEX ions, and only affects the local electric field distribution, but has no effect on the upstream plasma sheath. As the erosion progresses, the erosion position moves downstream along the grid aperture axis direction, and the erosion range becomes narrower. Regardless of the erosion phase, the erosion rate of the CEX ions located downstream of the decelerator grid is the largest. The erosion rate is related to the mean incident energy and angle, and their variation is closely related to the position and trajectory of CEX ions.

Heat loads on the accelerator grids of the ITER HNB prototype

Among the heating and current drive system which are being developed for the ITER experimental reactor, 2 neutral beam injectors with a power of 16.5 MW each will be based on the acceleration of H−/D− ions up to the energy of 870 keV / 1 MeV. Dealing with an accelerated beam power up to 40 MW, beam divergence, aiming and homogeneity have to be optimized to avoid damaging the beamline components or the grids of the accelerator. The design of the ITER HNB prototype, MITICA, was based on the coupling of several codes starting from experimental inputs available at that time. Heat loads, in particular, were estimated by the 2D version of the code Monte Carlo particle tracing code EAMCC. In the last years, a 3D version of the same code was developed and the operation of negative ion sources for fusion increased the knowledge concerning those parameters which are fundamental to estimate the expected heat loads. In this work, calculations are performed by EAMCC3D and the role of different source and accelerator parameters, such as the negative ion temperature and the beam halo, is highlighted.

Optimized design of the ion optics based on the over-perveance characteristics of the beam current

To illustrate the erosion mechanism of the edge aperture by high-energy ions, a three-dimension simulation model was established, and a particle-in-cell method was applied in tracking the hitting processes as well as a Monte Carlo collision method was used to deal with the particles’ collision. Numerical results showed that the potentials interacted among the sheaths of adjacent holes and the beam ions were over-focused in the accelerator gap, causing some ions to be intercepted on the downstream wall of the decelerator grid. This was a typical under-perveance characteristic of the beam current, operating at significantly less than the optimal perveance and corresponding to lower beam currents than optimal combination, which pushed the sheath to the left farther into the plasma. The farther the grid was, the more serious erosion patterns developed. In addition, the charge exchange ions were attracted and bombarded on the decelerator and the accelerator grids, respectively. The ion optics of the 30 cm ion thruster was optimized from four-zone to three-zone, where the plasma density upstream of the screen grid was increased by 1.5% and the perveance characteristics of the beam ions were improved such that many ions struck the edge hole. The background pressure was reduced from 3.0 × 10−3 to 8.0 × 10−4 Pa to decrease the erosion depths of the grids caused by the charge exchange ions, about 9.09% and two orders of magnitude, respectively, for the accelerator and decelerator grids.

Research and Analysis of Equivalent Circuit Model for Core Snubber

The ion source is the key to the NBI. It is very sensitive to overvoltage and overcurrent, and the breakdown frequently occurs between the acceleration grid. The core snubber is the most suitable surge suppression device to achieve DC fault protection. This paper introduces some basic concepts, comparing the core snubber’s material properties and important parameters. It also introduces the basic concept of an equivalent circuit model for snubber, including the derivation process and theoretical analysis. Then the characteristics of the snubber and surge suppression analysis are introduced. Finally, this paper looks forward to the future research direction of the snubber.

SPIDER, the Negative Ion Source Prototype for ITER: Overview of Operations and Cesium Injection

An overview of the recent operations and the main results of cesium injection in the Source for the Production of Ions of Deuterium Extracted from Rf plasma (SPIDER) negative ion source are described in this contribution. In experiments without cesium injection, all SPIDER plants were tested to verify the basic expectations on the operational parameters (e.g., electron cooling effectiveness of magnetic filter field) and to determine its operational region. For beam properties, it was shown that the current density varies across the beam in the vertical direction. In preliminary cesium experiments, the expected increase of negative ion current and simultaneous decrease of co-extracted electrons were found, along with the influence of the control parameters (polarization of the plasma electrodes, magnetic filter field) on the SPIDER beam uniformity in the horizontal and vertical directions. It was shown that non-Gaussian tails can be identified in the angular distribution on the plane perpendicular to the beam propagation direction. Stray particles, nonhomogeneous beam and large divergence might result in unexpected heat and particle loads over ITER neutral beam injector (NBI) accelerator grids; it is the goal of SPIDER to assess and possibly to identify suitable methods for controlling these beam features. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, is outlined. Such improvements are expected to allow SPIDER to pursue the ITER requirements in terms of negative ion current, electron-to-ion ratio, and beam duration.

