Electrostatic nuclear accelerator
Encyclopedia
An electrostatic nuclear accelerator is one of the two main types of particle accelerators
, where charged particles can be accelerated by subjection to a static high voltage potential. The static high voltage method is contrasted with the dynamic fields used in oscillating field particle accelerators. Owing to their simpler design, historically these accelerators were developed earlier. These machines are operated at lower energy than some larger oscillating field accelerators, and to the extent that the energy regime scales with the cost of these machines, in broad terms these machines are less expensive than higher energy machines, and as such they are much more common. Many universities world wide have electrostatic accelerators for research purposes.
Although these machines accelerate atomic nuclei, the scope of application is not limited to the nuclear sciences of nuclear physics
, nuclear astrophysics
and nuclear chemistry
. Indeed, those applications are outweighed by other uses of nuclear beams. Of the approximately 26,000 accelerators worldwide, ~44% are for radiotherapy, ~41% for ion implantation
, ~9% for industrial processing and research, ~4% for biomedical and other low-energy research (less than 1% are higher energy machines).
These accelerators are being used for nuclear medicine
in medical physics
, sample analysis using techniques such as PIXE
in the material sciences, depth profiling in solid state physics
, and to a lesser extent secondary ion mass spectrometry in geologic
and cosmochemical
works, and even neutron beams can be made from the charged particles emerging from these accelerators to perform neutron crystallography
in condensed matter physics
. The principles used in electrostatic nuclear accelerators could be used to accelerate any charged particles, but particle physics
operates at much higher energy regimes than these machines can achieve, and there are various better methods suited for making electron beams
, so these accelerators are used for accelerating nuclei.
terminal kept at a static potential on the order of millions of volts, charged particle
s can be accelerated. In simple language, an electrostatic generator
is basically a giant capacitor
(although lacking plates). The high voltage is achieved either using the methods of Cockcroft & Walton
or Van de Graaff
, with the accelerators often being named after these inventors. Van de Graaff's original design
places electrons on an insulating sheet, or belt, with a metal comb, and then the sheet physically transports the immobilized electrons to the terminal. Although at high voltage, the terminal is a conductor, and there is a corresponding comb inside the conductor which can pick up the electrons off the sheet; owing to Gauss's law
, there is no electric field inside a conductor, so the electrons are not repulsed by the platform once they are inside. The belt is similar in style to a conventional conveyor belt
, with one major exception: it is seamless. Thus, if the belt is broken, the accelerator must be disassembled to some degree in order to replace the belt, which, owing to its constant rotation and being made typically of a rubber
, is not a particularly uncommon occurrence. The practical difficulty with belts led to a different medium for physically transporting the charges: a chain of pellets. Unlike a normal chain, this one is non-conducting from one end to the other, as both insulators and conductors are used in its construction. These type of accelerators are usually called Pelletron
s.
Once the platform can be electrically charged by one of the above means, some source of positive ions
is placed on the platform at the end of the beam line, which is why it's called the terminal. However, as the ion source is kept at a high potential, one cannot access the ion source for control or maintenance directly. Thus, methods such as plastic rods connected to various levers inside the terminal can branch out and be toggled remotely. Omitting practical problems, if the platform is positively charged, it will repel the ions of the same electric polarity, accelerating them. As E=qV, where E is the emerging energy, q is the ionic charge, and V is the terminal voltage, the maximum energy of particles accelerated in this manner is practically limited by the discharge limit of the high voltage platform, about 12 MV under ambient atmospheric conditions. This limit can be increased, for example, by keeping the HV platform in a tank of an insulating gas with a higher dielectric constant
than air, such as SF6
which has dielectric constant roughly 2.5 times that of air. However, even in a tank of SF6 the maximum attainable voltage is around 30 MV. There could be other gases with even better insulating powers, but SF6 is also chemically inert
and non-toxic
. To increase the maximum acceleration energy further, the tandem
concept was invented to use the same high voltage twice.
