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 project can be understood through its two separate functions: electron emission and electron acceleration. In summary, to 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 undergo Joule heating. Thermionic emission will take place once the filament is sufficiently heated, hence introducing the electrons into the system.

The acceleration of electrons 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 would be 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 (anode) which is held at a positive potential difference with respect to 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 again once more 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 is connected to a current measurement device.

Team Members

The group consists of 4 team members, listed below in alphabetical order.

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

Idea

The project is divided into two parts.

Electron source

An electron source would be fabricated with the use of a Lenard/Crookes tube. This tube is an electrical experimental discharge tube within a vacuum system that generates electrons by ionisation of the remaining air molecules within the chamber. The project would hence be built in a vacuum chamber.

Collimated beam

The generated electrons would need to be collimated into a beam with the use of a homemade magnetic lens (a solenoid of some form). To observe this beam, a material capable of showing the electron spot would hence be needed in our project.

Concepts

Focusing Lens

Beam Resolution

One of the most important considerations in the scaling down portion is the Rayleigh Criterion. The Rayleigh Criterion relates the critical dimension (CD), or smallest feasible feature size resolvable by an optical system, to the imaging wavelength ($\lambda$) and numerical aperture (NA). This criteria is crucial as a result of a small CD, as shown in the equations below.

$NA=nsin(\theta )$

$CD=0.61{\frac {\lambda }{NA}}$

In the above equations, θ is the maximum half-angle of the light cone that enters or exits the lens, n is the refractive index of the medium, λ is the wavelength of light.

Wehnelt Cylinder

The Wehnelt cylinder is a negatively biased cylinder located right after the electron source. It affects the focusing of the electron beam and the electron emission from the source. When tuned appropriately, the Wehnelt cylinder, together with the anode, aids in the emission of electrons from the source by creating a net electric field from the attractive anode and repulsive Wehnelt cylinder. 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

Similar to the Wehnelt cylinder, the Einzel lens is also an electrostatic lens. In this project, the Einzel lens would be 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 lens is actively biased (positively). The other components of the lens are held at ground potential.

Schematic of Electron gun

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 would be positively biased up to 1 kV above ground while the Wenhelt Cylinder will be negatively biased down to 1 kV below ground (i.e. -1 kV). These values are simply ranges and specific potential values were determined through simulations. The components would be biased to recreate the conditions in the simulations.

Each of these potentials would be achieved with separate high voltage Power Supply Units (PSUs). Due to the shortage of devices, only 2 PSUs were used. This was achieved by biasing both the Anode and Einzel lens to the same potentials, thus allowing one PSU to be used for both. The simulations suggested that this configuration would provide sufficient acceleration and focusing effects on the electron beam.

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

Updated practical schematic of Electron gun for operation with only two High Voltage Power Supplies

Simulated Proof of Concept

Simulations were performed in a simulation software, SIMION, based on the above-mentioned proposed configuration.

Simulation Parameters

Wehnelt	-200 V
Anode	1000 V
Middle Einzel	500 V

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)

The above shows an ideal situation. However, in 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, with all simulations at top-down cross-sectional view.

Revised simulation at 0 V

Revised simulation at 200 V

Revised simulation at 400 V

Revised simulation at 600 V

Revised simulation at 800 V

Revised simulation at 900 V

Revised simulation at 1000 V

As shown, even with the compacting of the electron gun, a focused beam spot is still able to be produced with the following parameters.

Simulation Parameters

Item	Original	Revised
Wehnelt	-200 V	-200 V
Anode	1000 V	1000 V
Middle Einzel	500 V	900-1000 V

Design of the electron gun's "central column"

Based on the schematic of the simulation, the development of a "central column" was designed using a 3D Computer-Aided Design (CAD) software, SOLIDWORKS. It consists of 4 main components as described below.

