Negative Ion Beam Injector with Pre-Acceleration and Temperature Control
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Solution Overview
Problem
Current negative ion-based neutral beam injectors face challenges in achieving stable operation at high energies (500-1000 keV) and continuous wave operation with sufficient current density, due to issues like cesium accumulation, low ion current density, and inefficient neutralization, which limits their application in next-generation fusion devices.
Innovation Solution
The design incorporates an ion source with a cesiated molybdenum surface, external magnetic fields for electron deflection, a transition zone with bending magnets to separate co-streaming particles, and a neutralizer with both plasma and photon technologies to enhance neutralization efficiency, along with elevated temperature operation to prevent cesium accumulation and improve beam focusing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If negative ions are used to achieve high energy neutral beams (500-1000 keV), then the beam energy and neutralization efficiency are improved, but the ion current density and operational stability deteriorate
Solution Approach 1:
The patent employs multiple parameter changes to resolve the contradiction: (1) Using cesiated molybdenum surfaces to enhance negative ion production efficiency and current density; (2) Operating at elevated temperatures (150-200°C) to prevent cesium accumulation and maintain source stability; (3) Implementing a two-stage acceleration system with pre-acceleration to 120 keV followed by high-voltage acceleration to achieve 500-1000 keV final energy; (4) Using external magnetic fields for electron deflection and beam focusing to improve beam quality and reduce divergence
Solution Approach 2:
The patent introduces several intermediary elements: (1) A transition zone with bending magnets that separates co-streaming particles and directs negative ions into the accelerator while removing electrons and neutrals; (2) A neutralizer cell that converts accelerated negative ions to neutral atoms through charge exchange reactions; (3) Cesium as a surface coating material that enhances negative ion production through surface ionization processes
2Productivity
If cesium is used to enhance negative ion production, then the ion current density is improved, but cesium accumulation on surfaces causes operational instability
Solution Approach 1:
The patent maintains operational stability while using cesium by controlling the temperature parameter. The ion source and plasma electrode are heated to elevated temperatures (150-200°C), which prevents excessive cesium accumulation on surfaces while maintaining the cesiated molybdenum surface needed for high negative ion production efficiency. This temperature control balances cesium coverage to optimize both current density and stability
Solution Approach 2:
The patent implements temperature control mechanisms that act as feedback systems to maintain stable cesium coverage on molybdenum surfaces. By monitoring and controlling the temperature of the ion source and plasma electrode, the system maintains optimal cesium layer thickness that ensures high negative ion production without excessive accumulation that would cause instability
3Quantity of substance
If a thick gas cell is used for neutralization, then the neutral fraction is improved at low energies, but the neutral fraction drops rapidly at energies greater than 60 keV
Solution Approach 1:
The patent performs preliminary acceleration of negative ions to 120 keV in a pre-accelerator stage before injection into the high-voltage accelerator. This preliminary action allows the use of a thinner neutralizer cell at high energies, since the charge exchange cross-section is more favorable at these pre-accelerated energies, thereby maintaining high neutral fractions while enabling operation at 500-1000 keV final beam energies
Solution Approach 2:
The patent replaces the conventional thick gas cell neutralization approach with a two-stage process: (1) Pre-acceleration to 120 keV to optimize charge exchange conditions; (2) Use of a thinner neutralizer cell at high voltage, leveraging the energy-dependent cross-section characteristics to achieve efficient neutralization at MeV energies where traditional thick cells would be ineffective
4Device complexity
If negative ions are extracted from plasma without suppression measures, then the ion source simplicity is improved, but the accompanying electron current increases significantly
Solution Approach 1:
The patent applies local quality enhancement by using cesiated molybdenum surfaces specifically at the plasma electrode and ion extraction region. This localized cesiation creates a surface with enhanced negative ion production properties, allowing negative ions to be extracted with reduced accompanying electron current compared to conventional plasma sources, while maintaining relatively simple source architecture
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enables the production of a 5 MW neutral beam with 0.5-1.0 MeV energy and high overall energetic efficiency, addressing the limitations of current injectors by increasing ion current density and neutralization efficiency while maintaining reliable operation.
Implementation Method 1
a cesiated molybdenum surface to convert the positive ions and neutral atoms formed by the plasma drivers into negative ions
Implementation Method 2
The ions produced by the ion source are pre-accelerated to 120 keV before injection into a high energy accelerator by an electrostatic multi aperture grid pre-accelerator
Implementation Method 3
The 120 keV beam from the ion source passes through a pair of deflecting magnets, which enable the beam to shift off axis before entering the high energy accelerator
Implementation Method 4
The plasma drivers and the internal walls of a plasma box of the ion source are maintained at elevated temperature
Implementation Method 5
After acceleration to full energy, the beam enters the neutralizer where it is partially converted into a neutral beam
Implementation Method 6
The plasma drivers and the internal walls of a plasma box of the ion source are maintained at elevated temperature (150-200° C.) to prevent cesium accumulation on their surfaces
Data Source
AI summary
A negative ion-based beam injector comprising a negative ion source and an accelerator. The ions produced by the ion source are pre-accelerated before injection into a high energy accelerator by an electrostatic multi-aperture grid pre-accelerator, which is used to extract ion beams from the plasma and accelerate to some fraction of the required beam energy. The beam from the ion source passes through a pair of deflecting magnets, which enable the beam to shift off axis before entering the high energy accelerator. The negative ion-based beam injector can be combined with a neutralizer to produce about a 5 MW neutral beam with energy of about 0.50 to 1.0 MeV. After acceleration to full energy, the beam enters the neutralizer where it is partially converted into a neutral beam. The remaining ion species are separated by a magnet and directed into electrostatic energy converters. The neutral beam passes through a gate valve and enters a plasma chamber.