Plasma acceleration
Encyclopedia
Plasma Wakefield acceleration is a technique for accelerating charged particle
s, such as electron
s, positron
s and ion
s, using an electric field
associated with an electron plasma wave
. The wave is created either using electron pulses or through the passage of a very brief laser
pulses, a technique known as laser plasma acceleration. These techniques appear to offer a way to build high performance particle accelerator
s of much smaller size than conventional devices at the expense of coherency. Current experimental devices show accelerating gradients several orders of magnitude better than current particle accelerators. For example, an experimental laser plasma accelerator at Lawrence Berkeley National Laboratory
accelerates electrons to 1 GeV over about 3.3 cm (5.4x1020 gn
), whereas the SLAC conventional accelerator requires 64 m to reach the same energy. A recent experiment performed by a team at SLAC achieved an energy gain to 42 GeV
over 85 cm using a plasma wakefield accelerator (8.9x1020 gn). Once fully developed, the technology could replace many of the traditional RF accelerators currently found in hospitals and research facilities.
consists of fluid of positive and negative charged particles, generally created by heating a dilute gas. Under normal conditions the plasma will be macroscopically neutral, an equal mix of electron
s and ion
s in equilibrium. However, if an external electric field is applied, the plasma electrons, which are very light, will separate from the massive ions. A particle injected into such a plasma would be accelerated by the charge separation, but since the magnitude of this separation is generally similar to that of the external field, nothing is gained in comparison to a system that simply applies the field directly to the particle, which is the case in existing accelerator designs.
pulses or electron bunches. Plasma electrons are driven from the center of wake by the exciting fields (electron or laser). Plasma ions are too massive to move significantly and are assumed to be stationary. As the exciting fields pass, the plasma electrons are attracted back to the center of the wake by the positive plasma ions that remained there. This forms a full wake. As they reach the center, they create an area of dense negative charge. This negative charge then creates a charge separation or gradient between the back of the wake, where there are many electrons, and the middle of the wake, where there are mostly ions. Any electrons in between these two areas will be accelerated.
Similar to a beam-driven wake, a laser pulse can be used to excite the plasma wake. As the pulse travels through the plasma, the electric field of the light separates the electrons and nucleons in the same way that an external field would.
In the linear regime, plasma electrons aren't completely removed from the center of the wake. In this case, the linear plasma wave equation can be applied. However, the wake appears very similar to the blowout regime, and the physics of acceleration is the same.
can travel at speeds much higher than the wave they surf on by traveling across it. Accelerators designed to take advantage of this technique have been referred to colloquially as "surfatron"s.
. In RF accelerators, the field has an upper limit determined by the threshold for dielectric breakdown
of the acceleration tube. This limits the amount of acceleration over any given area, requiring very long accelerators to reach high energies. In contrast, the maximum field in a plasma is defined by mechanical qualities and turbulence, but is generally several orders of magnitude stronger than with RF accelerators. It is hoped that a compact particle accelerator can be created based on plasma acceleration techniques or accelerators for much higher energy can be built, if long accelerators are realizable with an accelerating field of 10 GV/m.
Plasma acceleration is categorized into several types according to how the electron plasma wave is formed:
The concept of plasma acceleration was first proposed by Toshiki Tajima and John Dawson in a theoretical article published in 1979. The first experimental demonstration of wakefield acceleration, which was performed with PWFA, was reported by a research group at Argonne National Laboratory
in 1988.
In this equation,
is the electric field
,
is the speed of light
in vacuum,
is the mass of the electron
,
is the plasma density (in particles per cube metre), and 
is the permittivity of free space.
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, such as electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s and ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, using an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
associated with an electron plasma wave
Plasma oscillation
Plasma oscillations, also known as "Langmuir waves" , are rapid oscillations of the electron density in conducting media such as plasmas or metals. The oscillations can be described as an instability in the dielectric function of a free electron gas. The frequency only depends weakly on the...
. The wave is created either using electron pulses or through the passage of a very brief laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulses, a technique known as laser plasma acceleration. These techniques appear to offer a way to build high performance particle accelerator
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...
s of much smaller size than conventional devices at the expense of coherency. Current experimental devices show accelerating gradients several orders of magnitude better than current particle accelerators. For example, an experimental laser plasma accelerator at Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
accelerates electrons to 1 GeV over about 3.3 cm (5.4x1020 gn
Standard gravity
Standard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...
), whereas the SLAC conventional accelerator requires 64 m to reach the same energy. A recent experiment performed by a team at SLAC achieved an energy gain to 42 GeV
GEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...
over 85 cm using a plasma wakefield accelerator (8.9x1020 gn). Once fully developed, the technology could replace many of the traditional RF accelerators currently found in hospitals and research facilities.
