lecture 1 overview of accelerators , nov 7, 2012

2012-11-07 Overview of Accelerators Experimental tools in Physics

Lecture 1 Overview of accelerators Jan Pallon, Nov 7, 2012 Lecture 2 Accelerator Mass Spectrometry (AMS) Kristina Stenström, Nov 27 (Tuesday) – tasks distributed. Lecture 3 Electrostatic accelerators Jan Pallon, Nov 28 – tasks distributed Lecture 4 Large accelerators Anders Oskarsson, Dec 5 Lecture 5 Accelerators for synchrotron light production Sverker Werin, Dec 12. Room SAHARA at MAX lab


Overview of Accelerators

Jan Pallon based on material from Ragnar Hellborg, Lund University, Sweden

G.Agricola ”De Re Metallica” 1556


Outline • Pre-accelerator development • (1) Direct voltage technique – one step acceleration • (2) Resonance acceleration • (3) Phase-stabilised acceleration • (4) Alternating gradient focusing • (5) Colliding beam systems • Applications


Pre-accelerator development

• Wilhelm Conrad Röntgen Röntgen

• Discovered the X-rays 1895

• Henri Becquerel photopaper

• Discovered the natural radioactivity 1896

• Marie and Pierre Curie Curies in lab

• Worked with radiation phenomena

• Joseph John Thomson Thomson

• Discovered the electron 1897



• Wilhelm Conrad Röntgen Röntgen

• Discovered the X-rays 1895

• Henri Becquerel photopaper

• Discovered the natural radioactivity 1896

• Marie and Pierre Curie Curies in lab

• Worked with radiation phenomena

• Joseph John Thomson Thomson

• Discovered the electron 1897



• Rutherford – 1911 Rutherford

• First use of a ”beam” of ions to investigate the inner structure of the atoms. Alpha particles from Ra and Th.

• Rutherford – 1919 • Disintegration of the nitrogen nucleus by alpha bombardment • Alpha + N O + H

• Quantum mechanics • During the 20s

• Cockcroft-Walton – 1932 acc

• First nuclear reaction with an • accelerator Li +p 2He


John Cockcroft, Ernest Rutherford, and E.T.S. Walton


Demand for accelerators • The first uses of energetic charged particles in “a beam” was when

the inner structure of the atoms was investigated • The existence of a positively charged nucleus having a diameter of

less than 10-13 m was demonstrated.

• in 1919 when Rutherford achieved the disintegration of the nitrogen nucleus by alpha-particle bombardment. These two famous experiments conceived by Rutherford demonstrated the demand for beams of particles with much higher intensities, well defined energy and the possibility to freely choose the particle species and their energy, i.e. the demand for accelerators.

• During the 20's the X-ray technique developed rapidly and DC equipment for producing voltages of a few hundred kV became available.

• Unfortunately, higher voltages were limited by corona discharging and insulating problems. The MV range seemed at that time to be impossible to reach.


• At the end of the 20's the development of the quantum mechanics showed that charged particles could penetrate through the potential wall around an atom and therefore particle energies of 0.5 MeV or less could be enough for splitting light atoms. This was a more moderate goal and accelerator development started in different laboratories.

• The first persons to reach the goal of initiating a nuclear reaction by a beam from an accelerator were Cockcroft and Walton at the Cavendish

• The years around 1930 can be taken as the starting point of the accelerator era and people at different laboratories did development work along different principles.



• Rutherford – 1911 Rutherford

• First use of a ”beam” of ions to investigate the inner structure of the atoms. Alpha particles from Ra and Th.

• Rutherford – 1919 • Disintegration of the nitrogen nucleus by alpha bombardment • Alpha + N O + H

• Quantum mechanics • During the 20s

• Cockcroft-Walton – 1932 acc

• First nuclear reaction with an • accelerator Li +p 2He


E.T.S. Walton


Accelerator development

• A factor of ten in increase of energy per seven years from 1930 up till now Livingston

• Higher Energy - Closer to Big Bang

CERN


The tremendous progress in the construction of accelerators since the 30's is illustrated in Fig.1.1 showing an exponential increase of about an order of magnitude in beam energy per seven years!


• Accelerators can be classified into different principle designs, but all principles are of course based on the only known method to accelerate a particle: to charge it and then apply an electrical field.


(1) Direct voltage technique acceleration in one step

• Cascade accelerators Asym

• Rectifying AC voltage • Electrostatic accelerators • Mechanical system carries the charge Princip

–Robert Van de Graaff » patentappl

–Ray Herb » First tank


Direct Voltage Technique

• In accelerators along this principle the particle (after ionization) is accelerated through an accelerator tube in one step.

