Negative Extraction Research Articles

On the road to ITER NBIs: SPIDER improvement after first operation and MITICA construction progress

To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 16.5 MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1 MeV D- ions for ≤1 h, 46A/870 keV H- ions for ≤1000 s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100 keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics.This paper will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters, including investigation of RF-plasma coupling efficiency and magnetic filter field effectiveness in reducing co-extracted electrons. SPIDER is progressing towards the first caesium injection, which aims at increasing the negative ion density. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out programmed modifications, will be outlined.The installation of each MITICA power supply and auxiliary system is completed; in-vessel mechanical components are under procurement by Fusion for Energy (F4E). Integration, commissioning and test of the power supplies, procured by F4E and QST, as the Japanese Domestic Agency (JADA), will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. In 2020 integrated tests of the power supplies on the accelerator dummy load started, including the assessment of their resilience to accelerator grid breakdowns using a short-circuit device located in vacuum.The aggressive programme, to validate the NBI design at NBTF and to meet ITER schedule (requiring NBIs in operation in 2032), will be outlined. Unfortunately, in 2020 the coronavirus disease infection affected the NBTF activities. A solution to proceed with integrated power tests despite the coronavirus is presented.

Development of a set of movable electrostatic probes to characterize the plasma in the ITER neutral beam negative-ion source prototype

We present the development and first use of a set of movable electrostatic probes on the full-scale ITER heating neutral beam prototype negative-ion source SPIDER. The probes access the ion source plasma from the multi-aperture accelerator aiming at the study of the plasma formation and expansion from the RF drivers through the transverse magnetic filter. The magnetic filter separates the relatively high electron-temperature region for plasma formation and hydrogen dissociation form the negative-ion extraction region in which low electron-temperature is required to avoid negative-ion destruction, but also causes non-uniformities and drifts in the large plasma discharge which will be studied by this setup. The set of electrostatic probes encompasses eight RF-compensated Langmuir probes, one double probe, one Mach probe for the assessment of plasma drift velocities, and two gridded retarding field energy analyzers to measure the positive-ion energy distribution function. Electric aspects as well as mechanical constraints given by the large in-vacuum movable structure, and thermal requirements of these relatively heat-flux components made the design challenging. Prototyping and commissioning of the measurement system is discussed offering examples of measured characteristics with the various probes.

Extraction of negative hydrogen ions produced with aluminum plasma grid

Negative hydrogen ions are produced and extracted using an aluminum plasma grid, without cesium through irradiation with hydrogen plasma. The negative ion amount increases sharply, up to ∼115 μA·cm−2, at a discharge power of 1 kW with an electric field downstream of the grid. These negative ions are extracted with high purity. In addition, it is suggested that negative ions are produced by positive hydrogen ions and hydrogen atoms. As high-density plasmas enhance the negative ion production, the current density of negative ions is expected to increase with plasma discharge power.

First results of negative ion extraction with Cs for CRAFT prototype negative beam source

In order to understand the physics and pre-study the engineering issues for radio frequency (RF) negative beam source, a prototype source with a single driver and three-electrode accelerator was developed. Recently, the beam source was tested on the RF source test facility with RF plasma generation, negative ion production and extraction. A magnetic filter system and a Cs injection system were employed to enhance the negative ion production. As a result, a long pulse of 105 s negative ion beam with current density of 153 A m−2 was repeatedly extracted successfully. The source pressure is 0.6 Pa and the ratio of co-extracted electron and negative ion current is around 0.3. The details of design and experimental results of beam source were shown in this letter.

Contrast Experiments of Pig Positive and Negative Hydrogen Ion Sources for Neutron Tubes

Recent studies reported that PIG negative hydrogen ions extraction for neutron tube is superiority to positive hydrogen ions extraction. Present maturing products of neutron tube on market almost use PIG ion source and extract positive hydrogen ions. If negative hydrogen ions extraction can be applied on present products, the combination property will be greatly improved. In this study, a contrast experiment of PIG positive and negative hydrogen ion source for D–T neutron tube is put into effect. The relationships between the current of ion source, the beam current and the voltage extraction are investigated. The experiment results demonstrate that the negative ion extraction has higher yield of neutron for unit target current. We also point out that the ratio of yield to target current is a considerable parameter to reflect the overall performance of neutron tube.

