LINEAR PARTICLE ACCELERATOR

LINEAR PARTICLE ACCELERATOR

www.Vidyarthiplus.com


• A linear particle accelerator (linac) is a type of particle accelerator.

• greatly increases the velocity of charged subatomic particles or ions.

• Subjects the charged particles to a series of oscillating electric potentials along a linear beam line;

• this method of particle acceleration was invented by Leo Szilard.



• Linacs have many applications: • they generate X-rays and high energy electrons for medicinal

purposes in radiation therapy, • serve as particle injectors for higher-energy accelerators, and • are used directly to achieve the highest kinetic energy for

light particles (electrons and positrons) for particle physics.



• The design of a linac depends on the type of particle that is being accelerated:

• electrons, • protons or • ions. • Linac range in size from a cathode ray tube to the 2-mile

(3.2 km) long linac at the SLAC National Accelerator Laboratory in Menlo Park, California.



CONSTRUCTION AND OPERATION

• A linear particle accelerator consists of the following elements:

• The particle source. • The design of the source depends on the particle that is being

moved. • Electrons are generated by a cold cathode, a hot cathode, a

photocathode, or radio frequency (RF) ion sources. • Protons are generated in an ion source, which can have many

different designs. • If heavier particles are to be accelerated, (e.g., uranium ions),

a specialized ion source is needed.



• A high voltage source for the initial injection of particles. • A hollow pipe vacuum chamber. • The length will vary with the application. • Within the chamber, electrically isolated cylindrical electrodes

are placed, whose length varies with the distance along the pipe.

• The length of each electrode is determined by the • frequency and power of the driving power source and • the nature of the particle to be accelerated, with shorter

segments near the source and longer segments near the target.



• The mass of the particle has a large effect on the length of the cylindrical electrodes;

• for example an electron is considerably lighter than a proton and so will generally require a much smaller section of cylindrical electrodes as it accelerates very quickly.



• RF energy, used to energize the cylindrical electrodes. • A very high power accelerator will use one source for each

electrode. • The sources must operate at precise power, frequency and

phase appropriate to the particle type to be accelerated to obtain maximum device power.

• An appropriate target. • If electrons are accelerated to produce X-rays then a water

cooled tungsten target is used. • Various target materials are used when protons or other

nuclei are accelerated, depending upon the specific investigation.



• As the particle bunch passes through the tube it is unaffected. • The frequency of the driving signal and the spacing of the

gaps between electrodes are designed so that the maximum voltage differential appears as the particle crosses the gap.

• This accelerates the particle, imparting energy to it in the form of increased velocity.

• Additional magnetic or electrostatic lens elements may be included to ensure that the beam remains in the center of the pipe and its electrodes.



SCHEMA OF A LINEAR ACCELERATOR www.Vidyarthiplus.com


Quadrupole magnets surrounding the linac of the Australian Synchrotron are used to help focus the electron beam



Advantages

• Linacs of appropriate design are capable of accelerating heavy ions to energies to extremely high levels.

• High power linacs are also being developed for production of electrons at high speeds.

• Linacs are also capable of prodigious output, producing a nearly continuous stream of particles.



Medical Linacs

• Linac-based radiation therapy for cancer therapy began with treatment of the first patient in 1953 in London at Hammersmith Hospital.

• An 8 megavolt machine was the first dedicated medical linac. • A short while later in 1955, 6 megavolt linac therapy from a

different machine was being used in the United States.



• Medical linacs use monoenergetic electron beams between 4 and 25 MeV.

• Gives an X-ray output with a spectrum of energies. • The electrons or X-rays can be used to treat both benign and

malignant disease. • The LINAC produces a reliable, flexible and accurate radiation

beam. • The device can simply be powered off when not in use; • there is no source requiring heavy shielding – although the

treatment room itself requires considerable shielding of the walls, doors, ceiling and etc to prevent escape of scattered radiation.



Gordon Isaacs, the first patient treated for retinoblastoma with linear accelerator radiation therapy (in this case an electron

beam), in 1957, in the U.S



Disadvantages

• The device length limits the locations where one may be placed.

