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Transcript
Introduction to particle accelerators Walter Scandale CERN - AT department Roma, marzo 2006 Lecture I - what are accelerators ? topics Fundamental discoveries in accelerator physics and technology Historical perspective Accelerator typologies Sources Linear accelerators Circular accelerators Special accelerators Synchrotrons Fixed target versus colliders Lepton versus hadron colliders Some relevant numbers Introductory remarks Particle accelerators are black boxes producing either flux of particles impinging on a fixed target or debris of interactions emerging from colliding particles In trying to clarify what the black boxes are one can list the technological problems describe the basic physics and mathematics involved Most of the phenomena in a particle accelerator can be described in terms of classical mechanics, electro-dynamics and restricted relativity quantum mechanic is required in a couple of cases just for leptons (synchrotron radiation, pinch effect) However there are some complications: many non-linear phenomena many particles interacting to each other and with a complex surroundings the observables are averaged over large ensembles of particles to handle high energy high intensity beams a complex technology is required In ten hours we can only superficially fly over the problems just to have a preliminary feeling of them The everyone’s accelerator Important discoveries 1900 to 1925 radioactive source experiments à la Rutherford -> request for higher energy beams; 1928 to 1932 electrostatic acceleration -> Cockcroft & Walton -> voltage multiplication using diodes and oscillating voltage (700 kV); Van der Graaf -> voltage charging through mechanical belt (1.2 MV); 1928 resonant acceleration -> Ising establish the concept, Wideroe builds the first linac; 1929 cyclotron -> small prototype by Livingstone (PhD thesis), large scale by Lawrence; 1942 magnetic induction -> Kerst build the betatron; 1944 synchrotron -> MacMillan, Oliphant and Veksel invent the RF phase stability (longitudinal focusing); 1946 proton linac -> Alvarez build an RF structure with drift tubes (progressive wave in 2p mode); 1950 strong focusing -> Christofilos patent the alternate gradient concept (transverse strong focusing); 1951 tandem -> Alvarez upgrade the electrostatic acceleration concept and build a tandem; 1955 AGS -> Courant, Snider and Livingstone build the alternate gradient Cosmotron in Brookhaven; 1956 collider -> Kerst discuss the concept of colliding beams; 1961 e+e- collider -> Touschek invent the concept of particle-antiparticle collider; 1967 electron cooling -> Budker proposes the e-cooling to increase the proton beam density; 1968 stochastic cooling -> Van der Meer proposes the stochastic cooling to compress the phase space; 1970 RFQ -> Kapchinski & Telyakov build the radiofrequency quadrupole; 1980 to now superconducting magnets -> developed in various laboratories to increase the beam energy; 1980 to now superconducting RF -> developed in various lab to increase the RF gradient. The Livingstone’s diagram In 1950 Livingstone plotted the accelerator energy expressed in a semilogarithmic scale as a function of the year of construction observing a linear growth. The energy increase by a factor 33 every decade, mostly due to discoveries and technological advances. Sources (1/3) Penning source Magnetron source Ion sources: positive formed from electron bombardment of a gas extracted from the resulting plasma: species ranging from H to U (multiply charged) negative ions sources ion sources: principal interest is in H-, for charge exchange injection surface sources: in a plasma, H picks up electrons from an activated surface volume sources: electron attachment or recombination in H plasma polarized ion sources: e.g., optically pumped source -> some penalty in intensity, relatively high (> 65 %) polarization Volume source QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Surface source QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Electron sources electron Sources (2/3) production mechanism: thermo ionic emission (pulse duration controlled by a pulsed grid) photocathode irradiation by pulsed laser (laser pulse width determines the pulse duration) initial acceleration methods DC HV guns -> 50-500 keV acceleration RF guns: cathode forms one wall of the RF cavity -> rapid acceleration to > 10 MeV in a few cells -> mitigates space charge effects, -> makes for low emittance NLC Electron Source layout, for polarized and un-polarized sources QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Sources (3/3) Positron sources “conventional” positron source: can get from 10-3 :1 up to ~1:1 positron/electron as electron energy rises from 0.2 to 20 GeV target 0.2 to 20 GeV e- positron e+ matching solenoid e+ RF linac solenoid production through high energy photons: ehigh energy e- g eg helical undulator sweep magnet Antiproton sources 80÷150 GeV p+ target p- horn lens To a storage ring with stochastic cooling p+/p- yield typically ≈ 10-5 e+ converter RF linac e+ solenoid m source is similar to p- source Linear accelerators (1/2) electrostatic accelerators