• Direct voltage accelerators are often identified with the type of high voltage generator used. The high voltage can be generated by rectifying AC voltage (often called cascade generator)

• or using electrostatic charging in which a mechanical system carries the charge to the high voltage terminal (called electrostatic accelerators).


Electrostatic accelerators

• In 1929 Robert Van de Graaff demonstrated the first generator model of this type. An electrostatic charging belt is used to produce the high voltage. The reasons why this type became so popular are: all types of ions can be accelerated, the ion energy can be changed continuously, the high-voltage stability is extremely good and therefore the ion energy has a very low energy spread.


Robert Van de Graaff/ Karl Compton


Medicine acc. Harvard Medical School in Boston 1937 electron acc 1.2 MV. X-rays up to 40 R/min per mA (0.01 C/kg air)


Introduction of the tank

• All modern accelerators of the electrostatic type are enclosed in a tank with gas of high pressure to reduce the size

• and to be independent on moisture in the air.


(2) Resonance acceleration The second principle is the use of resonance acceleration by using a radio-

frequency field • Gustaf Ising, Sweden 1924 Isings

• Rolf Wideröe, Norway 1928 Wideröe

• Principle design of the linear acc princip

• Ernest Lawrence in • Berkeley Lawnotation


In 1924, Gustaf Ising from Sweden proposed a method of particle acceleration that would give particles more energy than that provided by the maximum voltage in the system.


In 1928 Rolf Wideroe from Norway built the first linear accelerator by using a radiofrequency field over two gaps and accelerated sodium and potassium.


The principle of a linear accelerator is shown. The beam travels through a series of hollow tubular electrodes connected alternately to opposite poles of the RF voltage source. Particles are accelerated as they cross the gap between the electrodes. Upon entering the interior of an electrode, the particle drifts in a field-free region for a time equal to half the period of the RF voltage. In this way the polarity of the voltage is reversed during the time the particle is within the drift tube, and it is then accelerated as it crosses the next gap.


In 1929 Ernest Lawrence at the University of California at Berkeley discovered Wideroe’s article and realized that if the ions could somehow be returned to the first gap again and again multiple acceleration could take place. In 1930 Lawrence proposed the application of the Wideroe resonance principle but now inside a homogenous magnetic field such that the particle would be bent back to the same RF gap twice for each period of the radio-frequency field.


The Lawrence type of accelerator is called cyclotron and the principle is illustrated:

Within a flat, cylindrical vacuum chamber placed between the poles of a dipole magnet are two D-shaped electrodes consisting of hollow, flat half-cylinders. Energies possible to obtain with protons are up to 20 - 30 MeV. The advantage of a cyclotron compared to an electrostatic accelerator is that a much higher beam current (tens of mA) is available from a cyclotron. The disadvantages of the cyclotron are that the beam is pulsed, it is difficult to change the beam energy and normally this change cannot be done continuously. The energy resolution of the beam is also much worse compared to the electrostatic accelerator


The first successful cyclotron, the 4.5-inch model built by Lawrence and Livingston.


Lawrence is at the control panel of his 37 inch cyclotron.


Starting in 1936 Lawrence acc 8 MeV deuterons and provided most of the world’s supply of radio isotopes. It was also a good neutron source and already in 1936 used for patient treatment d,n reaction on Be target


More resonance accelerators

• Microtron (Veksler and Schwinger 1944) Orbits

share one common point princip

• RFQ (Kapchinski and Teplynkov 1970) Focusing, bunching, acceleration princip

• Alvarez structure, 1947 • resonator tubes princip


Another principle is used in the Microtron. The particle moves in a circular orbit between the pole pieces of a magnet. The orbits share one common point at which an RF acceleration resonator is located. The increase of mass at each gap transit is so large that the revolution time increases by one radio-frequency period. This is impossible to achieve for protons or heavy ions but is practicable for electrons. The principle was suggested by Vladimir Veksler and by Julian Schwinger in 1944. The beam current in a microtron is of the order of mA and the usual operating energy is in the 5 - 50 MeV range. Microtrons are mostly used as injectors or for industrial radiography.


A rather new type of low-energy accelerator for very high currents is the RFQ, first proposed by I.M. Kapchinski and V.A. Teplyakov in 1970. It combines the action of focusing and bunching the beam, in addition to acceleration proper. The bunching effect is very efficient and close to 100%.

Focusing is ensured by a transverse electrical gradient. A 1 - 2 m long RFQ can accelerate ions from an energy of a few tens of kV up to several MV. The RFQs are often today used as part of the injector of big accelerator. It is a compact and rather simple accelerator.


In 1947 Alvarez built a machine with a different structure. A set of resonator tubes which have a radio frequency voltage of the same phase applied to them. Inside each tube a potential distribution exist. The acceleration takes place in the tube.


