Introduction to CRTs

A cathode ray tube (CRT) is a vacuum tube used as a display in a computer monitor or TV. In a CRT, a beam of electrons can be focused on a phosphorescent viewing screen and rapidly varied in position and intensity to produce an image. It uses a large evacuated glass envelope which has a very distinctive funnel shape. Other common technologies used today include LCD’s and plasma displays. Let’s start by looking at the CRT, however.

How does a CRT Work?

There are three main sections in the operation of a CRT; the electron gun, the deflection system and the screen.

The electron gun begins with the heater, which is a filament of metal (usually thoriated tungsten, – the same material used in everyday household light bulb filaments). The filament indirectly heats up the cathode (composed of an oxide-coated nickel-cylinder), setting free the electrons from the surface and causing them to flow. This happens because when the power is connected and the temperature of the cathode is increased, the electrons are given sufficient thermal (heat) energy from external sources to break the bond that binds them to the metal nuclei. When this occurs, the heat energy transferred to the electron exceeds the work function of the metal. This phenomenon where electrons escape from the surface of metals is known as “thermionic emission”. Consequently, when the power is disconnected and the cathode is not heated, no electron beam is observed and no spot appears on the screen.

After the heater comes a control grid, which is generally placed directly on top of the cathode. It controls the number of electrons that are fired across the aperture and as such, controls the brightness of the CRT. On a more fundamental level, it controls the cathode current, responsible for the rate of emission of electrons or the intensity of the electron beam.

This is what the “ray” is – the stream of electrons which naturally pour off the heated cathode into the vacuum. Electrons are negative, so the high positive potential of the anode relative to the cathode attracts the electrons ejected from the cathode. The focusing anode produces a narrow, sharply-focused beam. Subsequently, the accelerating anode turns the electrons into a high speed-beam of electrons; for every volt they are accelerated through, they gain some 600km/s.

Sometimes, there is also a grid in the CRT following the accelerating anode which controls the astigmatism of the beam

By and large, the sole purpose of the electron gun is to take the many electrons emitted by the cathode and provide a focused electron beam which is accelerated towards the phosphor screen.

The deflection system consists of two sets of electromagnets; the vertical plates control the vertical position of the beam while the horizontal plates control the horizontal positioning. The electromagnets are in the form of steering coils and generate magnetic fields to direct the electrons to the screen. These steering coils are simply copper windings and wrap around the tube to steer the beam away from always landing in a tiny dot right in the center of the screen. By controlling the voltages in the coils, the electron beam can be positioned at any point on the screen.

The magnetic field created by the steering coil causes a force to act on the electrons which is perpendicular to the direction of travel and the magnetic field. This causes a charged particle in a magnetic field to follow a circular path. The faster the motion of the particle, the larger the circle traced out for a given field or, conversely, the larger the magnetic field required for a given radius of curvature of the beam. Without control over the particle energy and the magnetic field, this is deemed impossible.

We can use the fact that magnetic fields bend the path of a moving charged particle to our advantage by using it to control where the electron beam is aiming. A larger magnetic field is required to bend a faster-moving particle. The strength of the LHC (Large Hadron Collider) bending magnets is 8.36T (Tesla).

The beam then flies through the vacuum in the tube to the flat screen at the other end of the tube. The screen is at the viewing end of the tube and is coated with phosphors, which emit light in the form of photons when they are struck by electron beams and become “excited”. This is due to a property of the phosphors known as chemiluminescence.

The chemical composition of the phosphor determines the color of light that is produced. To make a colour screen, phosphors of red, green, and blue are clustered together in one “pixel” on the screen. The electron beam must strike each of the red, green and blue phosphors individually, causing them to glow at each pixel location. The resulting colour displayed on the screen is derived by the intensity of these electron beams as they strike each phosphor; the strength of the electron beam determines how brightly the phosphor will glow.

