Linear Accelerators (linacs) PDF

Linear Accelerators (LINACS) ❑The linac injection system is the source of electrons, a simple electrostatic accelerator referred to as the electron gun. ❑Two types of electron gun are in use in medical linacs: ❑ Diode type ❑ Triode type ❑Both electron gun types contain: ❑...

Linear Accelerators (LINACS) ❑The linac injection system is the source of electrons, a simple electrostatic accelerator referred to as the electron gun. ❑Two types of electron gun are in use in medical linacs: ❑ Diode type ❑ Triode type ❑Both electron gun types contain: ❑ Heated filament cathode ❑ Perforated grounded anode ❑ Triode gun also incorporates a grid M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ The radiofrequency power generation system produces the microwave used in the accelerating waveguide to accelerate electrons to the desired kinetic energy and consists of two major components: RF power source: ▪ magnetron: Producing Microwaves ▪ Klystron: amplifying Microwaves Pulsed modulator:produces the high voltage, high current, short duration pulses required by the RF power source and the injection system M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Waveguides are evacuated or gas filled metallic structures of rectangular or circular cross-section used in transmission of microwaves. ❑ Two types of waveguide are used in linacs: ❑ Radiofrequency power transmission waveguides (gas filled SF6) for transmission of the RF power from the power source to the accelerating waveguide. ❑ Accelerating waveguides (evacuated to about 10−6 tor) for acceleration of electrons. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Accelerating waveguide is obtained from a cylindrical uniform waveguide by adding a series of disks with circular holes at the centre, placed at equal distances along the tube to form a series of cavities. ❑ The accelerating waveguide is evacuated to allow free propagation of electrons. ❑ The cavities serve two purposes: ❑ To couple and distribute microwave power between cavities. ❑ To provide a suitable electric field pattern for electron acceleration. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ In the travelling wave accelerating structure the microwaves enter on the gun side and propagate toward the high energy end of the waveguide. ❑ Only one in four cavities is at any given moment suitable for acceleration. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ In the standing wave accelerating structure each end of the accelerating waveguide is terminated with a conducting disk to reflect the microwave power producing a standing wave in the waveguide. M.Sc. Medical Physics Dr. Khalid I Hussein  Linear Accelerators (LINACS) ❑ Auxiliary service consists of four systems that are not directly involved with electron acceleration: ❑Vacuum pumping system producing high vacuum in the accelerating waveguide. ❑Water cooling system for cooling the accelerating waveguide, target, circulator and RF generator. ❑Air pressure system for pneumatic movement of the target and other beam shaping components. ❑Shielding against leakage radiation produced by target, beam transport system and RF generator. M.Sc. Medical Physics Dr. Khalid I Hussein