(3) Phase-Stabilised Acceleration

• Synchro-Cyclotron, McMillan and Veksler 1945 - Modulated RF field, First Synchro-Cyclotron in Berkeley 1948, 350 MeV for studies of Pi-mesons.

•The increase in particle mass due to the relativistic effects limits the energy that can be reached in a cyclotron. The remedy is to modulate the applied RF field to keep in step with the cyclotron frequency.


Phase-Stabilized Acceleration 1

• In 1945 the third principle, which is to use phase-stabilized acceleration, was proposed independently by Edvin McMillan and Vladimir Veksler.

• In this type of accelerator, called synchro-cyclotrons, the frequency of the applied RF-field is decreasing with particle energy to compensate for the changing mass.

• This means that the particles travel through the synchro-cyclotron in bunches.

• The frequency is swept from its maximum value (when the bunch is near the center, the particles are only slightly accelerated, and the relativistic increase in mass is slight) to its minimum value (when the bunch is ready to exit the accelerator, the maximum energy is attained, and the mass has its largest value).


• A maximum energy up to 1 GeV for protons has been obtained.

• The disadvantage of a synchro-cyclotron compared to a cyclotron is the reduced current.

• Only one bunch at a time is sent through the synchro-cyclotron compared to lots of pulses through the cyclotron, therefore the beam current is reduced to a mean value of microA or even less.

• The first synchro-cyclotron was built in Berkeley 1948. It could accelerate protons to 350 MeV and was the first machine for studies of pi-mesons.


In Uppsala beginning of 1950s, 650 tons diameter 230 cm



• Synchro-Cyclotron, McMillan and Veksler 1945 • Modulated RF field, First SC in Berkeley 1948, 350 MeV for studies of

Pi-mesons foto

• AVF-Cyclotron (Isochronous cyclotron) foto

• Increasing magn field with increasing radius


Phase-Stabilized Acceleration 2

• Another way of overcoming the problem connected with the lack of resonance due to the increase of the relativistic mass in a homogeneous magnet can be to use an Azimuthally-Varying-Field cyclotron (also called Isochronous cyclotron) having an increasing magnetic field with increasing radius.

• Vertical focusing is obtained by radial or spiral ridges built on to the poles to create alternative high and low field sectors. Focusing forces giving axial stability arise at each sector boundary.

• The stable orbits in an AVF cyclotron are not circles; the particles perform radial oscillations about the circular orbit.


Triumf 520 MeV

• The maximum energy obtainable with an AVF accelerator is about the same as for synchro-cyclotrons. (1 GeV)

• An advantage of AVF is the larger possible beam current (of the order of 100 microA), depending on the fact that not only one bunch per time is possible but many pulses at the same time. In this way it is possible to operate the cyclotron with fixed frequency even to relativistic energies.




• Synchro-Cyclotron, McMillan and Veksler 1945 • Modulated RF field, First SC in Berkeley 1948, 350 MeV for studies of

Pi-mesons foto

• AVF-Cyclotron (Isochronous cycl.) foto

• Increasing magn field with increasing radius

• Synchrotron, McMillan and Veksler princip

• In 1954 the 6.4 GeV Bevatron for production of the anti-protons


In the synchrotron, also invented by McMillan and Veksler -- the massive magnet is replaced by a ring of bending magnets. Both the RF-field and the magnetic field are varied. The principle is shown in the figure


The first electron synchrotron was built by Govard and Barnes in Great Britain 1946, 8 MeV.


In 1954 the Bevatron at Berkeley was completed (10 000 tons iron). Its energy was 6 GeV, enough to demonstrate the existence of the antiproton. 500 kV CW plus 10 MeV linear acc. Energies up to 100 GeV can economically be obtained with a conventional synchrotron.


(4) Alternating Gradient Focusing dramatically reduced size, Livingston end of 1940s

• Sector design princip

• Field increasing and decreasing with r, resp

• Separate focusing princip

• From around 1970


The fourth principle uses alternating gradients for magnetic focusing. The use of this principle dramatically reduced the size of the magnets for large accelerators, allowing a much larger energy to become economically achievable. It was Livingston who at the end of the 40's considered the possibility of building a synchrotron with successive magnetic

sectors facing inwards to and outwards from the center in order to compensate for the effects of magnetic leakage. The field at the center of the beam tube has the same value in all sectors, but in one sector it decreases with r and in the neighboring sectors it increases. This means that in addition to bending the particle trajectories, the magnets have a strong lens effect. The principle is shown above.