Tandems locate the ion source outside the terminal, which means that accessing the ion source while the terminal is at high voltage is significantly less difficult, especially if the terminal is inside a gas tank. So then a cation beam from a sputtering ion source is injected from a relatively lower voltage platform towards the now improperly-named high voltage terminal, which will be positively charged by the arriving cations. Inside the terminal, the beam impinges on a thin foil (on the order of micrograms per square centimeter), often carbon
or beryllium
, stripping electrons from the ion beam so that they become anions. As it is difficult to make anions of more than -1 charge state, then the energy of particles emerging from a tandem is E=(q+1)V, where we have added the second acceleration potential from that cation to the positive charge state q emerging from the stripper foil; we are adding these different charge signs together because we are increasing the energy of the nucleus in each phase. In this sense, we can see clearly that a tandem can double the maximum energy of a proton beam, whose maximum charge state is merely +1, but the advantage gained by a tandem has diminishing returns as we go to higher mass, as, for example, one might easily get a 6+ charge state of a silicon
beam.
It is not possible to make every element into a cation easily, so it is very rare for tandems to accelerate any noble gases
heavier than helium
. Mercury
also is not known to be accelerated by tandems. Although it is widely remarked by those working with tandems that acceleration of heavy noble gases with a tandem is impossible, KrF- and XeF- have been successfully produced and accelerated with a tandem, although at beam currents well below the mA scale and using an ion production procedure much more complicated than is customary. It is not uncommon to make compounds in order to get cations, however, and TiH2 might be extracted as TiH- and used to produce a proton beam, because these simple, and often weakly bound chemicals, will be broken apart at the terminal stripper foil. Cation ion beam production was a major subject of study for tandem accelerator application, and one can find recipes and yields for most elements in the Negative Ion Cookbook. Tandems can also be operated in terminal mode, where they function like a single-ended electrostatic accelerator, which is a more common and practical way to make beams of noble gases.
The name 'tandem' originates from this dual-use of the same high voltage, although tandems may also be named in the same style of conventional electrostatic accelerators based on the method of charging the terminal.
Geometry=
One trick which has to be considered with electrostatic accelerators is that usually vacuum beam lines are made of steel. However, one cannot very well connect a conducting pipe of steel from the high voltage terminal to the ground. Thus, many rings of a strong glass, like Pyrex
, are assembled together in such a manner that their interface is a vacuum seal, like a copper gasket
; a single long glass tube could implode under vacuum or fracture supporting its own weight. Importantly for the physics, these inter-spaced conducting rings help to make a more uniform electric field along the accelerating column. This beam line of glass rings is simply supported by compression at either end of the terminal. As the glass is non-conducting, it could be supported from the ground, but such supports near the terminal could induce a discharge of the terminal, depending on the design. Sometimes the compression is not sufficient, and the entire beam line may collapse and shatter. This idea is especially important to the design of tandems, because they naturally have longer beam lines, and the beam line must run through the terminal.
Most often electrostatic accelerators are arranged in a horizontal line. However, some tandems may have a "U" shape, and in principle the beam can be turned to any direction with a magnetic dipole at the terminal. Some electrostatic accelerators are arranged vertically, where either the ion source or, in the case of a "U" shaped vertical tandem, the terminal, is at the top of a tower. A tower arrangement can be a way to save space, and also the beam line connecting to the terminal made of glass rings can take some advantage of gravity as a natural source of compression.
were named in some way referring to the method or type of acceleration. Terminal accelerators pre-date both linear accelerator technology and the nomenclature, so it would be confusing and incorrect to categorize them with a newer technology which is quite different. Linear accelerators use an array of oscillating electric fields, historically arranged in a line, but nothing would prevent a person from using magnets in between the columns of linear accelerators to form some other geometric shape. Oscillating field accelerators do not actually produce beams of particles, but rather packets of particles, unlike electrostatic accelerators which can have a beam current that is constant in time. Thus, the naming scheme for accelerators is based on the method of acceleration, or the physics, and not its geometry, which can be a point of confusion. In fact, it was the oscillating field design of the linear accelerator which inspired Lawrence
to construct the cyclotron
, which accelerates particles in a spiral, thus taking up a considerably smaller amount of space. A linear accelerator has more in common with a cyclotron than an electrostatic terminal accelerator.
(ev) was invented for use in accelerator-based sciences, because if q is entered in integer units of the elementary charge
and V in volts
, the energy is given in eV; if we wanted to convert the energy into joules
, we need to multiply by the elementary charge in Coulombs on both sides of the equation, yielding a very small number. Usually, the high voltage is quoted in MV and the beam energy then in MeV. With more sophisticated, higher energy machines, those working in particle physics
are just accustomed to discussing mass and energy in units MeV or GeV, but the relationship of the accelerator state to the beam energy is not as simple as merely knowing the terminal voltage and the particle species accelerated.