negative ion source high voltage terminal V ≤ 10 MV - Charging belt RF linac Wideroe (1928) n+ Analysing (n-1)+ Magnet Stripping foil tandem Van der Graaf, pelletron (n+1)+ positive ion beam energy = 2qV n+ V=V0*sin(t) Focusing magnets Alvarez (1946) Linear accelerators (2/2) Induction linac: the beam forms the secondary circuit of a high-current pulse transformer very low rep rates (a few Hz) magnetic core intermediate voltages (30-50 MeV) accelerating gaps very high peak currents (>10 kA) in short (0.1÷1 µs) pulses RFQ (RF quadrupole) electric quadrupole, with a sinusoidal varying voltage on its electrodes; the electrode tips are modulated in the longitudinal direction; solenoid this modulation results in a longitudinal accelerating field; pulser it is a capable of a few MeV of acceleration; typically used between the ion source and the Alvarez linac in proton RF linacs. Klystron - a microwave generator The e- beam enters in an RF cavity with Lcavity ≈ lRF In the cavity there is a velocity modulation of the e- beam In the drift region the velocity modulation induces a beam bunching The bunched beam induces a wake modulation in the second cavity The initial RF power is amplified in the second cavity The residual e- beam is absorbed in a stopper If the two cavities are coupled we have instead an oscillator QuickTi me™ a nd a TIFF (Uncompre ssed ) decomp resso r are need ed to se e th is p icture. velocity modulation -> e- beam bunching -> coherent emission Other RF power amplifier: the magnetron, the travelling wave tube (TWT) A microwave oven magnetron Circular accelerators (1/4) Betatron The betatron accelerate e- at relativistic speeds It is essentially a transformer with a doughnut shaped vacuum tube as its secondary coil The magnetic field B0 makes the electrons moving in a circle, The change of magnetic flux within the orbit ∆ = πr2∆ <B> produces an accelerating electric field E U 1 d r d B E <B> = 2·B0 -> stable obit along a fixed radius r at all energies (Wideroe condition) 2pr 2pr dt 2 dt Energies up to 300 MeV have been obtained. Betatrons are still used in industry and medicine as they are the very compact accelerators for electrons. Cyclotrons are similarly compact but cannot accelerate electrons to useful energies. p erB0 dp erdB0 1 B B 0 2 1 p eE dp edE 2 erd B Cross section of a Betatron Steel r <B> B0 Bguide= <B>/2 p=erBguide Coil Vacuum chamber Bguide = 1/2 Baverage Principle of Betatron Acceleration Circular accelerators (2/4) Cyclotron The cyclotron accelerate ions at non-relativistic speeds A constant magnetic field imposes circular orbits; The RF accelerating field in the Dee’s gap can be substantially reduced respect to a linear accelerator; The acceleration process is resonant and similar to a parametric resonator. Used in the industry and medicine to accelerate protons and ions The centripetal force is the Lorenz force: F = eE+evB The instantaneous radius of curvature r is: evB = mv2/r -> r = p/eB The cyclotron frequency is: = v/r = eB/m The maximal kinetic energy depends on the magnetic field and radius: Ecin = 1/2 mv2 = 1/2 e2B2r2/m Isochronous Circular accelerators (3/4) cyclotron The isochronous cyclotron (sector cyclotron) accelerate ions at relativistic speeds B varies with the azimuth: mimic the alternate gradient principle, focusing the beam also in the vertical direction B varies with r shape B(r) to keep constant whilst m and the kinetic energy Ecin increase above the relativistic limit The PSI Ring Cyclotron: a separated sector cyclotron with a fixed beam energy of 590 MeV, commissioned in 1974, produces a proton beam with the highest power in the world. The protons are accelerated in the ring cyclotron to almost 0.8c, corresponding to an energy of 590 MeV. The proton current amounts presently to almost 2 mA, which results in a beam power of over 1 MW. The principle components of the ring cyclotron are eight sector magnets, with a total weight of 2000 t, and four accelerator cavities (50 MHz frequency) each having a peak voltage of 730 kV. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Synchro-cyclotron Circular accelerators (4/4) A synchrocyclotron accelerate ions at relativistic speed It is simply a cyclotron with the accelerating supply frequency decreasing as the particles become relativistic and begin to lag behind. Although in principle they can be scaled up to any energy they are not built any more as the synchrotron is a more versatile machine at high energies. 