Accelerator Grid Research Articles

The 6,500-H Life Test Results of 30 cm Diameter Ion Thruster

The life test of the 30 cm diameter ion thruster developed by the Lanzhou institute of physics began in Apr. 2018 and ended in Jan. 2020, lasting 6,500 h. This paper introduces the results of the 6,500-h life test of the 30 cm diameter ion thruster completed on the ground, including the ground facility for life test, the variations of the thrusters working performance, the times of the breakdown and power restart, and the erosion of the key components. The results show that the performance parameters, such as thrust, specific impulse and efficiency, do not change obviously during the test and magnetic field of the discharge chamber has no change. With the increase of test time, breakdown times increased significantly, whereas the power restart time decreased correspondingly. The diameter of the cathode orifice decreases gradually and there is a blockage risk of orifice. However, the diameter of the keeper orifice increases and presents an inverted cone erosion pattern. The diameter of the decelerator grid aperture expanded from 1.6 to 1.8 mm from 0 to 1,000 h and slightly enlarged after that. The aperture diameter of the accelerator grid presented linear enlargement but the pits-and-grooves erosion is obvious.

Accelerator Grid Life Modeling of T6 Ion Thruster for BepiColombo

The T6 ion thruster is providing in-space propulsion for the BepiColombo mission, currently en route to Mercury. Simulations of the accelerator grid erosion during flight operation were carried out using the CEX2D and CEX3D codes in order to verify that the grid lifetime will be sufficient to impart the 5.75 total impulse per thruster required for the mission. Before calculating in-space erosion rates, the models were benchmarked against data from long-duration wear tests. Two-dimensional simulations tracked the time-dependent grid geometry and calculated changes in the electron backstreaming limit as the grids eroded. Sensitivity studies assessed the impact of uncertainty in the operating screen-to-accelerator grid gap on . Three-dimensional simulations employed a particle-in-cell model for the charge exchange ions and were able to accurately reproduce the nonaxisymmetric pits and grooves erosion pattern observed on the downstream face of the accelerator grid following ground testing. Charge exchange ion space charge effects were found to broaden the pit erosion features in flight, further reducing the peak erosion rate relative to that observed on the ground. The simulations predicted that T6 will have sufficient margin against both electron backstreaming and accelerator grid wearthrough over the duration of the mission.

Expansion of alternatively extracted ion–ion beam in a low pressure collisional medium

The aim of this paper is to investigate, through particle-in-cell with Monte Carlo collisions (PIC-MCC) simulations, the effects of collisions on the expansion of an ion–ion beam formed by the alternate extraction of oppositely charged ions. This beam is extracted from an ion–ion plasma, formed in the downstream of a radiofrequency magnetically filtered iodine electronegative plasma, by the mean of two extraction grids. In this grid system, the screen grid is biased with a square voltage waveform, in the low MHz range, while the acceleration grid is grounded. The collisionless interaction between the extracted ion packets involves and enhances electrostatic waves propagating at beam velocity. Our results show that in addition to these electrostatic waves, the presence of significant fraction of low energy ions, produced by charge exchange, beside extracted fast ions gives rise to two-stream instability. Furthermore, space and time dependent electric field might accelerate the slow ions, produced by charge exchange collision. This acceleration, involving Landau damping and its inverse mechanism, is mainly due to the interaction between ions and potential patterns inherent to two-stream instability.

Design and development of acceleration grid power supply-conversion system for CFETR N-NBI

The acceleration grid power supply (AGPS) is developed to provide accelerator power for China Fusion Experimental Rector (CFETR) Negative-ion Neutral Beam Injection (N-NBI) system. The rated output parameters of this AGPS are -1000kV/80A/80MW. The preliminary design of the AGPS is proposed in this paper. To improve the steady-state and dynamic performance of the AGPS, a new CFETR AGPS conversion system (AGPS-CS) based on three-level neutral point clamped (3L-NPC) rectifier and five-level neutral-point-clamped/cascaded H-bridge (5L-NPC/H) inverter is presented. The parameters of AGPS-CS are obtained theoretically, and simulations are used to verify its operational performance and show the effectiveness of the proposed topology. Results indicate that the proposed design provides good input-output features and can meet the requirements of the CFETR N-NBI system. This solution can also be used as a reference for the design of large-power high-voltage power supply.