Design of infrared radiation diagnostic calorimeter for the prototype radio frequency driven negative ion source for neutral beam injection.

In order to study the generation and extraction of negative ions for neutral beam injection application, a prototype radio frequency driven negative ion source and the corresponding test bench are under construction at the Institute of Plasma Physics, Chinese Academy of Sciences. A new design of infrared radiation diagnostic calorimeter for testing beam characteristics is put forward. Compared with the conventional calorimeter, the calorimeter adopts the block structure (8 × 28 tungsten hexahedron blocks) and modularization design (4 modules), so it has higher precision and good scalability. The thermal performance of the calorimeter is assessed using a finite element method. Simulation results show that the design can be achieved to operate in the stable-state mode at the maximum thermal flux 6.45 MW/m2 and meet the full requirement of beam diagnosis.

Simulations of negative ion extraction and transport for developing novel remote reactive ion processing system

A novel remote dry processing of specific reactive negative ions with a deflection and focusing system is being developed to precisely control their energy, flux and reaching position on the nanoscale. The trajectory simulations of the negative ion extraction and transport for the design of this processing apparatus have been carried out using SIMION software. It is found from the simulation results that the ion beams with good focusing characteristics, high directivity and appropriate current amount can be successfully extracted from the plasma source and transported to the reactor chamber through the deflection and focusing system. The simulation results of O− ion beam exhibit that the extraction and transport characteristics of the flat intermediate electrode are superior to those of the oblique one.

Study on production and extraction of negative impurity ions in a caesiated negative ion source

Doppler shift spectroscopy (DSS) diagnostics, performed on several neutral beam injectors based on positive ion sources, shows the appearance of a shifted peak corresponding to a hydrogen bearing impurity species (mostly due to water vapour: H2O). However, for DSS experiments based on negative ion sources, we are reporting the detection of this peak for the first time. This peak appears in the DSS spectrum when the observations are recorded with a high-speed turbo molecular pump (pressure during beam pulse ~1 × 10−4 mbar) and disappear when a high-speed cryo pump is used (pressure during beam pulse ~1 × 10−5 mbar). A detailed study on this peak using DSS and a residual gas analyser (RGA) was carried out in a ROBIN (RF operated beam source in India for negative ion research) test stand. In these investigations, it was established that the appearance of the peak is due to emissions from fast neutrals produced from the dissociation of fast hydrogen bearing impurity ions such as H2O−/OH−, extracted with energy E. For the present experiments, negative ions of H2O can be formed either by dissociative attachment of (H2O−)* leading to the formation of H−, O− and OH− fragments or via ion–molecule reactions between H−/O− with a H2O molecule. In this work, a study on these formation mechanisms is presented. There is some evidence that the amount of impurity present in the ion source affects the ratio of the main species. An estimation of relative impurity content in the ion source is done using the intensity ratio of a fast hydrogen peak and a hydrogen peak originating from the negative ion impurity. For these computations, the Balmer-alpha excitation cross-section at low energies (~1.5 keV) was reviewed, since the published data is available only for higher energy ranges, and suitable cross sections are used.

Preliminary design of diagnostic system for negative neutral beam injector at ASIPP

According to the latest physics design of the China Fusion Engineering Test Reactor (CFETR), two neutral beam injectors (NBIs), which deliver a total of 40 MW in not less than 3600 s with 1 MeV D0, are demanded to support current drive and plasma rotation. To minimize the risks and time to provide the CFETR with reliable NBIs, a negative NBI test facility will be developed at the Institute of Plasma Physics, Chinese Academy of Science. Its mission is to understand the characteristics of the RF driven ion source and negative ion generation and extraction and to improve RF efficiency and beam quality. In order to achieve this goal, a set of diagnostic tools will be used in this test facility. For source diagnostics, optical emission spectroscopy, cavity ring-down spectroscopy, laser absorption spectroscopy, and electrostatic probes are planned to be used. Beam emission spectroscopy, W-wire calorimeters, 1D carbon fiber composite diagnostic calorimeters, beam dump with thermocouples, and water-flow calorimetry are used to assess the beam properties. The design of the diagnostic system is presented.

