Researchers from the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and the University of California, Los Angeles have demonstrated a new, efficient way to accelerate positrons, the antimatter opposites of electrons. The method may help boost the energy and shrink the size of future linear particle colliders – powerful accelerators that could be used to unravel the properties of nature’s fundamental building blocks.
The scientists had previously shown that boosting the energy of charged particles by having them “surf” a wave of ionized gas, or plasma, works well for electrons. While this method by itself could lead to smaller accelerators, electrons are only half the equation for future colliders. Now the researchers have hit another milestone by applying the technique to positrons at SLAC’s Facility for Advanced Accelerator Experimental Tests (FACET), a DOE Office of Science User Facility.
“Together with our previous achievement, the new study is a very important step toward making smaller, less expensive next-generation electron-positron colliders,” said SLAC’s Mark Hogan, co-author of the study published today in Nature. “FACET is the only place in the world where we can accelerate positrons and electrons with this method.”
Simulation of high-energy positron acceleration in an ionized gas, or plasma – a new method that could help power next-generation particle colliders. The image shows the formation of a high-density plasma (green/orange color) around a positron beam moving from the bottom right to the top left. Plasma electrons pass by the positron beam on wave-like trajectories (lines). (W. An/UCLA)
Future particle colliders will require highly efficient acceleration methods for both electrons and positrons. Plasma wakefield acceleration of both particle types, as shown in this simulation, could lead to smaller and more powerful colliders than today’s machines. (F. Tsung/W. An/UCLA/SLAC National Accelerator Laboratory)
Nature - Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield
Shrinking Particle Colliders
Researchers study matter’s fundamental components and the forces between them by smashing highly energetic particle beams into one another. Collisions between electrons and positrons are especially appealing, because unlike the protons being collided at CERN’s Large Hadron Collider – where the Higgs boson was discovered in 2012 – these particles aren’t made of smaller constituent parts.Abstract
“These collisions are simpler and easier to study,” said SLAC’s Michael Peskin, a theoretical physicist not involved in the study. “Also, new, exotic particles would be produced at roughly the same rate as known particles; at the LHC they are a billion times more rare.”
However, current technology to build electron-positron colliders for next-generation experiments would require accelerators that are tens of kilometers long. Plasma wakefield acceleration is one way researchers hope to build shorter, more economical accelerators.
Previous work showed that the method works efficiently for electrons: When one of FACET’s tightly focused bundles of electrons enters an ionized gas, it creates a plasma “wake” that researchers use to accelerate a trailing second electron bunch.
Electrical breakdown sets a limit on the kinetic energy that particles in a conventional radio-frequency accelerator can reach. New accelerator concepts must be developed to achieve higher energies and to make future particle colliders more compact and affordable. The plasma wakefield accelerator (PWFA) embodies one such concept, in which the electric field of a plasma wake excited by a bunch of charged particles (such as electrons) is used to accelerate a trailing bunch of particles. To apply plasma acceleration to electron–positron colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas. Although substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch, such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFAs where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered—‘self-loaded’—so that about a billion positrons gain five gigaelectronvolts of energy with a narrow energy spread over a distance of just 1.3 meters. They extract about 30 per cent of the wake’s energy and form a spectrally distinct bunch with a root-mean-square energy spread as low as 1.8 per cent. This ability to transfer energy efficiently from the front to the rear within a single positron bunch makes the PWFA scheme very attractive as an energy booster to an electron–positron collider.
Plasma acceleration
From Wikipedia, the free encyclopedia
Plasma acceleration is a technique for accelerating charged particles, such as electrons, positrons and ions, using an electric field associated with electron plasma wave
or other high-gradient plasma structures (like shock and sheath
fields). The plasma acceleration structures are created either using
ultra-short laser
pulses or energetic particle beams that are matched to the plasma
parameters. These techniques offer a way to build high performance particle accelerators
of much smaller size than conventional devices. The basic concepts of
plasma acceleration and its possibilities were originally conceived by Toshiki Tajima and Prof. John M. Dawson of UCLA in 1979.[1] Initial designs of experiment for "wakefield" were conceived at UCLA by the group of Prof. Chan Joshi.[2] Current experimental devices show accelerating gradients several orders of magnitude better than current particle accelerators.
Plasma accelerators have immense promise for innovation of affordable and compact accelerators for various applications ranging from high energy physics to medical and industrial applications. Medical applications include betatron and free-electron light sources for diagnostics or radiation therapy and protons sources for hadron therapy. Plasma accelerators generally use wakefields generated by plasma density waves. However, plasma accelerators can operate in many different regimes depending upon the characteristics of the plasmas used.
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),[3] and one at the SLAC conventional accelerator (highest electron energy accelerator) requires 64 m to reach the same energy. Similarly, using plasmas an energy gain of more than 40 GeV was achieved using the SLAC SLC beam (42 GeV) in just 85 cm using a plasma wakefield accelerator (8.9x1020 gn).[4] Once fully developed, the technology could replace many of the traditional RF accelerators currently found in particle colliders, hospitals and research facilities.
