ACCELERATORSACCELERATORS

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““High Energy Physics High Energy Physics Phenomenology”Phenomenology”

Mikhail YurovMikhail Yurov

Kyungpook National UniversityKyungpook National UniversityOctober 25October 25thth

Introduction

The purpose of an accelerator of charged particle is to direct against a target a beam of a specific kind of particles of a chosen energy. There are many varieties of methods for accomplishing this task, all using various arrangements of electric and magnetic fields.

The design of accelerators varies greatly with purpose for which they will be used. We can somewhat loosely classify accelerators as low, medium, or high energy.

Low-energy accelerators are used to produce beams in the 10-100-MeV range, often for reaction or scattering studies to explain the structure of specific final states, perhaps even individual excited states.

TRIUMF cyclotronTRIUMF cyclotron

LINAC 400 MeVLINAC 400 MeV

Medium-energy accelerators operate in the range of roughly 100-1000 MeV. At this energies, collisions of nucleons with nuclei can release π mesons, and so these accelerators are often used to study the role of meson exchange in the nuclear force.

High energy accelerators produce beams of 1 GeV and above. Their purpose is less to investigate nuclear structure than to produce new varieties of particles and study their properties.

Let’s consider some of the equipment that is a necessary part of any accelerators facility.

First comes the ion source, from which originates the beam of ions or electrons that is to be accelerated. In its basic operation (for ions), a gas is ionized, usually by subjecting it to an electric discharge, and the positively charged ions are extracted by acceleration toward a negative electrode at potential of order 10 kV.

The beam transport (or beam optics) system consist of a number of electric or magnetic devices that focus the beam and bend or deflect it along the desired path. In analogy with optics, focusing devices are often called lenses, but they consist of magnetic fields, rather than glass. Bending magnets can also analyze beam into its component (because the radius of curvature of the path of the particle in a magnetic field depends on the momentum).

magnetic quadrupole lensmagnetic quadrupole lens

Targets for accelerated beams are as varied as the uses to which the accelerator is put.

Finally, essential parts of any accelerator facility are the detecting and analyzing devices used to record the identity, energy, time, and direction of the reaction product.

Let’s consider some of the techniques used to boost the particle to the desired energy.

Electrostatic Accelerators

The simplest way to accelerate a charge particle is to “drop” it through a constant potential difference V; if the particle has a charge q, it acquires a kinetic energy of qV.

The technology of electrostatic accelerators consists entirely in establishing and maintaining a high-voltage terminal to accelerate the charge particle from ion source.

The earliest development of this type for nuclear physics applications was in 1932 by Cockcroft and Walton. Their device was eventually to reach potential of 800 kV.

The ladder network was used for voltage multiplication. It can supply voltage in the 1-MV range. This technique of voltage multiplication was used by Cockcroft and Walton to perform the first nuclear disintegration using artificially accelerated particles:

p + 7Li → 4He + 4He

Because of its simplicity of design, the Cockcroft and Walton accelerator is in use today to provide sources of neutrons (2H + 2H → n + 3He can be done successfully at a few hundred keV) and also as an injector of particles, especially protons, for high energy accelerators.

The most common type of electrostatic accelerator in use today in nuclear physics laboratories is based on the Van de Graaff generator.

The basic principle of operation: when a charged inner conductor and a hollow outer conducting shell are placed in electrical contact, all of the charge from the inner conductor will flow to the outer one.

Charge is sprayed onto the belt by a corona discharge; a large potential difference (+20kV) at the corona points ionizes the air and repels the positive ions, where they are intercepted by moving belt.

The attached charge is carried mechanically into a high-voltage metal terminal. The charge flows off the belt when it comes in contact with a metal brush and is deposited on the terminal.

The charge is transferred to the terminal by a continuously moving belt, made of insulating material.

An ion source is located inside the terminal, and ions “fall to ground” through the potential difference V.

Van de Graaff accelerators are excellent research tools because they provide a steady-state beam with good energy regulation.

Terminal voltage are constant to within ±0.1% (±1-10 keV). The output beam energy of a Van de Graaff accelerator can be extended through the tandem configuration.

Negative ions produced by a source at ground potential are accelerated to a positive high-voltage terminal and pass through a stripping cell.

