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 2  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 semi- logarithmic 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. Recent signs of saturation ?

Ion sources: u positive ions sources n formed from electron bombardment of a gas n extracted from the resulting plasma: species ranging from H to U (multiply charged) u negative ion sources: principal interest is in H-, for charge exchange injection n surface sources: in a plasma, H picks up electrons from an activated surface n volume sources: electron attachment or recombination in H plasma n polarized ion sources: e.g., optically pumped source -> some penalty in intensity, relatively high (> 65 %) polarization Sources (1/3) Penning source Surface source Volume source Magnetron source

Electron sources u electron production mechanism: n thermo ionic emission (pulse duration controlled by a pulsed grid) n photocathode irradiation by pulsed laser (laser pulse width determines the pulse duration) u initial acceleration methods n DC HV guns -> keV acceleration n 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 Sources (2/3) NLC Electron Source layout, for polarized and un-polarized sources

Positron sources u “conventional” positron source : can get from :1 up to ~1:1 positron/electron as electron energy rises from 0.2 to 20 GeV u positron production through high energy photons: RF linac solenoid helical undulatorsweep magnet converter high energy e - e-e- e-e-  e+e+ e+e+ Antiproton sources matching solenoid RF linac solenoid target 0.2 to 20 GeV e - e+e+ e+e+ horn lens 80÷150 GeV p + targetp-p- To a storage ring with stochastic cooling p + /p - yield typically ≈ Sources (3/3)   source is similar to p - source

Linear accelerators (1/2) u electrostatic accelerators positive ion beam energy = 2qV n+n+ Analysing Magnet Charging belt negative ion source high voltage terminal V ≤ 10 MV Stripping foil - n+n+ (n-1) + (n+1) + u RF linac tandem Van der Graaf, pelletron Wideroe (1928) V=V 0 *sin(  t) Alvarez (1946) V=V 0 *sin(  t) Focusing magnets

u RFQ (RF quadrupole) n electric quadrupole, with a sinusoidal varying voltage on its electrodes; n the electrode tips are modulated in the longitudinal direction; n this modulation results in a longitudinal accelerating field; n it is a capable of a few MeV of acceleration; n typically used between the ion source and the Alvarez linac in proton RF linacs. u Induction linac: the beam forms the secondary circuit of a high-current pulse transformer n very low rep rates (a few Hz) n intermediate voltages (30-50 MeV) n very high peak currents (>10 kA) in short (0.1÷1 µs) pulses solenoid pulser accelerating gaps magnetic core Linear accelerators (2/2)

Klystron - a microwave generator  The e - beam enters in an RF cavity with L cavity ≈ RF  u In the cavity there is a velocity modulation of the e - beam u In the drift region the velocity modulation induces a beam bunching  u The bunched beam induces a wake modulation in the second cavity  u The initial RF power is amplified in the second cavity  u The residual e - beam is absorbed in a stopper  u If the two cavities are coupled we have instead an oscillator velocity modulation -> e - beam bunching -> coherent emission Other RF power amplifier: u the magnetron, u the travelling wave tube (TWT) A microwave oven magnetron

Circular accelerators (1/4) u Betatron n The betatron accelerate e - at relativistic speeds n It is essentially a transformer with a doughnut shaped vacuum tube as its secondary coil n The magnetic field B 0 makes the electrons moving in a circle, The change of magnetic flux within the orbit ∆  = π  2 ∆ produces an accelerating electric field E = 2·B 0 -> stable obit along a fixed radius  at all energies (Wideroe condition) n Energies up to 300 MeV have been obtained. n 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. Principle of Betatron AccelerationCross section of a Betatron B guide = /2 p=erB guide B guide = 1/2 B average Coil Steel Vacuum chamber B0B0 