Computational characterization of plasma transport across magnetic filter in ROBIN using PIC-MCC simulation

The RF based negative ion source experimental facility ROBIN has been setup to understand and investigate the different issues related to production, transport and extraction of negative hydrogen ions in negative ion sources for fusion applications. The source consists of a driver, an expansion chamber, a magnetic filter and extraction system consisting of different grids. The first phase of the ROBIN experiments have been conducted for hydrogen plasma generation and measurement of different plasma parameters without negative ion generation/ extraction. We require accurate characterization of plasma to completely understand the experimental results, the complex physics, and plasma dynamics. As a first step in this direction, we have developed an in-house parallel 2D-3v Particle-In-Cell with Monte Carlo Collision (PIC-MCC) code to understand the transport across magnetic filter under conditions similar to real ROBIN operational parameters (magnetic field, pressure, power, density etc.). Simulation results from our 2D-3v PIC-MCC code reproduces the qualitative as well as quantitative behaviors observed during the first phase of the ROBIN experiments such as decrement in electron temperature and similar plasma density profile across the magnetic filter. Our exhaustive PIC simulations helps us in quantifying the plasma transport across magnetic filter as well as the actual input power that is coupled to the plasma. Electron energy distribution function (EEDF) obtained from our simulations shows a Maxwellian distribution and is in agreement with similar experimental calculations reported in the literature. Several case studies have been performed to understand the role of the magnetic filter profile on plasma transport, which will help in planning future experiments by using the magnetic filter as a switching mechanism to achieve the required density and electron temperature profiles for efficient operation of ROBIN.

Generation of hydrogen ionic plasma superimposed with positive ion beam

In this study, a hydrogen ionic plasma with relatively low residual fractional electron concentration (ne/n+ ∼ 10−2) is generated using an aluminum plasma grid for the production of negative hydrogen ions and a control grid for negative ion extraction and electron removal. The ionic plasma is composed of negative and positive ions, containing molecular ions. Negative ions are in part produced using positive ions with several electron volts. A positive ion beam with 50 eV or more contributes to increase the density of the ionic plasma. The positive ion beam energy and the control grid bias voltage are tuned in such a way that a high-density ionic plasma is maintained.

Negative ion beam source as a complex system: identification of main processes and key interdependence

Complex network theory offers new tools to tackle controllability of systems such as processes affecting generation, extraction and acceleration of negative ions in ion beam sources. A model has therefore been developed and tested for the negative ion beam source NIO1. Preliminary results show good agreement between model predictions and experimental behaviour of the source. A subset of driver processes which in principle allow the system to be controlled has been identified. One of the driver processes related to the meniscus formation has been found to play a key role in the global controllability of the system, so that a detailed discussion of the physics behind its formation and uniformity has been provided.

3D-PIC modelling of a low temperature plasma sheath with wall emission of negative particles and its application to NBI sources

The 3D particle-in-cell (PIC) code ONIX (orsay negative ion extraction) is a versatile tool for simulating the formation and extraction of negative hydrogen ions and co-extracted electrons in caesiated negative ion sources. Mandatory for modelling these processes is a self-consistent description of the plasma-wall interface, i.e. the plasma sheath, in the presence of surface emitting charged particles. For simplified test cases, the description of the plasma sheath by ONIX is critically validated. Additionally, a set of numerical parameters for reducing the computational cost while still accurately simulating the sheath are presented. Finally, the updated version of ONIX is applied to the plasma of a negative ion source test facility for the ITER NBI system. Results of basic investigations on the influence of the magnetic fields and the meniscus shape on the extraction of negative ions, co-extraction of electrons and the beamlet quality are presented.