In a black-and-white screen, there is one phosphor that glows white when struck. Conversely, in a colour screen, the phosphor-coated screen is simply a plate of glass coated inside with red, green and blue phosphor dots or stripes, and separate “electron guns”, which bombard their respective colours at a time in a prescribed sequence known as the raster scan. This works at a rate faster than our eyes can perceive. The electrons beams can also illuminate the three different colours together.

In a CRT, there is also a shadow mask with tiny holes in the metal plate to separate the coloured phosphors in the layer behind the front glass of the screen. The holes are strategically placed to ensure that electrons from each of the tube’s three cathode guns reach only the matching-coloured phosphors on display. Furthermore, the spacing of the holes and phosphors and the placement of guns is arranged so that each coloured gun has an unobstructed path to its respective coloured phosphors. All three beams pass through the same holes in the mask, but the angle of approach is different for each gun. The red, green, and blue phosphors for each pixel are usually arranged in a triangular shape (sometimes called a “triad”).

The picture of the screen is reliant on the electrons precisely striking phosphors on the back of the screen which emit different colours of light when struck. When a new magnetic field is introduced, electrons are forced to land in the wrong place, resulting in the distortion of the image and the formation of psychedelic colours.

Introduction to Particle Accelerators



When one thinks of particle accelerators, they are likely to imagine the world’s largest CERN’s Large Hadron Collider (LHC), which is deepening our understanding of what happened just after the Big Bang.

A particle accelerator is a machine that accelerates elementary particles, such as electrons or protons, to very high energies. On a basic level, they produce beams of charged particles that can be used for a variety of research purposes. The two basic types of particle accelerators include linear accelerators and circular accelerators. Linear accelerators propel particles along a straight beam line and are directed at fix-targets such as a thin piece of metal foil, whereas circular accelerators propel particles around a circular track and can be used for both colliding beam and fixed-target experiments. Collisions at accelerators can occur either against a fixed target, or between two beams of particles.

How does a particle accelerator work?

Firstly, a particle source provides the elementary particles to be accelerated. As for the LHC, beams of protons or lead nuclei are accelerated instead of electrons. The protons are produced with an ion-source known as a duoplasmatron. One might expect the LHC to require a large source of particles, but protons for beams in 27-kilometre ring come from a single bottle of hydrogen gas! It is replaced only twice per year to ensure that it is running at the correct pressure.

Using an electric field, a particle accelerator speeds up and increases the energy of this beam of particles which travel inside a vacuum in a metal beam pipe. The vacuum is crucial to maintaining an air and dust-free environment for the beam of particles to travel unobstructed and not collide with gas molecules. It then uses very large magnets known as dipoles to steer the particles, followed by quadrupole magnets to focus them. These superconducting electromagnets are quite incredible because they produce a very strong magnetic field, capable of bending the path of particles travelling at a speed close to that of light.

After the electric field strips hydrogen nuclei (consisting of one proton and one electron) of their electrons in the first part of the accelerator, electric fields along the accelerator switch from positive to negative at a certain frequency, creating radio waves that accelerate the particles in closely spaced bunches. This frequency can be controlled to ensure the particles don’t move along in a continuous stream.

Subsequently, the particles are directed at the fixed target or two beams of particles are collided. Then, particle detectors are placed around the collision point to record and reveal the particles and radiation that are produced by the collision between a beam of particles and the target.

Important features of a particle accelerator include:

Radiofrequency (RF) cavities – Specially designed metallic chambers spaced at intervals along the accelerator. These are shaped to resonate at specific frequencies, enabling radio waves to interact with passing particle bunches. Every time a beam passes the electric field in an RF cavity, some of the energy from the radio waves is transferred to the particles, pulling them forward.

Various types of magnets – Different magnets serve different functions around a circular accelerator. Dipole magnets, for example bend the path of a beam of particles that would otherwise travel linearly. On the other hand, quadruple magnets act like lenses to focus a beam, gathering the particles closer together. These are known as “lattice magnets” and they maintain the stability and precise alignment of the beam.

