Why do we accelerate particles?

Transcription

1 Why do we accelerate particles? (1) To take existing objects apart 1803 J. Dalton s indivisible atom atoms of one element can combine with atoms of other element to make compounds, e.g. water is made of oxygen and hydrogen (OH) 1896 M. & P. Curie find atoms decay 1897 J. J. Thomson discovers electron 1906 E. Rutherford: gold foil experiment Physicists break particles by shooting other particles on them

2 Why do we accelerate particles? (2) To create new particles 1905 A. Einstein: energy is matter E=mc P. Dirac: math problem predicts antimatter 1930 C. Anderson: discovers positron 1935 H. Yukawa: nuclear forces (forces between protons and neutrons in nuclei) require pion 1936 C. Anderson: discovers pion muon First experiments used cosmic rays that are accelerated for us by the Universe are still of interest as a source of extremely energetic particles not available in laboratories

3 Accelerator components Particle source Accelerating equipment Vacuum

4 Basic accelerator physics Lorentz Force: F = qe + q(v B) magnetic force: perpendicular to velocity, no acceleration (changes direction) electric force: acceleration

5 Accelerators: Cockroft-Walton A (series of) voltage gap(s) Maximum energy of a single gap is 200 kv, limited by discharge CW accelerator at Fermilab: 750 kv

6 Accelerators: Van de Graaf Van de Graaf generator: an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe 1: metallic sphere 2: electrode connected to 1 3: upper roller 4: belt (positive side) 5: belt (negative side) 6: lower roller 7: lower electrode (ground) 8: spherical device, used to discharge the main sphere 9: spark

7 Linear accelerators A simple concept: a series of cylindrical tubes connected to a high frequency oscillator the electric field is alternating doesn t it mean that acceleration is followed by deceleration?

8 Surfing the electromagnetic wave Charged particles ride the EM wave create standing wave use a radio frequency cavity make particles arrive on time Self-regulating: slow particle larger push fast particle small push

9 Surfing the electromagnetic wave

10 How to create a standing wave? Klystron (S. & R. Varian) electrons flow into cavity, excite eigen modes creates standing electromagnetic waves A similar device (magnetron) found in your microwave oven 325 MHz Klystron for Proton Driver Linac (Fermilab)

11 1929 E.O. Lawrence Cyclotron The physics: centripetal force mv 2 /r = Bqv Particles follow a spiral in a constant magnetic field A high frequency alternating voltage applied between D- electrodes causes acceleration as particles cross the gap Advantages: compact design (compared to linear accelerators), continuous stream of particles Limitations: synchronization lost as particle velocity approaches the speed of light the world largest cyclotron at TRIUMF (520 MeV protons)

12 Synchrotron The idea: both magnetic field strength and electric field frequency are synchronized with the traveling particle beam particle trajectories confined to a thin vacuum beamline no large magnets, expandable synchrotron radiation limits its use for electrons Currently, accelerators of this type provide highest particle energies in the world

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14 Linear vs circular accelerators Circular accelerators are compact, can re-use the same RF cavities Main issue: synchrotron radiation P~(E/m) 4 LEP LHC (not necessarily bad) European Synchrotron Radiation Facility Grenoble, France

15 E B Beam optics Dipole magnets: bend the (monochromatic) beam can be used to select certain beam momentum Quadrupole magnets: focusing in one direction, defocusing in perpendicular direction two Q magnets with perpendicular axes placed next to each other focus the beam in both directions

16 Hadron vs electron colliders electron proton Point-like particle yes no Uses full beam energy yes no Transverse energy sum zero zero Longitudinal energy sum zero non-zero Synchrotron radiation large small

17 Accelerators: performance vs time Energy increases by a factor of 10 every 7 years

18 Large Electron-Positron collider Location: CERN (Geneva, Switzerland) accelerated particles: electrons and positrons beam energy: GeV, beam current: 8 ma the ring radius: 4.5 km operation:

19 Tevatron Location: Fermilab (Batavia, IL) accelerated particles: protons and anti-protons beam energy: 1 TeV, beam current: 1 ma the ring radius: 1 km operation:

20 Large Hadron Collider Location: CERN (Geneva, Switzerland) accelerated particles: protons beam energy: TeV, beam current: 0.6 A the ring radius: 2.8 km in operation since 2008

21 Future of accelerators Future linear colliders: ILC (International Linear Collider) and CLIC (Compact Linear Collider): TeV Very Large Hadron Collider (not SLHC a.k.a HL- LHC): TeV Muon Collider: TeV lepton collider without synchrotron radiation capable of producing many more Higgs particles compared to an e + e collider Problem: muon cooling (MuCool, MICE)