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
, where charged particles can be accelerated by subjection to a static high voltage potential. The static high voltage method is contrasted with the dynamic fields used in oscillating field particle accelerators. Owing to their simpler design, historically these accelerators were developed earlier. These machines are operated at lower energy than some larger oscillating field accelerators, and to the extent that the energy regime scales with the cost of these machines, in broad terms these machines are less expensive than higher energy machines, and as such they are much more common. Many universities world wide have electrostatic accelerators for research purposes.
Although these machines accelerate atomic nuclei, the scope of application is not limited to the nuclear sciences of nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
, nuclear astrophysics
Nuclear astrophysics
Nuclear astrophysics is an interdisciplinary branch of physics involving close collaboration among researchers in various subfields of nuclear physics and astrophysics, with significant emphasis in areas such as stellar modeling, measurement and theoretical estimation of nuclear reaction rates,...
and nuclear chemistry
Nuclear chemistry
Nuclear chemistry is the subfield of chemistry dealing with radioactivity, nuclear processes and nuclear properties.It is the chemistry of radioactive elements such as the actinides, radium and radon together with the chemistry associated with equipment which are designed to perform nuclear...
. Indeed, those applications are outweighed by other uses of nuclear beams. Of the approximately 26,000 accelerators worldwide, ~44% are for radiotherapy, ~41% for ion implantation
Ion implantation
Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
, ~9% for industrial processing and research, ~4% for biomedical and other low-energy research (less than 1% are higher energy machines).
These accelerators are being used for nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
in medical physics
Medical physics
Medical physics is the application of physics to medicine. It generally concerns physics as applied to medical imaging and radiotherapy, although a medical physicist may also work in many other areas of healthcare...
, sample analysis using techniques such as PIXE
PIXE
Particle-induced X-ray emission or proton-induced X-ray emission is a technique used in the determining of the elemental make-up of a material or sample. When a material is exposed to an ion beam, atomic interactions occur that give off EM radiation of wavelengths in the x-ray part of the...
in the material sciences, depth profiling in solid state physics
Solid-state physics
Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from...
, and to a lesser extent secondary ion mass spectrometry in geologic
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
and cosmochemical
Cosmochemistry
Cosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples...
works, and even neutron beams can be made from the charged particles emerging from these accelerators to perform neutron crystallography
Neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material: A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of...
in condensed matter physics
Condensed matter physics
Condensed matter physics deals with the physical properties of condensed phases of matter. These properties appear when a number of atoms at the supramolecular and macromolecular scale interact strongly and adhere to each other or are otherwise highly concentrated in a system. The most familiar...
. The principles used in electrostatic nuclear accelerators could be used to accelerate any charged particles, but particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
operates at much higher energy regimes than these machines can achieve, and there are various better methods suited for making electron beams
Cathode ray
Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
, so these accelerators are used for accelerating nuclei.
Single-ended machines
Using a high voltageHigh voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
terminal kept at a static potential on the order of millions of volts, charged particle
Charged particle
In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s can be accelerated. In simple language, an electrostatic generator
Electrostatic generator
An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current...
is basically a giant capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
(although lacking plates). The high voltage is achieved either using the methods of Cockcroft & Walton
Cockcroft-Walton generator
The Cockcroft–Walton generator, or multiplier, is an electric circuit which generates a high DC voltage from a low voltage AC or pulsing DC input...
or Van de Graaff
Van de Graaff
Van de Graaff can refer to the physicist Robert J. Van de Graaff or machines named in honor of the physicist. The devices include:* Van de Graaff generator* The linear particle accelerator design that bears his name....
, with the accelerators often being named after these inventors. Van de Graaff's original design
Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented in 1929 by American physicist Robert J. Van de Graaff. The potential differences achieved in modern Van de Graaff...
places electrons on an insulating sheet, or belt, with a metal comb, and then the sheet physically transports the immobilized electrons to the terminal. Although at high voltage, the terminal is a conductor, and there is a corresponding comb inside the conductor which can pick up the electrons off the sheet; owing to Gauss's law
Gauss's law
In physics, Gauss's law, also known as Gauss's flux theorem, is a law relating the distribution of electric charge to the resulting electric field. Gauss's law states that:...