Lifetime Considerations for Electrospray Thrusters

Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.

RF Helicon-based Inductive Plasma Thruster (IPT) Design for an Atmosphere-Breathing Electric Propulsion system (ABEP)

Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend such missions lifetime, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planet with atmosphere, enabling new mission at low altitude ranges for longer times. Challenging is also the presence of reactive chemical species, such as atomic oxygen in Earth orbit. Such species cause erosion of (not only) propulsion system components, i.e. acceleration grids, electrodes, and discharge channels of conventional EP systems. IRS is developing within the DISCOVERER project, an intake and a thruster for an ABEP system. The paper describes the design and implementation of the RF helicon-based inductive plasma thruster (IPT). This paper deals in particular with the design and implementation of a novel antenna called the birdcage antenna, a device well known in magnetic resonance imaging (MRI), and also lately employed for helicon-wave based plasma sources in fusion research. This is aided by the numerical tool XFdtd®. The IPT is based on RF electrodeless operation aided by an externally applied static magnetic field. The IPT is composed by an antenna, a discharge channel, a movable injector, and a solenoid. By changing the operational parameters along with the novel antenna design, the aim is to minimize losses in the RF circuit, and accelerate a quasi-neutral plasma plume. This is also to be aided by the formation of helicon waves within the plasma that are to improve the overall efficiency and achieve higher exhaust velocities. Finally, the designed IPT with a particular focus on the birdcage antenna design procedure is presented.

Experimental and numerical investigation on the asymmetry of the current density extracted through a plasma meniscus in negative ion accelerator

In multi-beamlet negative ion accelerators for neutral beam injectors, the transverse magnetic field necessary for suppressing the co-extracted electrons induces a deflection of the negative ion beamlets that must be corrected. For the design, particle-tracing simulation codes are used to compute ion trajectories and optical properties of the beamlets in the acceleration stage. In these codes, uniform boundary conditions are normally assumed for the ion current density distribution at the surface (called meniscus), where negative ions are extracted from the plasma and form beamlets, which are accelerated across the apertures of the accelerator grids. Recently, experimental campaigns dedicated to the accurate measurement of the beamlet deflection in the acceleration phase revealed higher deflection than foreseen by simulations. In this work, we demonstrate that an agreement with the experimental data can be obtained by incorporating in the numerical simulations a non-radially symmetric distribution of the ion current density extracted across the meniscus surface. In the first part, the asymmetry of the ion current density at the meniscus is studied in an empirical way, by analysing and fitting the experimental results obtained with different operating parameters. In the second part, we show that the ion current asymmetry estimated by this procedure is well consistent with the flow pattern of H− ions calculated in the meniscus zone using a detailed particle in cell (PIC) model of the ion source in the presence of transverse magnetic field.

Design and control of the accelerator grid power supply-conversion system applied to CFETR N-NBI prototype

The acceleration grid power supply (AGPS) rated 200 kV/25 A is a key component devoted to supply the acceleration grids of the China fusion engineering test reactor negative-ion-based neutral beam injector (N-NBI) prototype system. This paper focused on the design and control of the AGPS conversion system (AGPS-CS), with emphasis on the requirement of the wide range output voltage and rise time. A voltage regulation switch at the front of step-down transformer is applied to optimize the grid current and DC-link voltage. Moreover, a new feedforward control strategy with piecewise PI compensator is proposed to improve the characteristics of AGPS. The simulation results of the proposed AGPS-CS are presented, proving the performance of the power supply to achieve the desired requirements.

Achievement of high power and long pulse negative ion beam acceleration for JT-60SA NBI.

