Linacs - Photons Flashcards
What are the main components of the body of the linac?
Electron gun
Accelerating waveguide
Microwave generator
Modulator
What is the role of the modulator?
To supply ~3us pulses to cathodes of the electron gun and microwave generator
What does the modulator contain?
A pulse forming network and thyratron with a PRF of ~200-300Hz
How does a Magnetron produce microwaves of suitable energy?
Electrons are thermionically emitted for a cathode.
These are accelerated across the evacuated space between the cathode and anode.
They follow a curved path in the permanent magnetic field, creating oscillating electric fields as they cross cavity openings.
Electrical field feed-back bunches the electrons.
Circulating bunches amplify the electric field.
The resulting microwaves are injected into the accelerating waveguide.
What is the peak microwave power of a Magnetron?
~3MV, so only allows acceleration up to ~6MV.
How does a Klystron produce microwaves of suitable energy?
Low power microwaves from a solid state pilot oscillator and thermionic emission electrons accelerated using a poitential difference of ~10keV are injected into the first cavity.
The microwaves accelerate or decelerate the electrons depending on the phase at injection, leading to bunch at the input frequency.
The electrons supply power to the microwaves by the electric field amplifying the input.
there is an increased electron density.
The catcher cavity is tuned to resonate at the bunch frequency so the electrons create an electric field in the cavity.
Microwaves are then injected into the waveguide as they have been amplified.
What is the peak microwave power of a Klystron?
~6MV, depending on the number of cavities. There are usually 4 cavities. This enables acceleration greater than 10MV.
How much energy of the electron bunches is wasted in a Klystron?
~40%
How does the electron gun work?
Electrons are emitted by thermionic emission, heater ~1000celcius.
A concave cathode focuses the emissions. ~20keV
Theere is a constant cloud of electrons around the cathode.
A grid has a high switching of potential to permit a phase shift between the electron and microwave pulses. The grid is held at a high negative potential to stop electrons returning to the anode as it is held at ground potential.
All in a high vacuum.
How does the accelerating waveguide work?
The field of microwaves accelerate electrons.
Initial buncher sections, first 1.5-2.5 cavities, produce differential acceleration of the electrons to create bunching by varying the microwave phase. This accounts for the majority of the acceleration.
After this the cavities lengthen to a point, then energy is gained through increasing the mass when v~c.
What is the accelerating waveguide made of?
copper due to good electrical conductivity.
What maintain the electrons in a focused beam?
Focussing coils.
What are the differences between standing waveguides and travelling waveguides?
Standing: Short, complex, microwaves injected anywhere, side coupled cavities. The electrons are accelerated along standing waves.
Travelling waveguide: long, simple, microwaves injected at the start. Electrons are accelerated along travelling waves.
What controls the microwave velocity in the waveguide?
Discs with holes in between cavities.
What is the purpose of the bending magnet?
To deflect the electrons to pass through the isocentre