Advanced NBI beam characterization capabilities at the recently improved test facility BATMAN Upgrade

The test facility BATMAN was dedicated since its start in 1996 to the development of radio frequency driven negative hydrogen ion sources for ITER NBI with focus on formation and extraction of negative ions, technological developments and improved concepts. During 2017, the test facility was upgraded in order to replace the former extraction system with a new ITER-like extraction system comparable in size to one ITER beamlet group and having the option to flow a current through the grid creating a magnetic filter field as foreseen for the ITER sources. In addition, the beam diagnostics has been extended: beam emission spectroscopy is located at two positions from the grounded grid (26 cm and 129 cm) with spatial resolution in vertical direction. A newly developed tungsten wire calorimeter is placed just 19 cm downstream the grounded grid to provide quantitative measurements of individual beamlets, whereas the tungsten wire calorimeter at 180 cm distance is still in use for qualitative beam profile diagnostics. Together with a beam dump calorimeter with a crosswise arrangement of thermocouples, beam divergence and uniformity can be studied. This is accompanied by modeling the beamlet transport from the extraction system up to the calorimeter. Results from the first experimental campaign are reported, being very promising for detailed understanding of features measured on large beams.

Extraction of positive and negative ion beams from large area plasma sources

In this study we investigate extraction and acceleration of positive and negative ions from a pulsed inductively coupled plasma (ICP) in oxygen and hydrogen. Experiments were performed in a directed ribbon beam system where ions can be produced at an incident angle non-normal to the target. Positive and negative ions were transported to the wafer. The RF plasma source, extraction electrodes and wafer were biased synchronously. The energy of extracted ions closely follows the amplitude of the applied bias voltage and it ranges from few hundreds of volts to 10 kV. The peak beam current density can reach 100 A m−2. The ion beam angles, ion current, and ion composition are reported. The injection physics requires correlating the positive and negative ion and electron densities near the extraction opening with the extracted currents. The plasma was modeled using CRTRS, a plasma fluid code that self-consistently solves for ICP power deposition, electrostatic potential, and plasma dynamics. A new perspective on the possible production of angled ion beams on surfaces is discussed.

Hefei utility negative ions test equipment with RF source: commissioning and first results

In order to study the key technology and physics of RF driven negative ion source for neutral beam injector in China, the Hefei utility negative ions test equipment with RF source was developed at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). Its negative ion source can be equipped with single or double RF drivers. There is a plasma expansion chamber with depth of 19 mm and an enhanced filter field. A three electrodes negative ion accelerator was employed to extract and accelerate the negative ions, which are plasma grid, extraction grid and ground grid. And there are several diagnostic tools for the plasma and beam parameters measurement. The characteristics of plasma generation, negative ion production and extraction were studied on the test equipment. The negative ion beam was extracted from the RF driven negative ion source for the first time. The detailed structure and main results are presented in this article.

Review of particle-in-cell modeling for the extraction region of large negative hydrogen ion sources for fusion.

Particle-in-cell (PIC) codes are used since the early 1960s for calculating self-consistently the motion of charged particles in plasmas, taking into account external electric and magnetic fields as well as the fields created by the particles itself. Due to the used very small time steps (in the order of the inverse plasma frequency) and mesh size, the computational requirements can be very high and they drastically increase with increasing plasma density and size of the calculation domain. Thus, usually small computational domains and/or reduced dimensionality are used. In the last years, the available central processing unit (CPU) power strongly increased. Together with a massive parallelization of the codes, it is now possible to describe in 3D the extraction of charged particles from a plasma, using calculation domains with an edge length of several centimeters, consisting of one extraction aperture, the plasma in direct vicinity of the aperture, and a part of the extraction system. Large negative hydrogen or deuterium ion sources are essential parts of the neutral beam injection (NBI) system in future fusion devices like the international fusion experiment ITER and the demonstration reactor (DEMO). For ITER NBI RF driven sources with a source area of 0.9 × 1.9 m2 and 1280 extraction apertures will be used. The extraction of negative ions is accompanied by the co-extraction of electrons which are deflected onto an electron dump. Typically, the maximum negative extracted ion current is limited by the amount and the temporal instability of the co-extracted electrons, especially for operation in deuterium. Different PIC codes are available for the extraction region of large driven negative ion sources for fusion. Additionally, some effort is ongoing in developing codes that describe in a simplified manner (coarser mesh or reduced dimensionality) the plasma of the whole ion source. The presentation first gives a brief overview of the current status of the ion source development for ITER NBI and of the PIC method. Different PIC codes for the extraction region are introduced as well as the coupling to codes describing the whole source (PIC codes or fluid codes). Presented and discussed are different physical and numerical aspects of applying PIC codes to negative hydrogen ion sources for fusion as well as selected code results. The main focus of future calculations will be the meniscus formation and identifying measures for reducing the co-extracted electrons, in particular for deuterium operation. The recent results of the 3D PIC code ONIX (calculation domain: one extraction aperture and its vicinity) for the ITER prototype source (1/8 size of the ITER NBI source) are presented.