Long pulse acceleration of hydrogen negative ion beams with the power density over 70 MW/m2 and the pulse length over 100 s has been demonstrated for the first time by using a multi-aperture 3-stage accelerator. Such long pulse acceleration was achieved by integrating the design of beam optics and voltage holding capability to meet the requirements of JT-60SA. By using the newly designed accelerator for JT-60SA, voltage holding at 500 kV with beam acceleration was stably sustained even after 5 g of cesium was seeded, and heat load on each acceleration grid was reduced below the allowable level for long pulse, less than 5% of total acceleration power. As a result, 500 keV, 154 A/m2 for 118 s beam acceleration was achieved, which satisfies the requirement of the negative ion source for JT-60SA. This pulse length of such high-power density beams is longest in the world. In addition, the result contributes to the long pulse acceleration of multi-stage electrostatic accelerators, such as 1 MeV negative ion accelerator for ITER.

Grid Gap Variation of Ion Thruster During Startup in Orbit

The grids play a central role in ion thrusters and considerably affect both the performance and lifetime. In this article, the primary goal is to study the variation of the grid gap during the startup process of an ion thruster in orbit. Therefore, we established a detailed model of ion thrusters using ANSYS. We have carried out several experiments to valid both thermal and mechanical performances of the model. The results reveal that the temperature of the ion thruster in orbit periodically varies. The grid gap between the screen grid and the accelerator grid decreases at first and then increases during the startup process, in which the minimum depends on the initial temperature and the thermal control. Higher temperature and more thermal control will raise the minimum gap. To launch a 30-cm-diameter ion thruster from the lowest temperature point and ensure a safe gap, 50 °C thermal control would do a great help.

Tests Design and Models Validation for the Acceleration Grid Power Supply Conversion System of the MITICA Test Facility

Tests Design and Models Validation for the Acceleration Grid Power Supply Conversion System of the MITICA Test Facility

Ion beam and discharge characteristics of cold cathode ion source

In this work, some developments in the acceleration system of cold cathode ion source have been constructed to produce broad beam to be used in different industrial applications. An electrostatic probe with electrical circuit is constructed for study the extracted ion beam distribution. Broad beam 25 mm with ion current in the range of 1 mA is extracted from the constructed extraction system. The obtained optimum distance between the extraction grid and acceleration grid is 3 mm. The characteristics are measured to investigate the ion beam current I b as a function of different parameters (discharge voltage V d , gas pressure P, magnetic field intensity B and acceleration voltage V acc ). The magnetic field is collimated and intensifies the plasma that enhances the extracted beam current. The obtained cold cathode ion source can be used in different applications like surface etching, surface modification and deposition due to its long life and compactness.

Structure design and analysis of RF ion source for negative ion source test facility.

A negative ion source acts as a critical part in a neutral beam injector (NBI). A high current ion source is required for the high-power NBI. In this paper, a prototype radio frequency (RF) ion source and its test facility are developed in the Institute of Plasma Physics, Chinese Academy of Sciences, to demonstrate the key technology of the high power negative ion source. The structure design of the RF negative ion source is presented, involving the designs of the ion source plasma generator and accelerator. The detailed structure design and analysis of the key parts of the ion source are also presented, such as the Faraday shield (FS) and accelerator grids. The fluid-thermal-structural coupling characteristics of the FS and grid are explored with different mechanisms of fluid pressure, RF power, and the structure type on the thermal stress. Then, the processing and manufacturing scheme of the FS and grids are also given. Finally, the results were presented with a manufactured three cooling channel FS. The experimental results prove that the developed structure design of the RF ion source is effective and reliable, and the correctness of finite element analysis is also verified by experimental data comparison.

