Shielding Electrode for Electron Gun Vacuum Stability
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Solution Overview
Problem
The stability of the electron beam current in charged particle radiation devices is affected by the degree of vacuum, with residual gas adsorption leading to variations and the generation of electron stimulated desorption (ESD) gas, which deteriorates the vacuum and causes instability in the emission current.
Innovation Solution
A charged particle radiation device with a cylindrical shielding electrode that surrounds the charged particle source to shield back scattered charged particles, ensuring a stable vacuum environment and reducing ESD gas generation by intercepting and redirecting back scattered electrons, thereby maintaining a high degree of vacuum and stabilizing the emission current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a high positive voltage is applied to the extraction electrode to concentrate the electric field and emit electron beam, then the electron beam emission is enabled, but back scattered electrons are generated which collide with the electron source and generate ESD gas that deteriorates the vacuum
Solution Approach 1:
A shielding electrode is introduced as an intermediary component between the extraction electrode and the electron source. This shielding electrode intercepts back scattered electrons before they reach the electron source, preventing ESD gas generation. The shielding electrode is held at a potential between the extraction electrode potential and the electron source potential, creating an electric field that directs back scattered electrons toward the shielding electrode rather than the electron source.
2Reliability
If the degree of vacuum is enhanced to reduce residual gas and stabilize emission current, then the emission current stability is improved, but the device complexity and cost increase due to requiring ultrahigh vacuum pumps and stricter vacuum maintenance
Solution Approach 1:
The invention converts the harmful effect of back scattered electrons into a beneficial outcome by using them to illuminate the shielding electrode, which then emits secondary electrons that can be utilized. More importantly, by preventing ESD gas generation through the shielding electrode, the vacuum system operates at a more moderate vacuum level (10^-4 to 10^-6 Pa range) rather than requiring ultrahigh vacuum (10^-7 Pa or below), thereby reducing system complexity while maintaining emission current stability.
3Reliability
If the electron source surface is periodically purified by flashing or field evaporation to remove adsorbed gas, then the emission current stability is maintained, but the operation time and energy consumption increase
Solution Approach 1:
The shielding electrode performs a preliminary protective action by intercepting back scattered electrons before they can reach the electron source and cause gas adsorption. This preventive measure eliminates the need for periodic purification operations such as flashing or field evaporation, as the electron source surface remains clean without requiring active intervention. The shielding electrode continuously prevents contamination throughout operation.
4Object-affected harmful factors
If the electron source is surrounded by a closed structure to shield back scattered electrons, then ESD gas generation is reduced, but the vacuum evacuation route is narrowed and conductance is lowered
Solution Approach 1:
Rather than completely enclosing the electron source with a closed structure, the shielding electrode is designed as an open cylindrical structure with a specific diameter (typically 0.5 to 2 times the extraction electrode diameter) and length. This localized shielding approach provides protection against back scattered electrons in the critical region near the electron source while leaving the vacuum evacuation route sufficiently open. The shielding electrode position and dimensions are optimized to balance shielding effectiveness with vacuum conductance.
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
The shielding electrode enhances the degree of vacuum around the charged particle source, reducing gas adsorption and ESD gas generation, leading to a more stable and prolonged emission current with reduced frequency of purification operations.
Implementation Method 1
a shielding electrode held at a potential between the extraction electrode potential and the charged particle source potential, thereby shielding against back scattered charged particles
Implementation Method 2
the pressure within the electron gun is kept at or below 10^-7 to maintain a high degree of vacuum
Implementation Method 3
a cold field emission electron gun and a Schottky electron gun; each causes an electric field to concentrate on the tip of an electron beam by applying a high positive voltage (extraction voltage) to an extraction electrode opposed to an electron source having a sharp tip
Data Source
AI summary
An electron gun that serves to reduce the quantity of electron stimulated desorption and accomplishes vacuum evacuation efficiently with a sufficient degree of vacuum. An electron source 1 and an extraction electrode 6 are provided for emitting an electron beam 7 from the electron source 1. A first vacuum chamber 16 containing the electron source 1 is connected to a second vacuum chamber 9 via an aperture 8 provided in the extraction electrode 6. Each vacuum chamber is differentially evacuated with an independent vacuum evacuation means, and the generation of electron stimulated desorption gas 11 is reduced by securing a wide route of vacuum evacuation around the electron source 1 and intercepting the procession of back scattered electrons 12 emitted from the area with the electron beam 7 on the extraction electrode 6 by using a shielding electrode 22 given a prescribed potential.