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Fundamental Concepts of Particle AcceleratorsI : Dawn of Particle Accelerator Technology
Koji TAKATA
KEK
[email protected]://research.kek.jp/people/takata/home.html
Accelerator Course, Sokendai
Second Term, JFY2012
Oct. 25, 2012
Contents
§1 Dawn of Particle Accelerator Technology
§2 High-Energy Beam Dynamics: (1)
§3 High-Energy Beam Dynamics: (2)
§4 RF Acceleration
§5 Future of the High Energy Accelerators
§6 References
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 2 / 20
Dawn of Particle Accelerator Technology
Contents
1 discovery of artificial nuclear disintegration(1919 - 1932)and birth of particle accelerators
2 various types of early accelerators
3 from DC acceleration to RF acceleration
4 problems in RF acceleration
5 Great Progress Just after World War II(1941 - 1945)
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 3 / 20
Discovery of artificial nuclear disintegration(1919 - 1932)and the birth of
particle accelerators (1)
Ernest Rutherford (Cavendish Lab, Cambridge, UK)discoverednuclear disintegration by the alpha (α) rays (1917 - 1919).
• He confirmed that protons were produced in a nitrogen-gas filledcontainer in which a radioactive source emitting alpha rays was placed.
α+ 147N → p+ 16
8O
This discovery provoked strong demands to artificially generate highenergy beams to study in more detail the nuclear disintegrationphenomena.
Thus started the race for developing high energy accelerators, andRutherford himself was a great advocator.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 4 / 20
Discovery of artificial nuclear disintegration(1919 - 1932)and birth of particle accelerators (2)
The first disintegration of atomic nuclei with accelerator beams wasachieved at the Cavendish Laboratory in 1932 by John D. Cockcroftand Ernest T. S. Walton, who used 800 kV proton beams acceleratedby a DC voltage-multiplier.
p+ 73Li → α+ α
They revised the multiplier circuit first invented by H. Greinacher(1919).
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 5 / 20
DC HV Accelerators
DC Generators:two major methods
Cockcroft & Walton’s 800 kV voltage-multiplier circuit withcapacitors and rectifier tubes.
Van de Graaff’s 1.5 MV belt-charged generator (1931).
Electrostatic accelerators are still in use for the mass spectroscopy,because of their fine and stable tunability of the acceleration voltage.
• analysis of the ratio 14C/12C : an important tool for archaeology.
• the time after a creature stopped breathing is estimated in 14C’s halfdecay time 5, 730 years.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 6 / 20
Cockcroft & Walton’s voltage-multiplier circuit
6V 0
V(3+cos ωt)V(1+cos ωt)V cos ωt
AC
V(5+cos ωt)
4V2V0
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 7 / 20
Cockcroft around 1932
Ref.:E. Segre, From X-rays to Quarks, page 227,
(W. H. Freeman and Company, 1980).
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 8 / 20
Van de Graaff’s 1.5MV Belt-charged Generator
Insulating Belt
High Voltage for Acceleration
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 9 / 20
HV Limits in Electrostatic Accelerators
DC acceleration is limited by high-voltage breakdown (BD).
• Typical BD voltages for a 1cm gap of parallel metal plates
Ambience Typical BD Voltages
Air (1 atm) ≈ 30 kV
SF6 gas (1 atm) ≈ 80 kV
SF6 gas (7 atm) ≈ 360 kV
Transformer oil ≈ 150 kV
Ultra High Vacuum ≈ 220 kV
• Wider gaps do not make drastic improvement in BD limits.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 10 / 20
High Voltage Breakdown Demonstration for a Van de Graaff generator
Ref. :“van der graaf generator” in “ http://en.wikipedia.org/wiki/”
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 11 / 20
Intermediate stage towards RF Acceleration:D. W. Kerst’s betatron (1940)
Electric field due to time variation of the magnetic flux Φ.
• The AC transformers work on this principle.
• Faraday’s law in Maxwell’s equation:
∇×E = −∂B
∂t.
• Integrate the tangential component of the electric field Ealong a closed boundary C of an area S:∮
CE · dl = − ∂
∂t
∫∫SB · n dxdy = − ∂
∂tΦ,
where dl: line element of the curve C, andn: unit normal-vector of the area dS = dxdy.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 12 / 20
Kerst’s First Publication of the Betatron
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 13 / 20
First Linear Accelerator (Linac) by Wideroe
Proposal by Gustaf Ising (Sweden, 1925).
Trial study by Rolf Wideroe (Norway/Germany, 1928).
VRF ∼ 25 kV (1MHz) per gap ×2 with a drift tube.
He convinced that the scheme can be repeated any number of timesto reach ever higher beam energies.
RF
BeamIon So urce
Drift Tube
This is the prototype of the present-day drift tube linacs (DTL).
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 14 / 20
Ernest Lawrence’s Cyclotron (1931)
Trial study of the multiple RF acceleration of charged particlesmoving on a circular orbit in a magnetic field.
• The first circular accelerator.
• Multiple acceleration at the cyclotron frequency
ωc = eB⊥/m.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 15 / 20
Early Cyclotrons
Lawrence with the first cyclotron
Ref. :Segre, E. From X-rays to Quarks,page 229 (W. H. Freeman and
Company, 1980)
A cyclotron at RIKEN, Japan, acceleratedprotons to 9 MeV and
deuterons to 14 MeV (1939).
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 16 / 20
Circular Motion of Particles in the Cyclotron
�
RF Generator
rn rn+1(> rn)
Electric FieldMagnetic Field
dee
dee
dee
dee
beam
Circular orbit of particles with charge eand mass m in magnetic field B(assuming β = v/c ≪ 1).
• orbit radius: r = mvc|e|B .
• revolution frequency: fc =|e|B2πm .
• f depends only on B and neither onr nor on v.
• cyclotron frequency: ωc = 2πfc.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 17 / 20
Demonstration of the Circular Orbit of Electron Beams in a Magnetic Field
Ref. :“http://en.wikipedia.org/wiki/Cyclotron”
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 18 / 20
Problems in RF Acceleration
1 Linacs:
• poor RF power sources: electron tube technology was not yet matured.
2 Cyclotrons:
• relativistic increase of particle mass:
→ decrease of ωc,
→ asynchronism with RF.
3 Betatrons:
• It was very difficult to inject and trap electron beams correctly on thecircular orbit in the donut.
• Indispensable was the analysis of the transverse oscillations of particles.
• It led to the present-day theory of the betatron oscillations.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 19 / 20
Great Progress Just after World War II
1 Discovery of the phase stability principle in RF acceleration.
• Vladimir Veksler (1944) and Edwin M. McMillan (1945).
• Cyclotrons.→ synchrocyclotron, and eventually
→ synchrotron.
2 Strong focusing: new idea for the transverse beam focusing.
• Christofilos (1950) and Courant-Livingston-Snyder (1952).
3 Radars in practical use quickened the development of high power
microwave tubes.
• magnetrons and klystrons.
Koji Takata (KEK) Fund. Conc. Part. Acc. 1 Acc. Course, Oct. 2012 20 / 20