Collisions in the cell remove electrons, and some of the initial negative ions are converted to positive ions.

They are further accelerated traveling from the high-voltage terminal back to ground.

Recent advances in accelerator technology have enabled tandem Van de Graaff accelerators to achieve terminal voltage in excess of 20 million volts.

Cyclotron Accelerators

The term circular accelerator refers to any machine in which beams describe a closed orbit. The earliest and simplest of these accelerators is the cyclotron.

The beam is bent into a circular path by a magnetic field, and the particles orbit inside semicircular metal chambers called “dees”. The dees are connected to a source of alternating voltage.

When the particles are inside the dees, they feel no electric field and follow a circular path under the influence of the magnetic field. In the gap between the dees the particles feel an accelerating voltage and gain a small energy each cycle.

A cyclotron has constant magnetic field magnitude and constant rf frequency.

Beam energy is limited by relativistic effects, which destroy synchronization between particle orbits and rf field. Therefore, the cyclotron is useful only for ion acceleration.

The virtue of cyclotrons is that they generate a continuous train of beam micro pulses.

Cyclotron are characterized by large-area magnetic fields to confine ions from zero energy to the output energy.

In the basic design of the fixed-field, fixed-frequency cyclotron, there is no acceptable way to compensate for the relativistic effect, and this provide an ultimate limit on the size of such machines. For proton, an energy of about 40 MeV is the maximum that can be achieved.

The CIME cyclotron (Cyclotron pour The CIME cyclotron (Cyclotron pour Ions de Moyenne Energie)Ions de Moyenne Energie)

Synchrotrons

Synchrotrons are the present standard accelerators for particle physics research. They are cycled machines.

Both the magnitude of the magnetic field and rf frequency are varied to maintain a synchronous particle at a constant orbit radius.

The constant-radius feature is very important; bending and focusing fields need extend over only a small ring-shaped volume. This minimizes the cost of the magnets, allowing construction of large-diameter machines.

Particles follow a circular path and are accelerated by a resonant electric field as they cross a gap during each orbit.

As the energy increases, the frequency of the ac voltage across the gap must increase to maintain the resonance; the magnetic field must increase to keep the radius constant.

All modern synchrotrons use transverse focusing systems composed of strong lenses in a focusing-defocusing array. Strong focusing minimizes the beam cross section, reducing the magnet size.

Synchrotrons are used to accelerate both ions and electrons, although electron machines are limited in energy by emission of synchrotron radiation.

The main limits on achievable energy for ions are the cost of machine.

Linear Accelerators

In a linear accelerator particle receive many individual accelerations by an ac voltage, as in a cyclotron, the difference being that they travel in straight line in a linac.

The beam travels through a series of hollow tubular electrodes connected alternately to opposite poles of the ac 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 ac voltage.

The operation of such an accelerator is dependent on the condition that the entrance of the particle into each gap be in resonance with electric field across the gap.

The basic accelerator described so far can be considered to be a cavity in which a resonant electromagnetic standing wave is present. For high energies and high currents, it is more efficient to use a traveling wave, in which we imagine the particles to travel the length of the accelerator riding the crest of a traveling wave.

The problem is to construct a cavity in which the phase velocity of the traveling wave exactly matches the velocity of a particle. This is done using the “disk-loaded” configuration. The dimension of the disk determine the phase velocity of the wave.

ππ mode: fields alternate in adjacent cavities mode: fields alternate in adjacent cavities 0 mode: fields are the same in adjacent cavities0 mode: fields are the same in adjacent cavities

Colliding-beam acceleratorsIn the quest for higher energies to study

the production of new and exotic species of particle, the goal of the accelerator designer is to convert as much of the incident kinetic energy as possible into mass energy of the new particles

During the last few decades colliding-beam machines have become dominant. In these accelerators, two counter-rotating beams of particles collide in several intersection regions around the ring.

To counteract small reaction rate, the reactions are made to occur by first passing each beam through a storage ring. In a storage ring, many accelerator pulses can be kept circulating for times of the order of 1 day. At the same time, the beams are focused to occupy a far smaller area than they did upon leaving the accelerator.

Arrangement of Intersecting Arrangement of Intersecting Storage Ring, CernStorage Ring, Cern