Linear accelerator, Types, Construction

A linear accelerator accelerates charged particles in a straight line, with the largest being 3.2 km long and capable of 50 GeV energy. It uses the Faraday Cage Effect but faces limitations.

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Definition?

The particle accelerator in which charged particles are accelerated in a straight line is called a linear accelerator. The beam of particles produced by a particle accelerator generally comprises protons, electrons, or charged subatomic particles. The largest linear accelerator in the world is 3.2 km long. This accelerator can accelerate electrons up to 50 GeV energy. A linear accelerator is generally used for external beam radiation treatment for patients with cancer.

Principle:

A charged particle gets accelerated to a very high energy if passed through some potential difference repeatedly.

Construction:

The linear accelerator consists of the following parts.

1. Particle Source

The particle source provides the particles such as protons or electrons that are required to be accelerated. The design of a particle source depends on the nature of these particles. A cold cathode, hot cathode, photocathode, or radio frequency (RF) ion source generates the electrons. The protons are generated in an ion source which can have many different designs. A specialized ion source is needed if heavier particles such as uranium ions are to be accelerated.

2. High Voltage Source

The high voltage source is needed for the initial injection of particles.

3. Hollow Pipe Vacuum Chamber

Its length varies with the application. The pipe may be 0.5 m to 1.5 m long, device is to be used for the production of X-rays. The pipe may be about 10 m long, device is used as an injector for a synchrotron. The pipe may be several thousand meters long. If the device is to be used as the primary accelerator for nuclear particle investigations.

4. Cylindrical Electrodes

The cylindrical electrodes are placed within the evacuated chamber whose length varies with the distance along the pipe. These electrodes are called drift tubes.

5. Radio-frequency Source

One or more radio frequency generator is used to energize the drift tubes. In high high-power accelerator, one source of RF is used for each tube.

6. Appropriate Target

A water-cooled tungsten target is used, if electrons are accelerated to produce. X-rays. The various target materials are used depending upon the nature of the investigation when protons or other nuclei are accelerated.

Block Diagram:

The block diagram of the linear accelerator is shown in Fig. It consists of n-drift tubes Their axes are along the straight line. The length of tubes is increasing. The odd tubes are connected to one terminal and even tubes are connected to another terminal radiofrequency source. These tubes are inside an evacuated glass chamber. When one set of tubes is positive, then the other set is negative.

Working:

The charged particles are injected into the first drift tube at that moment when it has a polarity opposite to that of particles. The frequency of an oscillator is so adjusted that the polarity of drift tubes gets reversed at the moment when the particle reaches the gap between the drift tubes. Two types of focusing are required for the proper functioning of linear accelerators.

1. Radial Focusing:

The focusing of the charged particles towards the common axis of drift tubes while crossing the gap between two tubes is called radial focusing. The radial focusing is required because charged particles may deviate from a straight path when reach the gaps due to curved electric lines of an electric field.

2. Phase Focusing:

The process of making all the charged particles reach the successive gaps with a constant phase difference between them is called phase focusing. Phase focusing is required because the phase difference between particles may change in gaps due to different energies gained by particles. The drift tube acts as a Faraday cage when particle bunch passes through the tube. The frequency of the driving signal and the spacing of the gaps between drift tubes is so designed that maximum voltage difference appears as the particle crosses the gap. This accelerates the particle and increases its energy and velocity. Consider first drift tube has positive potential while the second drift tube has negative potential when a positively charged particle enters into the first gap. The particle gets accelerated when it is present in the first gap because a positively charged drift tube pushes it while a negatively charged drift tube attracts it. The particle gains relatively greater energy and velocity before entering into the second drift tube. In this way, the particle gets accelerated whenever crosses the gap. The length of the next drift tubes is increased in order to compensate for the velocity of the charged particle. The particles get an increment of energy whenever they enter into a gap and finally hit the target with high energy.

Faraday Cage Effect:

According to the Faraday Cage Effect, a conductor's external surface is charged with electricity. Consequently, there isn't an electrostatic field inside the conductor. This is done in order to prevent the particles from being affected by the alternating potential when it is pointing in the opposite direction of their travel. The particle travels between drift tubes in a manner that is in phase with the accelerating field. Only when the particle is in the portion of the cycle that accelerates is it exposed to the field. As the particles travel faster and must remain inside the tube for the entire half cycle, the tube sections must get larger.

Disadvantages:

The disadvantages of linear accelerators are

The device's length restricts its possible placement locations.

· A great number of driver devices associated with power supplies are required. It increases the construction and maintenance expenses.

If conducting material makes up the walls of the accelerating cavities and the accelerating fields are strong, the wall resistivity rapidly transforms electric energy into heat.