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Energy Recovery Linear Accelerator

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This photocathode gun, which is part of the ERL injector, is where the electrons are created.

X-ray beams from charged particle accelerators have become an essential tool in today's investigation of all types of materials, from airplane wings to cell membranes and from pollutants in leaves to matter under earth-core pressures. The development of a new type of accelerator, called Energy Recovery Linear accelerator or ERL, envisioned and invented at Cornell, that provides more brilliant beams in shorter pulses will move such investigations to new frontiers.

The ERL proposed to be built at Cornell will serve as an upgrade to the Cornell High Energy Synchrotron Source (CHESS) and will be the first-ever ERL x-ray source.

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Introduction to the ERL

Basic principle of operation

Electro-magnetic fields in microwave cavities can accelerate as well as decelerate electrons. In an ERL, electrons are injected in these cavities at the proper time for acceleration, which produces a high energy electron beam. This electron beam is then forced to oscillate to the right and left of their mean path in an arrangement of many horse-shoe magnets of alternating orientation called an undulator. As a consequence of this oscillation, X-rays are produced and the beam loses about 0.1% of its energy. The 99.9% of its remaining energy can be recaptured into the electro-magnetic fields when the electrons are re-injected into the linac at the proper time for deceleration. Subsequently this energy is available for the acceleration of another bunch of electrons.

Every bunch of electrons therefore first traverses the linac for acceleration, then a return loop containing undulators leads it from the end of the linac back to its beginning, and finally the bunch traverses the same linac for deceleration. While a conventional storage-ring X-ray source recycles electrons at high energy billions of times, unfortunately compromising the beam size, the ERL thus sends each bunch of electrons with its very small size through the undulators only once, but it reuses its energy multiple times. The very small beamsize is key to moving materials investigations to new frontiers.

Because the energy of the electrons is periodically taken from and then put back into the electro-magnetic fields in microwave cavities, these cavities must constantly be operated. Conventional cavities made of some conducting material, e.g. copper, cannot be operated constantly with high fields since they would become too hot. This is the reason why the ERL will employ novel superconducting microwave cavities which are cooled to -456 Fahrenheit to produce hardly any heat when operated continuously at high fields.

Currently a prototyping facility is being set up to show that the desired very small beam sizes can be produced and injected into an ERL.

ERL plans at Cornell

The design of the Cornell X-ray ERL should be made cost efficient by reusing as much infrastructure of the existing CESR ring as beneficial. The operation of CHESS should be disrupted as little as possible while building and commissioning the ERL, and the facility should provide space for a sufficient number of X-ray beam lines. While it could have turned out that reusing CESR imposes too many constraints, quite contrary it has been found that the flexibility of CESR's magnet arrangement holds several advantages for an ERL design. In order to extend the space for cavities, to make space for possible upgrades, and to minimize the impact on CHESS operation, the layout in the figure below has been devised.

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Layout of the existing CESR ring (6) and the ERL upgrade

Electrons from an injector that is optimized for very narrow and short electron pulses (1) would be accelerated to the right in a first linear accelerator or linac (2). A return loop (3) would send them into a second linac which is located in the same straight tunnel (4) and accelerates to the final high energy. An arc (5) injects the electrons into the already existing CESR ring (6) where they travel clockwise until another arc (7) injects them back into the first linac, where they are decelerated to half their energy. The return loop leads the electrons to the second linac section where they are decelerated back to their low injection energy with which they are finally dumped (8).

The South half of the CESR tunnel would contain undulators and would reuse the current facilities of CHESS. Additionally, new user areas could be created in the North section of CESR (at the top of the figure) and in straight sections of the linac tunnel. The location of the linac at a hillside is chosen in such a way that no existing building foundations interfere and that X-ray beam lines with easy access can be added between the linac and CESR. A return arc is also shown which connects the arcs (5) and (7) so that electrons can return to the linacs after acceleration without passing through CESR. This connection has been chosen so that the ERL could be built and commissioned while CESR is still used as a storage ring x-ray source. Another advantage of this upgrade plan is that most of the CESR tunnel is reused, which creates space for a large number of insertion devices.

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Sketch of the cross-section of the tunnel containing the two linacs

The tunnel extension shown in the figure above has a section of 250 m with two linacs side by side. A sketch of a possible tunnel cross-section is shown in the adjacent figure. A straight tunnel housing two linacs, reduces tunnel cost as well as the required length of cryogenic lines and cables. The tunnel is laid out longer than required for the two linacs, so that an extension of the facility by extra undulators or by a Free Electron Laser can upgrade the facility at a later time.

Want to learn more? This PDF file has more details.