, there is no electric field inside a conductor, so the electrons are not repulsed by the platform once they are inside. The belt is similar in style to a conventional conveyor belt
Conveyor belt
A conveyor belt consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley...
, with one major exception: it is seamless. Thus, if the belt is broken, the accelerator must be disassembled to some degree in order to replace the belt, which, owing to its constant rotation and being made typically of a rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
, is not a particularly uncommon occurrence. The practical difficulty with belts led to a different medium for physically transporting the charges: a chain of pellets. Unlike a normal chain, this one is non-conducting from one end to the other, as both insulators and conductors are used in its construction. These type of accelerators are usually called Pelletron
Pelletron
A pelletron is a type of electrostatic particle accelerator similar to a Van de Graaff generator. Pelletrons have been built in many sizes, from small units producing voltages up to 500 kilovolts and beam energies up to 1 megaelectronvolt of kinetic energy, to the largest system, which has...
s.
Once the platform can be electrically charged by one of the above means, some source of positive ions
Ion source
An ion source is an electro-magnetic device that is used to create charged particles. These are used primarily to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.- Electron ionization :...
is placed on the platform at the end of the beam line, which is why it's called the terminal. However, as the ion source is kept at a high potential, one cannot access the ion source for control or maintenance directly. Thus, methods such as plastic rods connected to various levers inside the terminal can branch out and be toggled remotely. Omitting practical problems, if the platform is positively charged, it will repel the ions of the same electric polarity, accelerating them. As E=qV, where E is the emerging energy, q is the ionic charge, and V is the terminal voltage, the maximum energy of particles accelerated in this manner is practically limited by the discharge limit of the high voltage platform, about 12 MV under ambient atmospheric conditions. This limit can be increased, for example, by keeping the HV platform in a tank of an insulating gas with a higher dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
than air, such as SF6
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
which has dielectric constant roughly 2.5 times that of air. However, even in a tank of SF6 the maximum attainable voltage is around 30 MV. There could be other gases with even better insulating powers, but SF6 is also chemically inert
Inert
-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...
and non-toxic
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...
. To increase the maximum acceleration energy further, the tandem
Tandem
Tandem is an arrangement where a team of machines, animals or people are lined up one behind another, all facing in the same direction....
concept was invented to use the same high voltage twice.
Tandem accelerators
Conventionally, positively charged ions are accelerated because this is the polarity of the atomic nucleus. However, if one wants to use the same static electric potential twice to accelerate ions, then the polarity of the ions' charge must from cations to anions or vice-versa while they are inside the conductor where they will feel no electric force. It turns out to be simple to remove, or strip, electrons from an energetic ion. One of the properties of ion interaction with matter is the exchange of electrons, which is a way the ion can lose energy by depositing it within the matter, something we should intuitively expect of a projectile shot at a solid. However, as the target becomes thinner or the projectile becomes more energetic, the amount of energy deposited in the foil becomes less and less.Tandems locate the ion source outside the terminal, which means that accessing the ion source while the terminal is at high voltage is significantly less difficult, especially if the terminal is inside a gas tank. So then a cation beam from a sputtering ion source is injected from a relatively lower voltage platform towards the now improperly-named high voltage terminal, which will be positively charged by the arriving cations. Inside the terminal, the beam impinges on a thin foil (on the order of micrograms per square centimeter), often carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
or beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, stripping electrons from the ion beam so that they become anions. As it is difficult to make anions of more than -1 charge state, then the energy of particles emerging from a tandem is E=(q+1)V, where we have added the second acceleration potential from that cation to the positive charge state q emerging from the stripper foil; we are adding these different charge signs together because we are increasing the energy of the nucleus in each phase. In this sense, we can see clearly that a tandem can double the maximum energy of a proton beam, whose maximum charge state is merely +1, but the advantage gained by a tandem has diminishing returns as we go to higher mass, as, for example, one might easily get a 6+ charge state of a silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
beam.