This revolutionized the approach to accelerator design and made it possible to arrive at compact design, also for machines with much higher energies than could be envisaged before. The energy range for AG synchrotrons is 100 GeV to 1 TeV. For synchrotrons from around 1970 a major advance in design has been the separation of the bending and focusing functions, so that dipole magnets bend the beam and quadrupole magnets do the focusing as illustrated:


(5) Colliding Beam System introduced in the late 60s

only centre of mass energy useful

• ISR at CERN, 28 GeV protons (1972-83) • 56 GeV center of mass energy corresponds to 1700 GeV • Cooling the beam • Superconducting magnets

• LHC, 1200 magnets, 27 km, 7+7 TeV LHC


Colliding Beam System The next large energy step was made by a colliding beam system introduced in the late 60's. In particle collisions only the center-of-mass energy is useful. For fixed target accelerators this means that the main part of the particle energy will be wasted as kinetic energy of colliding particles and their reaction products. On the contrary, if two particles with the same momentum that move in opposite directions are made to collide, all the available energy can be made use of in the interaction. An example is the CERN Intersecting Storage Rings (ISR) in operation 1972--83 with two beams of 28 GeV in two interlacing storage rings 300 m making collision at 8 positions. The 56 GeV center of mass energy is equivalent to a beam energy of 1700 GeV against a fixed target.


From a pre-accelerator a weak current is injected into a storage ring over a long period of time (in the order of a day). At the same time, the beam is focused to occupy a far smaller area than it did upon leaving the pre-accelerator. In this way the particle density is considerably increased and a circulating current equivalent to several amperes can be obtained


The ISR was the highest energy machine in the world until the SPS at CERN started operating as a proton-antiproton collider in 1981 at 2 times 270 GeV. Anti-protons were produced in a fixed target by irradiation with 26 GeV protons. The beam of antiprotons is of very low intensity and the beam also has an extremely low quality as the antiprotons are produced with spread in direction as well as in energy.

However, one of the exciting developments that originated within the ISR project was ``cooling" of the beam. This is a method to reduce the beam dimensions (the phase-space) and energy spread, and thus increase the beam density. Carlo Rubbia and Simon Van der Meer at CERN cooled antiprotons to dimensions and intensities comparable to a proton beam and accumulated them over long periods.

Then they accelerated them to about 300 GeV in the SPS to make them collide with protons of the same energy. This became a success and the particles W and Z that mediate the weak interaction were identified in 1982--83.

Another major discovery made by a colliding beam accelerator is the top quark, at the Tevatron collider at the Fermi Laboratory using 900 GeV protons and 900 GeV antiprotons


By the use of super-conducting magnets the mass of magnets can be radically reduced by at least one order of magnitude. The reduction in the dimensions, weight, cost and supply power of superconducting magnets is very attractive and a much higher magnetic field is available. In this way the accelerator will be more compact and cheaper. The present state of the art is the Large Hadron Collider under construction. More than 1200 superconducting magnets are used. Two proton beams of 7 TeV each will circulate in the 27 km long path.


Nobel Awards to Accelerator Pioneers

• Lawrence 1939 • Cockcroft-Walton 1951 • McMillan 1951 • Schwinger 1965 • Alvarez 1968 • Rubbia-Van de Meer 1984


Applications (1) - Energy

– Transmutation of Nuclear Waste ADT

– Heating for Plasma Ignition in a Tokamak


Applications (2) - Industrial Processing

– Ion Implantation – Thin Layer Activation – Surface engineering – Radiation Processing – Ion Beam Processing – Micro Lithography – Micro Machining – Sterilisation, Disinfection – Food Preservation – Contra Band Detection Princip


(3) Research Applications Analytical Applications

– Like PIXE, RBS, NRA, AMS (also micro-beam technique) IBA

– Neutron production – For condensed matter science ESS

– Imaging with Synchrotron Radiation – Laser Beam Production – FEL FEL


European Spallation Source, ESS


Free Electron Lasers are high power sources of electromagnetic radiation, utilizing accelerated electrons, which are oscillating transverse to their propagation axis. Fig wiggler


FEL: 6 MV vertical accelerator


Stanford Linear Accelerator Center 's (SLAC) two-mile-long linear accelerator (or linac) has begun a new phase of its career, with the creation of the Linac Coherent Light Source (LCLS).

http://lcls.slac.stanford.edu/


Groundbreaking for the LCLS officially took place in October of 2006, and the first X-ray pulses streamed through the machine in April 2009. The first LCLS experiments are set to begin in September 2009.

A Femto-second Camera for Molecular Movies By sequencing together images of the ultra small, taken with the ultrafast pulses of the LCLS, scientists are for the first time creating molecular movies, revealing the frenetic action of the atomic world for us to see.


• Understanding the precise dynamics at work on these scales will forever change our understanding of chemistry, physics and materials science.



The cyclotron as seen by . . . " cartoon series, by Dave Judd and Ronn MacKenzie

http://www.lbl.gov/image-gallery/image-library.html


The cyclotron as seen by . . . " cartoon series, by Dave Judd and Ronn MacKenzie

lecture 1 overview of accelerators , nov 7, 2012

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