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Defining Linear Particle Accelerators Engineering Essay

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Published: Mon, 5 Dec 2016

Abstract of work done As linear particle accelerator was invented in 1928 by Rolf Wideroe which is (often shortened to linac) is a type of particle accelerator that greatly increases the velocity of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beamline

In my term paper I have included the different types of accelerators and their uses as well as on the Linear accelerators that how it helps to accelerate the particle linearly. Their importance in medical physics and radiotherapy. Included the different components of accelerator and their workings. And also enclosed the images required to make the things to be very clear with proper description.

ACCELERATORS

A particle accelerator is a device that uses electric fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field.

In the early 20th century, cyclotrons were commonly referred to as atom smashers. Despite the fact that modern colliders actually propel subatomic particles-atoms themselves now being relatively simple to disassemble without an accelerator-the term persists in popular usage when referring to particle accelerators in general.

Uses

Beamlines leading from the Van de Graaf accelerator to various experiments, in Paris.

In early particle accelerators a Cockcroft-Walton voltage multiplier was responsible for voltage multiplying. This piece of the accelerator helped in the development of the atomic bomb. Built in 1937 by Philips of Eindhoven it currently resides in the National Science Museum in London, England.

The now disused Koffler particle accelerator at the Weizmann Institute, Rehovot, Israel

Beams of high-energy particles are useful for both fundamental and applied research in the sciences, and also in many technical and industrial fields unrelated to fundamental research. It has been estimated that there are approximately 26,000 accelerators worldwide. Of these, only ~1% are the research machines with energies above 1 GeV (that are the main focus of this article), ~44% are for radiotherapy, ~41% for ion implantation, ~9% for for industrial processing and research, and ~4% for biomedical and other low-energy research.

Types of Accelerators:

Cockroft-Walton accelerators: high DC voltage device which accelerates ions through steps of voltage created by a voltage divider.

Van de Graaf accelerators: charge is transported by an insulating belt to a conductor which builds in voltage as a result of charge collection.

Cyclotrons: An oscillating electric field repetitively accelerates charged particles across the gap between semicircular magnetic field regions.

Synchrocyclotrons: cyclotrons with variable-frequency accelerating voltages to track relativistic effects.

Betatrons: electron accelerators in a circular geometry with acceleration achieved by magnetic flux increase.

Synchrotrons: large ring accelerators where the particles move in an evacuated tube at constant radius, accelerated by radio frequency applications with synchronous magnetic field increases to maintain the constant radius.

Linear Accelerators: linear arrays of radio frequency acceleration cells.

Low-energy machines

Everyday examples of particle accelerators are cathode ray tubes found in television sets and X-ray generators. These low-energy accelerators use a single pair of electrodes with a DC voltage of a few thousand volts between them. In an X-ray generator, the target itself is one of the electrodes. A low-energy particle accelerator called an ion implanter is used in the manufacture of integrated circuits.

High-energy machines

DC accelerator types capable of accelerating particles to speeds sufficient to cause nuclear reactions are Cockcroft-Walton generators or voltage multipliers, which convert AC to high voltage DC, or Van de Graaff generators that use static electricity carried by belts.

Electrostatic particle accelerators

This method is still extremely popular today, the number of electrostatic accelerators greatly out-numbering any other class, they are more suited towards lower energy studies owing to the practical voltage limit of about 30 MeV (when the accelerator is placed in a gas tank).

Oscillating field particle accelerators

Due to the high voltage ceiling imposed by electrical discharge, in order to accelerate particles to higher energies, techniques involving more than one lower, but oscillating, high voltage sources. These electrodes can either be arranged to accelerate particles in a line or circle, depending on whether the particles are subject to a magnetic field while they are accelerated, causing their trajectories to arc.

Applications

By far the most common use of particle accelerators is for basic research on the composition of matter. The quantities of energy released in such machines are unmatched anywhere on Earth. At these energy levels, new forms of matter are produced that do not exist under ordinary conditions. These forms of matter provide clues about the ultimate structure of matter.

Accelerators have also found some important applications in medical and industrial settings. As particles travel through an accelerator, they give off a form of radiation known as synchrotron radiation. This form of radiation is somewhat similar to X rays and has been used for similar purposes

Applications of Particle Accelerators in Medical Physics.

THE ROLE OF PARTICLE ACCELERATORS COMBATING CANCER.