It is not possible to make every element into a cation easily, so it is very rare for tandems to accelerate any noble gases
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
heavier than helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. Mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
also is not known to be accelerated by tandems. Although it is widely remarked by those working with tandems that acceleration of heavy noble gases with a tandem is impossible, KrF- and XeF- have been successfully produced and accelerated with a tandem, although at beam currents well below the mA scale and using an ion production procedure much more complicated than is customary. It is not uncommon to make compounds in order to get cations, however, and TiH2 might be extracted as TiH- and used to produce a proton beam, because these simple, and often weakly bound chemicals, will be broken apart at the terminal stripper foil. Cation ion beam production was a major subject of study for tandem accelerator application, and one can find recipes and yields for most elements in the Negative Ion Cookbook. Tandems can also be operated in terminal mode, where they function like a single-ended electrostatic accelerator, which is a more common and practical way to make beams of noble gases.
The name 'tandem' originates from this dual-use of the same high voltage, although tandems may also be named in the same style of conventional electrostatic accelerators based on the method of charging the terminal.
Geometry=
One trick which has to be considered with electrostatic accelerators is that usually vacuum beam lines are made of steel. However, one cannot very well connect a conducting pipe of steel from the high voltage terminal to the ground. Thus, many rings of a strong glass, like Pyrex
Pyrex
Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
, are assembled together in such a manner that their interface is a vacuum seal, like a copper gasket
Gasket
thumb|sright|250px|Some seals and gaskets1. [[o-ring]]2. fiber [[Washer |washer]]3. paper gaskets4. [[cylinder head]] [[head gasket|gasket]]...
; a single long glass tube could implode under vacuum or fracture supporting its own weight. Importantly for the physics, these inter-spaced conducting rings help to make a more uniform electric field along the accelerating column. This beam line of glass rings is simply supported by compression at either end of the terminal. As the glass is non-conducting, it could be supported from the ground, but such supports near the terminal could induce a discharge of the terminal, depending on the design. Sometimes the compression is not sufficient, and the entire beam line may collapse and shatter. This idea is especially important to the design of tandems, because they naturally have longer beam lines, and the beam line must run through the terminal.
Most often electrostatic accelerators are arranged in a horizontal line. However, some tandems may have a "U" shape, and in principle the beam can be turned to any direction with a magnetic dipole at the terminal. Some electrostatic accelerators are arranged vertically, where either the ion source or, in the case of a "U" shaped vertical tandem, the terminal, is at the top of a tower. A tower arrangement can be a way to save space, and also the beam line connecting to the terminal made of glass rings can take some advantage of gravity as a natural source of compression.
Confusion with Linear Accelerators
Electrostatic accelerators are often confused with linear accelerators simply because they can (but do not always) accelerate particles in a line. As we can see even early in their history, acceleratorsParticle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
were named in some way referring to the method or type of acceleration. Terminal accelerators pre-date both linear accelerator technology and the nomenclature, so it would be confusing and incorrect to categorize them with a newer technology which is quite different. Linear accelerators use an array of oscillating electric fields, historically arranged in a line, but nothing would prevent a person from using magnets in between the columns of linear accelerators to form some other geometric shape. Oscillating field accelerators do not actually produce beams of particles, but rather packets of particles, unlike electrostatic accelerators which can have a beam current that is constant in time. Thus, the naming scheme for accelerators is based on the method of acceleration, or the physics, and not its geometry, which can be a point of confusion. In fact, it was the oscillating field design of the linear accelerator which inspired Lawrence
Ernest Lawrence
Ernest Orlando Lawrence was an American physicist and Nobel Laureate, known for his invention, utilization, and improvement of the cyclotron atom-smasher beginning in 1929, based on his studies of the works of Rolf Widerøe, and his later work in uranium-isotope separation for the Manhattan Project...
to construct the cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
, which accelerates particles in a spiral, thus taking up a considerably smaller amount of space. A linear accelerator has more in common with a cyclotron than an electrostatic terminal accelerator.
Understanding the origin of the electron volt
The relation E=qV also indicates very simply why the electronvoltElectronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
(ev) was invented for use in accelerator-based sciences, because if q is entered in integer units of the elementary charge
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
and V in volts
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
, the energy is given in eV; if we wanted to convert the energy into joules
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
, we need to multiply by the elementary charge in Coulombs on both sides of the equation, yielding a very small number. Usually, the high voltage is quoted in MV and the beam energy then in MeV. With more sophisticated, higher energy machines, those working in particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
are just accustomed to discussing mass and energy in units MeV or GeV, but the relationship of the accelerator state to the beam energy is not as simple as merely knowing the terminal voltage and the particle species accelerated.