Numerical study of the collisionless interaction between positive and negative ion beams

We study, through two dimensional Particle-In-Cell simulations, the expansion of an ion-ion beam in vacuum. This beam is generated by a continuous extraction of positive and negative ions from two adjacent gridded ion sources. The grid systems are biased to extract and accelerate, continuously, positive and negative ions from two distinct ion sources. The ion sources are positioned such that their grid systems form an angle θ. In this work, we study two configurations, θ = π and θ = π/2. The proposed device constitutes an alternative approach to the usual positive ion beam neutralized by electrons. This work aims, on the one hand, to demonstrate that the neutralization of a continuously extracted ion beam space charge might be achieved by the use of oppositely charged ions. On the other hand, this work investigates the physical properties of the generated ion-ion beam. Our results show that, for θ = π, potential barriers form in the close vicinity of the acceleration grids. These potential barriers oppose to extraction and induce an ion backflow. This backflow increases with the increase in extracted ion current density. It represents ∼60% to ∼80% of extracted ions for an extracted ion current density ranging from 1 to 10 A/m2. Moreover, for θ = π/2, the potential barriers are located downstream the grids, typically at one source diameter. For this configuration, we found that the backflow is drastically reduced to about 25% of the extracted ions.

Modeling of negative ion extraction from a magnetized plasma source: Derivation of scaling laws and description of the origins of aberrations in the ion beam

We model the extraction of negative ions from a high brightness high power magnetized negative ion source. The model is a Particle-In-Cell (PIC) algorithm with Monte-Carlo Collisions. The negative ions are generated only on the plasma grid surface (which separates the plasma from the electrostatic accelerator downstream). The scope of this work is to derive scaling laws for the negative ion beam properties versus the extraction voltage (potential of the first grid of the accelerator) and plasma density and investigate the origins of aberrations on the ion beam. We show that a given value of the negative ion beam perveance correlates rather well with the beam profile on the extraction grid independent of the simulated plasma density. Furthermore, the extracted beam current may be scaled to any value of the plasma density. The scaling factor must be derived numerically but the overall gain of computational cost compared to performing a PIC simulation at the real plasma density is significant. Aberrations appear for a meniscus curvature radius of the order of the radius of the grid aperture. These aberrations cannot be cancelled out by switching to a chamfered grid aperture (as in the case of positive ions).

Massive parallel 3D PIC simulation of negative ion extraction

The 3D PIC-MCC code ONIX is dedicated to modeling Negative hydrogen/deuterium Ion (NI) extraction and co-extraction of electrons from radio-frequency driven, low pressure plasma sources. It provides valuable insight on the complex phenomena involved in the extraction process. In previous calculations, a mesh size larger than the Debye length was used, implying numerical electron heating. Important steps have been achieved in terms of computation performance and parallelization efficiency allowing successful massive parallel calculations (4096 cores), imperative to resolve the Debye length. In addition, the numerical algorithms have been improved in terms of grid treatment, i.e., the electric field near the complex geometry boundaries (plasma grid) is calculated more accurately. The revised model preserves the full 3D treatment, but can take advantage of a highly refined mesh. ONIX was used to investigate the role of the mesh size, the re-injection scheme for lost particles (extracted or wall absorbed), and the electron thermalization process on the calculated extracted current and plasma characteristics. It is demonstrated that all numerical schemes give the same NI current distribution for extracted ions. Concerning the electrons, the pair-injection technique is found well-adapted to simulate the sheath in front of the plasma grid.