Progress of ITER-India activities for ITER deliverables—challenges and mitigation measures

The responsibilities of ITER-India include a mix of precision, heavy, R&D intensive and interface intensive systems, under the built-to-print and functional systems categories. In several systems, the components are the first or largest of their kind. The uniqueness of component specifications and adherence to the desired quality standards considering the harsh nuclear environment in which the components need to operate and survive the lifetime of ITER leads to a challenging situation—namely that neither the existing laboratories nor potential suppliers have ever done or encountered such a scale-up (either in size, capacity, precision, quality etc) and in many cases do not have even the research and development (R&D) infrastructure to match ITER’s requirements. To bridge this gap, ITER-India adopted a two-step approach of first manufacturing prototypes as per ITER-specific standards and then subjecting them all to critical evaluation before launching bulk production. Some of the key areas of development include demonstrations of the following: (a) 1.5 MW radiofrequency power in the 35–65 MHz frequency range under Diacrode and Tetrode tube technologies for the ion cyclotron resonance heating system; (b) a low loss multi-process pipe cryogenic transfer line and the development of a 3.3 kg s−1 cold circulator for the cryoline and cryo-distribution system; (c) an angled accelerator grid with precision in positioning of apertures within 50 μm, as a first of its kind development, and special, ITER grade, copper alloy development for the diagnostic neutral beam (DNB) system. Full scale test facilities have been built for the performance assessment of the DNB, the electron cyclotron and ion cyclotron sources, cryogenic transfer lines and diagnostics systems. A summary is presented of the technical features of the major areas of procurements, notable achievements in R&D and manufacturing and their application in the Indian R&D related to fusion.

Mechanical analysis of C/C composite grids in ion optical system

The ion thruster is an engine with high specific impulse for satellites and spacecrafts, which uses electric energy to boost the spacecraft. The ion optical system, also known as gate assemblies which consist of acceleration and screen grids, is the key component of the ion thruster. In this paper, the static mechanical properties of the C/C composite grids are evaluated based on the structural design. Representative volume element (RVE) is adopted to simplify the braded composite structure as a continuum material. The dynamical behavior of the 100mm ion thruster optics in the launch environment (1 000g shock-load) is numerically modeled and simulated with the half-sine pulse method. The impact response of the C/C and molybdenum gate assemblies on the stress distribution and deformation is investigated. The simulated results indicate that the magnitudes of the normal displacement of the composite grids subject to the uniformly distributed load are on the same level as molybdenum grids although the normal stiffness of the composite grids is much smaller. When subject to impact loading, the stress distribution in the C/C composite grids is similar to molybdenum grids while the stress magnitude is much smaller. This finding shows that the C/C gate assemblies outperform molybdenum grids and meet the requirement of long lifetime service in space travel.

Feedback control of stored energy and rotation with variable beam energy and perveance on DIII-D

Design and testing of a new method for real-time control on DIII-D using the new voltage and perveance variation capability of DIII-D’s neutral beam injection system is presented. The approach enables control of power, torque, stored energy, and rotation without requiring pulse-width modulation of the beams. Such modulations are perturbative to experiments, can cause beam reliability problems, and could potentially damage or fatigue accelerator grids, especially looking forward to active control of long-pulse devices and reactors. The control algorithm accounts for beam line loss mechanisms and includes approaches that address the complexity of having three manipulated variables for each of the eight beams and compensate for the rate and magnitude limits affecting the voltage and perveance actuators. Results are presented that demonstrate the performance of the approach in the first feedback control experiments to use the expanded controllability of the DIII-D beams.

Numerical analyses and tests for optimized and enhanced heat transfer solutions in DEMO

A set of novel design solutions for high performance cooling systems have been developed and tested by Consorzio RFX, achieving, with experimental tests, the challenging heat transfer conditions foreseen for Heating and Current Drive Systems of present and future nuclear fusion devices. The project, called Multi-design Innovative Cooling Research & Optimization (MICRO), has the triple objective of:•Verifying the full qualification of the manufacturing process and assess the cooling performance of the present solutions applied on the acceleration grids designed for ITER Heating Neutral Beam Injectors and MITICA experiment, which have similar heat loads as for DEMO1.•Identifying alternative designs realizable with presently qualified manufacturing processes for improving heat transfer mechanisms with limited pressure drops.•Developing further optimized solutions by means of the investigation of the interrelated effects of geometric parameters and thermofluid dynamic properties of the coolant fluid.The main aim of present work is to extend the fatigue life-cycle of high thermal stress components. In the first part of the paper the design description and constraints are presented together with the results obtained from numerical analyses carried out on the full-scaled model of the electrostatic grids; the outcomes of the experimental campaign, together with the characterization of the properties and performance of a set of alternative dielectric fluids, are shown in the second one.

