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Virtual tour of the E157 experiment

[ Why ] [ High Energy ] [ High accelerating gradient ] [ E-157 experimental concept ] [ Location ] [ Plasma chamber ] [ Beam-plasma interaction ] [ Energy gain/loss ] [ Plasma focusing ] [ Summary ]

WHY   In the last 50 years particle accelerators have become important tools to experimentally study the basic constituents of matter (elementary particles) and the fundamental laws of nature. Many important discoveries revolutionized our understanding of the nature of the universe and spawned many technological applications. Though tremendous progress has been made, our present theory of the physical world is not complete. In order to experimentally pursue the quest for the grand unified theory ever more powerful accelerators are needed. As present accelerator technologies start to reach their limits, the E-157 experiment aims at studying and extending a new plasma-based accelerator technology. [ top ]

HIGH ENERGY     An accelerator collects a large number (billions) of well-known elementary particles (e.g. electrons or positrons or protons, ...) into a limited volume, called a "bunch" of particles. One or several bunches constitute the particle beam that is being accelerated. In the process of acceleration the energy of every particle in the beam is increased tremendously. The final beam energy of the accelerator defines its physics potential. Due to Einstein's famous formula E = m c2, a particle with mass m has the energy E, with c just being the light velocity. Present accelerators explore particle masses of up to about 150 GeV/c2. A new particle is produced, for example, if an electron and positron, both with an accelerated energy of 45.6 GeV collide and convert (annihilate) into a Z-Boson with a mass of 91.2 GeV/c2. In order to advance science into unknown regions beyond our present knowledge, accelerators with a larger final beam energy are required. [ top ]

HIGH ACCELERATING GRADIENT     The accelerating gradient in an accelerator describes the energy gain per acceleration length. A given technology provides some maximum gradient. Neglecting more subtle problems and considering a linear (straight) accelerator, a higher final beam energy could be achieved by building a longer accelerator. However, this approach faces practical limits in space and cost. Plasma-based acceleration potentially provides accelerating gradients of up to 100 GeV/m. This is more than 1000 times higher than what is achieved with conventional technology. Proof-of-principle experiments have demonstrated those gradients over mm-lengths. If plasma-based accelerator technology can be extended into the meter range with a well-controlled beam-plasma interaction, a new generation of short and ultra-powerful accelerators will become available. [ top

Schematic Layout of the E-157 Plasma Wakefield Acceleration Experiment
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E-157 EXPERIMENTAL CONCEPT     The E-157 experiment aims at using the existing SLAC facilities to study plasma wakefield acceleration at the forefront of advanced accelerator research. The limitations of the existing SLAC facilities limit the achievable accelerating gradient to about 1 GeV/m. However, the length of acceleration will be 1 m, surpassing previous experiments by 2-3 orders of magnitude. As an appropiate sequence of driver and witness bunch can presently not be obtained from the SLAC linac, a single bunch is used to both drive wakefields and to witness their accelerating effect. Therefore the experiment does not provide a useful accelerated beam. However, it will address the questions of both long acceleration length and control of beam quality. We hope that the results of this and other experiments will ultimately allow to design and engineer an actual 1 m long plasma acceleration module, that provides a beam with high energy and good quality. [ top
Schematic layout and photograph of SLAC facilities
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LOCATION     The E-157 experiment will use the existing SLAC facilities with only minor modifications. It is going to be installed in the Final Focus Test Beam (FFTB) facility. The experiment will be performed with the high energy, high power electron beam from the 3 km long SLAC linac. The schematic layout of the used facilities and a photograph of the relevant SLAC area is shown to the right. The E-157 experiment will occupy approximately a 3 m space in the FFTB beamline at IP-1, replacing the present E-144 (non-linear Compton scattering) setup and two weak magnets. [ top ]

PLASMA CHAMBER     A 1 m long plasma chamber is filled with Lithium gas. Since Lithium is a solid at room temperature a few grams of Lithium are heated to about 600 degrees Celsius to produce the required gas with about 100000 billion atoms per cubic cm. The following picture shows a 0.5 m prototype of the Lithium plasma chamber at a UCLA lab. 

Photograph of a 0.5 m long plasma chamber prototype at a UCLA lab.
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    A laser with the right photon energy is sent through the Lithium gas to fully singly ionize the gas and to produce the plasma. The plasma contains an equal number of electrons and positively charged ions. Because of their smaller mass the plasma electrons are much more mobile than the heavy ions. 
Principle of beam-driven plasma wakefield acceleration
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BEAM - PLASMA INTERACTION     Once the plasma is established, a 0.6 mm long bunch of 40 billion electrons is sent into the plasma with light velocity. The electrons are confined to a small area with a radius of about 0.04 mm; then the bunch density is about ten times larger than the plasma density. The space charge of the electron bunch expels the plasma electrons (-), driving oscillations of plasma electrons as they experience the restoring force of the plasma ions (+). The longitudinal density modulations of plasma electrons excite strong accelerating fields. Those fields can produce acceleration orders of magnitudes larger than acceleration in conventional accelerating structures. [ top
Simulated change of energy in SLC bunch
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ENERGY GAIN / LOSS     Parts of the SLAC bunch are decelerated, the electrons loose energy to the plasma electrons. However, electrons in the tail of the bunch experience acceleration of up to 0.5-2.0 GeV, depending on the actual bunch length. A simulation for a conservative SLC bunch length of 0.6 mm is shown below. The longitudinal electron distribution is shown in light blue. Beam deceleration (energy loss) is indicated by green, acceleration (energy gain) by red. The beam acceleration is evaluated in 1 ps bins, resulting in the energy spread per slice as indicated by the dashed line. We note that maximum beam acceleration will only affect a small part of the beam outside of 3 standard deviations. This imposes a challenge for measuring purposes. A different choice of plasma density will change the number of accelerated particles with a given acceleration. [ top
Simulated beam size oscillations in plasma cell
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PLASMA FOCUSING     The injected electron bunch does not only experience large acceleration but also super-strong focusing of several thousand Tesla per m. As a result the injected SLAC beam with a radial size of about 0.04 mm will be focused to a much smaller beam size, leading to periodic oscillations in beam size. If the bunch is extracted at minimum beam size, it's size will grow rapidly downstream. It is best extracted at maximum beam size, allowing for the best beam diagnostic downstream. [ top ]

SUMMARY     The experiment E-157 will carefully study beam-plasma interaction over lengths that are more than 100 times beyond those realized today. We will measure ultra-high gradient acceleration and beam focusing. Due to the limitations of the SLAC beam, the experiment does not provide a useful accelerated beam. However, it addresses the important questions of how to extend plasma wakefield acceleration to the meter scale and how to preserve the beam quality. We hope that the results of this and other experiments will allow to design and engineer an actual 1 m long plasma acceleration module, that provides a beam with high energy and good quality. Such a module will certainly find a broad range of applications in basic and applied research. [ top

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Last updated: November 08, 1997.