DC active power filter based on model predictive control for DC bus overvoltage suppression of accelerator grid power supply

The China Fusion Engineering Test Reactor plans to build a 200 kV/25 A acceleration grid power supply (AGPS) for the negative-ion-based neutral beam injector prototype system. The AGPS uses a rectifier-inverter-isolated step-up structure. There is a DC bus between the rectifier and the inverter. In order to limit DC bus voltage ripple and transient fluctuations, a large number of capacitors are used, which degrades the reliability of the power supply and occupies a large amount of space. This work finds that due to the difference in the turn-off time of the rectifier and the inverter, the capacitance mainly depends on the rectifier current when the inverter is turned off. On this basis, an active power filter (APF) scheme is proposed to absorb the current. To enhance the dynamic response ability of the APF, model predictive control is adopted. In this paper, the circuit structure of the APF is introduced, the prediction model is deduced, the corresponding control strategy and signal detection method are proposed, and the simulation and experimental results show that APF can track the transient current of the DC bus and reduce the voltage fluctuation significantly.

Modelling activity in support of MITICA high voltage system protections

MITICA, the full-scale prototype of ITER Heating Neutral Beam (HNBs), has challenging requirements on its power supplies. In particular its unconventional high voltage system is designed to operate at high power and voltage (up to -1 MV dc and about 54 MW), with many interfaces and high voltage insulation technologies co-existing in a single plant. Moreover, the electrical system must manage frequent breakdowns between the accelerator grids during the operation, creating electrical fast transients characterized by extremely high voltage and current peaks. In the conceptual design phase suitable design provisions were conceived to limit the energy in the grids and to limit the overvoltage on specific components in the case of grids-breakdown. The integration tests of the power supplies showed the capability of the system to generate and sustain voltage as high as 700 kV; at higher voltage the voltage withstanding capability was not confirmed as some breakdowns occurred in the plant. Although the location of the breakdowns has not been identified yet, an impact of such breakdowns was highlighted as a couple of the components of the plant suffered a damage. This paper is focused on the analyses performed for the understanding of the fault occurred on the secondary bushing of the 1 MV insulating transformer dedicated to the supply of the Ion Source and Extraction Power Supply system (ISEPS). An extensive activity of modelling and simulation has been developed for explaining the dynamic occurred. The model is based on the stray parameters of the components, which were not easy to be estimated in the design phase, but which turn out to have relevant effects in local fast transient phenomena. After reaching a full understanding of the phenomena, the models have been used to support the conceptual design of an additional protection system for the transformer, for MITICA but also in view of the ITER installation.

Start-up process of ion optics in ion thrusters

In this study, a particle-in-cell model of ion optics was established to simulate the changing trend of an ion beam shape over time during an ion optic start-up. This model was established to study the effect of ion beam shape change on the accel grid during ion optical start-up and to discuss the start-up strategy of ion optics. The results show that the ions at the edge of the beam have a radial velocity component toward the centerline of the ion optics owing to the electric field; this causes the ion beam to shrink gradually toward the centerline over time. Furthermore, the accel grid can be prevented from being bombarded by ions by design at the operating voltage. However, the grid may still be bombarded by ions during the low-voltage start-up process, thereby affecting its life. Thus, the ion dynamics must be considered during the start-up process.

Development of HVDC Gas-Insulated Components for the Power Supply of Neutral Beam Injectors

The development of High Voltage Direct Current Gas-Insulated components that will be employed for the power supply of Neutral Beam Injectors, requires specific numerical tools to analyze the electric field distribution, with the purpose of supporting the engineering and operational phases.The tools must comply with the nonlinearity of the material properties of solid insulators and the transport mechanisms in the dielectric gas. Despite its high Global Warming Potential, SF6 will be considered as the baseline dielectric gas for the development of HVDC gas-insulated components to be used for the Acceleration Grid Power Supply of Neutral Beam Injectors. Attention especially to the mechanisms driving to an increase of leakage current flowing in the gas, thus giving rise to possible discharge phenomena is here placed, mainly focusing on radiation and injection regimes. The described methodologies are adopted for the numerical modeling of the 500 kV DC gas-insulated components that will be used for the power supply of the Neutral Beam Injector of the Divertor Tokamak Test facility. In particular, a conceptual design of the SF6 Transmission Line and the SF6-air bushing is conceived.