The Westinghouse Atom Smasher, an early Van de Graaff accelerator built 1937 at the Westinghouse Research Center in Forest Hills, Pennsylvania. The cutaway shows the fabric belts that carry charge up to the mushroom-shaped high voltage electrode. To improve insulation the machine was enclosed in a 65 ft. pressure vessel which was pressurized to 120 psi during operation. The high pressure air increased the voltage on the machine from 1 MV to 5 MV.750 keV Cockcroft-Walton accelerator initial stage of the KEK accelerator in Tsukuba, Japan. The high voltage generator is right, the ion source and beam tube is at left
Owing to their simpler design, electrostatic types were the first particle accelerators. The two most common types are the Van de Graaf generator invented by Robert Van de Graaff in 1929, and the Cockcroft-Walton accelerator invented by John Cockcroft and Ernest Walton in 1932. The maximum particle energy produced by electrostatic accelerators is limited by the maximum voltage which can be achieved the machine. This is in turn limited by insulation breakdown to a few megavolts. Oscillating accelerators do not have this limitation, so they can achieve higher particle energies than electrostatic machines.
The advantages of electrostatic accelerators over oscillating field machines include lower cost, the ability to produce continuous beams, and higher beam currents that make them useful to industry. As such, they are by far the most widely used particle accelerators, with industrial applications such as plastic shrink wrap production, high power X-ray machines, radiation therapy in medicine, radioisotope production, ion implanters in semiconductor production, and sterilization. Many universities worldwide have electrostatic accelerators for research purposes. High energy oscillating field accelerators usually incorporate an electrostatic machine as their first stage, to accelerate particles to a high enough velocity to inject into the main accelerator.
Applications
Electrostatic accelerators have a wide array of applications in science and industry. In the realm of fundamental research, they are used to provide beams of atomic nuclei for research at energies up to several hundreds of MeV.
Although these machines primarily accelerate atomic nuclei, there are a number of compact machines used to accelerate electrons for industrial purposes including sterilization of medical instruments, x-ray production, and silicon wafer production.[1]
A special application of electrostatic particle accelerator are dust accelerators in which nanometer to micrometer sized electrically charged dust particles are accelerated to speeds up to 100km/s.[2] Dust accelerators are used for impact cratering studies,[3] calibration of impact ionization dust detectors,[4] and meteor studies.[5]
Single-ended machines
Using a high voltage terminal kept at a static potential on the order of millions of volts, charged particles 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 types of accelerators are usually called Pelletrons.
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.
Tandem accelerators
Electrostatic Van de Graaff Tandem nuclear accelerator at NCSRD in Greece
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 change from anions to cations 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 an anion beam from a sputtering ion source is injected from a relatively lower voltage platform towards the high voltage terminal. 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 cations. 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 anion 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 an anion easily, so it is very rare for tandems to accelerate any noble gases heavier than helium, although KrF− and XeF− have been successfully produced and accelerated with a tandem.[6] It is not uncommon to make compounds in order to get anions, 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. Anion 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.[7] 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.
The MP Tandem van de Graaff is a type of Tandem accelerator. Ten of these were installed in the 20th century; six in North America[8] and four in Europe.[9]
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.
Particle energy
In a single-ended electrostatic accelerator the charged particle is accelerated through a single potential difference between two electrodes, so the output particle energy is equal to the charge on the particle multiplied by the accelerating voltage
In a tandem accelerator the particle is accelerated twice by the same voltage, so the output energy is , as the anion form is singly charged. If the charge is in conventional units of coulombs and the potential is in volts the particle energy will be given in joules. However, because the charge on elementary particles is so small (the charge on the electron is 1.6x10−19 coulombs), the energy in joules is a very small number.