Influence of the Decelerator Grid on the Optical Performance of the Ion Thruster

In order to reduce the erosion of the ion thruster accelerator grid, which is caused by charge-exchange (CEX) ions, the 2-grid optical system is added to a decelerator grid to block the reflux CEX ions. The previous experiment and simulation results have proven that the decelerator grid can effectively reduce the Pit and Groove erosion. However, the influence of the decelerator grid on the optical performance has not yet been studied well. In this paper, a three-dimensional Immersed Finite Element Method-Particle in Cell-Monte Carlo Collision (IFE-PIC-MCC) algorithm was adopted to investigate the effect of the decelerator grid on the optical performance under crossover and normal circumstances. Results show that the decelerator grid has no effect on the focusing state and the distribution of beam ions. It also has little effect on the CEX ions from the upstream and extraction (center) regions. However, it has great influence on the downstream CEX ions. When the upstream plasma number density is small, the decelerator grid will cause most of the downstream reflux CEX ions to impinge on the accelerator grid aperture barrel, resulting in the significant increase of the Barrel erosion of the accelerator grid. With the increase of the upstream plasma number density, the downstream reflux CEX ions tend to impact the downstream surface of the decelerator grid, which means the decelerator grid begins to block the downstream backflow of CEX ions.

Design of acceleration grid power supply for CFETR negative-ion-based neutral beam injection prototype

The planned China Fusion Engineering Test Reactor (CFETR) needs one or more Negative-ion-based Neutral Beam Injectors (N-NBIs) with a beam energy of 500 keV to heat plasma and drive current. In order to prepare for CFETR, a prototype of CFETR N-NBI is being designed to accelerate hydrogen negative ions up to 200 keV with a beam current as high as 20 A for 3600 s at max, and an Acceleration Grid Power Supply (AGPS) rated at −200 kV/25 A is needed. A single stage inverter-type high voltage power supply is adopted as AGPS of the N-NBI prototype. The AGPS mainly includes a 12 pulse rectifier based on thyristor, a dual-dc-link system rated at 5 kV, a three phase three level neutral point clamped dc/ac inverter based on injection enhanced gate transistor, a three-phase set-up transformer, a high voltage uncontrolled rectifier based on avalanche diode and a high voltage R-C dc-filter. This paper describes the design of the AGPS for CFETR N-NBI prototype, focusing on the inverter, the step-up transformer and the HV diode rectifier.

Grid Deformation Real-Time Measurement System of Ion Thruster Based on Videometrics

In order to conduct high-precision measurement of the LIPS-300 ion thruster grid deformation in a vacuum, high-temperature, and plasma environment, a noncontact videometrics system using a telemicroscope was designed. Based on the captured image, the interactive partitioning edge detection method (IPEDM) was used to obtain stable and clear edges of multiple circular cooperative targets. Meanwhile, magnification factor calibration, rotation angle correction, and subpixel-level grid deformation measurement were performed with cooperative targets. The measurement results show that under the power of 750 W in the discharge chamber, the maximum thermal deformation of the screen grid is 1120 μm, and the gap between the screen grid and the accelerator grid is reduced by 420 μm. An accuracy assessment of the system shows that the grid deformation measurement accuracy is better than 12 μm, and the system satisfies the requirement of high-precision real-time measurements of the grid thermal deformation of the ion thruster under the discharge-chamber-running condition and the plasma-beam-extraction condition.