Introduzione agli acceleratori - prima parte Corsi di Dottorato -Bologna Corso di Fisica delle Alte Energie Maggio 2014 Per Grafstrom University of Bologna and CERN
Organizzazione Introduction Lorenz force: first and second term, basis of it all. Phase stability Weak and strong focussing Quadrupoles and FODO cells RF cavities Fixed target versus collider 2
Gli acceleratori nel mondo An accelerator is a device that produce particles (protons,electrons,ions) of a certain energy normally measured in potenses of ev Example: 1 Mev corresponds to the energy a charged particle has acquired after passsage in a voltage drop of 1 million Volts There are about 15000 accelerators in the world used for many different aplications.the wast majority are used for ion implementation, surface modifications, sterilization or medical application. Material science, biology and medicine are the dominating sciences related to accelerators. However here will concentrate on the accelerators used in Particle Physics. These are a small minorities of the accelerators existing in the world. 3
Gli acceleratori nel mondo Insert the table 4
Perché acceleratori in fisica delle alte energie? 5
Wavelength versus energy 6
Understanding the Universe Unification? Electroweak Transition Particle Physics and Philosophy Maria in der Aue, March 2011, P. Jenni (CERN) Experimental Methods in Particle Physics 7
Big Bang Proton Atom Radius of Earth Earth to Sun Radius of Galaxies LHC Universe Super-Microscope Study physics laws of first moments after Big Bang increasing Symbiosis between Particle Physics, Particle Physics and Philosophy Astrophysics Maria in der Aue, March and 2011, Cosmology AMS Hubble VLT ALMA P. Jenni (CERN) Experimental Methods in Particle Physics 8
La storia.. non è solo la storia The most important step in the development also describes the underlying principles of accelerators today. Factor 10 each 7 year! 9
La base di tutto-the the Lorenz force We could stop here. the rest are details. but the devil in the details il diavolo è nei dettagli 10
Utilizzare il primo termine in forza di Lorenz: F=q E One gap solution Cookroft Walton 11
Due possibilità Il passo successivo. Many gaps- one after each other LINEAR accelerator The same gap- many times CIRCULAR accelerator 12
Il primo acceleratore lineare - Wideroe The particles gain energy at each gap Length of drift tubes follow increasing velocity Spacing regular a v approach c 13
Utilizzare il secondo termine della forza di Lorenz: F= q( v x B ) r =mv/bq T =2π r/v = 2π m/ Bq T independent of v 14
Un semplice acceleratore circolare-cyclotroncyclotron T independent of v the particle always arrives at the correct time to the accelerating gap 15
Il primo ciclotrone : Lawrence and Livingstone 1.2 MeV 1932 However.max energy ~ 10 MeV. 16
Einstein sostituire Newton..... T= 2π m/bq T=2π m /Bq T non indipendente di v più 17
Relativity velocity CPS c PSB 1 mv 2 2 Newton: E = Einstein: energy increases not velocity energy SPS / LHC } E = mc 2 R. Steerenberg, 10-Jan-2011 AXEL - 2011 18
Come essere in tempo...to to the gap.? Vary the frequency of the voltage at the gap to compensate for the increase of mass. To start with this was thought to be impossible because of the precision of the frequency change that was thought necessary However. self-regulating The principle of phase stability discovered in the 1940th Synchrocyclotron 19
Da sincrociclotrone a sincrotrone r =mv/bq the radius increase with energy Il raggio aumenta con l energia Too much iron in the end Troppo ferro, alla fine Max energy in range 500 MeV to 1 GeV 20
La soluzione: il sincrotrone Acceleration gap Only magnetic field along one well defined track. No iron in the middle Change the field to keep constant radius Injector 21
Come essere stabile nello spazio The particles have to be confined both radially and vertically. The particles oscillate around its ideal path. The oscillations must be stable and not diverge 22
Come essere stabile nello spazio? Weak focusing in the 40 th Strong focusing discovered in the 50 th Alternate gradient Separate bending and focusing functions Opened the door to modern accelerators 23
Weak focusing RADIAL VERTICAL Can be shown that we have weak focussing in both planes if the following condition is fullfilled: B= B 0 (r 0 /r) n where 0 n 1 weak focussing implies stable but LARGE amplitudes in the oscillations 24