The Texas Petawatt laser facility at the University of Texas at Austin accelerated electrons to 2 GeV over about 2 cm (1.6x1021 gn).[5] This record was broken (by more than 2x) in 2014 by the scientists at the BELLA (laser) Center at the Lawrence Berkeley National Laboratory, when they produced electron beams up to 4.25 GeV.[6]
In late 2014, researchers from SLAC National Accelerator Laboratory using the Facility for Advanced Accelerator Experimental Tests (FACET) published proof of the viability of plasma acceleration technology. It was shown to be able to achieve 400 to 500 times higher energy transfer compared to a general linear accelerator design. [7][8]
Plasma accelerators have immense promise for innovation of affordable and compact accelerators for various applications ranging from high energy physics to medical and industrial applications. Medical applications include betatron and free-electron light sources for diagnostics or radiation therapy and protons sources for hadron therapy. Plasma accelerators generally use wakefields generated by plasma density waves. However, plasma accelerators can operate in many different regimes depending upon the characteristics of the plasmas used.
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),[3] and one at the SLAC conventional accelerator (highest electron energy accelerator) requires 64 m to reach the same energy. Similarly, using plasmas an energy gain of more than 40 GeV was achieved using the SLAC SLC beam (42 GeV) in just 85 cm using a plasma wakefield accelerator (8.9x1020 gn).[4] Once fully developed, the technology could replace many of the traditional RF accelerators currently found in particle colliders, hospitals and research facilities.
The Texas Petawatt laser facility at the University of Texas at Austin accelerated electrons to 2 GeV over about 2 cm (1.6x1021 gn).[5] This record was broken (by more than 2x) in 2014 by the scientists at the BELLA (laser) Center at the Lawrence Berkeley National Laboratory, when they produced electron beams up to 4.25 GeV.[6]
In late 2014, researchers from SLAC National Accelerator Laboratory using the Facility for Advanced Accelerator Experimental Tests (FACET) published proof of the viability of plasma acceleration technology. It was shown to be able to achieve 400 to 500 times higher energy transfer compared to a general linear accelerator design. [7][8]
Contents
Concept
A plasma consists of fluid of positive and negative charged particles, generally created by heating or photo-ionizing (direct / tunneling / multi-photon / barrier-suppression) a dilute gas. Under normal conditions the plasma will be macroscopically neutral (or quasi-neutral), an equal mix of electrons and ions in equilibrium. However, if a strong enough external electric or electromagnetic field is applied, the plasma electrons, which are very light in comparison to the background ions (at least by a factor of 1836), will separate spatially from the massive ions creating a charge imbalance in the perturbed region. A particle injected into such a plasma would be accelerated by the charge separation field, but since the magnitude of this separation is generally similar to that of the external field, apparently nothing is gained in comparison to a conventional system that simply applies the field directly to the particle. But, the plasma medium acts as the most efficient transformer (currently known) of the transverse field of an electromagnetic wave into longitudinal fields of a plasma wave. In existing accelerator technology various appropriately designed materials are used to convert from transverse propagating extremely intense fields into longitudinal fields that the particles can get a kick from. This process is achieved using two approaches: standing-wave structures (such as resonant cavities) or traveling-wave structures such as disc-loaded waveguides etc. But, the limitation of materials interacting with higher and higher fields is that they eventually get destroyed through ionization and breakdown. Here the plasma accelerator science provides the breakthrough to generate, sustain, and exploit the highest fields ever produced by science in the laboratory.What makes the system useful is the possibility of introducing waves of very high charge separation that propagate through the plasma similar to the traveling-wave concept in the conventional accelerator. The accelerator thereby phase-locks a particle bunch on a wave and this loaded space-charge wave accelerates them to higher velocities while retaining the bunch properties. Currently, plasma wakes are excited by appropriately shaped laser pulses or electron bunches. Plasma electrons are driven out and away from the center of wake by the ponderomotive force or the electrostatic fields from the exciting fields (electron or laser). Plasma ions are too massive to move significantly and are assumed to be stationary at the time-scales of plasma electron response to the exciting fields. As the exciting fields pass through the plasma, the plasma electrons experience a massive attractive force back to the center of the wake by the positive plasma ions chamber, bubble or column that have remained positioned there, as they were originally in the unexcited plasma. This forms a full wake of an extremely high longitudinal (accelerating) and transverse (focusing) electric field. The positive charge from ions in the charge-separation region then creates a huge 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 (in self-injection mechanism). In the external bunch injection schemes the electrons are strategically injected to arrive at the evacuated region during maximum excursion or expulsion of the plasma electrons.
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.
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.
