Electron Gun

Electron Guns have been used since the late 1940s in cathode ray tube (CRT) televisions and monitors.

The design of our electron gun can be understood through its two separate functions: electron emission and electron acceleration. Namely, to first introduce electrons into the system and then to accelerate these electrons, in a controlled manner, towards a target.

The introduction of electrons is done through thermionic emission. An electric current is passed through a filament which will then undergo Joule heating. Thermionic emission will take place once the filament is sufficiently heated. This introduces electrons into the system.

The acceleration is achieved by differentially biasing electrodes within the electron gun to create an electric field that will accelerate the electrons towards the target in a controlled manner. The last clause requires more than mere potential differences but electrostatic lenses such that the electron beam can be coaxed into a desired trajectory. Broadly, both these objectives are achieved by having an acceleration section followed by a focusing section.

Directly following thermionic emission, the electrons are subject to an accelerating potential. This accelerating potential is provided by an electrode (termed the anode) which is held at a positive potential difference from the filament. This causes electrons emitted from the filament to be accelerated towards the plate. Working in tandem with the anode is the Wenhelt Cylinder. Once accelerated past the anode, the beam is focused, again, with an Einzel lens.

These two loosely defined steps should result in an electron beam that is focused on a target. The target could be anything from a screen coated in phosphorous (to show the beam spot) to a Faraday Cup which can then be connected to a current measurement device.

Team Members

Aliki Sofia Rotelli (A0236272H)
Lai Tian Hao (A0236351L)
Lim En Liang, Irvin (A0173028J)
Tan Chuan Jie (A0154805E)

Idea

There are two parts to this project. The first would be to create the electron source which seems to be done with the use of a Lenard/Crookes tube [1]. Secondly, we want to focus this electron beam into a spot with the use of a homemade magnetic lens (probably a solenoid of some form). We could perhaps perform a measurement of the focal length with respect to the magnetic field strength of the lens.

Concepts

Focusing Lens

Beam Resolution

One of the most important considerations scaling down is the Rayleigh criteria. The Rayleigh criteria relates the critical dimension (CD), or smallest feasible feature size resolvable by an optical system, to the imaging wavelength (𝜆) and numerical aperture (NA). This criteria is crucial. As a result of decreasing the critical dimension.

$CriticalDimension={\frac {(0.61)\lambda }{(NA=n_{1}Sin(\theta _{1}))}}$

The Wehnelt cylinder

The Wehnhelt cylinder is a negatively biased cylinder located right after the filament. It influences the focusing of the electron beam and the electron emission from the filament. When tuned appropriately, the Wenhelt cylinder, together with the anode, aids in the emission of electrons from the filament by creating a net electric field from the attractive (to the electrons) anode and repulsive Wehnelt. Being negatively biased, the Wehnelt cylinder also acts as a convergent electrostatic lens which focuses the electron beam path from the filament to the anode.

Einzel Lens

Like the Wehnelt Cylinder, the Einzel lens is also an electrostatic lens. In this electron gun, the Einzel lens is made of three cylindrical tubes in series along the direction of the electron beam. Due to the electrostatic symmetry, the ions do not lose energy through the focusing process. In operation only the middle Einzel is actively biased (positively). The other components of the lens are held at ground potential.

Schematic of Electron gun

The electrical schematic is concerned with controlling the potentials of critical components. These components are the Wenhelt Cylinder, Anode and Einzel Lens. A separate distinction would be the polarity of these components. Both the Anode and Einzel lens will be positively biased up to 1kV above ground while the Wenhelt Cylinder will be negatively biased down to 1kV below ground (i.e. -1kV). These values are simply ranges and specific potential values were determined through simulations and the components were biased to recreate the conditions in these simulations.

Each of these potentials are achieved with a separate High Voltage Power Supply Unit. As will be detailed, due to the shortage of devices only two Power Supply Units were used. This was achieved by biasing both the Anode and Einzel lens to the same potentials, thus allowing one Power Supply Unit to be used for both of them. This was only possible as simulations suggested that this provided sufficient acceleration and focusing effects on the electron beam.

The actual practical set-up is more accurately depicted below.

Simulated Proof of Concept

Simulations were performed in SIMION based on a proposed arrangement of the components mentioned above.
With parameters:

Wehnelt: -200V
Anode: 1000V
Middle Einzel: 500V

A Simulation of the focused electron beam with Wehnelt cylinder and Einzel lens (Top-Down Cross-Sectional View)
A Simulation of the focused electron beam with Wehnelt cylinder and Einzel lens (Isometric View)
A Simulation of the focused electron beam with Wehnelt cylinder and Einzel lens (Potential Field)

These are ideal situations and in actual practical set-ups the space is constrained and all the items had to be placed closer to each other. This results in the set-ups depicted below.

Design of the electron gun's "central column"

Based on the schematic of the simulation, the development of a "central column" was designed using SOLIDWORKS, which consists of:

Acrylic plate
Einzel lens
Anode
Wehnelt cylinder

Acrylic Plate

The acrylic plate was created in a disk shape with a 100mm outside diameter and a 28mm inner diameter. In addition, the disk's inner ring features a 1mm by 32mm (depth/diameter) recessed on both sides. The recess was created to allow for the secure installation of subsequent elements.

