Lecture 2: Linear Accelerators

Question 1

When were cathode ray tubes invented?

Answer 1

1857

Question 2

How were cathode ray tubes invented?

Answer 2

They were developed from Geissler tubes.

Question 3

What is a cathode ray tube?

Answer 3

A tube of low pressure gas with electrodes at each end. When a voltage is applied to the electrode, electrons and ions generated by direct ionisation diffuse towards the electrodes, ionising more gas atoms and causing a current to flow. When electrons recombine with the ions, photons of light are emitted.

Question 4

How were Crookes tubes invented?

Answer 4

They were developed from cathode ray tubes.

Question 5

What are Crookes tubes?

Answer 5

Tubes consisting of a higher voltage cathode and an anode in a lower pressure tube (than the CRT). Ions collide with the cathode, releasing electrons which are accelerated to give a cathode ray. They can ionise the atoms in the glass of the tube which causes it to glow.

Question 6

When did Wilhelmina Roentgen investigate the cathode ray tube?

Answer 6

1895

Question 7

What did Wilhelmina Roentgen discover when investigating the cathode ray tube?

Answer 7

That he could use the cathode ray tube to create an x-ray image.

Question 8

What is the purpose of an electrostatic accelerator?

Answer 8

To generate high voltages to accelerate charged particles.

Question 9

Give two examples of electrostatic accelerators

Answer 9

• Van de Graaff generator
• Cockcroft-Walton generator

Question 10

What is the maximum voltage that a Van de Graaff generator is capable of producing?

Answer 10

~ 15 MV

Question 11

What is the maximum particle energy that a Van de Graaff generator can achieve?

Answer 11

~ 15 MeV

Question 12

What is the maximum particle energy that a Crockcroft-Walton generator can achieve?

Answer 12

A few MeV

Question 13

What electrical component is a Van de Graaff generator similar to?

Answer 13

A capacitor

Question 14

What is the equation for capacitance?

Answer 14

V = voltage
Q = charge
C = capacitance

Question 15

What is the equation for capacitance when voltage is changing?

Answer 15

V = voltage
t = time
I = current
C = capacitance

Question 16

Give two problems with the use of high voltages in electrostatic accelerators

Answer 16

• They are dangerous
• They tend to arc to ground

Question 17

What are the two main types of linear accelerator?

Answer 17

• Standing wave LINAC
• Travelling wave LINAC

Question 18

Describe how standing wave LINACs work

Answer 18

Particles are accelerated from the negative end of one tube to the positive end of an adjacent tube. This works because the adjacent tubes have out-of-phase voltages and the voltages in each tube are reversed whilst the particles are in the tubes.

Question 19

Describe how the voltage changes for the tubes in a standing wave LINAC

Answer 19

Adjacent tubes are out of phase with one another.

Question 20

What are the 7 things that a standing wave LINAC needs to operate?

Answer 20

• A voltage gap to pull the particle through the series of tubes
• Tubes connected to an RF voltage ~ 200 MHz
• Tubes that act as Faraday cages (constant velocity inside the tube)
• Synchronised voltage sources for all tubes
• Potentials that reverse whilst the particle is inside the tube.
• A positive voltage in the tube that the particle is accelerating towards.
• Increasing tube length of successive tubes.

Question 21

What is the equation for the time in a single tube for a standing wave LINAC?

Answer 21

t = time in a single tube
f = frequency
l_i = length of tube
v_i = velocity of particle in the tube

Question 22

Describe how travelling wave LINACs work

Answer 22

Electrons are accelerated towards a positive E-field (standing waves in cylindrical wave guides) and ‘ride’ the wave close to the speed of light.

Question 23

Why can’t electromagnetic waves in free space be used in a travelling wave LINAC?

Answer 23

They are transverse so don’t propagate in the correct direction and they travel at the speed of light so the electrons cannot keep up with the wave.

Question 24

What energy are particles injected into a standing wave LINAC at?

Answer 24

~ 1 MeV

Question 25

What energy are electrons injecting into a travelling wave LINAC at?

Answer 25

~ 100 keV

Question 26

What is the main medical use of LINACs

Answer 26

Radiotherapy

Question 27

How do medical LINACs work for radiotherapy?

Answer 27

1) An electrode gun (cathode ray tube with a hole in the anode) produces electrons.
2) A Klystron microwave electron accelerator (travelling wave LINAC) causes electrons to bunch and accelerates them to ~0.6c.
3) Standing wave RF accelerator brings the electrons to ~c.
4) 270 degree bending magnet focuses the electrons on the target plane.
5) Electrons are fired at the target to produce gamma rays.
6) A collimator focuses the rays.

Question 28

Why do RT beams rotate when being used for medical purposes?

Answer 28

They rotate so that healthy cells aren’t damaged as heavily as cancerous cells. A smaller dose can be delivered at a greater number of angles.