Since all elementary particles have charges which are multiples of the elementary charge on the electron, coulombs, particle physicists use a different unit to express particle energies, the electron volt (eV) which makes it easier to calculate. The electronvolt is equal to the energy a particle with a charge of 1e gains passing through a potential difference of one volt. In the above equation, if is measured in elementary charges e and is in volts, the particle energy is given in eV. For example, if an alpha particle which has a charge of 2e is accelerated through a voltage difference of one million volts (1MV), it will have an energy of two million electron volts, abbreviated 2MeV. The accelerating voltage on electrostatic machines is in the range 0.1 to 25MV and the charge on particles is a few elementary charges, so the particle energy is in the low MeV range. More powerful accelerators can produce energies in the giga electron volt (GeV) range.
Related Research Articles
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In an electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons.
A fusor is a device that uses an electric field to heat ions to a temperature at which they undergo nuclear fusion. The machine induces a potential difference between two metal cages, inside a vacuum. Positive ions fall down this voltage drop, building up speed. If they collide in the center, they can fuse. This is one kind of an inertial electrostatic confinement device – a branch of fusion research.
A cyclotron is a type of particle accelerator invented by Ernest Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. A cyclotron accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying electric field. Lawrence was awarded the 1939 Nobel Prize in Physics for this invention.
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct current (DC) electricity at low current levels. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved by modern Van de Graaff generators can be as much as 5 megavolts. A tabletop version can produce on the order of 100 kV and can store enough energy to produce visible electric sparks. Small Van de Graaff machines are produced for entertainment, and for physics education to teach electrostatics; larger ones are displayed in some science museums.
In electromagnetism and electronics, electromotive force is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical transducers provide an emf by converting other forms of energy into electrical energy. Other electrical equipment also produce an emf, such as batteries, which convert chemical energy, and generators, which convert mechanical energy. This energy conversion is achieved by physical forces applying physical work on electric charges. However, electromotive force itself is not a physical force, and ISO/IEC standards have deprecated the term in favor of source voltage or source tension instead.
Robert Jemison Van de Graaff was an American physicist, noted for his design and construction of high-voltage Van de Graaff generators. He spent most of his career in the Massachusetts Institute of Technology (MIT).
A linear particle accelerator is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beamline. The principles for such machines were proposed by Gustav Ising in 1924, while the first machine that worked was constructed by Rolf Widerøe in 1928 at the RWTH Aachen University. Linacs have many applications: they generate X-rays and high energy electrons for medicinal purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles for particle physics.
Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic confinement fusion (MCF) designs. Most IEC devices directly accelerate their fuel to fusion conditions, thereby avoiding energy losses seen during the longer heating stages of MCF devices. In theory, this makes them more suitable for using alternative aneutronic fusion fuels, which offer a number of major practical benefits and makes IEC devices one of the more widely studied approaches to fusion.
A synchrocyclotron is a special type of cyclotron, patented by Edwin McMillan in 1952, in which the frequency of the driving RF electric field is varied to compensate for relativistic effects as the particles' velocity begins to approach the speed of light. This is in contrast to the classical cyclotron, where this frequency is constant.
A Pelletron is a type of electrostatic generator, structurally similar to a Van de Graaff generator. Pelletrons have been built in many sizes, from small units producing voltages up to 500 kilovolts (kV) and beam energies up to 1 megaelectronvolt (MeV) of kinetic energy, to the largest system, which has reached a DC voltage of over 25 megavolts and produced ion beams with energies over 900 MeV.
Electron optics is a mathematical framework for the calculation of electron trajectories in the presence of electromagnetic fields. The term optics is used because magnetic and electrostatic lenses act upon a charged particle beam similarly to optical lenses upon a light beam.
The Cockcroft–Walton (CW) generator, or multiplier, is an electric circuit that generates a high DC voltage from a low-voltage AC. It was named after the British and Irish physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton, who in 1932 used this circuit design to power their particle accelerator, performing the first artificial nuclear disintegration in history. They used this voltage multiplier cascade for most of their research, which in 1951 won them the Nobel Prize in Physics for "Transmutation of atomic nuclei by artificially accelerated atomic particles".
