Linear accelerator 1

What is the basic principle of the linear accelerator

• accelerates particles in straight lines

• electrons accelerated

• microwaves are the driving force

What is a magnetron

• device used to generate microwaves

• operated in high power pulsed mode

• microwave frequency can drift as magnetron temperature changes

What is a klystron

• microwave amplifier

• low power microwaves generate by a solid state system uses as input

  • produces high power microwaves

• stable frequency

  • used for standing wave waveguides

How do magnetrons work

• thermionic emission to a cathode → supply of electrons created

• electrons travel in a radial direction

• interact with a magnetic field

• rotate around chambers

How does a klystron work

• electron gun produces electron flow

• bunching cavities regulate electron speed

  • they arrive arrive in bunches at output activity

• electron bunches cause oscillations in output cavity

  • creating high energy microwaves

• microwaves flow into waveguide for transport to accelerator

• electrons are absorbed and energy converted to heat

What is the microwave process

• electrons emitted by cathode

• in 1^st chamber, velocity is modulated

• 2^nd cavity has more intense modulation

  • bunching the electron tighter together

• 3rd chamber then slows the electrons

• loss of energy transferred into microwaves

  • forced down cavity catcher

How are microwaves channelled

• through a simple copper pipe

  • filled with sulphur hexa-fluoride

  • ideal medium to carry the microwave to accelerator waveguide

• prevents arcing

  • result of strong charged fields within pipe

What is vacuum

• as air is pumped out of chamber, pressure drops

• 3 stages

  • rough vacuum

  • high vacuum ultra-high

How do electron guns produce electrons

• via thermionic emission cathode

• produces a narrow beam of electrons

  • repel each other

  • need to be focussed

How is electron production achieved

• initially passing electrons through a ring of high negative potential

  • compress the beam into a tight circle

• pulled towards anode

  • beam increases velocity

  • enters waveguide

How does an accelerator work

• microwaves from klystron fed into accelerator structure

  • via waveguides

• pattern of alternating electro-magnetic waves develop

  • forces electrons to travel down the accelerator waveguide

How does a pulsed modulator work

• simultaneously delivers high voltage pulses to the klystron or magnetron and the electron gun

  • each pulse is 2 microseconds long

What is the structure of a linear accelerator

• hollow tube

  • made from conducting material → copper

  • must be efficient in wave transmission & minimal loss of electron energy

• composed of internal irises of varying apertures and spaced at varying distances

What are travelling waves

• bunches of electrons injected and accelerated in the copper structure of the linac

What is the speed of wave determined by

• cavity geometry

• size of holes separating the cavities

What are the components of a linear accelerator