Evaluation of Devices’ Impact on Society

Altogether, a CRT is simply a linear particle accelerator that creates an image on a fluorescent screen by accelerating and deflecting a beam of electrons in a vacuum. All though CRTs are several orders of magnitude less powerful than the LHC, the principles of operation are very similar. Moreover, a CRT is a prime example of a particle accelerator found commonly around the house in old-fashioned TVs and computer monitors.

Advantages of the CRT consist of:

Without sacrificing the image quality, they can operate at any resolution, geometry and aspect ratio.

Exceptional viewing angles.

Provides long life service.

Can maintain good brightness.

As of recent, however, CRTs have become fairly outdated, being replaced by newer and more efficient technologies such as LCDs and plasma displays. A disadvantage of the CRT is that even an evacuated glass envelope (capable of holding a vacuum) the size of a modern LCD or a plasma display, would weigh a lot and use lots of power, yet still not provide as great of a picture compared with the modern alternatives. Apart from being large, heavy and bulky, a CRT is also deep (refers to length from screen face to rear end) and relatively fragile. On the other hand, LCDs are smaller, lighter, cheaper and easier to build, store and transport. Furthermore, they are easier to display in shops and cheaper to power. These qualities allow LCDs to be very beneficial for manufacturers to both build and sell to consumers. The uses for the analogue output of the CRT are losing ground as LCDs continue to improve, with faster response times, better contrast, higher resolutions and convenience of the form factor of their LCD displays.

Compared with particle accelerators, the applications of CRTs are limited. In a CRT, the electrons can be deflected by a magnetic (or in the case of oscilloscopes, an electrical) field before they strike the phosphorescent screen, creating an image. For instance, this image could be electrical waveforms (on an oscilloscope), radio wave echoes of aircraft or ships (on a radar screen) or pictures on an old-fashioned television screen or computer monitor. This single function cannot be duplicated by any other tube or transistor; namely, the ability to convert electronic signals to visual displays, such as pictures, radar sweeps, or electronic waves is unique to CRTs . Previously, CRTs have been also been used as memory devices in which the light emitted from the fluorescent material (if any at all) does not hold any meaning to the visual observes yet the visible pattern on the tube face can cryptically represent the stored data.

Despite the huge impact of CRTs, it is unquestionable that particle accelerators have had a bigger impact on society and will continue to do so in the future. Particle accelerators touch nearly every part of our life and have diverse applications, ranging from medical and scientific research to consumer product development and national security. Since the early days of the cathode ray tube in the 1890s, particle accelerators have made significant contributions to scientific and technological innovation. Today, there are more than 30,000 particle accelerators in operation around the world.

All applications of particle accelerators include:

Semi-conductors: Accelerator technology is used to implant ions in silicon chips increasing effectiveness in electronic products such as computers and smart phones.

Clean air and water: Research conducted shows that blasts of electrons from an accelerator can effectively clean up dirty water, sewage sludge and polluted gases from smokestacks.

Medical diagnostics: Accelerators can produce a range of radioisotopes for medical diagnostics and treatments used at hospitals worldwide in millions of procedures annually.

Pharmaceutical research: Powerful X-ray breams from synchrotron light sources allow scientists to analyse protein structures quickly and accurately, leading to the development of new drugs to treat major diseases such as cancer, diabetes, malaria and AIDS.

Nuclear energy: Potential to treat nuclear waste by enabling the use of an alternative fuel such as thorium to produce nuclear energy.

Shrink wrap: Particle accelerators are used to produce the sturdy, heat-shrinkable film that keep items and baked goods fresh and protect board games, DVDs and CDs.

DNA research: The 2009 Nobel Prize in Chemistry was awarded for analyzing and defining how the ribosome translates DNA information into life. This was made possible through synchrotron light sources. This research could lead to the development of improved antibiotics.

Cancer therapy: A particle beam has been deemed as the best tool for treating certain kinds of cancer, with fewer side effects than traditional treatments.