Einzel lens

The three components of the Einzel was designed with dimensions:

Outer diameter: 32mm
Inner diameter: 28mm
Length: 30mm

Anode

The anode was designed with dimensions:

Outer diameter: 32mm
Apeture (diameter): 8mm
Thickness: 2mm

Wehnelt cylinder

Outer diameter: 32mm
Inner diameter: 28mm
Aperture (diameter): 8mm
Length: 30mm

"Central column" design

Central column with chamber design

Equipment & Components

The following items were considered for the system:

High Vacuum (HV) Components

Roughing Pump
Turbo-Molecular Pump

Electrical Components

High Voltage Power Supply Unit (PSU) (up to 5kV)
Current Controlled PSU

Beam Focusing Components

Einzel Lens

Measurement Components

High Vacuum Digital Gauge, with range $(10^{-3}$ $(10^{-3}$ ~ $10^{-7})$ $10^{-7})$ mbars
- Edwards PRM10 (Pirani Gauge)
- Edwards D145-41-000 (Penning Gauge)

Other Components

Filament (Possible choices)
- Tungsten (W) (Used in our system)
- Lanthanum Hexaboride (LaB6)
- Cerium Hexaboride (CeB6)
Phosphors
- Zinc sulfide(ZnS) (Copper/Silver Activated)
CF-40 Copper Gaskets

Setup

Manufacturing of central column

All of the items listed below were made in the workshop.

Acrylic Plate

The laser cutter is unable to make cuts at varying heights. To work around this limitation, a different approach was devised. First, a disk was cut out of a 5mm thick acrylic sheet with an outer radius of 100mm and an inner diameter of 32mm. Second, a 32mm/28mm (outer/inner) diameter ring was cut. Finally, the pieces were put together with an adhesive compound that allowed for a 1mm recess on both sides of the disk.

[Insert photo of acrylic plate]

Nine additional disks were manufactured in the same manner to obtain a total of ten disk plates.

Lathe Machine: Drilling

The three components of the Einzel lens, anode, and Wehnelt cylinder were sculpted out from a 42mm by 350mm (diameter/length) aluminum rod with the use of a lathe (a machine that rotates a workpiece about an axis of rotation to perform an operation such as cutting, sanding, drilling, etc.). After securely clamping down the aluminum rod, the rotational gears were set to “low” with a rotational speed of 105/610 rounds per minute (RPM) based on drill bit size: (Large/Small), and a drill bit secured within the “Tail Stock”.

Lathe Machine: "Facing"

After achieving the structure's inner diameter, a "facing" tool was used to remove the surface material from the outside construction, "thinning" it from 42mm to 32mm. Rotational gear was set to low-610 RPM when "facing."

Manufactured items

The following items were manufactured for the system.

Manufactured wehnelt cylinder
Manufactured Anode
Manufactured Einzel

Other than the manufactured items mentioned above, the filament was also prepared. The glass of the bulb was broken by lightly tapping the bulb with a metal hammer in a plastic bag. Broken glass fragments are then disposed of, resulting in the following:

Chamber Setup

Note: Latex gloves were worn during the setup of the external ports of the chamber and the assembly of the centre column.

M5 nuts were utilized as a locking mechanism to guarantee that acrylic plates were firmly secured in place (Top and bottom of the acrylic plates).

The wehnelt cylinder was then carefully lowed into the chamber to ensure no contact between it and the filament.

Placement of the wehnelt cylinder within the central column stack.
Placement of the wehnelt cylinder within the central column stack (side profile).

The rest of the components including the anode and three einzel lens were then assembled within the chamber to achieve a full stack similar to the illustration of the schematic diagram.

With the central column setup completed, the phosphor was then mounted on the chamber lid with the use of Kapton tape. (Note: During the process of mounting, glass slide shattered causing a slight misalignment of the phosphor with respect to the center of the column.)

Phosphor on a microscope glass mounted on with Kapton tape (bottom side).
Phosphor on a microscope glass mounted on with Kapton tape (top side).

Fully Assembled System

Measurements/Test

Phosphor Characterization

The phosphor powder used in this project was kindly provided by Nejilock and has a composition of >99% zinc sulfide. The powder can be characterised using a fluorescent microscope. The powder is excited with UV light and the emission is collected. The spectrum of wavelengths emitted is presented in the figure below. The majority of the emission is in the 450-550 nm region, meaning the powder will have a blue/green glow.

In order to insert the phosphor powder into our system, we had to devise a way to attach it to a glass slide. The most successful method found was to create a paste by adding transparent nail polish and thinner to the powder. The paste can then be applied onto a small section of the glass slide. Once the thinner evaporates, the powder will be stuck to the glass.

Pressure Characterization

The roughing pump and the turbomolecular pump were both characterized while they were connected to the chamber. Do note that for a turbo-roughing system, vacuum pressure criteria of approximately $10^{-3}$ mbar must first be achieved using the roughing pump before the turbomolecular pump can be turned on. This is to ensure that the turbines blades of the turbo remain undamaged. Maximum vacuum pressure achieved with this setup is $1.5\times 10^{-6}$ mbar after 16 hours and $5.2\times 10^{-7}$ mbar after 48hours of pump down. With an initial maximum of $6.5\times 10^{-6}$ mbar at 41mins.

The plot of pressure in mbars as a function of time
The plot of pressure in mbars as a function of time

Vacuum pressure after 48hours of pumping
Turbo-molecular pump
Roughing pump backing the turbo pump

Conductance Test

After all of the components were assembled, conductance tests of individual components were performed to ensure proper connections between the components and the feedthroughs.

Demonstration of checking for proper connection with a multimeter
Feedthrough connections

Experiment/Results

Similar to the parameters simulated:

The Wehnelt cylinder at potential: -200V
Anode and the first and third Einzel lens at the potential: 1kV

Power supply to wehnelt cylinder at -200V and 1kV to anode and einzel lens.

Filament power supply:
- Voltage: 10V
- Current: 0.1104A

A tinge of fluorescence can be spotted which is illuminated from the phosphor coating. With a more prominent glow located at the top right quadrant of the phosphor coating. This coincides with the slight misalignment of the glass slides during installation.

Electron Gun

Contents

Team Members

Idea