Basic concept
A plasmaPlasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
consists of fluid of positive and negative charged particles, generally created by heating a dilute gas. Under normal conditions the plasma will be macroscopically neutral, an equal mix of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s in equilibrium. However, if an external electric field is applied, the plasma electrons, which are very light, will separate from the massive ions. A particle injected into such a plasma would be accelerated by the charge separation, but since the magnitude of this separation is generally similar to that of the external field, nothing is gained in comparison to a system that simply applies the field directly to the particle, which is the case in existing accelerator designs.
Overview
What makes the system potentially useful is the possibility of introducing waves of very high charge separation that travel through the plasma. Currently, plasma wakes are excited by laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulses or electron bunches. Plasma electrons are driven from the center of wake by the exciting fields (electron or laser). Plasma ions are too massive to move significantly and are assumed to be stationary. As the exciting fields pass, the plasma electrons are attracted back to the center of the wake by the positive plasma ions that remained there. This forms a full wake. As they reach the center, they create an area of dense negative charge. This negative charge then creates a charge separation or gradient between the back of the wake, where there are many electrons, and the middle of the wake, where there are mostly ions. Any electrons in between these two areas will be accelerated.
Exciting the Plasma Wake
A beam-driven wake can be created by sending a relativistic proton or electron bunch into an appropriate plasma or gas. In some cases, the gas can be ionized by the electron bunch, so that the electron bunch both creates the plasma and the wake. This requires an electron bunch with relatively high charge and thus strong fields. The high fields of the electron bunch then push the plasma electrons out from the center, creating the wake.Similar to a beam-driven wake, a laser pulse can be used to excite the plasma wake. As the pulse travels through the plasma, the electric field of the light separates the electrons and nucleons in the same way that an external field would.
Blowout vs. Linear Regime
If the fields are strong enough, all of the ionized plasma electrons can be removed from the center of the wake: this is known as the "blowout regime". Although the particles are not moving very quickly during this period, macroscopically it appears that a "bubble" of charge is moving through the plasma at close to the speed of light. The bubble is the region cleared of electrons that is thus positively charged, followed by the region where the electrons fall back into the center and is thus negatively charged. This leads to a small area of very strong potential gradient following the laser pulse.In the linear regime, plasma electrons aren't completely removed from the center of the wake. In this case, the linear plasma wave equation can be applied. However, the wake appears very similar to the blowout regime, and the physics of acceleration is the same.
Acceleration
It is this "wakefield" that is used for particle acceleration. A particle injected into the plasma near the high-density area will experience an acceleration toward (or away) from it, an acceleration that continues as the wakefield travels through the column, until the particle eventually reaches the speed of the wakefield. Even higher energies can be reached by injecting the particle to travel across the face of the wakefield, much like a surferSurfing
Surfing' is a surface water sport in which the surfer rides a surfboard on the crest and face of a wave which is carrying the surfer towards the shore...
can travel at speeds much higher than the wave they surf on by traveling across it. Accelerators designed to take advantage of this technique have been referred to colloquially as "surfatron"s.
Comparison with RF Acceleration
The advantage of plasma acceleration is that its acceleration field can be much stronger than that of conventional radio-frequency (RF) 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...
. In RF accelerators, the field has an upper limit determined by the threshold for dielectric breakdown
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
of the acceleration tube. This limits the amount of acceleration over any given area, requiring very long accelerators to reach high energies. In contrast, the maximum field in a plasma is defined by mechanical qualities and turbulence, but is generally several orders of magnitude stronger than with RF accelerators. It is hoped that a compact particle accelerator can be created based on plasma acceleration techniques or accelerators for much higher energy can be built, if long accelerators are realizable with an accelerating field of 10 GV/m.
Plasma acceleration is categorized into several types according to how the electron plasma wave is formed:
- plasma wakefield acceleration (PWFA): The electron plasma wave is formed by an electron bunch
- laser wakefield acceleration (LWFA): A laser pulse is introduced to form an electron plasma wave.
- laser beat-wave acceleration (LBWA): The electron plasma wave arises based on different frequency generation of two laser pulses.
- self-modulated laser wakefield acceleration (SMLWFA): The formation of an electron plasma wave is achieved by a laser pulse modulated by stimulated Raman forward scatteringRaman scatteringRaman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
instability.
The concept of plasma acceleration was first proposed by Toshiki Tajima and John Dawson in a theoretical article published in 1979. The first experimental demonstration of wakefield acceleration, which was performed with PWFA, was reported by a research group at Argonne National Laboratory
Argonne National Laboratory
Argonne National Laboratory is the first science and engineering research national laboratory in the United States, receiving this designation on July 1, 1946. It is the largest national laboratory by size and scope in the Midwest...
in 1988.
Formula
The acceleration gradient for a linear plasma wave is:In this equation,
is the electric fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
,
is the speed of lightSpeed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in vacuum,
is the mass of the electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
,
is the plasma density (in particles per cube metre), and is the permittivity of free space.