Post-deflection acceleration refers to the process of accelerating an electron beam after it has been deflected by an electric or magnetic field. This acceleration is commonly used in cathode ray tubes (CRTs), electron microscopes, and other electron beam devices.
In CRTs, for example, a cathode emits a beam of electrons that is accelerated towards a phosphor-coated screen. The beam passes through a series of electric and magnetic fields, which control its trajectory and focus it onto specific areas of the screen. These fields can deflect the electron beam horizontally and vertically to create the desired image.
During the deflection process, the electron beam experiences a change in velocity due to the electric or magnetic forces acting on it. However, this deflection also causes the beam to lose kinetic energy. To compensate for this energy loss and maintain a consistent beam intensity, post-deflection acceleration is applied.
The post-deflection acceleration is typically achieved by using an additional electric field. This field is designed to increase the kinetic energy of the electrons, allowing them to regain the energy lost during deflection. The electrons are accelerated towards a positively charged electrode, often called an anode, which provides the necessary electric potential for acceleration.
By applying the appropriate post-deflection acceleration, the electron beam can maintain its intensity and focus on the desired target, such as the phosphor screen in a CRT. This ensures that the resulting image or display is bright and clear.
In summary, post-deflection acceleration is the process of re-energizing an electron beam after it has been deflected by electric or magnetic fields. It helps maintain the intensity and focus of the beam, ensuring accurate imaging in various electron beam applications.
According to the equation in electronstatic deflection system ,
D = lEd/d/2dEd
The value of acceleration voltage Ea should be low. This voltage is usually kept below 4KV. Althrough this voltages gives good sensitivity but it reduces brightness. It maximum frequency to be displayed is below 10 HH2 then mono acceleration tubes.are used however, If frequency above 10HH2 are to be displayed post. post deflection acceleration (PDA) tubes are used.
⟶ There are two types of PDA structure.
a) Spiral acceleration.
b) Mesh acceleration.
★ Spiral acceleration :- Spiral accelerator uses high resistance narrow spiral of graphite which is painted over a considerable length of the inside of the envelope tuned. The spiral is connected to the aluminium film on the phospher, if present. A voltage of about 10KV is applied to this spiral.
★ Mesh acceleration :- mesh accelerator uses spherical mesh into the lelix tube this shopes the acceleratiing field and prevents it from effecting original beam deflection. Both the system spiral PDA , are limited in scan angle to about 35˙ to 40˙ , A high expansion mesh system beam deflection up to 90˙ can be obtained. However, spot size also increase.
Imagine an electron beam being emitted from a source, such as a cathode. The beam consists of a stream of electrons moving in a particular direction. To accelerate the electron beam, an electric field can be applied.
In a simple setup, two electrodes can be used: a cathode and an anode. The cathode is negatively charged, while the anode is positively charged. The electric field is created between these electrodes, causing the electrons in the beam to experience a force in the direction from the cathode to the anode.
As the electrons move through the electric field, they gain kinetic energy and accelerate. The magnitude of the acceleration depends on the strength of the electric field and the charge-to-mass ratio of the electrons. The electric field provides the necessary potential difference to accelerate the electrons to the desired velocity.
It's important to note that the actual configuration and design of an electron beam acceleration system can vary depending on the specific application, such as CRTs or electron microscopes. Different devices may use additional components like magnetic fields or complex electrode arrangements to control the trajectory and focus of the electron beam.
While I cannot provide a visual representation here, you may find it helpful to search for diagrams or illustrations of electron beam acceleration systems online, which can give you a clearer picture of how it works in different contexts.
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