ACCELERATORS FOR RADIOTHERAPY

HADRONTHERAPY FACILITIES

ACCELERATOR BASED FACILITY FOR RADIOISOTOPES PRODUCTION- Both Radioisotopes and enriched stable isotopes are essential to a wide variety of applications in medicine, where they are used in the diagnosis and treatment of illness.

LINEAR ACCELERATORS

 

A Linear Accelerators or LINAC is a particle accelerator which accelerates charged particles – electrons, protons or heavy ions – in a straight line.

Charged particles enter on the left and are accelerated towards the first drift tube by an electric field. Once inside the drift tube, they are shielded from the field and drift through at a constant velocity. When they arrive at the next gap, the field accelerates them again until they reach the next drift tube. This continues, with the particles picking up more and more energy in each gap, until they shoot out of the accelerator on the right.

The drift tubes are necessary because an alternating field is used and without them, the field would alternately accelerate and decelerate the particles. The drift tubes shield the particles for the length of time that the field would be decelerating

 

The linear accelerator, or linac, is the electromagnetic catapult that brings electrons from a standing start to relativistic velocity–a velocity near the speed of light. Here is a photo of the FNRF linac.

 

 

The linac is ~2.5 meters long–not a great distance in which to get even an electron from zero to almost 300,000 kilometers per second.

Linacs have many applications, from the generation of X-rays for medicinal purposes, to being an injector for a higher-energy accelerators, to the investigation of the properties of subatomic particles. The design of a linac depends on the type of particle that is being accelerated: electrons, protons or ions. They range in size from a cathode ray tube to the 2-mile (3.2 km) long Stanford Linear Accelerator Center in Stanford, California also called Linac,

type of particle accelerator that imparts a series of relatively small increases in energy to subatomic particles as they pass through a sequence of alternating electric fields set up in a linear structure. The small accelerations add together to give the particles a greater energy than could be achieved by the voltage used in one section alone.

PARTS OF LINEAR ACCELERATOR

The major parts of a linear accelerator are:

The electron gun

The buncher

The linac itself

Each part is responsible for a stage in the acceleration of the electrons.

 

The Electron Gun

 

The electron gun (see electron gun theory), located at the left in the drawing, is where electron acceleration begins. The electrons start out attached to the molecules in a plate of barium aluminate or other thermionic materials such as thorium. This is the cathode of the electron gun. A cathode is a surface that has a negative electrical charge. In linac electron guns this charge is usually created by heating the cathode. Barium aluminate is a “thermionic” material; this means that it’s electrons tend to break free of their atoms when heated. These electrons “boil” near the surface of the cathode.

 

The gate is like a switch. It consists of a copper screen, or “grid,” and is an anode. An anode is a surface with a positive electrical charge. Every 500 millionth of a second the gate is given a strong positive charge that causes electrons to fly toward it from the cathode in tremendous numbers. As these electrons reach the gate, they become attracted even more strongly by the main anode, and pass through the gate.

 

Because the gate is pulsing at a rate of 500 million times per second (500 MHz), the electrons arrive at the anode in loose bunches, a 500 millionth of a second apart. The anode is a torus (a doughnut) shaped to create an electromagnetic field that guides most of the electrons through the hole into the next part of the accelerator, called the buncher.

 

The Buncher

 

The purpose of the buncher is to accelerate the pulsing electrons as they come out of the electron gun and pack them into bunches. To do this the buncher receives powerful microwave radiation from the klystron. The microwaves accelerate the electrons in somewhat the same way that ocean waves accelerate surfers on surfboards. Look at the following graph:

 

 

The yellow-orange disks are electrons in the buncher. The curve is the microwave radiation in the buncher. The electrons receive more energy from the wave–more acceleration–depending on how near they are to the crest of the wave, so the electrons riding higher on the wave catch up with the slower ones riding lower. The right-hand wave shows the same group of electrons a split second later. On the front of the wave, the two faster electrons have almost caught up with the slower electron. They won’t pass it though, because they are now lower on the wave and therefore receive less acceleration.

 

The higher electron on the back of the wave gets just enough acceleration to match the speed of the wave, and is in the same position as it was on the left-hand wave. This represents the last electron in the bunch. The lower electron on the back of the wave gets too little energy to keep up with the bunch and ends up even lower on the right-hand wave. Eventually it will fall back to the electron bunch forming one wave behind.