Wake created by an electron beam in a plasma
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) accelerators. 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:
- 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. The "Surfatron" is an improvement on this technique.[9]
- self-modulated laser wakefield acceleration (SMLWFA): The formation of an electron plasma wave is achieved by a laser pulse modulated by stimulated Raman forward scattering instability.
Formula
The acceleration gradient for a linear plasma wave is:
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. Dielectric wall accelerator
From Wikipedia, the free encyclopedia
Patent sketch for the DWA
Possible uses of this concept include its application in external beam radiotherapy (EBRT) using protons or ions.
Contents
Operation
An external alternating-current power supply provides an electromagnetic wave that is transmitted to the accelerator tube using a waveguide. The power supply is switched on only a very short time (pulsed operation).[2]Electromagnetic induction creates a traveling electric field, which accelerates charged particles. The traveling wave overlaps with the position of the charged particles, leading to their acceleration inside as they pass through the tube's vacuum channel.[2] The field inside the tube is negative just ahead of the proton and positive just behind the proton. Because protons are positively charged, they accelerate toward the negative and away from the positive. The power supply switches the polarity of the sections, so they stay synchronized with the passing proton.
Construction
The accelerator tube is made from sheets of fused silica, only 250 µm thick. After polishing, the sheets are coated with 0.5 µm of chromium and 2.5 µm of gold. About 80 layers of the sheets are stacked together, and then heated in a brazing furnace, where they fuse together. The stacked assembly is then machined into a hollow cylinder. Fused silica is pure transparent quartz glass, a dielectric, which is why the machine is called a "dielectric wall accelerator."A sketch of one of the assembled modules of the accelerator is shown in the patent sketch. The module is about 3 cm long, and the beam traveles upward. The dielectric wall is seen as item number 81. It is surrounded by a pulse forming device called a Blumlein. In figure 8A, the power supply charges the Blumlein. In figure 8B, silicon carbide switches surrounding the Blumlein close, shorting out the edge of the Blumlein. The energy stored in the Blumlein rushes toward the dielectric wall as a high voltage pulse.
Usage in Proton Therapy
The dose produced by a native and by a modified proton beam in passing
through tissue, compared to the absorption of a photon beam
Protons are many times heavier than electrons, which makes them easier to control and allows them to more precisely target a tumor. Protons are charged particles and are accelerated to a predetermined energy level which, using the bragg peak delivers the energy directly within 1–2 mm inside the tumor and stops. IMRT photons primarily indirectly ionizes the single strand DNA nuecleotide using the free radical method to disrupt cell life. Tumors are notorious for having a poor blood supply called cell (hypoxia). This requires the use of drugs to oxygenate the tumor. Since the tumor is fast growing it has a poor blood supply meaning any drugs directed at them will have difficulty getting to them.
Advantages and Limitations
The DWA addresses the main issues with the current proton therapy systems—cost and size Video. Depending on the desired final beam energy, the conventional medical accelerator solutions (cyclotrons and small synchrotrons) can have large cost factors and space requirements, which could be circumvented by DWAs. The cost estimate for a DWA is about 20 million US dollars.[citation needed]DWAs are expected to reach acceleration gradients around 100 MV/m.[2]
The system is a spinoff of a DOE device to inspect nuclear weapons. This system requires several new advances because of the high energies, like e.g. high gradient insulators.[4] A wide band-gap photoconductive switch, about 4,000 are needed. A Symmetric Blumlein, typical width 1mm.
Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield
- Nature
- 524,
- 442–445
- doi:10.1038/nature14890
- Received
- Accepted
- Published online
Electrical
breakdown sets a limit on the kinetic energy that particles in a
conventional radio-frequency accelerator can reach. New accelerator
concepts must be developed to achieve higher energies and to make future
particle colliders more compact and affordable. The plasma wakefield
accelerator (PWFA) embodies one such concept, in which the electric
field of a plasma wake excited by a bunch of charged particles (such as
electrons) is used to accelerate a trailing bunch of particles. To apply
plasma acceleration to electron–positron colliders, it is imperative
that both the electrons and their antimatter counterpart, the positrons,
are efficiently accelerated at high fields using plasmas1.
Although substantial progress has recently been reported on high-field,
high-efficiency acceleration of electrons in a PWFA powered by an
electron bunch2,
such an electron-driven wake is unsuitable for the acceleration and
focusing of a positron bunch. Here we demonstrate a new regime of PWFAs
where particles in the front of a single positron bunch transfer their
energy to a substantial number of those in the rear of the same bunch by
exciting a wakefield in the plasma. In the process, the accelerating
field is altered—‘self-loaded’—so that about a billion positrons gain
five gigaelectronvolts of energy with a narrow energy spread over a
distance of just 1.3 metres. They extract about 30 per cent of the
wake’s energy and form a spectrally distinct bunch with a
root-mean-square energy spread as low as 1.8 per cent. This ability to
transfer energy efficiently from the front to the rear within a single
positron bunch makes the PWFA scheme very attractive as an energy
booster to an electron–positron collider.
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