Focusing Electrode (Wehnelt Cylinder): Diameter 5-50 mm

Q: What is the primary function of the Wehnelt Cylinder?

It controls electron beam focus and regulates emission current through electrostatic repulsion, creating the initial crossover point in electron optical systems.

Q: Why are high melting point materials used?

To withstand thermal loads from proximity to the hot cathode and prevent deformation that would alter electrostatic field geometry.

Q: How does voltage affect beam characteristics?

More negative voltage increases beam convergence (smaller spot) but reduces current; optimal settings balance resolution and signal intensity.

Component Specifications

Definition

The Focusing Electrode, commonly known as the Wehnelt Cylinder, is a critical component in electron gun assemblies used in electron microscopes, cathode ray tubes, and electron beam lithography systems. This cylindrical electrode surrounds the cathode and applies a negative bias voltage relative to it, creating an electrostatic field that shapes and focuses the emitted electron beam while regulating beam current. Its precise geometry and voltage control determine the beam's convergence angle, spot size, and stability, directly impacting imaging resolution and system performance.

Working Principle

The Wehnelt Cylinder operates on electrostatic lens principles. By applying a negative voltage relative to the cathode, it creates an equipotential surface that repels electrons, forming a converging electrostatic field. This field focuses diverging electrons from the cathode into a crossover point, controlling both beam convergence and emission current through space-charge effects. The electrode's cylindrical shape and precise positioning optimize field symmetry for uniform focusing.

Materials

High-purity molybdenum, tungsten, or nickel alloys with low outgassing rates, high melting points (>2000°C), and excellent thermal stability. Surface finish: <0.5 μm roughness. Coating: Optional graphite or lanthanum hexaboride for enhanced emission stability.

Technical Parameters

Parameter	Typical range	Notes & selection driver
Length	10-100 mm
Diameter	5-50 mm
Voltage Range	-50 to -500 V relative to cathode
Alignment Tolerance	±0.01 mm
Vacuum Compatibility	<10^-6 Torr
Operating Temperature	Up to 1500°C

Ranges are indicative industry figures for RFQ preparation, not a supplier commitment. Confirm every value and standard with the legal manufacturer before ordering.

Standards

ISO 16700, DIN 50300

Parent Products

This component is used in the following industrial products

Electron Gun

A device that generates, accelerates, and focuses a beam of electrons for medical radiation therapy applications.

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Engineering Analysis

Risks & Mitigation

Thermal deformation altering focus
Voltage instability causing beam drift
Contamination from outgassing
Misalignment degrading resolution

FMEA Triads

Trigger: Material creep at high temperature
Failure: Cylinder deformation changes electrostatic field
Mitigation: Use refractory alloys with creep resistance; implement active cooling

Trigger: Voltage supply ripple
Failure: Beam position instability and focus fluctuation
Mitigation: Install filtered DC power supplies with <0.1% ripple; use feedback stabilization

Trigger: Contamination buildup on surface
Failure: Field distortion and arcing
Mitigation: Regular in-situ cleaning; maintain ultra-high vacuum (<10^-8 Torr)

Industrial Ecosystem

Compatible With

Typical Suppliers & Equivalents

Compliance & Inspection

Tolerance

±0.005 mm dimensional, ±1% voltage stability

Test Method

Electron beam profiling using Faraday cup arrays; field simulation with finite element analysis

Procurement Evaluation Criteria

A practical evidence checklist for RFQ preparation and supplier evaluation.

Technical documentation

Request current drawings, revision history, and a signed specification sheet.

Manufacturing capability

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Inspection readiness

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Manufacturers of Focusing Electrode (Wehnelt Cylinder)

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Frequently Asked Questions

What is the primary function of the Wehnelt Cylinder?