Acrylic Plate

Schematic design of Acylic plates

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

Einzel Lens

Schematic design of Einzel lens

The three components of the Einzel was designed with dimensions:

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

Anode

Schematic design of Anode

The anode was designed with dimensions:

• Outer diameter: 32 mm
• Aperture (diameter): 8 mm
• Thickness: 2 mm

Wehnelt Cylinder

Schematic design of Wehnelt cylinder

The Wehnelt cylinder was designed with dimensions:

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

"Central Column" design

Central Column Schematic Design of Electron Gun

"Central Column" with Chamber design

Schematic diagram overview of electron gun

Equipment & Components

To build the electron gun, the following items were considered for the system.

High Vacuum (HV) Components

• Roughing Pump (RP)
• Turbo-Molecular Pump (TMP)

Electrical Components

• 2 x PHYWE High voltage power supply with digital display (10 kV DC, 2 mA)
• Current Controlled PSU

Beam Focusing Components

• Einzel Lens

Measurement Components

• High Vacuum Digital Gauge, with range $(10^{-3}$ ~ $10^{-7})$ 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

Main Set-Up

Manufacturing of central column

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

Acrylic Plate

The laser cutter was 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 5 mm thick acrylic sheet with an outer radius of 100 mm and an inner diameter of 32 mm. Next, a 32 mm/28 mm (outer/inner) diameter ring was cut. Finally, the pieces were put together with an adhesive compound that allowed for a 1 mm recess on both sides of the disk.

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”.

A drill bit with a diameter of 28mm was used to remove aluminum rod material with a lathe (GIF).

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."

Rotational gear: low-610 RPM, Removal of the surface material of aluminum rod ("Facing") (GIF).

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:

Broken bulb with exposed filament

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).

Assembly of first component (Filament).

• 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.

Assembled components which includes: filament, wehnelt cylinder, anode and einzel lens.

• 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

Fully assembled setup

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.

The wavelength spectrum of the phosphor, excited by UV source.

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 (RP) and the Turbomolecular Pump (TMP) were both characterized.

For a turbo-roughing system, a vacuum pressure of approximately $10^{-3}$ mbar must first be achieved using the RP before the TMP can be turned on, so as to not damage the turbine blades of the TMP.

Various pressures at different timings are recorded into the table below.

Chamber pressure

Time	Pressure
41 min	$6.5\times 10^{-6}$ mbar
16 h	$1.5\times 10^{-6}$ mbar
48 h	$5.2\times 10^{-7}$ mbar

The graphs of pressure against time for both RP and TMP were also plotted into logarithmic graphs, as shown below. The RP was transitioned into the TMP at approximately the 1064th second at a pressure of $5.2\times 10^{-3}$ mbar.

RP pressure graph

TMP pressure graph

Photos

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:

• Wehnelt cylinder potential: -200V
• Anode: 1kV
• Einzel Lens: 1kV

As mentioned, because of the identical potential for the Anode and Einzel, the same power supply was able to be used for both of them while a separate one was used for the Wenhelt Cylinder.

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

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

Power supply for tungsten filament.

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.

Phosphor glow due to focused electron beam

Problems and Discussion

Dim luminescence of phosphorus

When the setup was running, the glow of the phosphor was barely observed. This could be attributed to the thickness of the phosphorus and the spot size.

As mentioned previously, the spot size of the beam would be a few hundred microns. With such a spot size and a thick phosphorus layer, when the beam hits the bottom of the layer, it would illuminate only slightly. The phosphor would thus only appear to slightly "glow" and not allowing us to see the spot as expected.

Filament alignment

As the setup was built from the bottom-up and due to limitations of the size and the holder, the filament could not be adjusted freely, thereby resulting in possibilities of misaligning the filament.

After the setup was complete, it was observed that the filament was biased to one side. This would affect the results as the simulation assumes the filament to be at the centre of the Wehnelt Cylinder.

High Voltage power supplies

In the current setup, only 2 power supplies were used to vary the potential differences of the 3 Einzel lenses and the Wehnelt Cylinder. An extra power supply would enable the Einzel lens potential differences to be tuned separately, providing better fine-tuning abilities.