• The construction and maintenance expense of this portion is large.



BETATRON



• A betatron is a cyclic particle accelerator developed by Donald Kerst at the University of Illinois in 1940 to accelerate electrons.

• The betatron is essentially a transformer with a torus-shaped vacuum tube as its secondary coil.

• An alternating current in the primary coils accelerates electrons in the vacuum around a circular path.

• The betatron was the first important machine for producing high energy electrons.





A 6 MeV betatron (1942) www.Vidyarthiplus.com


OPERATING PRINCIPLE

• In a betatron, the changing magnetic field from the primary coil accelerates electrons injected into the vacuum torus.

• This causes them to circle round the torus in the same manner as current is induced in the secondary coil of a transformer (Faraday's Law).



Faradays Law of Electromagnetic Induction

• The phenomenon by which an emf is induced in a conductor when it is cut by magnetic flux is known as electromagnetic induction.



Faradays First Law

• It states that, When ever a conductor cuts a magnetic field or viceversa an emf is induced in it and it sets up in such a direction so as to oppose the cause of it.



Faradays Second Law

• It states that the magnitude of induced emf is equal to the rate of change of flux linkage.

• Mathematically • e = -NdØ/dt • e – induced emf • N- number of turns of coil • dØ/dt – rate of change of flux • the minus sign represents that the induced emf or current

sets up in a direction so as to oppose the cause of it.



Induced Emf

• Induced emf could be classified into • dynamically induced emf and • statically induced emf.



Dynamically Induced EMF • This is the emf induced due to the motion of a conductor in a

magnetic field. • Mathematically • e = Blv volts • e-induced emf • B – flux density of magnetic field in Tesla • l = length of conductor in meters • v- velocity of conductor in m/s



• if the conductor moves in an angle θ, the induced emf could be represented as

• e= Blvsinθ • the direction of induced emf is given by flemmings right hand

rule.



Flemings Right Hand Rule www.Vidyarthiplus.com


• According to this rule, • extend the thumb, forefinger, and the middle finger of the left

hand in such a way that all the three are mutually perpendicular to each another.

• If the forefinger points in the direction of the magnetic field and

• the middle finger in the direction of the current, then, • the thumb points in the direction of the force exerted on the

conductor. • Devices that use current carrying conductors and magnetic

fields include electric motors, generators, loudspeakers and microphones.



Statically Induced EMF

• The emf produced in a conductor due to the change in magnetic field is called statically induce emf .

• It could be classified into two • 1)self induced emf and • 2)mutual induced emf





The stable orbit for the electrons satisfies



• where • θ is the flux within the area enclosed by the

electron orbit, • r is the radius of the electron orbit, and • H is the magnetic field at r.



In other words, the magnetic field at the orbit must be half the average magnetic field over its circular cross section:

This condition is often called Wideroe's condition



Applications • provide high energy beams of electrons—up to about 300

MeV. • If the electron beam is directed at a metal plate, the betatron

can be used as a source of energetic x-rays or gamma rays; • may be used in industrial and medical applications

(historically in radiation oncology). • A small version of a Betatron was also used to provide

electrons converted into neutrons by a target to provide prompt initiation of some nuclear weapons.



Limitations

• The maximum energy that betatron can impart is limited by • the strength of the magnetic field due practical size of the

magnet core. • The next generation of accelerators, the synchrotrons,

overcome these limitations.



• http://www.youtube.com/watch?v=hy9atKAqAf4

• http://www.youtube.com/watch?v=k27PZCUPeiE

• http://www.youtube.com/watch?v=oD2B1Ba2N3U

• http://www.youtube.com/watch?v=_moypMx05Fw (IMRT for Cancer Therapy)



http://www.youtube.com/watch?v=hy9atKAqAf4

http://www.youtube.com/watch?v=hy9atKAqAf4

http://www.youtube.com/watch?v=k27PZCUPeiE

http://www.youtube.com/watch?v=k27PZCUPeiE

http://www.youtube.com/watch?v=_moypMx05Fw




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LINEAR PARTICLE ACCELERATOR