2pr 2pm0g Trev v eB RF ec 2 B m 0c 2 E cin The revolution period increases with the energy since the path length increases faster than the speed The radiofrequency decreases with the energy: a variable capacity modifies the RF resonant circuit Microtron Special accelerators A microtron accelerate e- at relativistic speed It is simply a cyclotron for e- containing an RF linac and a bending field region Turn after turn Trev increases by a multiple of TRF so that the e- are always in phase with the accelerating RF 2pm0 T RF eBh k h 2 E m c 0 cin hk 2p 2 T rev,1 ec 2 B m0c E cin hTRF first turn T 2p E kT change per turn rev cin RF 2 ec B Cebaf concept: disentangle RF for magnet The orbits are separated by large space A magnetic system for each consecutive orbit Orbit lengths shaped to keep synchronicity for an optimal RF system (large k) within limited space and costs Special accelerators FFAG Fixed Field Alternate Gradient A FFAG accelerate e- at relativistic speed recent versions accelerate p or m beams at high rate It allows strong focussing, RF synchronisation, fixed B field -> fast cycling m complex A modern synchrotron Main components of a modern accelerator Source of charged particles; Acceleration element (RF cavities); Guiding magnets (quadrupole, dipoles, correctors); Vacuum system; Beam diagnostics; Physics detectors in an experimental area Extraction devices special magnets high voltage septa high power targets Fixed target versus collider rings Collider Fixed target Advantage A l beam population N1 N1 particles target density r cross section no. of target particles N2 = rlA effective interaction area Aeff = N2 = rlA probability of interaction P = Aeff/A = rl reaction rate R = P•dN1/dt = rl•dN1/dt Luminosity bunch population in beam 1 N1 bunch population in beam 2 N2 rms beam radius beam areap2 L = R/ = rl•dN1/dt = N2/A•dN1/dt L = fN1N2/4p2 Synchrotron radiation Polarized light Fan in the bending plane Energy loss per turn e 2 3g 4 U 30 r E 4 GeV U MeV 0.0885 rm Lepton versus hadron circular colliders RF is a major concern (At the parton level ) -> magnets are a major concern Type of accelerators (1990) Main accelerators for research Colliders in operation (2001) Type e+e- two rings e+e- single ring e+e- single ring e+e- two rings e+e- two rings e+e- single ring Pbar-p single ring ep two rings e+e- linear collider Facility DAFNE (Italy) BEPC (China) CESR (US) SLAC PEP-II (US) KEK-B (Japan) CERN LEP (Europe) Fermilab Tevatron (US) DESY HERA (Germany) SLAC SLC (US) Ecm (GeV) 1.05 3.1 10.4 10.4 10.4 200 1800 300 100 Luminosity (1033cm-2 s-1) 0.01 0.05 0.8 0.6 0.3 0.05 0.02 0.02 0.002 Main accelerators for research Colliders under investigation or in construction (2006) Type pp two rings e+-e- linear collider µ+-µ- single ring pp two rings Facility CERN LHC NLC – JLC -TESLA - CLIC Muon collider VLHC Ecm (GeV) 14000 500-3000 100-3000 100000 Luminosity (1033cm-2 s-1) 10 10 0.1-100 10 Accelerators in operation for nuclear physics research (2006) Type AuAu two ring collider Electron Microtron Electron linac Proton synchrotron Isochronous he avy-ion cyclotron Isochronous cyclotron Isochronous cyclotron Facility Ecm (GeV) Luminosity (1033cm-2 s-1) BNL RHIC (US) 100/nucleon 10-6 CEBAF (US) 4 Bates (US) 0.3-1.1 IUCF (US) 0.5 MSU NSCL (US.) 0.5 TRIUMF(Canada) 0.5 PSI (Switzerland) 0.5 - Other applications Field Accelerator Topics of study Atomic Physics Low energy ion beams Condensed matter physics Condensed matter physics Material science Synchrotron radiation sources Spallation neutron sources Ion beams Atomic collision processes - study of excited states - electron-ion collisions - ele ctronic stopping power in solids X-ray studies of crystal structure Chem istry and biology Synchrotron radiation sources Neutron scattering studies of metals and crystals - liquids and amorphous materials Proton and X-ray activation analysis of materials - X-ray emission studies accelerator mass spectrometry Chem ical bonding studies: dynamics and kinetics - protein and virus crystallography biological dynamics Other applications Oil well logging with neutron sources from small linacs Archaeological Medical dating with accelerator mass spectrometry diagnostics using accelerator-produced radioisotopes Radiation therapy for cancer: X-rays from electron linacs, neutrontherapy from proton linacs, proton therapy; pion and heavy-ion therapy Ion implantation with positive ion beams Radiation processing with proton or electron beams: polymerization,vulcanization and curing, sterilization of food, insect sterilization,production of micro-porous membranes X-ray microlithography using synchrotron radiation Inertial confinement fusion using heavy-ion beams as the driver Muon-catalyzed Tritium fusion production, and radioactive waste incineration, using high energy proton beams How many accelerators today? CATEGORY Ion implanters and surface modifications Accelerators in industry Accelerators in non-nuclear research Radiotherapy Medical isotopes production Hadrontherapy Synchrotron radiation sources Research in nuclear and particle physics TOTAL NUMBER 7'000 1'500 1'000 5'000 200 20 70 110 15'000 Lecture I - what are accelerators ? reminder The accelerators are basic tools for physics discovery: new ideas and technological breakthrough sustained an impressive exponential progress of their performance for more than 80 years Many different type of accelerator are used for particle and nuclear physics research, however the large majority of the existing accelerators is used for a multitude of practical applications The synchrotrons are the backbone of accelerator complex, however old ideas and concepts are still revisited and upgraded to achieve more demanding requirements Colliders are the master tool in the quest of the highest energy, whilst fixed target operation allow reaching the highest rates Hadron and lepton colliders play complementary roles