Accelerators

1
Accelerators

• Weve seen a number of examples of technology
  transfer in particle detector development from
  HEP (basic science) to industry (medical, )
• Particle accelerators provide another such
  example
• There are currently more than 30,000 particle
  accelerators in use throughout the world with
  only a small fraction being used in HEP/nuclear
  research

2
Accelerators

• Circa 2000

3
Accelerators

• A brief history

4
Accelerators

• A brief history
• Electrostatic (Cockcroft-Walton, van de Graaf)
• Linac (linear accelerator)
• Circular (cyclotron, betatron, synchrotron)
• Development of strong focusing
• Colliding beams (present day)
• Plasma wakefield, ???

5
Accelerators

• Moores law et/C

6
Accelerators

• Moores law

7
Linac

• Linac linear accelerator
• Applications in both high energy physics and
  radiation therapy

8
Linac

• Linacs are single pass accelerators for
  electrons, protons, or heavy ions
• Thus the KE of the beam is limited by length of
  the accelerator
• Medical (4-25 MeV) 0.5-1.5 m
• SLAC (50 GeV) 3.2 km
• ILC (250 GeV) - 11 km
• Linac static field, induction (time varying B
  field), RF
• Operate in the microwave region
• Typical RF for medical linacs 2.8 GHz
• Typical accelerating gradients are 1 MV/m 100
  MV/m

9
Linac

• Brief history
• Invented by Wideroe (Germany) in 1928
• Accelerated potassium ions to 50 keV using 1 MHz
  AC
• First realization of a linac by Sloan (USA) in
  1931
• No further progress until post-WWII when high
  power RF generators became available
• Modern design of enclosing drift tubes in a
  cavity (resonator) developed by Alvarez (USA)
• Accelerated 32 MeV protons in 1946 using 200 MHz
  12 m long linac
• Electron linac developed by Hansen and Ginzton
  (at Stanford) around the same period
• Evolved into SLAC laboratory and led to the birth
  of medical linacs (Kaplan and Varian Medical
  Systems)

10
Linac

• Wideroes linac

11
Linac

• Alvarez drift tube linac
• First stage of Fermilab linac

12
Linac

• A linac uses an oscillating EM field in a
  resonant cavity or waveguide in order to
  accelerate particles
• Why not just use EM field in free space to
  produce acceleration?
• We need a metal cavity (boundary conditions) to
  produce a configuration of waves that is useful
• Standing wave structures
• Traveling wave structures

13
LINAC

• Medical linacs can be either type

14
Waveguides
15
Waveguides

• Cyclindrical wave guide

16
TM Modes
TM01 mode
17
Waveguides
18
Waveguides

• Phase and group velocity

19
Waveguides

• Phase and group velocity

20
Waveguides

• The phase velocity can be slowed by fitting the
  guide with conducting irises or discs
• The derivation is complicated but alternatively
  think of the waveguide as a transmission line
• Conducting irises in a waveguide in TM0,1 mode
  act as discrete capacitors with separation d in
  parallel with C0

21
Waveguides

• Disc loaded waveguide

22
Traveling Wave Linac

• Notes
• Injection energy of electrons at 50 kV (v0.4c)
• The electrons become relativistic in the first
  portion of the waveguide
• The first section of the waveguide is described
  as the buncher section where electrons are
  accelerated/deaccelerated
• The final energy is determined by the length of
  the waveguide
• In a traveling wave system, the microwaves must
  enter the waveguide at the electron gun end and
  must either pass out at the high energy end or be
  absorbed without reflection

23
Traveling Wave Linac
24
Standing Wave Linac

• Notes
• In this case one terminates the waveguide with a
  conducting disc thus causing a p/2 reflection
• Standing waves form in the cavities (antinodes
  and nodes)
• Particles will gain or receive zero energy in
  alternating cavities
• Moreover, since the node cavities dont
  contribute to the energy, these cavities can be
  moved off to the side (side coupling)
• The RF power can be supplied to any cavity
• Standing wave linacs are shorter than traveling
  wave linacs because of the side coupling and also
  because the electric field is not attenuated

25
Standing Wave Linac
26
Standing Wave Linac

• Side coupled cavities

27
Electron Source

• Based on thermionic emission
• Cathode must be insulated because waveguide is at
  ground
• Dose rate can be regulated controlling the
  cathode temperature
• Direct or indirect heating
• The latter does not allow quick changes of
  electron emission but has a longer lifetime

28
RF Generation

• Magnetron
• As seen in your microwave oven!
• Operation
• Central cathode that also serves as filament
• Magnetic field causes electrons to spiral outward
• As the electrons pass the cavity they induce a
  resonant, RF field in the cavity through the
  oscillation of charges around the cavity
• The RF field can then be extracted with a short
  antenna attached to one of the spokes

29
RF Generation

• Magnetron

30
RF Generation

• Magnetron

31
RF Generation

• Klystron
• Used in HEP and gt 6 MeV medical linacs
• Operation effectively an RF amplifier
• DC beam produced at high voltage
• Low power RF excites input cavity
• Electrons are accelerated or deaccelerated in the
  input cavity
• Velocity modulation becomes time modulation
  during drift
• Bunched beam excites output cavity
• Spent beam is stopped

32
RF Generation

• Klystron

33
Medical Linac

• Block diagram

34
Medical Linac
35
Medical Linac
36
Cyclotron

• The first circular accelerator was the cyclotron
• Developed by Lawrence in 1931 (for 25)
• Grad student Livingston built it for his thesis
• About 4 inches in diameter

37
Cyclotron

• Principle of operation
• Particle acceleration is achieved using an RF
  field between dees with a constant magnetic
  field to guide the particles

38
Cyclotron

• Principle of operation

39
Cyclotron

• Why dont the particles hit the pole pieces?
• The fringe field (gradient) provides vertical and
  (less obviously) horizontal focusing

40
Cyclotron

• TRIUMF in Canada has the worlds largest cyclotron

41
Cyclotron

• TRIUMF

42
Cyclotron

• NSCL cyclotron at Michigan State

43
Cyclotron
44
Betatron

• Since electrons quickly become relativistic they
  could not be accelerated in cyclotrons
• Kerst and Serber invented the betatron for this
  purpose (1940)
• Principle of operation
• Electrons are accelerated with induced electric
  fields produced by changing magnetic fields
  (Faradays law)
• The magnetic field also served to guide the
  particles and its gradients provided focusing

45
Betatron

• Principle of operation

Steel
r
Coil
ltBgt
B0
Vacuum chamber
Bguide 1/2 Baverage
46
Betatron

• Principle of operation

47
TM Modes
48
TE Modes
Dipole mode
Quadrupole mode used in RFQs
49
Waveguides