Ready 1953. First machine above 1 GeV Energy 3.3 GeV Brookhaven National laboratory First external beams Discovered K L The Cosmotron HOWEVER: Weak focussing implied big magnets because of the large amplitudes. Going higher in energy would just require too much steel!! 25
Strong focusing The principle of strong focusing was discovered in the 50th. Recall the condition for weak focusing...but of course strong defocus in the radial direction IMPORTANT DISCOVERY: Combine n 1 and n 0 and the net effect is FOCUSING 26
Strong focusing machines The two first and most famous machines based on strong focussing: The PS at CERN 28 GeV in 1959 The AGS at Brookhaven 33 GeV in 1960 27
Separate bending and focusing functions It was soon realized that there are advantages in separating the bending and focusing functions. Instead of using bending magnets with positive or negative field gradients separate the functions by using simple bending magnets with n=0 in combination with separate quadrupoles What is a quadrupole? Che cosa è un quadrupolo? 28
Quadrupole Magnet A Quadrupole has 4 poles, 2 north and 2 south Magnetic field They are symmetrically arranged around the centre of the magnet There is no magnetic field along the central axis. Hyperbolic contour x y = constant R. Steerenberg, 10- Jan-2011 AXEL - 2011 29
Quadrupole fields Magnetic field On the x-axis (horizontal) the field is vertical and given by: B y x On the y-axis (vertical) the field is horizontal and given by: B x y The field gradient, K is defined as: ( By) d ( Tm ) 1 dx 30
Types of quadrupoles Force on particles This is a: Focusing Quadrupole (QF) It focuses the beam horizontally and defocuses the beam vertically. Rotating this magnet by 90º will give a: Defocusing Quadrupole (QD) R. Steerenberg, 10- Jan-2011 AXEL - 2011 31
Focusing and Stable motion Using a combination of focusing (QF) and defocusing (QD) will keep the beams focused in both planes when the position in the accelerator, type and strength of the quadrupoles are well chosen. By now our accelerator is composed of: Dipoles, constrain the beam to some closed path (orbit). Focusing and Defocusing Quadrupoles, provide horizontal and vertical focusing in order to constrain the beam in transverse directions. A combination of focusing and defocusing sections that is very often used is the so called: FODO lattice. This is a configuration of magnets where focusing and defocusing magnets alternate and are separated by non-focusing drift spaces. R. Steerenberg, 10- Jan-2011 AXEL - 2011 32
FODO cell The FODO cell is defined as follows: QF QD QF L1 L2 Centre of first QF FODO cell Or like this Centre of second QF R. Steerenberg, 10- Jan-2011 AXEL - 2011 33
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The mechanical equivalent-l'equivalente meccanico The gutter below illustrates how the particles in our accelerator behave due to the quadrupolar fields. Whenever a particle beam diverges too far away from the central orbit the quadrupoles focus them back towards the central orbit. R. Steerenberg, 10- Jan-2011 AXEL - 2011 35
Abbiamo anche bisogno di accelerazione We talked about bending and focussing BUT of course we also need acceleration Provided by RF cavities 36
Che cosa è una cavità a RadioFrequenza? Basically hollow metal box providing electric field, in the right direction, with the right strength and the right time The cavity becomes an resonating structure and grow stronger and stronger at the right Frequency. Noise do not resonate. 37
Molti insieme 38
Osservare! The RF cavity requires a bunched beam 39
Ingredienti di base nel moderno acceleratore Bending magnets: dipoles to keep the particles in their circular track Quadrupoles: to keep the particles together and focussed in small areas. Accelerating gaps or RF cavities. 40
Pedagogical sketch of a hadron machines Collimator Dipole Quad RF cavities Quad Dipole Collision/ Experiment Injection Collimator Collimator Quad Dipole Collimator Collimator Quad Collimator Dipole Dump/extraction line Dump/extraction line Goal: producing the highest number of collisions at the highest energy, in the safest way Particle Physics and Philosophy Maria in der Aue, March 2011, P. Jenni (CERN) Experimental Methods in Particle Physics 42
Distinzione importante Fixed target versus collider 43
Different approaches: fixed target vs collider Fixed target Storage ring/collider 44
La storia.. non è solo storia... Factor 10 each 7 year! 45