Electron scattering occurs when electrons are displaced from their original trajectory. This is due to the electrostatic forces within matter interaction or, if an external magnetic field is present, the electron may be deflected by the Lorentz force. This scattering typically happens with solids such as metals, semiconductors and insulators; and is a limiting factor in integrated circuits and transistors.
The mass-to-charge ratio (m/Q) is a physical quantity relating the mass (quantity of matter) and the electric charge of a given particle, expressed in units of kilograms per coulomb (kg/C). It is most widely used in the electrodynamics of charged particles, e.g. in electron optics and ion optics.
Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral-beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 1990s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative-ion-based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NBI designed for ITER is a substantial challenge (D0, 1 MeV, 40 A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), ion cyclotron resonance heating (ICRH), and lower hybrid resonance heating (LH).
The Alternating Gradient Synchrotron (AGS) is a particle accelerator located at the Brookhaven National Laboratory in Long Island, New York, United States.
The Triangle Universities Nuclear Laboratory, abbreviated as TUNL, is a tripartite research consortium operated by Duke University, the University of North Carolina at Chapel Hill, North Carolina State University and North Carolina Central University. The laboratory is located on the West Campus of Duke University in Durham, North Carolina. Researchers are now drawn from several other universities around the United States in addition to members from the founding universities. TUNL also participates in long term collaborations with universities and laboratories around the world. Funding for TUNL comes primarily from the United States Department of Energy Office of Nuclear Physics.
The Denys Wilkinson Building is a prominent 1960s building in Oxford, England, designed by Philip Dowson at Arup in 1967.
A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies to contain them in well-defined beams. Small accelerators are used for fundamental research in particle physics. Accelerators are also used as synchrotron light sources for the study of condensed matter physics. Smaller particle accelerators are used in a wide variety of applications, including particle therapy for oncological purposes, radioisotope production for medical diagnostics, ion implanters for the manufacture of semiconductors, and accelerator mass spectrometers for measurements of rare isotopes such as radiocarbon.
The Dynamitron is an electrostatic particle accelerator invented by Marshal Cleland in 1956 at Washington University in St. Louis and manufactured by IBA Industrial. It is similar to a Cockcroft-Walton accelerator, using a series capacitor-diode ladder network to generate a high DC voltage on an electrode, which accelerates particles through an evacuated beam tube between the electrode and a target at ground potential. However instead of being powered at one end as in the Cockcroft-Walton, the capacitive ladder is charged in parallel electrostatically by a high frequency oscillating voltage applied between two long half-cylindrical electrodes on either side of the ladder column, which induce voltage in semicircular corona rings attached to each end of the diode rectifier tubes. in combination with an inductor this structure forms a resonant tank circuit for the oscillator providing the voltage, at a frequency of 100 kHz. The alternating voltage across the diodes pumps charge in one direction up the series stack of diodes to the high voltage electrode as in the Cockcroft-Walton circuit. The advantage of this design is that it can produce higher beam currents than the Cockcroft-Walton design, up to hundreds of milliamperes, and the high frequency reduces the ripple on the accelerating voltage. The accelerator stack is inside a tank of pressurized sulfur hexafluoride gas for insulation. It can accelerate either electrons or positive ions, and tandem versions have been built.
↑ Minehara, Eisuke; Abe, Shinichi; Yoshida, Tadashi; Sato, Yutaka; Kanda, Mamoru; Kobayashi, Chiaki; Hanashima, Susumu (1984). "On the production of the KrF- and XeF- Ion beams for the tandem electrostatic accelerators". Nuclear Instruments and Methods in Physics Research Section B. 5 (2): 217. Bibcode:1984NIMPB...5..217M. doi:10.1016/0168-583X(84)90513-5.
↑ Middleton, R: A Negative Ion Cookbook, University of Pennsylvania, unpublished, 1989 Online pdf
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