 

The Linac

 

The linac itself is just an extension of the buncher. It receives additional RF power to continue accelerating the electrons and compacting them into tighter bunches. Electrons enter the linac from the buncher at a velocity of 0.6c–that’s 60% of the speed of light. By the time the electrons leave the linac, they are traveling very close to the speed of light.

Applications

A linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator can also be used in stereotactic radiosurgery similar to that achieved using the gamma knife on targets within the brain. The linear accelerator is used to treat all parts/organs of the body. It delivers a uniform dose of high-energy x-ray to the region of the patient’s tumor. These x-rays can destroy the cancer cells while sparing the surrounding normal tissue. The LINAC is used to treat all body sites with cancer and used in not only external beam radiation therapy, but also for Stereotactic Radiosurgery and Stereotactic Body Radiotherapy.

A linear accelerator is also used for Intensity-Modulated Radiation Therapy (IMRT), Image Guided Radiation Therapy (IGRT), Stereotactic Radiosurgery (SRS) and Stereotactic Body Radio Therapy (SBRT).

How does the equipment work?

1. A powerful radio frequency system, similar to a radar transmitter, produces high electric fields in the gaps between electrodes.

2. The electric fields in each gap oscillate together at the frequency of the rf power.

3. The charged particles arrive in bunches, timed to enter the first gap when the field is accelerating.

4. When the field is reversed, i.e. decelerating, the particles are hidden in the bore of the drift tube, shielded from the electric field.

5. The drift tube length and spacing increases to keep pace with the increasing particle velocity as they gain energy.

6. The bunches are timed to arrive in the center of the gap, as the field is increasing, so that those arriving early gain less energy, and those arriving late gain more energy.

7. The beam is focused in the transverse direction by strong permanent magnet quadrupoles inside each drift tube.

The linear accelerator uses microwave technology (similar to that used for radar) to accelerate electrons in a part of the accelerator called the “wave guide”, then allows these electrons to collide with a heavy metal target. As a result of the collisions, high-energy x-rays are produced from the target. These high energy x-rays will be directed to the patient’s tumor and shaped as they exit the machine to conform to the shape of the patient’s tumor. The beam may be shaped either by blocks that are placed in the head of the machine or by a multileaf collimator that is incorporated into the head of the machine. The beam comes out of a part of the accelerator called a gantry, which rotates around the patient. The patient lies on a moveable treatment couch and lasers are used to make sure the patient is in the proper position. The treatment couch can move in many directions including up, down, right, left, in and out. Radiation can be delivered to the tumor from any angle by rotating the gantry and moving the treatment couch.

Linear Accelerator – Example 1

(Cathode Ray Tube)

The cathode ray tube is a linear accelerator found in many TVs, computer monitors, etc.

Linear Accelerator – Example 2

(Stanford Linear Accelerator)

Stanford Linear Accelerator

The largest linac in the world is the Stanford Linear Accelerator, located at the Stanford Linear Accelerator Center (SLAC) in Stanford, California. An underground tunnel 3 kilometers (2 miles) in length passes beneath U.S. Highway 101 and holds 82,650 drift tubes along with the magnetic, electrical, and auxiliary equipment needed for the machine’s operation. Electrons accelerated in the SLAC linac leave the end of the machine traveling at nearly the speed of light with a maximum energy of about 32 GeV (gigaelectron volts).

Components

Accelerator

The main accelerator is an RF linear accelerator that can accelerate electrons and positrons up to 50 GeV. At 2.0 miles (about 3.2 kilometers) long, the accelerator is the longest linear accelerator in the world, and is claimed to be “the world’s straightest object.” The main accelerator is buried 30 feet (about 10 meters) below ground and passes underneath Interstate Highway 280. The above-ground klystron gallery atop the beamline is the longest building in the United States.

SLAC pit and detector

The Stanford Linear Collider was a linear accelerator that collided electrons and positrons at SLAC. The center of mass energy was about 90 GeV, equal to the mass of the Z boson, which the accelerator was designed to study. Grad student Barrett D. Milliken discovered the first Z event on 12 April 1989 while poring over the previous day’s computer data from the Mark II detector. The bulk of the data was collected by the Stanford Large Detector, which came online in 1991. Although largely overshadowed by the Large Electron-Positron Collider at CERN, which began running in 1989, the highly polarized electron beam at SLC (close to 80%) made certain unique measurements possible.

Presently no beam enters the south and north arcs in the machine, which leads to the Final Focus, therefore this section is mothballed to run beam into the PEP2 section from the beam switchyard.


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