1. Particle Accelerators
The particle accelerator plays the same role that the telescope plays for astronomy or the microscope plays for biology
Wave nature of matter:
Higher energy means smaller wavelength and the ability to resolve smaller structure
For example, visible light with l~10-5m cannot resolve an individual nucleus
1/2/2019
Physics 590B - Fall 2014

2. Crooke’s Tube
A Crooke’s Tube is a partially evacuated glass cylinder with metal electrodes at either end
An applied high voltage causes electrons (cathode rays) to travel from the cathode to the anode
Electrons are produced by ionization of the residual air in the tube
“Maltese Cross” tube
1/2/2019
Physics 590B - Fall 2014

3. Cockroft-Walton Accelerator
The Cockroft-Walton accelerator was the first transmutation of an element by artificially accelerated particles (1932):
Achieved ~800kV
Limited by spark discharge to surroundings
Still in use today as injectors or neutron sources
C-W from the BNL AGS
(AC ripple on accelerating voltage)
Cockroft, Rutherford and Walton
1/2/2019
Physics 590B - Fall 2014

4. Van de Graaf Accelerator
The Van De Graaf is the most common electrostatic accelerator in use today
Main idea is to enclose the high potential in an inert atmosphere to prevent discharge
Tandem Van De Graff using the same accelerating voltage twice by stripping electrons from a negatively charged beam
10-12 MeV range
Robert Van de Graaf
1/2/2019
Physics 590B - Fall 2014

5. Large Potentials
All of the accelerators we have discussed so far require large potentials
Widerӧe (1929) suggested a series of potentials:
Zero net effect! ab is cancelled by bc
However, if we reverse the field at just the right time… a b c d E + - + - a b c d a b c d + - - + - + + -
(metal beam tube prevents the particle seeing changing field)
1/2/2019
Physics 590B - Fall 2014

6. Cyclotrons
A cyclotron uses magnetic fields to confine the particles to an orbit
Relativity comes into play:
Synchrocyclotron (w varies)
Isochronous cyclotron (B varies)
cyclotron frequency
constant!
Limited to ~25MeV for protons
1/2/2019
Physics 590B - Fall 2014

7. Synchrotrons Essentially a linear accelerator bent back on itself
Replace the large magnet in a cyclotron with a string of magnets
The particle is accelerated in the ring, and the magnetic field is adjusted to keep the orbit at the same radius
Particles are injected every 1-3s (from another accelerator)
Lots of energy required to ramp the magnets up and down
Very high energies possible
LHC – “last dipole” installation
FNAL – BOONE beamline
1/2/2019
Physics 590B - Fall 2014

8. Phase Stability (I)
One of the advantages of this approach is that it will provide automatic regulation of the accelerated beam
Particles from the source, having just the right velocity and phase, will be captured and accelerated
An accelerator of this type will provide “bunches” of beam (not continuous)
One of the advantages of this approach is that it will provide automatic regulation of the accelerated beam
Particles from the source, having just the right velocity and phase, will be captured and accelerated
An accelerator of this type will provide “bunches” of beam (not continuous)
One of the advantages of this approach is that it will provide automatic regulation of the accelerated beam
Particles from the source, having just the right velocity and phase, will be captured and accelerated
An accelerator of this type will provide “bunches” of beam (not continuous) V V V V
STABLE
STABLE
STABLE
STABLE
slow
slow
slow
slow
fast
fast
fast
fast
UNSTABLE
UNSTABLE
UNSTABLE
UNSTABLE t t t t
1/2/2019
Physics 590B - Fall 2014

9. Phase Stability (II)
Relativistic effects change the phase of the accelerating voltage for which the beam is stable.
Consider a high energy beam where the energy is many times the mass of the particles
The speed of particles is essentially v=c (constant)
A “fast” particle arrives late because its orbit is larger
A “slow” particle arrives early because its orbit is smaller
The accelerator must “jump” from one phase to the next at transition.
The CERN SPS was the first accelerator to demonstrate this.
RHIC was the first superconducting machine to go through transition.
fast - unstable
fast - stable
1/2/2019
Physics 590B - Fall 2014

10. Weak and Strong Focusing
To perform experiments we want narrow, well-defined beams
We’ve discussed longitudinal stability, what about lateral?
“Weak focusing” synchrotrons required very large, strong magnets to keep the beam focused
Practical limit on the size a machine could be made
By introducing a gradient in the magnetic fields, the beam could be alternately focused in horizontal and vertical directions
Smaller magnets required
Gradient field can be broken into dipole + quadrupole + sextupole +…
Very small beampipes and apertures
No theoretical limit to beam energies
1/2/2019
Physics 590B - Fall 2014

11. The “B” Factories
High luminosity colliders designed to produce high statistics samples of “b” quarks for study
Run continuously for long periods of time in a “factory” mode.
Electron+ Positron machines
Operate at the Upsilon(4s) resonance (b-bbar)
SLAC/PEP-II (US)
BaBar
KEK-B (Japan)
Belle
1/2/2019
Physics 590B - Fall 2014

12. Relativistic Heavy Ion Collider
RHIC pC “CNI” polarimeters
absolute pH
polarimeter
BRAHMS
& PP2PP
PHOBOS
RHIC
Siberian
Snakes
PHENIX
STAR
Siberian Snakes
Spin Rotators
5% Snake
LINAC
BOOSTER
AGS pC “CNI” polarimeter
Pol. Proton Source
AGS
200 MeV polarimeter
Rf Dipoles
20% Snake
1/2/2019
Physics 590B - Fall 2014

13. Large Hadron Collider
The highest energy particle accelerator in the world – the Large Hadron Collider – is operating now in Geneva, Switzerland .
Currently operating at 7TeV, plan is to ultimately go to 14TeV.
SPS
LHC PS
1/2/2019
Physics 590B - Fall 2014

14. The Future - Next Linear Collider?
Precision tests of the Standard Model best done in a very clean environment – electron/positron collisions.
However, very difficult to accelerate e+/e- in a synchrotron
Energy lost due to synchrotron radiation (Bremmstrahlung)
Must be replaced
Goes into the superconducting magnets!
Lots of ongoing development and design
1/2/2019
Physics 590B - Fall 2014

15. BACKUP
1/2/2019
Physics 590B - Fall 2014

16. Alternating Gaps
The time between the gaps must be ½ the period of the AC voltage
Assume a fixed frequency and potential:
Example: protons with Ln = 0.1m, neV0= 10MeV, f = 2.5MHz
Very high frequencies (for the time) required!
WWII development of klystrons made this practical
n = gap number
V0 = accelerating potential
Length of nth gap grows like n1/2
1/2/2019
Physics 590B - Fall 2014

17. The Future - Muon Collider?
What if we use muons instead of electrons?
Won’t have Bremmstrahlung problems, but
No “source” of muons
Have to generate them from secondary beams from pion decays
Currently being developed as a possiblity for FNAL.
Must generate high intensity muon beams, capture, cool and collide them with high luminosity.
1/2/2019
Physics 590B - Fall 2014

18. DESY
The HERA collider interacted beams of electrons/positrons at 30GeV with proton beams at 820GeV.
Measured much of what we know about parton distributions at low-x
Asymmetric collisions
Center of mass moving in the lab frame
Much easier for certain experimental observables
1/2/2019
Physics 590B - Fall 2014