u Cyclotron n The cyclotron accelerate ions at non-relativistic speeds n A constant magnetic field imposes circular orbits; n The RF accelerating field in the Dee’s gap can be substantially reduced respect to a linear accelerator; n The acceleration process is resonant and similar to a parametric resonator. n Used in the industry and medicine to accelerate protons and ions  The centripetal force is the Lorenz force: F = eE+ev  B  The instantaneous radius of curvature  is: evB = mv 2 /  ->  = p/eB  The cyclotron frequency  is:  = v/  = eB/m  The maximal kinetic energy depends on the magnetic field and radius: E cin = 1/2 mv 2 = 1/2 e 2 B 2  2 /m Circular accelerators (2/4)

u Isochronous cyclotron n The isochronous cyclotron (sector cyclotron) accelerate ions at relativistic speeds n B varies with the azimuth: n mimic the alternate gradient principle, focusing the beam also in the vertical direction B varies with  shape B(  ) to keep  constant whilst m and the kinetic energy E cin 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. u The protons are accelerated in the ring cyclotron to almost 0.8  c, corresponding to an energy of 590 MeV. u The proton current amounts presently to almost 2 mA, which results in a beam power of over 1 MW. u 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. Circular accelerators (3/4)

u Synchro-cyclotron n A synchrocyclotron accelerate ions at relativistic speed n It is simply a cyclotron with the accelerating supply frequency decreasing as the particles become relativistic and begin to lag behind. n 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. Circular accelerators (4/4) 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

Special accelerators u Microtron n A microtron accelerate e - at relativistic speed n It is simply a cyclotron for e - containing an RF linac and a bending field region n Turn after turn T rev increases by a multiple of T RF so that the e - are always in phase with the accelerating RF first turn change per turn Cebaf concept: disentangle RF for magnet n The orbits are separated by large space n A magnetic system for each consecutive orbit n Orbit lengths shaped to keep synchronicity for an optimal RF system (large k) within limited space and costs

Special accelerators u FFAG Fixed Field Alternate Gradient A FFAG accelerate e - at relativistic speed recent versions accelerate p or  beams at high rate n It allows strong focussing, RF synchronisation, fixed B field -> fast cycling  complex

A modern synchrotron Extraction devices uspecial magnets uhigh voltage septa uhigh power targets Main components of a modern accelerator u Source of charged particles; u Acceleration element (RF cavities); u Guiding magnets (quadrupole, dipoles, correctors); u Vacuum system; u Beam diagnostics; u Physics detectors in an experimental area

Fixed target versus collider rings N 1 particles beam population N 1 target density  cross section  no. of target particles N 2 =  l A effective interaction area A eff =  N2 =  l A probability of interactionP = A eff /A =  l reaction rateR = PdN 1 /dt =  l dN 1 /dt Fixed target A l Collider Advantage Luminosity bunch population in beam 1 N 1 bunch population in beam 2 N 2 rms beam radius  beam area  2 L = R/  =  l dN 1 /dt = N 2 /AdN 1 /dt L = fN 1 N 2 /4  2

Synchrotron radiation Energy loss per turn u Polarized light u Fan in the bending plane

Lepton versus hadron circular colliders -> (At the parton level ) RF is a major concern magnets are a major concern

Type of accelerators (1990)

Main accelerators for research Colliders in operation (2001)

Main accelerators for research Colliders under investigation or in construction (2006) Accelerators in operation for nuclear physics research (2006)

Other applications

 Oil well logging with neutron sources from small linacs  Archaeological dating with accelerator mass spectrometry  Medical diagnostics using accelerator-produced radioisotopes  Radiation therapy for cancer: X-rays from electron linacs, neutron- therapy 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 fusion  Tritium production, and radioactive waste incineration, using high energy proton beams

CATEGORY NUMBER Ion implanters and surface modifications7'000 Accelerators in industry1'500 Accelerators in non-nuclear research1'000 Radiotherapy5'000 Medical isotopes production200 Hadrontherapy20 Synchrotron radiation sources70 Research in nuclear and particle physics110 TOTAL 15'000 How many accelerators today?

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